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eHam Forums => Mobile Ham => Topic started by: ZENKI on April 04, 2014, 01:32:02 AM



Title: Ground Breaking Mobile Antenna Articles in QEX
Post by: ZENKI on April 04, 2014, 01:32:02 AM
For those interested in the performance and design of mobile antennas you should read the ground breaking mobile antenna articles in QEX  in the last 2 Issues. The articles are by Barry W9UCW. These articles will surely test and challenge your understanding about the many technical parameters that makes a  very efficient mobile antenna. The articles covers  coil current, coil Q, capacity  hat positioning, coil current, coil location and everything else related to mobile antenna performance. It certainly will challenge the very firm views that some have about mobile antennas like helical whips and other low Q antennas. The data is supported by field strength measurements and I am sure the conclusions will be controversially debated. Some of the best data that I have seen  for mobile antennas and ground loss and antenna efficiency.



Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 04, 2014, 06:00:52 AM
For those interested in the performance and design of mobile antennas you should read the ground breaking mobile antenna articles in QEX  in the last 2 Issues.

For those of us who don't subscribe to QEX, could you list a few of the "ground breaking" concepts that are not reflected in the 1980's 75m mobile shootout results at the bottom of the following web page?

http://w5dxp.com/shootout.htm


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: K0BG on April 04, 2014, 06:31:05 AM
I beg to differ with you, as the article has a whole lot of errors, not the least of which is the way the measurements were done.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: ZENKI on April 04, 2014, 05:02:35 PM
1. High Q coils did not outperform a LOW Q coil in field strength tests.(no difference!)
2. Coil positioning or coil distance from the base had more impact on efficiency
3. Antenna matching  with a antenna tuner  from  the resonant matched  tuning had a major impact on gain.
4.  A helical whips  performance on both 80 meter and  20 meters could perform better than  a poorly positioned coil or a based loaded antenna.
5.  Length of the mast below the coil had a huge impact on performance.
6. Size of the  vehicle  has the most impact on ground resistance
7. Multiple resonators  had no impact on performance.
8. Magnet mounts  reduced field strength  significantly. If you do use a mag mount use as many magnets as possible.(Mag mount is -2db below direct car connection 14mhz) 80 meters was  4db down.
9. Cap hats and coil metal ends had little impact on field strength
10. No field strength advantage to using  cap hat loading only over a coil loaded vertical
11. Keeping a cap hat wires horizontal is important
12. Alternate coil loading like ferrite shows promising results Ferrite loaded antenna  was 1.3db down from a high q coil antenna!
13. Base matching is important for performance
14. Coil current measurements were performed! "Current tapers  from the bottom to the top of the loading coil"
          Coil Q had little impact on this current taper
          current taper varies depending on what section of the 1/4  wave is being replaced
          There is no link between coil Q and current taper
           The coil current conclusions  are numerous and cant be summarized here that will do the article justice.
I am sure the claims will be very well debated!
15. Measured Ground resistance data provided
16. Reactance and capacitance  data provided for various mag mounts

I probably have left out critical data  from the article which should be read to confirm the above summary of the conclusions by the author.
The articles are however well worth reading especially since field strength data was made.



For those interested in the performance and design of mobile antennas you should read the ground breaking mobile antenna articles in QEX  in the last 2 Issues.

For those of us who don't subscribe to QEX, could you list a few of the "ground breaking" concepts that are not reflected in the 1980's 75m mobile shootout results at the bottom of the following web page?

http://w5dxp.com/shootout.htm



Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: ZENKI on April 04, 2014, 05:06:45 PM
There well maybe  errors. You are a respected author, and I am sure QEX would welcome  any corrections. QEX is peer reviewed.
I would not be surprised if there is a flood of letter to  the editors in next months QEX.

I and many hams  are interested in the subject matter and I am sure your views on how to do measurements correctly would be of interest to many.
I beg to differ with you, as the article has a whole lot of errors, not the least of which is the way the measurements were done.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: KB4QAA on April 04, 2014, 05:40:53 PM
Wow, Zenki didn't even complain about IMD performance!  :)


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: ZENKI on April 04, 2014, 09:41:16 PM
Thanks for reminding me about IMD.

The question and importance of minimizing IMD  when installing mobile radios and amplifiers should be taken seriously.
The  most critical aspect for mobile installations is having a stiff DC supply and using a radio that has good inherent  IMD performance.
Its good to note that the IMD performance of  the Icom 7100 has improved over its predecessors. Icom should be congratulated for improving the transmit performance of their radios.

Choosing a well designed legal amp would be a wise choice. Unfortunately the message that intelligent hams should not use illegal CB amplifiers with poor IMD performance has to be rammed home. Listening to these dumb CB operators on the ham bands continuing their use of substandard equipment that causes splatter  its clear that there are number of hams who dont want to change their filthy operating styles and equipment choices. 

At the end of the day splatter is QRM just like power line noise and  other EMC threats that  ruins the enjoyment of the ham radio bands. If you want to be a idiot and continue to promote and encourage moronic equipment choices you are welcome to do so. Maybe you can tell the ham world why deliberate generated QRM and more noise and interference is good thing for the ham bands.  Improved and enforced IMD performance standards would be a wonderful precedent for the ham radio service.  It would help if you would ask the ARRL to test the transmitters with the same thoroughness that they review receivers. Its only when hams realize that they being short changed by buying expensive radios with excellent receivers and very poor transmitters that hams might start asking questions. We could have ZERO splatter on the ham bands and there is no technical reasons why this should not be possible. Maybe one day Elecraft and Icom will start using pre-distortion technology in their radios  which would contribute to the minimization of IMD. This would be a good choice for most ham manufacturers since they  are not smart enough to design a transmitter without ALC faults and cant seem to design a basic PA that has  very good IMD performance.

I am always interested in opposing views. Maybe you can  enlighten hams why we should have the same standards as the CB bands and use equipment  and amplifiers that routinely causes 30khz worth of splatter and spews spurious products  as high as 15 meters. I would like to try and understand the superiority of  this CB level engineering that hams seem to be proud and confident of.  I would really like to know how I can copy weak through this mess, does the splatter enhance weak signal reception?javascript:void(0);
Wow, Zenki didn't even complain about IMD performance!  :)


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: KM3F on April 04, 2014, 10:00:17 PM
Not that this is of any real value;  my mobile antenna is a dummy load by many accounts, is a old system, a  Hussler, all stock with 40m load coil.
Mounted on a custom made mount welded to the Receiver hitch and trailer  ball insert, grounded to the frame with copper braid.
I did this so I can tow a trailer and tip the antenna over to open the tail gate without losing the use of the antenna and remove it by pulling the hitch mount pin If I must do heavy loading/unloading in the box.
The antenna will come off either by the whole hitch insert or separately.
The truck is  Ford F150 Super Crew
For a dummy load it sure loses some of the 100 watt power to the air when I can work DX into Europe in the evenings and NVIS in the daytime.
Until I can find something to replace it with worth changing to, it stays as is..
Good luck.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: ZENKI on April 05, 2014, 04:44:08 AM
I work more hams running so called "dummy load" mobile antennas than  screwdrivers. I am also in this camp using a compromise mobile antennas because  I have no intention or desire to install a heavy and clumsy screwdriver. I chose my bands and my antennas and I know what efficiency I can get away with. I enjoy  mobile operation on 20 and 40 meters a lot and I dont feel the need or desire to install a  big massive screwdriver. If you pay attention to finer details even a  humble helical whip can  deliver minimum acceptable performance. The article  mentioned above has provided some data that supports my anecdotal observations and likewise shootouts that  have proven that a well installed  simple mobile can get close or equal to the big heavy metal screwdrivers. While I do envy  those that have these perfect big screwdriver installations. Its just not going to be the reality  in many parts of the world where much smaller cars and design standards regulation restrict what you can attach to the car. There are laws that protect pedestrians from harm from any protrusion that can harm them and which impacts on the cars safety systems. There is also the problem  that  small  hatches and other compact cars are becoming the norm and SUV and 4 wheel drives are  small segment of the market  which is the exact opposite of the US car market.

There are many hams working DX on 160 meters with  antennas with less than 1% efficiency, you should not feel bad about running an antenna that has got  something above 50% efficiency especially when you are getting the result. Theres no arguing with K0BG's philosophy about  there being only one right in mobile  installation methods. However this reality also confronts us in normal ham operation. We dont all run 200 ft towers with stacked yagis and  those stations with a hexbeam on a 20ft pole have just as much ham radio fun as the guy with the 200 ft tower. I always believe in the science which is always right however we are hams and have to be pragmatic about what we do and what we chose for antennas. The most important point is that we enjoy what we doing with what we have and dont need to have antenna envy syndrome.


Not that this is of any real value;  my mobile antenna is a dummy load by many accounts, is a old system, a  Hussler, all stock with 40m load coil.
Mounted on a custom made mount welded to the Receiver hitch and trailer  ball insert, grounded to the frame with copper braid.
I did this so I can tow a trailer and tip the antenna over to open the tail gate without losing the use of the antenna and remove it by pulling the hitch mount pin If I must do heavy loading/unloading in the box.
The antenna will come off either by the whole hitch insert or separately.
The truck is  Ford F150 Super Crew
For a dummy load it sure loses some of the 100 watt power to the air when I can work DX into Europe in the evenings and NVIS in the daytime.
Until I can find something to replace it with worth changing to, it stays as is..
Good luck.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 05, 2014, 10:17:53 AM
1. High Q coils did not outperform a LOW Q coil in field strength tests.(no difference!)

A 75m Texas Bugcatcher (high-Q coil) outperforms a 75m hamstick (low-Q coil) by about 10 dB - measured results.

Quote
2. Coil positioning or coil distance from the base had more impact on efficiency

Already known for decades that center loading is better than base loading.

Quote
5.  Length of the mast below the coil had a huge impact on performance.

Already known for decades because that's where the high current area exists.

Quote
6. Size of the  vehicle  has the most impact on ground resistance

Already known for decades

Quote
8. Magnet mounts  reduced field strength  significantly.

Already known for decades

Quote
9. Cap hats and coil metal ends had little impact on field strength

In the measured CA mobile 75m shootouts, a good capacity hat increased the radiated signal by about 2 dB having essentially the same effect as lengthening the element below the coil.

Quote
14. Coil current measurements were performed! "Current tapers  from the bottom to the top of the loading coil"

It is apparent from The ARRL Antenna Book that "Current taper" in a shortened  loaded standing wave monopole is not well understood by the ARRL (or the average ham). In a high-Q loading coil, virtually all of the current taper is caused by the phase difference between the forward traveling wave and the reflected traveling wave on the standing-wave antenna. The total current is the phasor sum of the forward current and the reflected current and is primarily a standing wave. The purpose of the loading coil is to put the reflected wave in phase with the forward wave at the antenna feedpoint resulting in antenna resonance. Here's a representation of the forward and reflected current components:

(http://www.w5dxp.com/coil.gif)

If1 is the forward current flowing into the bottom of the loading coil from the feedpoint. Ir1 is the reflected current flowing out of the bottom of the loading coil back toward the feedpoint. If2 is the forward current current flowing out of the top of the loading coil toward the tip of the whip. Ir2 is the reflected current flowing into the top of the loading coil after being reflected from the tip of the whip.

Here's what the total current phasors look like assuming the antenna is base-loaded, the phase shift through the loading coil is 45 degrees, and for the purpose of simplicity, that the entire antenna, including the coil, is lossless.

(http://www.w5dxp.com/phasor.gif)

Even assuming the coil is lossless and the SWR on the standing wave antenna is infinite, the current taper still occurs. The total current at the top of the coil is 71% of the total current at the bottom of the coil as it is in the model that follows.

The current taper is not primarily a loss of current to radiation or losses. It is primarily a simple phasing function that occurs between the forward current wave and the reflected current wave on the standing-wave antenna. That's why it is known as a standing-wave antenna.The results are very similar whether we assume the system is lossless or not.

If we extend the whip above the coil by 1/4WL, the current taper will reverse itself resulting in a greater magnitude of total current at the top of the coil than at the bottom of the coil, i.e. apparently more current flowing out of the top of the coil than is flowing into the bottom of the coil. That should be enough to to convince one that current taper is primarily the result of phasing between the forward current and reflected current, not the result of losses or radiation. Here's an EZNEC model of both antennas.

(http://www.w5dxp.com/test316.GIF)

How can the current into the bottom of the coil on the long antenna be 1.3 amps while the current out of the top of the coil is 2.1 amps??? Doesn't that violate some current law??? Yes, in the lumped circuit model but NOT in the distributed network model.

Some well-known gurus have promoted the lumped circuit concept that the RF current into a coil is equal to the current out of the coil in magnitude and phase but that concept just doesn't work for distributed networks! A large 75m air-core mobile loading coil, e.g. a 75m Texas Bugcatcher coil, is a distributed network which is an appreciable percentage of a wavelength long (usually in the ballpark of 10% of a wavelength).

What we can correctly state is that the forward current into a loading coil is approximately equal in magnitude to the forward current out of the loading coil and the reflected current into that loading coil is approximately equal in magnitude to the reflected current out of the loading coil. Most of the current taper from end to end in a large air-core loading coil is the result of the respective phase shifts (delays) in the forward and reflected currents flowing through the coil, not the result of losses or radiation.

We can also correctly state that the phase shift in the total standing wave current from end to end in the coil is very close to zero. Anyone familiar with a standing wave knows that a pure standing wave doesn't change phase over each half-wavelength of wire (or loading coil). In the phasor diagram above, one can see that the phase of the total current at the bottom of the coil is the same as the phase of the total current at the top of the coil.



Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 05, 2014, 02:04:21 PM
Because of so much confusion and false information about mobile loading coils, I am starting a new thread over on the TowerTalk/Antennas Towers and More forum titled, "Errors in The ARRL Antenna Book".


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: KB4QAA on April 05, 2014, 02:27:13 PM
Quote
Thanks for reminding me about IMD.

Argh!  LOL


catching up....
Quote
Maybe you can  enlighten hams why we should have the same standards as the CB bands and use equipment  and amplifiers that routinely causes 30khz worth of splatter and spews spurious products  as high as 15 meters. I would like to try and understand the superiority of  this CB level engineering that hams seem to be proud and confident of.  I would really like to know how I can copy weak through this mess, does the splatter enhance weak signal reception?javascript:void(0

I have never implied nor stated that I endorse anything but upholding the highest technical standards!


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: KB4QAA on April 05, 2014, 03:26:56 PM
Beautiful diagrams and description, Cecil!


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 05, 2014, 06:56:40 PM
Beautiful diagrams and description, Cecil!

Thanks, it's from an article that I wrote more than 10 years ago.

http://w5dxp.com/current.htm


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: AC5UP on April 06, 2014, 08:42:31 AM
How can the current into the bottom of the coil on the long antenna be 1.3 amps while the current out of the top of the coil is 2.1 amps??? Doesn't that violate some current law??? Yes, in the lumped circuit model but NOT in the distributed network model.

Which then makes me wonder.... Could there be an advantage in winding a loading coil with a variable pitch?

Let the turns per inch vary, with the lower current segment having a tighter pitch than the higher current segment (or vice versa). I want to say I've seen this in practice on the 'Kay-Dub' CB whips but I won't swear to it under oath...


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W8JI on April 07, 2014, 06:51:51 AM
1. High Q coils did not outperform a LOW Q coil in field strength tests.(no difference!)


We all know that can't be correct.
Quote
2. Coil positioning or coil distance from the base had more impact on efficiency

Everyone already should know that.

Quote
3. Antenna matching  with a antenna tuner  from  the resonant matched  tuning had a major impact on gain.

That statement is unclear.

Quote
4.  A helical whips  performance on both 80 meter and  20 meters could perform better than  a poorly positioned coil or a based loaded antenna.

So?


Quote
5.  Length of the mast below the coil had a huge impact on performance.

Everyone knows that already.
Quote
6. Size of the  vehicle  has the most impact on ground resistance

Someone would expect otherwise?
Quote
7. Multiple resonators  had no impact on performance.

That isn't true as written.

Quote
8. Magnet mounts  reduced field strength  significantly. If you do use a mag mount use as many magnets as possible.(Mag mount is -2db below direct car connection 14mhz) 80 meters was  4db down.

Who would think otherwise?

Quote
9. Cap hats and coil metal ends had little impact on field strength

That isn't universally true, but it can be true in some cases.

Quote
10. No field strength advantage to using  cap hat loading only over a coil loaded vertical

That's been dis proven hundreds of times by many people.
 
Quote
11. Keeping a cap hat wires horizontal is important

Why? Why can't they slope up?




Quote
12. Alternate coil loading like ferrite shows promising results Ferrite loaded antenna  was 1.3db down from a high q coil antenna!

This argument calls argument 1 a lie.
 
Quote
14. Coil current measurements were performed! "Current tapers  from the bottom to the top of the loading coil"
          Coil Q had little impact on this current taper
          current taper varies depending on what section of the 1/4  wave is being replaced
          There is no link between coil Q and current taper
           The coil current conclusions  are numerous and cant be summarized here that will do the article justice.
I am sure the claims will be very well debated!


The only way current can change is if the coil has displacement currents. It has NOTHING to do with with what amount of a 1/4 wave the coil replaces. That idea is just silly.

73 Tom



Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 07, 2014, 10:45:52 AM
The only way current can change is if the coil has displacement currents. It has NOTHING to do with with what amount of a 1/4 wave the coil replaces. That idea is just silly.

Tom, you and I agree on 99.9% of topics and I don't know what the QEX article said about current taper because I am in the act of borrowing those magazines from a fellow ham, have not read them yet, and therefore don't know the details. But it is obvious that if a coil is long enough it can support standing wave current loops and nodes. Otherwise, a helical antenna could not work. The question is: How long must a coil be before the current is not uniform as assumed in a lumped circuit analysis?

It is a well known technical fact that when the electrical length of an inductor results in non-uniform currents through the inductor that the limits of the lumped circuit model have been exceeded and we should switch to the distributed network model just as we do for wires and transmission lines that result in non-uniform currents through them. The distributed network model alleviates the need for the "displacement current" band-aid required by the lumped circuit model. (There are numerous technical papers indicating that displacement current was conjured up because of ignorance of the existence of photons.)

It appears to me that the logical mistake you are making is Petitio Principii, i.e. assuming the proof. You assume that the loading coils must obey the lumped circuit model and therefore they must. However, if the distributed network model is used (or Maxwell's equations) some interesting things appear that do exist in the real world but cannot exist in the lumped circuit model.

Taking the mobile (standing wave) antenna example from The ARRL Antenna Book, 20th edition, page 16-7 where FP is the feedpoint:

FP---15 deg base---+---loading coil---+---15 deg stinger---

We know that the antenna must be 1/4WL (90 deg) long electrically in order to put the reflected wave in phase with the forward wave to achieve resonance and that 90 degrees makes the antenna analysis a distributed network problem. How those phasing components are distributed along the 90 deg antenna is the question. We can do a similar thing with transmission lines to create a shortened 1/4WL (90 deg) stub which also requires a distributed network analysis. For simplicity, let's assume lossless lines with a velocity factor of 1.0.

FP---15 deg 50 ohm line---X---??? deg 450 ohm line---Y---15 deg 50 ohm line---

When we solve for the number of degrees occupied by the section of 450 ohm line in the center, we come up with a value of 20.8 degrees. So how can 15 degrees plus 20.8 degrees plus 15 degrees add up to 90 degrees? Answer: We have not taken all of the phase shifts into account. There is a negative phase shift at point X where the Z0 changes from 50 ohms to 450 ohms and there is a positive phase shift at point Y where the Z0 changes from 450 ohms to 50 ohms. So there is a total of five phase shifts which when added up equal 90 degrees. The same thing is true for the mobile antenna above.

A wire in space has a characteristic impedance of a few hundred ohms. If the wire is horizontal, we can use the single-wire transmission line equation to calculate that characteristic impedance. For instance, a #14 wire horizontal at a height of 30 ft has a characteristic impedance of 600 ohms. Again for simplicity, let's make the mobile antenna horizontal such that the base and stinger have a characteristic impedance of 500 ohms. We can use the Hamwaves inductance calculator to estimate the characteristic impedance of a 75m loading coil to be in the ballpark of 4500 ohms.

FP---15 deg base---+---??? deg loading-coil---+---15 deg 50 stinger---

One can see the similarity to the shortened stub above. All we have done is multiply the characteristic impedances by 10 so again the loading coil must occupy the same 20.8 degrees that it did in the shortened 1/4WL stub example. And again there are the two phase shifts at the impedance discontinuities that added into the other three phase shifts must add up to 90 degrees.

The Hamwaves inductance calculator is located at:

http://hamwaves.com/antennas/inductance.html

After entering the loading coil parameters, we obtain the characteristic impedance of the coil and beta, the axial propagation factor in radians/meter, for the coil. We take the axial propagation factor in radians/meter and multiply by 1.455 to get degrees/inch of delay in the coil. Multiplying degrees/inch by the length of the coil in inches gives the electrical length of the loading coil in degrees.

Tom, your picture used to be on that web page indicating that you wanted a more accurate inductance calculator but your picture and request seems to have been removed. Could the reason be that the more accurate inductance calculator uses the distributed network model for the coils which is incompatible with a lumped circuit analysis? That inductance calculator agrees in concept with the 75m Texas Bugcatcher loading coil measurements made at Louisiana Technical University.

http://w5dxp.com/coilmeas.htm


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W8JI on April 07, 2014, 02:20:45 PM
Cecil,

It is drag racing season, and I am behind on paying work (as usual) so I can't get into long things.

I have the QEX article, but I have not read it in detail. What I have gathered is that Barry seems to have an agenda that ALL loading coils, regardless of dimensions and external termination, behave like the missing section of a 1/4 wave.

I think everyone *sensible* agrees a perfect coil behaves as nothing but a lumped reactance and resistance in series. As the coil is made physically larger, or as shunting capacitance from windings to "ground" is added,  it behaves more and more like a transmission line. As a matter of fact the old TV set delay lines are just like long skinny inductors with a second bifilar winding to make them act like a transmission line. A helical antenna is another example that can be more like a transmission line than a loading coil.

A small dimension toroid with a lot less capacitance from windings to outside world than the whip or hat terminating it acts like a lumped component. I've measured them. They have almost perfectly the same (I can't measure the current change) on each terminal.

Depending on dimensions of what we measure, we can get almost any result we like. Barry obviously measured some stuff where hats did not impact field strength (contrary to hundreds of other measurements) and then summarized that hats NEVER affect FS.



There are probably 50 ways to convey this or express this, but many of the claims Zenki posted are just wrong. Here is some of the text. I can convert the article to plain text with a text scan converter.

Here is part of it:

Plain Language Conclusions:
1. The current tapers from the bottom to the top of loading coils used to resonate shorter than quarter wave length monopoles. The Q of the coil has little to no effect on the drop.

The amount of taper seems related to that portion of the quarter wave that has been replaced by the coil, but that is an over-simplification. The  reason  the  current  tapers,  other  than  a small  amount  of  conductor resistance and radiation, is that  the net current at any point is the “vector” sum of currents at that point. And, at any point along the monopole, or a series inductor,  there is a phase difference  between  the current coming from the source and the current reflected back from the open end or top of the monopole. The resultant net current is less as you move toward the open end of the monopole, where it is virtually zero, because at that end point,  the  forward  and  reflected  currents  are  equal  in  magnitude  and opposite in phase thus superposing to zero.

This information may answer the questions we had about the lack of impact of coil Q on field strength and the inability to confirm the published  formulas  to  “optimally”  locate  coils  in  the  mast.  It  may  also explain why capacity only loading is no better than top coil loading, all else remaining the same.


>>>>>>

If what Barry proposed above were true, a toroid would show significant current taper. It does not. Both Roy Lewallen and I have confirmed that.

There have to be displacement currents from the coil to space around the coil for there to be current taper. If there is no place to waves to stand, they will not stand.

73 Tom




 
 

     


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 07, 2014, 06:53:50 PM
What I have gathered is that Barry seems to have an agenda that ALL loading coils, regardless of dimensions and external termination, behave like the missing section of a 1/4 wave.

If you have read my previous postings you will see that you and I agree on a lot of points about what Barry is alleged to have said. I have not read the article so cannot comment yet about what Barry actually said.

Would you agree that in my previous shortened stub example that the 450 ohm line section in between the two 15 degree sections of 50 ohm line behaves like part of the missing section of the 1/4 wavelength stub?

Quote
I think everyone *sensible* agrees a perfect coil behaves as nothing but a lumped reactance and resistance in series.

That is a shortcut method that works for perfect (ideal) coils and near perfect coils. A 75m Texas Bugcatcher is nowhere near a perfect (ideal) coil. The Louisiana Tech grad students measured an electrical length of 41 degrees at 4 MHz. If you wouldn't use the lumped circuit model on a piece of transmission line that is 41 degrees long, why would you use the lumped circuit model on a 75m Texas Bugcatcher loading coil that is 41 degrees long?

The 1/4WL self-resonant frequency of a 75m Texas Bugcatcher loading coil is 8.2 MHz which means it is 90 degrees long at 8.2 MHz. Doesn't it make sense for it to be 41 degrees long at 49% that self-resonant frequency? It certainly doesn't make any sense at all to assert that it is 4.3 degrees long at half of 8.2 MHz where it is a 1/4WL helical monopole.

Quote
A small dimension toroid with a lot less capacitance from windings to outside world than the whip or hat terminating it acts like a lumped component.

I don't doubt it. My argument is that a humongous 75m air-core Texas Bugcatcher loading coil is obviously NOT a small dimension coil and certainly NOT a toroid so that argument is irrelevant.

Quote
If what Barry proposed above were true, a toroid would show significant current taper. It does not. Both Roy Lewallen and I have confirmed that.

Again, my argument is not with Barry because I am suffering from ignorance of what he wrote. Why do you and Roy hide behind small toroidal coils when the subject is large air-core 75m Texas Bugcatcher loading coils? When phase is important, anything longer than about 15 degrees requires a distributed network analysis for valid results. The toroidal coils that you guys tested are probably less than 15 degrees long but a 75m Texas Bugcatcher loading coil is certainly longer than 15 degrees on all HF frequencies.

Quote
There have to be displacement currents from the coil to space around the coil for there to be current taper.

That's only true if one presupposes the assumptions of the lumped circuit model. When the only tool one has is the lumped circuit model, all problems involving inductors look like lumped circuits even when they are nowhere near to being lumped circuits.:) Current taper in a standing wave antenna wire is caused primarily by the phasing between the forward current and the reflected current. The same is true for large loading coils. Do you know that the current phase shift over 30 degrees of a small wire dipole is in the ballpark of 1 degree? So why can't the phase shift for a 30 degree long coil mounted in that same dipole also be in the ballpark of 1 degree?


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 08, 2014, 06:38:46 AM
If there is no place (for) waves to stand, they will not stand.

This is one of those statements akin to, "All an antenna tuner does is make the transmitter happy." It is more of a myth than an accurate technical statement. Any wire that connects a source to a load has a physical length that prohibits RF signals from traveling faster than the speed of light which is a little less than one foot per nanosecond in free space. Even if the transmission line is one foot long, it takes time for the source signal to reach a mismatched load and during that time, there are no reflections. When the signal through that one foot of wire reaches the mismatched load after about one nanosecond, then and only then do reflections occur. During steady-state there is indeed a standing wave on any length of transmission line including short single-wire transmission lines over a ground plane. In the real world, there is no way to avoid a delay in a wire because RF signals cannot travel faster than the speed of light.

The problem is that the lumped circuit model presumes that electronic/electromagnetic signals travel faster than the speed of light without any real-world delay. If we use the lumped circuit model on a 75m Texas Bugcatcher loading coil, then we are presuming faster-than-light propagation of signals through the loading coil. That presumption is obviously false for all real world inductors. The reason that we get away with that presumption is that the delay through a small real world coil can be negligible and/or irrelevant so the lumped circuit model gives results close enough to reality to be a useful shortcut that avoids some messy mathematics.

It is when speed of light delays become non-negligible and therefore relevant that we need to discard the simple lumped circuit model in favor of the more complex distributed network model which is closer to Maxwell's equations. If both models yield results that are reasonably close to each other, we are free to use the lumped circuit model. If the two results are not reasonably close, the distributed network model results are always the more valid results. It is not simply two ways of looking at the same thing. It is often the choice between a valid model and an invalid model, e.g. the impedance transformation on a transmission line with reflections.

The number of electrical degrees occupied by a 75m mobile air-core loading coil is one of those cases where the results of a lumped circuit analysis differ drastically from the results of a distributed network analysis. Therefore, the results of the distributed network analysis is valid and the results of the lumped circuit analysis are invalid.

A free space distributed network analysis on a 75m Texas Bugcatcher loading coil indicates that it occupies 33.4 degrees at 4 MHz. University lab measurements on my real-world 75m Texas Bugcatcher loading coil indicate that it occupies 41 degrees at 4 MHz. The ~20% difference in those two values is explained by the real-world objects within the field of the coil during the measurements. The propagation delay through the loading coil will always be greater in the real world than it is in free space. Here's an article describing those Louisiana Tech U. measurements.

http://w5dxp.com/coilmeas.htm

Even with band-aid patches on the lumped circuit model, the electrical degrees occupied by the 75m Texas Bugcatcher coil comes out to be in the ballpark of 4 degrees. Since that value differs by a magnitude from the distributed network calculations and lab measurements, it must necessarily be an invalid value.

Depending upon its design, any real world 75m air-core mobile loading coil will occupy ~20-40 degrees of the mobile antenna. The base and stinger occupy some more of the antenna, 30 degrees in the previous example. The phase shifts caused by the impedance discontinuities at the bottom and top of the loading coil make up the remainder of the 90 degree resonant antenna.

Note: this discussion is about large 75m air-core mobile loading coils, NOT about small toroidal inductors. I don't know how many degrees a toroidal inductor occupies. It would be interesting to have a plot of propagation delay vs inductance for toroidal coils.

This is also not a condemnation of the lumped circuit model which is a very useful model. This is simply a request to recognize the system conditions that force us to switch models. One suggested rule of thumb is that when magnitudes are the only important consideration, anything longer than 60 degrees requires a switch to the distributed network model. However, if phase (and phase angles) are an important consideration, anything longer than 15 degrees requires a switch to the distributed network model. With transmission lines, its easy to follow that rule of thumb. With inductors, its not so easy to recognize when to switch.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: WS3N on April 08, 2014, 05:14:42 PM
The distributed network model alleviates the need for the "displacement current" band-aid required by the lumped circuit model. (There are numerous technical papers indicating that displacement current was conjured up because of ignorance of the existence of photons.)

I'm not sure what you're saying here. I doubt that you mean Maxwell was wrong to add the displacement current to Ampere's Law.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 08, 2014, 07:36:19 PM
I doubt that you mean Maxwell was wrong to add the displacement current to Ampere's Law.

Maxwell was not "wrong" at the time because photons had not yet been discovered and he was understandably ignorant of the fact that electromagnetic fields and waves consist of quantized photon particles. If he had known about photons and their properties, he would never have had to invent the concept of displacement current.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: WS3N on April 09, 2014, 04:39:20 AM
I doubt that you mean Maxwell was wrong to add the displacement current to Ampere's Law.

Maxwell was not "wrong" at the time because photons had not yet been discovered and he was understandably ignorant of the fact that electromagnetic fields and waves consist of quantized photon particles. If he had known about photons and their properties, he would never have had to invent the concept of displacement current.

What do photons have to do with it? Maxwell's equations would not "work" without the displacement current term. There would be no traveling-wave solutions.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 09, 2014, 05:33:20 AM
What do photons have to do with it?

http://photontheory.com/Kemp/Kemp.html

"Thus the displacement current will be called the photon current, because it too is quantized in nature."

"The displacement current or photon current is also shown to be equivalent to the energy of the photon; thereby making the displacement current the actual photon."

Quote
Maxwell's equations would not "work" without the displacement current term.

That's true, but if Maxwell had known about photons, he would not have named it "displacement" current. The way displacement current was explained to me at Texas A&M in the 50's was that the electrons were displaced around a sneak path while a field was being established between the two plates of a capacitor and I suspect that concept came from Maxwell. Today we know that RF photons flow through the capacitor's dielectric at the speed of light in the medium and electrons cannot move fast enough at RF to traverse that theorized displacement path.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: WS3N on April 09, 2014, 06:47:28 AM
When we solve for the number of degrees occupied by the section of 450 ohm line in the center, we come up with a value of 20.8 degrees. So how can 15 degrees plus 20.8 degrees plus 15 degrees add up to 90 degrees? Answer: We have not taken all of the phase shifts into account. There is a negative phase shift at point X where the Z0 changes from 50 ohms to 450 ohms and there is a positive phase shift at point Y where the Z0 changes from 450 ohms to 50 ohms. So there is a total of five phase shifts which when added up equal 90 degrees. The same thing is true for the mobile antenna above.

This is an interesting way to look at the solution. A more obvious way, at least for me, is to treat it as a simple boundary-value problem. The full antenna has a simple cosine solution between the base at 0 and the tip at 90. We want to remove the center 2/3 of the antenna but we must maintain the continuity of the solution (the current). When the center section is replaced by a coil, we still want the cosine solution in the sections [0, 15] and [75, 90]. At each end of the coil we must enforce the continuity of the current. We also require the voltage to be continuous at these points, but this is equivalent to a discontinuity in the slope of the current. It is these slope discontinuities (equivalent to your phase shifts) that allow the original solutions from the ends to be joined over a much shorter distance.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 09, 2014, 07:28:10 AM
It is these slope discontinuities (equivalent to your phase shifts) that allow the original solutions from the ends to be joined over a much shorter distance.

As long as we both are using valid models, the results will be the same. I have worked out what I think is an original method of solving these problems using the Smith chart illustrated by the following diagram.

(http://w5dxp.com/mobant.JPG)

The phase shift at an impedance discontinuity seems to be the difference between the impedance at that point normalized to the two different values of characteristic impedance existing at that point, i.e. the number of degrees between -j2374 ohms normalized to Z01=4747 ohms and Z02=475 ohms is 52.3 degrees. This works for transmission lines when the Z0s are known and it should work just as well for wires and coils if the Z0s are known.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: WS3N on April 09, 2014, 08:24:41 AM
What do photons have to do with it?

http://photontheory.com/Kemp/Kemp.html

"Thus the displacement current will be called the photon current, because it too is quantized in nature."

"The displacement current or photon current is also shown to be equivalent to the energy of the photon; thereby making the displacement current the actual photon."

Quote
Maxwell's equations would not "work" without the displacement current term.

That's true, but if Maxwell had known about photons, he would not have named it "displacement" current. The way displacement current was explained to me at Texas A&M in the 50's was that the electrons were displaced around a sneak path while a field was being established between the two plates of a capacitor and I suspect that concept came from Maxwell. Today we know that RF photons flow through the capacitor's dielectric at the speed of light in the medium and electrons cannot move fast enough at RF to traverse that theorized displacement path.

Yes, the name is an artifact of history. I don't remember the details of his thinking, something about vortices (because it's a curl) in the medium. Nevertheless, he wrote his set of equations long before they were shown to be correct for the quantum case, where they describe the photon probability amplitude, similar to the Schroedinger equation for nonrelativistic matter waves.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 09, 2014, 10:08:28 AM
I received the borrowed copies of QEX and have read the article. The title of this thread is somewhat misleading. Nowhere did Barry, w9ucw, refer to his articles as "ground breaking". IMO, there are two areas that bear some discussion.

1. High-Q coils vs low-Q coils - It's obvious that when Barry says "low-Q coil", he is not talking about truly low-Q coils (like a 75m hamstick coil) but is instead talking about relatively high-Q coils that have a lower Q than very high-Q coils. It would have been nice if he had measured and published the Qs of the test coils so we wouldn't be thinking he said that a coil with a Q of 1 is just as good as a coil with a Q of 300.

In one of the 1980s 75m mobile CA shootouts, three mobile antennas tied for top honors. One was a Texas Bugcatcher with a large top hat. One was a screwdriver antenna with a large top hat. The third was my junkbox antenna consisting of a CB whip top-loaded with a long, small diameter loading coil and some short horizontal stingers for a top hat. A picture of that Rube Goldberg junkbox antenna is the second one at:

http://w5dxp.com/shootout.htm

The first picture is a screwdriver antenna with a large top hat similar to the one that tied for top honors in the CA shootout.

The point is that the Texas Bugcatcher coil is a large diameter, large wire very high-Q loading coil. The screwdriver has a lower-Q long, skinny loading coil. As one can see in the third picture, my junkbox loading coil was even longer and skinnier and thus even lower-Q. This evidence from the 1980s CA shootout agrees with Barry's observation about higher-Q and lower-Q coils. But he should not have called them low-Q coils. A low-Q coil is the one in the 75m hamstick used for the CA shootout which was 10dB down from the top antennas. It's too bad that Barry didn't include a 75m hamstick in his mix of mobile antennas.

My guess as to the reason that the lower-Q coils were almost as effective as the high-Q coils is that the low-Q coils, because of their overall longer length compared to the shorter length high-Q coils, is that they have a higher radiation resistance because of their longer length which somewhat compensates for their lower Q. That also would help to explain why screwdriver antennas are as effective as bugcatcher antennas.

2. Current Droop - I agree with Barry that most of the current droop apparent in the measurements are due to phasing between the forward current and reflected current. Anyone who believes that a 75m air-core loading coil is a lumped circuit is confused about the nature of standing wave antennas. A good 75m mobile antenna may have a feedpoint impedance of 15 ohms caused by the in-phase superposition of the forward and reflected waves such that the feedpoint impedance equals (Vfor-Vref)/(Ifor+Iref). For that to be 15 ohms, the magnitude of the reflected wave would have to be about 95% of the forward wave, i.e. the SWR on the standing wave antenna is about 40:1 which makes any phase measurements irrelevant and immaterial because there is virtually no phase shift at all when the SWR is 40:1.

With the exception of Barry not measuring the Qs of his test coils, I think it is a good article.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: K0BG on April 09, 2014, 03:59:16 PM
A few things we want to keep in mind about loading coils......

The higher the reactance, the higher the length to diameter ratio. In other words, an 80 meter coil will be much longer than its diameter for optimal Q (≈4:1). On 20 meters, the ratio will be close to 1:1.

Large diameter coils—over about 3.5 inches or so—will have an excessive amount of distributed capacitance. This has two effects. First, the Q is lower, and the self-resonant point is also lower. Equate the latter as excessive loss (lower Q), as the operating frequency nears the self resonant point. If the self resonant point is exceeded, the coil looks more like a lossy capacitor, than an inductor.

Anything, even air, within the coil's field, will effect its Q. This especially includes, large metallic end caps, metallic shorting plungers, and incorrectly mounted cap hats (placed directly atop the coil instead at the top of the antenna).


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: WS3N on April 11, 2014, 08:19:36 AM
It is these slope discontinuities (equivalent to your phase shifts) that allow the original solutions from the ends to be joined over a much shorter distance.

As long as we both are using valid models, the results will be the same. I have worked out what I think is an original method of solving these problems using the Smith chart illustrated by the following diagram.

The phase shift at an impedance discontinuity seems to be the difference between the impedance at that point normalized to the two different values of characteristic impedance existing at that point, i.e. the number of degrees between -j2374 ohms normalized to Z01=4747 ohms and Z02=475 ohms is 52.3 degrees. This works for transmission lines when the Z0s are known and it should work just as well for wires and coils if the Z0s are known.

I can see how that works for two sections (coil and whip). What do you do for three sections, e.g. your stub made of 50/450/50 sections?


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 11, 2014, 01:26:54 PM
I can see how that works for two sections (coil and whip). What do you do for three sections, e.g. your stub made of 50/450/50 sections?

The principle is the same with one interesting difference. While the phase shift is positive at the 450-50 ohm impedance junction at the top of the 450 ohm section, the phase shift at the 50-450 ohm impedance discontinuity at the bottom of the 450 ohm section is negative and subtracts from the 15 degrees of 50 ohm line. In the previous example, the negative phase shift subtracts about 13 degrees from the 15 degree 50 ohm section tied to the feedpoint.

Have you ever noticed how little effect adding length to the base section under the coil has on the resonance frequency vs how much effect adding length to the stinger changes the resonant frequency? The negative phase shift at the bottom of the coil and the positive phase shift at the top of the coil are responsible for that phenomenon.

It is well known that for a fixed length mobile antenna, moving the loading coil from the base to the center of the antenna requires considerable increase in the inductance of the loading coil. That is because we are making the stinger shorter.

If we keep the same long stinger and simply add a base section under the loading coil, the resonant frequency doesn't change much because the negative phase shift at the bottom of the loading coil cancels out a large number of the added degrees. Adding 15 degrees of bottom section to the physical length of a resonant base-loaded antenna only adds about two degrees to the electrical length of the antenna.

Hoping that people would see the similarity between shortened mobile antennas and dual Z0 shortened stubs, I wrote the following article about five years ago but nobody seems to have had an epiphany because of it.:)

http://w5dxp.com/shrtstub.htm 

I know there's nothing new under the sun concerning antennas but as far as I know, the above information is original, i.e. I've never seen it explained anywhere else.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 14, 2014, 06:43:30 AM
Here's an interesting graph produced by EZNEC/AutoEZ that displays the amplitude and phase of the current through a 75m Bugcatcher mobile antenna using the helix function of EZNEC to simulate a 75m Texas Bugcatcher mobile loading coil. The antenna uses mininec ground and is resonant on 4 MHz.

(http://www.w5dxp.com/mob4MHz.png)

The current through the loading coil is about 33 degrees of a standing wave current waveform with a sky high SWR on the standing wave antenna. The very small phase shift through the coil happens because of the standing waves and is not indicative of the ~21.5 ns traveling wave propagation delay through the coil.

Note that the stinger is 16 degrees long but has only a 0.04 degree phase shift through it. Again, that is a characteristic of standing wave current and certainly not indicative of the traveling wave propagation delay through the stinger. 0.04 degrees of propagation delay in 16 degrees of stinger would be 400 times the speed of light.


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: KB1GMX on April 19, 2014, 02:33:33 PM
What amazes me is that it took three pages to realize that High-Q and Low-Q are
useless phrases.

Without knowing what is considered high and low in terms of Q there is no meaning
to any statement around that.  This is made worse in that Q is ratiometric number and
going from 50 to 100 is a larger change than going from 100 to 300.

I'd expect low means something south of 100 and high is something north of 400.  Most
coils tend to fall in that range so which means the difference form one to another is
measurable but not a large quantitiy.   Therein the 80/20 rule applies, where getting a
3db improvement is likely much easier and noticeable than getting that last db or fraction
there of.  When your in the realm of antennas especially in the 160 to 30M range the
initial problem is that the antenna is electrically small and the other half (car/truck
capacitive coupling to earth) represents  lot of lossy things that physics says just are
what they are. 

The usual loss area is not the antenna besides some awful constructions out there its
the radio to feed point interface.

The best example of this is a Workman antenna for 40M that's about seven or so feet tall.
When it failed from street salt I pulled the wire off to find it was enameled copper plated aluminum.
Rebuilt it exactly with same size enameled copper wire trying to duplicate it exactly netted
a noticeable increase in signal, half the swr bandwidth, and a much lower base impedance
measured.  A matching transformer corrected the SWR.   Since i had two of the antenna
one "fixed" and the other not it was easy to compare using a remoted spec analyser as RX
and was found to be nearly 3db, part of it being resistive loss, the other part being the coil was
also made with the same high resistance wire greatly lowering the measured Q.  The resistive
loss make the antenna match but it was a lossy match so lit looked good while doing nothing
to help radiation. 

So yes Q counts, its the total effective Q of the antenna including the coil and I cant see how a
antenna of stainless or aluminum with a really cute coil of fancy plated metal is that much better
that one using mostly copper with a simpler and still good quality coil. 

The law of diminishing returns prevails.

Allison



Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: NO9E on April 20, 2014, 06:00:31 PM
See Table 2 (80m) and 102" antenna with coil at top.

                                         Hi-Q  Lo-Q
Resistance at resonance        43.6     43.5
Field strength                        8.5       8.6
2:1 Bandwidth                      12       25

Wider bandwidth should go with higher resistance or lower efficiency. But both are same. How do we get a wider bandwidth for nothing? Is it real or a mistake? Perhaps I should ask Barry...

Ignacy, NO9E   


Title: RE: Ground Breaking Mobile Antenna Articles in QEX
Post by: W5DXP on April 22, 2014, 03:21:22 PM
How do we get a wider bandwidth for nothing?

Seems to me the question is: How do we get equally strong radiated signals from a "low-Q" and high-Q loading coil? It seems obvious that Barry's "low-Q" coils were not really low-Q, just lower Q than the high-Q coils. A 75m hamstick has a truly low-Q coil that is much lower Q than Barry's "low-Q" coils.

I suspect that the shorter, large diameter high-Q coils and the longer small diameter low-Q coils had the same loading reactance and when the longer small diameter low-Q coils were in place the antenna was physically longer resulting in a larger radiation resistance that compensates for the lower-Q.

I will try to contact Barry on this subject.