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Author Topic: Ground Breaking Mobile Antenna Articles in QEX  (Read 7936 times)
ZENKI
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« 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.

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W5DXP
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« Reply #1 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
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0BG
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« Reply #2 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.
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ZENKI
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« Reply #3 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

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ZENKI
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« Reply #4 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.
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KB4QAA
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« Reply #5 on: April 04, 2014, 05:40:53 PM »

Wow, Zenki didn't even complain about IMD performance!  Smiley
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ZENKI
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« Reply #6 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!  Smiley
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KM3F
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« Reply #7 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.
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ZENKI
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« Reply #8 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.
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W5DXP
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« Reply #9 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:



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.



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.



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.

« Last Edit: April 05, 2014, 10:36:24 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5DXP
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« Reply #10 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".
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KB4QAA
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Posts: 2369




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« Reply #11 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!
« Last Edit: April 05, 2014, 02:31:12 PM by KB4QAA » Logged
KB4QAA
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« Reply #12 on: April 05, 2014, 03:26:56 PM »

Beautiful diagrams and description, Cecil!
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W5DXP
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« Reply #13 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
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AC5UP
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« Reply #14 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...
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