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Author Topic: Sherwood mobile antenna article  (Read 3721 times)
WX7G
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« on: March 06, 2011, 08:14:06 AM »

http://www.sherweng.com/documents/HFMobileAntennas-sm.pdf

This antenna article mentions a reference antenna that by the description is a Hustler. It's interesting that the difference, on 80 and 40 meters, between the reference antenna and a maximized mobile antenna is 6 dB. That is about what I model going from a Hustler to an all out installation.

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K0BG
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« Reply #1 on: March 06, 2011, 09:23:19 AM »

Someone needs to do a series of tests which take all of the variables into account, and normalized to the extent possible. No shoot out can do that, Rob's article notwithstanding. This is an excerpt of an e-mail I recently received.



Quote
... having been an antenna engineer for over 40 years, the methodology employed to perform, accurate,repeatable HF antenna measurements is lacking.  The overall performance characteristics CANNOT be performed in a field test transmission i.e. via the inonsphere BUT must be performed on a TRUE calibrated antenna test facility (either a calibrated ground plane range or see below  items A-B) , with calibrated equipment, i.e. field intensity meter, spectrum / network analyzers in which the amplitude and phase taper of the illuminating RF energy is know to with a .25db peak to peak variation to accurately measure gain. 
 
Typically there are two methods to achieve this type of 'high quality' measurements;
 
        A)  an arch antenna range in which the actual vehicle is placed on a turntable and rotated in the azimuth plane while a receive antenna is placed on the arch and its elevation is varied from 0 degrees elevation up to 90 degrees.
 
        (The arch range functionality however at < 100 Mhz and below has some unique problems in that the mutual coupling between the test antenna (i.e. the mobile antenna) and the calibrated antenna on the arch, must be measured using a network analyzer using the S Parameters , S12/S21,  isolation measurements and placed in the software to allow for this correction vs. angle.)
 
        B)  Method # 2 utilizes an airborne asset with a calibrated receiver on board maintaining a precise distance and flown in pattern around the antenna structure; i.e. vehicle - antenna and  pattern are recorded as a function of angle to plot the elevation patterns.
 
It is essential that all measurements be performed in the far field; i.e. R>= 2D^2/lambda or at a minimum lambda/D, where D is the overall size of the test aperture.
 
 
Currently, I am building a facility for methodology "A" above on a 42 acre piece of property in South Central Va.  Initially this range will perform SCALE MODEL analysis with the appropriate dielectric and conductivity measurements performed at the 'scaled' frequencies; in this case linear modeling is appropriate; this is how aircraft antennas were developed  in the 1950's/60's using scale model of airframes and placed on an antenna range.  The same applies for mobile antenna.


Another issue almost everyone overlooks, is the change that even one dB increase in signal strength can make in the SNR of the receiver. Enough said.
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WX7G
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« Reply #2 on: March 06, 2011, 12:19:22 PM »

Poppycock. I perform radiated repeatable RF measurements for a living. It's not that hard. I used to teach a class titled "The Art of Signal Measurement" where I would teach the differences between ABSOLUTE and RELATIVE measurements. I taught the students how to perform a measurement error analysis to quantify (and thereby be able to control) the sources of error. The measurement I cite is a RELATIVE measurement and the author did a good job of making it so.

Some people like to make something very simple very complicated.

In the article the author placed antenna  A on the bumper of a car and measured the field strength. He then replaced that antenna with  antenna B and measured the field strength. He stated the difference as 6 dB. Pretty simple. This is a RELATIVE measurement between antenna A and antenna B. They are mounted the same way on the same vehicle. A RELATIVE measurement like this is simple to do with small measurement uncertainty. The azimuth radiation pattern will be the same for these two antennas. The elevation radiation pattern will be essentially the same for these two antennas. The other issues mentioned by the author of your email drop out of the error analysis. In other words, an error that affects the measurement of antenna A affects the measurement of antenna B equality. Therefore, such an error makes NO difference.



« Last Edit: March 06, 2011, 01:12:09 PM by WX7G » Logged
GOLDTR8
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« Reply #3 on: March 06, 2011, 02:19:18 PM »

I thought that it was a good read and informative.

There is always something that one can learn and via one bit at a time.
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K0BG
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« Reply #4 on: March 07, 2011, 06:09:03 AM »

Yes David, but if the receive antenna didn't move, they all you measured was the radiation at one TOA. That really doesn't tell you very much.
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W9PMZ
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« Reply #5 on: March 07, 2011, 06:24:07 AM »

Alan,

The measurement should be done based on ground wave propagation, so TOA does not matter.  If more that the ground wave is influencing the measurement the more the problems there will be; because then the measurement would be time dependent.

Remember that a linear time in-variant measurement should be made.

73,

Carl - W9PMZ
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W5DXP
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« Reply #6 on: March 07, 2011, 06:25:17 AM »

This antenna article mentions a reference antenna that by the description is a Hustler. It's interesting that the difference, on 80 and 40 meters, between the reference antenna and a maximized mobile antenna is 6 dB.

As another data point, in 198x CA 75m mobile shootouts, the Hustler system measured 6 dB below the bugcatchers and screwdrivers with no top hat. Adding a top hat to a bugcatcher/screwdriver gained 2 dB.

http://www.w5dxp.com/shootout.htm
--
73, Cecil, w5dxp.com
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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.
M6GOM
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« Reply #7 on: March 08, 2011, 02:10:23 PM »

Someone needs to do a series of tests which take all of the variables into account, and normalized to the extent possible. No shoot out can do that, Rob's article notwithstanding.

Indeed. There is an annual mobile antenna shootout that I believe you mention. I had a look and TBH I was horrified at the antenna installations of the participants. The results are about as much use as a chocolate fireguard. I could take mine there and win simply because I'd be the only participant with it mounted via a hole drilled in the middle of the car roof.
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K0BG
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« Reply #8 on: March 08, 2011, 06:44:37 PM »

David, WX7G, used the term poppycock with reference to my post about the correct way to do field strength measurements. Be that as it may, measuring just one specific plane in the overall pattern, regardless of the frequency, doesn't tell you much. A fact he challenged again. If you increase ground losses, it is indeed possible to increase the near field signal strength, which is easily explainable. Yet, David, and others consider near field measurements an accurate comparison. They're not, but you're not going to convince them otherwise.

A few posts ago, I suggested to David, WX7G, that he think outside the box. His response was to challenge me with the fact he had 9 or so patents covering antennas, and 6 in the works. All I could find was 2, and they had nothing to do with antennas. When I asked for the patent numbers, suddenly the references were edited out. That fact alone, is enough for me to question any, and everything he postulates. This includes any suggestion that antenna shootouts, or that antenna mast lengths have nothing to do with efficiency, field strength, or input impedance. His erroneous statements to the contrary, further erode his position.

I can't speak for you, Coner, but from here on in, I'm going to ignore his postulations.


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W9PMZ
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« Reply #9 on: March 09, 2011, 02:33:05 AM »

if a signal orginates from a source and is propagated to a destination, e.g. transmitter, coax, antenna, transmission medium, antenna, coax, receiver - this is a linear time in-variant system (LTI).

so in the transmission meduim, if it is LTI, then all of the parts in all of the planes are affected equally.  in other words if one plane is acted upon by 6dB then because it is LTI, then all of the other planes will be acted upon in the same manner by the same 6dB.

this is a relative measurement and not an absolute measurement.  relative measurements are easy to make, everyone does it all of the time.

73,

carl - w9pmz
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W5DXP
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« Reply #10 on: March 09, 2011, 05:48:10 AM »

Yet, David, and others consider near field measurements an accurate comparison. They're not, but you're not going to convince them otherwise.

For all of the 75m CA shootouts that I attended, the measurements were always made in the far-field - many wavelengths away from the source. All the entries were typically vertically polarized using vehicles as ground planes. The relative field-strength measurements for particular types of antennas were consistently correlated over a number of years. Samples with obvious problems were removed from the sample pool before summarizing the results.

Near-field measurements and dissimilar antenna comparisons would be suspect. I don't see any primary problem with the way the 75m CA shootouts were conducted by RF engineers who performed such activities for a living.

so in the transmission meduim, if it is LTI, then all of the parts in all of the planes are affected equally.

Does a Mini-Cooper and a 30 foot motorhome have the same take-off-angle?
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73, Cecil, w5dxp.com
« Last Edit: March 09, 2011, 05:58:12 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.
W9PMZ
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« Reply #11 on: March 09, 2011, 07:30:20 AM »

"Does a Mini-Cooper and a 30 foot motorhome have the same take-off-angle?"

The meduim is a linear path.  So my comment earlier to the comment that I made, that the comparison be done where the ground wave is dominat, then a comparison of source to point one and pont two can be done to evaluate the difference.

73,

Carl - W9PMZ
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W5DXP
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« Reply #12 on: March 09, 2011, 03:34:14 PM »

... the comparison be done where the ground wave is dominat,

Let me rephrase the question: Between a Mini-Cooper car and a 30 foot RV, is the ratio of sky-wave signal to ground-wave signal equal? What are the laws of physics governing that ratio?
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73, Cecil, w5dxp.com
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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.
W9PMZ
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« Reply #13 on: March 10, 2011, 02:01:12 AM »

"Between a Mini-Cooper car and a 30 foot RV,"

This is why shoot outs are meaningless exercises for antenna comparison.  In a shoot out all you are comparing is the installation, not the antenna.

If you wanted a shoot out that measured the performance of the antenna, the antennas under test would be mounted to the same platform and the measruement equipment would be the same.

73,

Carl - W9PMZ
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W5DXP
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« Reply #14 on: March 10, 2011, 06:48:56 AM »

If you wanted a shoot out that measured the performance of the antenna, the antennas under test would be mounted to the same platform and the measruement equipment would be the same.

In one of the 75m CA shootouts, I entered three configurations using my full-size GMC pickup. A 75m hamstick and a 11.5' whip/SG-230 combo were both 12 dB down from my top-rated, top-loaded antenna using the same measuring equipment. I'm convinced those results were valid. Most of the other entries were also mounted on pickups. Predictably, the small cars didn't yield very good results.
--
73, Cecil, w5dxp.com
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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.
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