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eHam Forums => Mobile Ham => Topic started by: KH6AQ on December 11, 2010, 04:48:58 PM



Title: HF mobile ant position
Post by: KH6AQ on December 11, 2010, 04:48:58 PM
The best kept secret in HF mobile antenna performance is the mounting position of the "antenna." The reason I put antenna in parentheses is because what we call the antenna is often really an ACED, or Antenna Current Excitation Device. The ACED actually excites antenna current onto the automobile itself, which then becomes a part - and sometimes the dominant part - of the actual antenna.

A good example of this is my compact car. I wanted good 7 MHz performance and settled on two options: A 5' tall Yaesu ATAS-120A screwdriver mounted on the roof, or a 10' Tarheel HP-100A mounted on a trailer hitch. The first option is small, unobtrusive, and costs $370 with a Diamond 400C mount. The second is large, is quite visible, and costs $700 with controller and trailer hitch mount. Both antennas top out at 10' above the pavement.

A NEC-2 model revealed that mounted as described the ATAS-120A would have a radiation efficiency of 9%. And mounted as described the Tarheel would also have a radiation efficiency of 9%. This is revealed by a NEC-2 model where the "ground" loss is 10 ohms and the coil Q of the ATAS-120A is only 90 while the HP-100A is a high 300. Due to the different mounting configurations the radiation resistance of the ATAS is 3.5 ohms while the Tarheel is 1.4 ohms.

Why is there the same efficiency from the smallest (and lowest Q) screwdriver antenna and from a much larger (and high-Q) screwdriver antenna? Antenna mounting is the answer. With the ATAS-120A mounted on the vehicle roof the ATAS and the vehicle form a 10' long antenna. With the Tarheel mounted near the pavement much of it's magnetic field is canceled by an opposite magnetic field from the vehicle. The opposite magnetic field is due to antenna displacement current causing vehicle conduction current in a downward direction (as the antenna has conduction current in the upward direction).

Placing the Tarheel parallel to the vehicle negates that portion of the Tarheel; the lower 5' of the Tarheel becomes ineffective and we are left with a 5' antenna.

So, the 10' Tarheel becomes a 5' antenna while the 5' ATAS becomes a 10' antenna.

The radiation efficiency of the actual ATAS-120A installation was indirectly measured by a VSWR bandwidth method. This measurement shows a radiation efficiency, at 7 MHz, of 11%.

Conclusions: Bigger isn't always better when it comes to mobile "antennas." And higher coil Q isn't always better. System performance can depend more on where the antenna is mounted than what type (length, coil Q) of antenna it is.

Tip: Avoid mounting the antenna parallel to the vehicle.




Title: RE: HF mobile ant position
Post by: K0BG on December 12, 2010, 06:52:53 AM
Your logic is flawed.


Title: RE: HF mobile ant position
Post by: AD4U on December 13, 2010, 08:22:59 AM
I am not qualified to comment on your theory or your logic.  I am in SC and you are in UT some 2000 miles apart.  As I previously stated I run an original Texas Bug Catcher on a F-250 Ford diesel truck and I am very happy with it.   However I do not think the Bug Catcher is 11% effiicent on 40 meters.  Even if it were, I do not know how to accurately measure it.

I wish we could park side by side - the only real test that matters in the real world - my full size original Texas Bug Catcher VS your ACED antenna "system" each running 100 watts and do as many tests as practical on any and all bands.

Dick  AD4U


Title: RE: HF mobile ant position
Post by: KH6AQ on December 13, 2010, 08:43:01 AM
Dick,

every measure of radiation efficiency is an indirect measurement. A field strength measurement is an indirect measurement. For amateur measurements (where an antenna test site with known characteristics is not known) an A-B comparison can be used. A full size monopole is often used. I have used this method to test antennas such as the Isotron and the PF antenna.

The most direct way to measure radiation efficiency of a mobile antenna is with an input impedance (Rin) measurement combined with a NEC model of the vehicle and antenna placed over "perfect" ground. This is used to determine the radiation resistance (Rr). Radiation efficiency = Rr/Rin. The formula used to calculate the radiation resistance of a monopole cannot be used because it does not account for the vehicle. Place the "antenna" on top of the vehicle and the vehicle adds to the antenna length. Place the "antenna" next to the vehicle and the vehicle subtracts from the antenna length. Neither has exactly a one-to-one correspondence and so a simulation must be used for a precise calculation of radiation resistance.

Another method - and this is a good check of the base impedance measurement - is to measure the VSWR bandwidth. This is useful for antennas that have a base matching reactance. With a base matching reactance this method is not perfect but gives useful data. The VSWR points must be selected to be close to what the the antenna would present without the base matching reactance. Loss resistance is then added to a NEC model until it exhibts the same VSWR bandwidth.

These are the two methods I've used for several physically short antennas I have designed for commercial use. Signal strength measurements against a reference antenna (quarter wavelength monopole) were also used. All three methods were in good agreement.

I'll take field strength measurements of my mobile antenna vs a 20' vertical having 64 radials. That reference antenna has a radiation efficiency of 50% (found by a base impedance measurement).




Title: RE: HF mobile ant position
Post by: K0BG on December 13, 2010, 11:58:08 AM
David, I just don't agree with your logic.

First, if I were to use your original hypotheses, then a base-station vertical mounted on the ground would have an effective height of about 8,000 miles! That's nonsense.

Next, You say field strength is an indirect measurement. It is, but only if you do it incorrectly. If you were correct, then folks like WB2WIK, who spent a life-time measuring antenna field strengths, would have wasted all his career.

Then you state:

Quote
The most direct way to measure radiation efficiency of a mobile antenna is with an input impedance (Rin) measurement combined with a NEC model of the vehicle and antenna placed over "perfect" ground. This is used to determine the radiation resistance (Rr). Radiation efficiency = Rr/Rin.


This isn't logical either. When you measure the input resistance, you measure everything; Rr, Rg, Rc, etc. They are in series with one another, and by factoring one out so to speak, you don't account for changes in the rest.

Probably the most inane comment is the one about measuring an antenna's bandwidth to determine if efficiency. I can easily build a model with great bandwidth, and poor efficiency, or even great efficiency. And, I can build a model with lousy bandwidth, and have either poor or good bandwidth. The input impedance notwithstanding in any case!

And again, considering the size of the vehicle into an antenna's efficiency calculation, is fraught with problems from the get go.



Title: RE: HF mobile ant position
Post by: AD5X on December 13, 2010, 04:32:42 PM
Alan - I think he meant to say that he uses EZNEC to calculate the radiation resistance by modeling the antenna over lossless ground.  Assuming the model is correct, you can then calculate the efficiency since the losses would be the total input resistance minus the radiation resistance.

Phil - AD5X


Title: RE: HF mobile ant position
Post by: K0BG on December 13, 2010, 05:51:07 PM
Well, Phil, it isn't as simple as that.

EZNEC will calculate anything you put into it. The problem is, however, that all of the losses are in series with one another, except for some coupling losses between the antenna and the body of the vehicle in question (those are typically small in comparison to the other losses). Since they are in series, changing one, affects the others, and you cannot measure that change with an impedance measurement. Nor can you assume what you measured is a positive effect. A good example of this, is the installation of a cap hat. Any place you put it above the coil, will indeed increase the capacitance equally. However, if it is placed very close to the coil, the added capacitance reduces the coil's Q. Placed correctly, the radiation resistance increases. Both increase the input impedance, but one is by additional loss, the other by an increase in radiation resistance. A fact you cannot measure, or calculate by EZENC (separate as it were).

Dr. Belrose, VE2CV, did point out a way to use EZNEC, and the actual input impedance of an antenna, to arrive at an efficiency level within reason, accuracy wise. That procedure was not in any way close to what David suggested.

As Tom Rauch, W8JI, has stated many times, you cannot assume any measurement you can't measure directly. In other words, there is no way to separate the components making up the input impedance by calculation (factoring out),and for darn sure, you can't assume them which he apparently has.

As I stated firstly, his logic is faulty.


Title: RE: HF mobile ant position
Post by: AD5X on December 14, 2010, 04:41:23 AM
Alan - I'm having a hard time agreeing with you on this.  Model an antenna on EZNEC.  Base loaded, center loaded, and add a capacity hat if you want and locate it wherever you want.  The inductor should be modeled as lossless of course.  EZNEC will give you the radiation resistance.  Now measure the actual resonant antenna input.  Subtract the radiation resistance from EZNEC, and the remainder will be all other losses.  This will show the effect of incorrectly placing the capacity hat.  With knowledge of the radiation resistance (if you believe in your EZNEC model), you can determine efficiency.  Where is the error here?

Phil - AD5X


Title: RE: HF mobile ant position
Post by: KG4RUL on December 14, 2010, 06:20:11 AM
Keep in mind that EZNEC is modeling software.  MANY assumptions are made in each analysis that can lead you down the primrose path.  Use EZNEC as a guide but, expect to make changes in a real-world situation.


Title: RE: HF mobile ant position
Post by: AD5X on December 14, 2010, 09:29:05 AM
Keep in mind that EZNEC is modeling software.  MANY assumptions are made in each analysis that can lead you down the primrose path.  Use EZNEC as a guide but, expect to make changes in a real-world situation.

In this case, however, the discussion is about modeling a simple antenna over perfect ground with lossless components in order to determine radiation resistance.  I suspect that the EZNEC results are probably pretty good in this case.

Phil - AD5X


Title: RE: HF mobile ant position
Post by: K0BG on December 14, 2010, 10:04:04 AM
Well, unfortunately, Dennis is correct. Modeling programs are tools, not panaceas.

In this case, the losses are in series. If you change one, you effect the others, and there is no way around it. In other words, the radiation resistance will change as a result of changing the ground losses. Fact is, I used to believe just like you Phil. That Rr is an exact number based on the overall length, diameter, etc. of the radiating element. But after reading the white papers done by both Rudy, N6LF, and Dr. Belrose, VE2CV, et. al., I've had to retrench my beliefs.

Even if you use the data collected (true or untrue), you can't apply it to the real world, because you have no way of accurately measuring the ground loss. And speaking of which, I have always believed that the ground losses increase with decreasing frequency. That is generally true. However, because there are standing waves between the body of the vehicle and the surface under it which cause the ground loss factor in the first place, you can't assume the losses are linear. In fact they are not. So, the methodology, or logic if you please, is flawed.


Title: RE: HF mobile ant position
Post by: KH6AQ on December 14, 2010, 10:07:27 AM
Phil you're right. For the antennas we are discussing, where the loss resistance is apportioned is not critcal. For example, let's look at a 7.5' center loaded 7 MHz vertical over perfect GND. We can add loss resistance to make the 3:1 VSWR bandwidth 290 kHz: With the loss resistance at the base it's 28 ohms and the radiation efficiency is 8.9%. With the loss resistance at the loading coil it's 25 ohms and the radiation efficiency is 9.8%. The difference in gain is only 0.4 dB.

So, the VSWR bandwidth method of determing radiation efficiency is robust; it is forgiving of incorrectly proportioning the loss resistance. Now our possible error is perhaps 0.2 dB in the example above. One does not need to know how much is ground loss and how much is inductor loss to arrive at an answer that is within a fraction of a decibel of the correct efficiency value.

And we can fairly accurately apportion the loss resistance. A program such as the K6STI coil program tells us the loss resistance of our inductor. Next we add base loss resistance until the modeled and measured VSWR bandwidth are the same.

The vehicle enters into this because it becomes part of the radiating structure. An HF mobile antenna is not quite a vertical. And it's not quite a ground plane. It's closer to a vertical dipole. One proof of this is to model a mobile antenna (vehicle and "antenna") with and without ground. The resonant frequency of the system does not change much; it is somewhat ground independant.

When the mobile system is configured as an "antenna" on top of the vehicle - at an instant in time - there is antenna current upward in the "antenna" and upward in the vehicle. The magnetic fields add, the system current-area is maximized and we have maximum radiation for a given antenna current.

When the mobile system is configured as an "antenna" on the bumper - at an instant in time -there is antenna current upward in the "antenna" yet downward in the vehicle. The magnetic fields partially cancel, the system current-area is minimized and we have minimum radiation for a given antenna current.

When one steps back and looks at a vehicle antenna system it's really quite simple. Placement of the "antenna" to maximize current-area is the most fundamental aspect of mobile antenna system design. If one does not understand an installation can be hit or miss. Hit it right and it works well. Hit is wrong and you can have an expensive installation with inferior performance.

Present day electrical engineering is based on modeling. That is, being able to predict how a circuit will function before it's built. Cut-and-try or lab-only measurements went out of style 100 years ago. Mathematical (paper and pencil) modeling gave way to computer modeling 40 years ago. I design circuits in SPICE, they are built and they work as designed. There is no cut-and-try. Complex integrated circuits are designed entirely in SPICE. Design, build, and they work the first time. The same thing applies to antenna design. And that is much, much simpler than SPICE circuit design. How complicated can a piece of wire be? And if one does want to accurately model real ground they need only use NEC-4.


Title: RE: HF mobile ant position
Post by: K0BG on December 14, 2010, 10:16:43 AM
Apparently, Dave, you didn't read what I said previously. If you really put some thought into it, you'll realize you can't use the VSWR as a measure of efficiency. But... you're going to have to think outside the box for once.


Title: RE: HF mobile ant position
Post by: NK5G on December 14, 2010, 10:59:11 AM
Your logic is flawed.

Sigh


Title: RE: HF mobile ant position
Post by: K0BG on December 14, 2010, 12:32:09 PM
What are the numbers? I'd like to look them up.


Title: RE: HF mobile ant position
Post by: W9PMZ on December 14, 2010, 12:58:52 PM
"because you have no way of accurately measuring the ground loss."

Just because you may not have a meter that when connected to two terminals results in the ground loss value does not mean you cannot infer this value by making other related measurements.

Engineers and sciencetists do this all the time...

73,

Carl - W9PMZ


Title: RE: HF mobile ant position
Post by: K0BG on December 14, 2010, 01:02:21 PM
I agree Carl, but the term accurate is what's in question. As I stated, we're trying to assume it is a linear function, when it is anything but linear.


Title: RE: HF mobile ant position
Post by: KH6AQ on December 14, 2010, 03:09:26 PM
Bottom line: The ground loss resistance does not need to be known to measure short antenna radiation efficiency. Only the total of all losses need be known. This is the measured input resistance.  

Loss in the ground near a short antenna manifests itself as an increase in the antenna input resistance. When we quote a number for this it is input referred, meaning it is equivalent to an actual resistor at the antenna input.

A second method to measure radiation efficiency of a short antenna is the measured VSWR bandwidth. One takes a lossless antenna simulation and adds series loss resistance at the antenna input until the simulated VSWR bandwidth is equal to the measured VSWR bandwidth. For a base-loaded antenna this is entirely accurate. For a center-loaded or top-loaded antenna an error is caused by the loss resistance being placed in one spot rather than two (at the input for GND loss and at the coil for coil loss). But, as pointed out in an earlier example this error can be less than 0.5 dB. Alternatively, one distributes the loss resistance more realistically by placing an estimated loss resistance at the center or top loading inductor.



Title: RE: HF mobile ant position
Post by: K0BG on December 14, 2010, 04:06:04 PM
This is from my web site:

Quote
In the Technical Correspondence section of the September 2006 issue of QST (page 57), are a few paragraphs written by Dr. Jack Belrose, VE2CV. Jack explains how to use an antenna analyzer and EZNEC to calculate the efficiency of a mobile antenna. The basic premise is to compare the measured input impedance of your mobile antenna, compare it to the modeled impedance given by EZNEC, and then adjusting the coil Q (resistive loss) until the two impedances (measured and calculated) equal. Then reading the programs calculated radiation efficiency.

This gives you a close approximation of the efficiency. This is essentially what you're trying to arrive at, but in all due respect David, you're going about it incorrectly.

Again, and again, you can not use the VSWR bandwidth to calculate efficiency. It just doesn't compute! There is even a formula out on the net (it was once on Phil's, AD5X, web site) which supposedly ended up with the figure. Tom Rauch, W8JI, responded to this very subject in a post a few months ago to an article about this very subject.

From me, not Tom, I suggested you think outside the box. Remember, the reactance doesn't change linearity on both sides of the resonant point. If for no other reason, this should give you some idea of the futility of using bandwidth. Apparently, it hasn't. What's more, the change in reactance to each side of resonance, it directly dependent on the antenna's structure. What you arrive at with once specific case, cannot be applied to another, wholly different design.

My point about the ground losses stands. They are caused by the standing waves between the superstructure of the vehicle in question, and the surface under it. It should be obvious that you can't measure them. Sure, I agree it is what causes the ground loss in the first place. However, there are a few gotchas. One, you can't measure them, and you can't assume them. They are frequency dependent, vehicle dependent, surface dependent, and are even effected by the existing STP.

Alas, there is no definitive way to measure, compare, or even speculate the efficiency of any given antenna, except to measure its actual field strength. But even that must be done in a all-factors-normalized fashion. Few, if any, amateurs have the wherewithal to do so. 

By the way, I'm still waiting for the list of patents you have. So far, I can only find two which relate to a timing circuit.


Title: RE: HF mobile ant position
Post by: KI4SDY on December 14, 2010, 05:56:19 PM
I am glad to see that someone is finally "footnoting" where he is copying his information from. That is the ethical thing to do. ;)


Title: RE: HF mobile ant position
Post by: W9PMZ on December 15, 2010, 02:04:14 AM
"Again, and again, you can not use the VSWR bandwidth to calculate efficiency"

So would someone please explain the method of the VSWR bandwidth method, and providing the equations?

Regarding ground loss.  Standing waves?  Isn't this just about the capacitance between "ground" and the vehicle?  If so, then the ground loss would be linear and dependent upon frequency.

73,

Carl - W9PMZ


Title: RE: HF mobile ant position
Post by: K0BG on December 15, 2010, 04:52:52 AM
Once again Carl, you're correct, but with a caveat. The coupling is not consistent, and changes with frequency, as we already know. However, the change isn't linear, and in fact it is possible to have a higher ground loss at the top of 40 meters, than you do at the bottom of 40 meters. It is also not uncommon to have more loss on 40, than you do on 80. Seems weird that's for sure.

I forgot. Here is what I say about the formula on my web site:

Quote
By the way, there is a formula circulating the Internet which states that antenna Q is equal to 360 times the frequency in MHz, divided by the 2:1 VSWR bandwidth in kHz. One has to assume they mean antenna system Q, but that's not a given. While this formula might give you a comparison between antenna A and antenna B (all else being equal), the actual Q of the antenna (system or otherwise) requires a textbook-full of formulas, and a lot more information than just the 2:1 bandwidth! Fact is, this formula is no more specific than the number of DX contacts a specific antenna garnered.
[/i]


Title: RE: HF mobile ant position
Post by: W9PMZ on December 15, 2010, 05:34:08 AM
"Remember, the reactance doesn't change linearity on both sides of the resonant point. If for no other reason, this should give you some idea of the futility of using bandwidth."

I think that there is a reason, if you look at the power response of the system you essentially see a bandpass response.  Circuits 101 tells you that the center of the response is the geometric mean and not the center of the two points that are used to define the start and stop.

73,

Carl - W9PMZ


Title: RE: HF mobile ant position
Post by: KH6AQ on December 15, 2010, 07:09:01 AM
I think an article written by me will help educate. Whille this subject got it's start at eham.com a better place to publish the article is at eeweb.com.


Title: RE: HF mobile ant position
Post by: K0BG on December 15, 2010, 07:36:22 AM
David, I'm still waiting for you to post your patent numbers.


Title: RE: HF mobile ant position
Post by: W9PMZ on December 15, 2010, 08:12:59 AM
 PAT. NO.  Title
1 6,675,313  Method and apparatus for reducing signal timing skew on a printed circuit board 
2 6,526,519  Method and apparatus for reducing signal timing skew on a printed circuit board 

found two...

maybe more but i am not really that familiar with the search tool


Title: RE: HF mobile ant position
Post by: K0BG on December 15, 2010, 11:35:21 AM
I found those two myself, but he earlier said he has 6 or 8 whatever it was, and several pending. He has since edited the post, and removed the reference. Apparently, he doesn't have the number he said he had. Hum....


Title: RE: HF mobile ant position
Post by: W9PMZ on December 15, 2010, 01:09:32 PM
maybe not the case, could have multiple names on the patent.  nonetheless...

WX7G makes the statement,

 "We can add loss resistance to make the 3:1 VSWR bandwidth 290 kHz:"

then K0BG makes the statement,

"By the way, there is a formula circulating the Internet which states that antenna Q is equal to 360 times the frequency in MHz, divided by the 2:1 VSWR bandwidth in kHz."

Not understanding fully either method at this time, perhaps apples are being compared to oranges...

73,

Carl - W9PMZ


Title: RE: HF mobile ant position
Post by: K0BG on December 15, 2010, 02:56:24 PM
Don't take it out of context Carl. The formula is bogus, and relates back to some article published on the Antennex site a few years ago.

I looked for it, but didn't spend a lot of time, but Tom, W8JI, commented on the formula a year or so ago. I don't remember who posted the article, but the subject was similar to this one. Tom's answer was very explicit, and if I do find it, I'll add it.


Title: RE: HF mobile ant position
Post by: N3OX on December 15, 2010, 05:29:21 PM
I think we need to differentiate between specific, oversimplified, faulty procedures and more sophisticated comparisions of feed measurements and models using a more general technique.  If I had a way to know the antenna's radiation resistance and reactance referred to the feedpoint at every frequency on the 40m band, and I did a high accuracy measurement of the feed impedance and compared to the KNOWN lossless case, I could get an excellent estimate for the efficiency of the antenna across the band.

This would be true even if the loss resistance was a function of frequency, because I'm not just simply comparing something like the 2:1 VSWR bandwidth... I'm actually predicting the resistance of the antenna in the absence of ohmic losses and measuring the resistance with ohmic losses.  I can calculate antenna efficiency as Rrad/(Rrad+Rloss) with Rrad (the radiation resistance) and Rloss (the loss resistance) referred to the same point in the antenna.  In this discussion, that point is the feedpoint.   Whether or not you can use this fact to estimate antenna efficiency hinges entirely on whether or not you have an accurate prediction of the radiation resistance or not.

Assumptions of how the loss resistance changes with frequency only cause errors if you make those assumptions.  So maybe the loss resistance doubles going from 7MHz to 14MHz.  Fine.   That can be incorporated into the model, and you can see if this is happening by comparing model to measurement.  If I recall correctly, a lot of the "you can't use feed impedance to measure efficiency" discussion centers around some examples with sparse radial systems where feed impedance AND bandwidth AND field strength went UP at the same time as radials were added.  I can imagine a few ways that this would happen (current max isn't at the base of the antenna for some reason, and moves closer as radials are added... ground system reactance requires re-resonating the radiator by changing its height, etc).  This is an important counterexample to the idea that you can ALWAYS associate REDUCED feedpoint impedance and narrowed bandwidth with a more efficient antenna.  You can't ALWAYS do that.

But I don't recall anyone bringing a detailed measurement/modeling comparison into those discussions.  I don't think the disagreement was with a model, I think the disagreement was with a too-simple assumption that more radials ONLY change the loss resistance.  I've already come up with a couple of model examples in EZNEC where I can get the total feed resistance AND the radiating efficiency to go up at the same time when I add radials close to the ground and re-resonate the vertical.  But this effect is associated with changes in the radiation resistance referred to the feedpoint as the antenna is re-configured physically and the current distribution changes.

It's important to constrain models with measurements to make sure you didn't leave something important out of the model.  And I think that it's important to really understand the quantitative limitations of what we can do when parts of the antenna are close to the ground.

But at the same time, I think we're getting a bit lost in the details and not really discussing what's at the heart of WX7G's claim: the radiation resistance of an antenna mounted down low on a vehicle is modified by the vertical component of the currents flowing on the vehicle.  This is a reasonable hypothesis with modeling and general theoretical evidence to back it up.

As far as actually doing the model/measurement comparison based on impedance?  You can imagine several levels of comparison.  The 2:1 SWR bandwidth (a two-point comparison) is simple, but you could also check the shape of the SWR curve in comparison with model.  In EZNEC, it would be very tedious to generate a model SWR curve with a frequency-dependent loss resistance for comparison to measurements, but it could be done.  In other software it would be straightforward.   


Title: RE: HF mobile ant position
Post by: K0BG on December 16, 2010, 06:11:03 AM
Exactly, Dan. There no simple, easy, quick fix, type of solution. And as you alluded to, you can't assume any parameter, you didn't (or can't) measure.


Title: RE: HF mobile ant position
Post by: N3OX on December 16, 2010, 07:49:21 AM
as you alluded to, you can't assume any parameter, you didn't (or can't) measure.

I  didn't intend to say that.  Sometimes parameters don't matter.

If I were modeling a high horizontal yagi, earth parameters would be irrelevant.  Unless it was a particularly low impedance design, tubing conductivity wouldn't matter.  What do I mean by they "don't matter?"  I mean that the errors in gain and front to back and impedance prediction would be tiny.  Very possibly swamped by construction realities.    There are some types of antenna where NEC-2 predictions are better than anything I could ever hope to measure.

I don't think that's the case for mobile antennas.  You need to measure SOMETHING about the parameters.  But if you have a sophisticated enough model, you can back out the major parameter (total loss resistance) from impedance measurements.  The only question is whether or not the model is sophisticated enough that all you need is a feed impedance measurement to make it the model and real system match. 

That is a "parameter measurement" but it's an indirect one that many people believe is useless.  I'm not so sure.  I think that feedpoint impedance compared to a good enough model is actually a very useful measurement.  It may be that for some (most?) hams, that would be a more reliable measurement than field strength.  Field strength measurements have errors too, very possibly larger errors than impedance comparisons between measurement and good models. 

I think the "impedance based" efficiency estimation fell out of favor based on a too-simple model of verticals over radial systems.  That model was "the feed impedance of the lossless system stays the same as you add radials."  That is false.  But it didn't invalidate Efficency = Rrad/(Rrad+Rloss)!  It just made it clear that you have to have a better way of knowing Rrad to use feed impedance measurements to calculate efficiency.

============

One last thing, that I want to set off down here so it doesn't get lost:  WX7G is saying that mounting an antenna low on the car adversely affects its radiation resistance.  This is consistent with your advice, Alan, that it's the "metal under the antenna, not to the side" that matters.

The vertical component of currents on the car flowing out of phase with the vertical antenna cancel in the far field with those from the antenna, reducing the radiation resistance.  Those in phase reinforce those from the antenna in the far field.  So a shorter antenna mounted higher might have more total radiation resistance. 

It's just like how the radiation resistance of a ground plane increases as you slope the radials further and further downward.  (Actually, like what happens going from UP-sloping radials to down-sloping ones.)  That's really the main point of the original post, and I think it's probably right.  Should we test it?  Sure.  Should we be skeptical?  Sure. You always have to be skeptical of everything, especially your own beliefs.  But let's discuss it directly.



Title: RE: HF mobile ant position
Post by: K0BG on December 16, 2010, 08:41:15 AM
I agree. I don't put much stock in shootouts, if for no other reason than the methodology typically used. And, I think you and Dr. Belrose are on the same thought vector.

His methodology was to compare the model with the measured input impedance, and then adjust the coil Q until the input values agreed. My only argument would be, do we adjust the coil's Q, or do we change the ground loss figure whatever it is. They are in series after all, so perhaps both, but to what standard if any. To expand on this a bit more, I suspect an efficiency plot (curve?) would be different depending on where the antenna was mounted (no doubt higher is better).

If nothing else, all this palaver is an interesting debate.