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Author Topic: Antenna shootout methods  (Read 8412 times)
W8JI
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« on: October 08, 2012, 12:54:19 PM »

Reading some stuff, it seems people misunderstand elevation patterns of mobile antennas. They think the zero angle null means the antennas have no groundwave, and peak radiation is always at some higher angle. They then wrongly conclude the antennas are being measured in a deep null area.

If that the primary complaint of people who think the measurement methods are unfair or inaccurate?

If so, how common is that error in perception?

Why is the website of one antenna manufacturer so nasty about people on a personal level??? It seems to rant about some particular unfairness or other people's intelligence. What is the unfairness or stupidity they refer to?

73 Tom
 
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W5DXP
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« Reply #1 on: October 08, 2012, 03:03:06 PM »

If that the primary complaint of people who think the measurement methods are unfair or inaccurate?

Years ago, when I was involved in some of the CA shootouts, the major complaint seemed to be that the type of vehicle had as much (if not more) effect than the antenna itself. GMC and Chevy pickups always seemed to win.Smiley
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K0BG
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« Reply #2 on: October 08, 2012, 03:30:19 PM »

Part of those complaints you refer to are directly aimed at me personally. I can elaborate, but let's leave sleeping dogs lie (pun intended).

Then along comes egos! When they get bruised every negative comment you ever heard of comes to the surface. Heck, I've heard and read words I didn't know were in the dictionary!

Part of the issue is the way antenna shootouts are conducted (which I think you are referring to). They're not scientifically normalized, the measuring methodology is poor at best, so the resulting data isn't much more than a brag tape! Adding great insult, is a total lack of theory or practicality. You of all people understand about nulls, shadowing, and other issues dealing with measuring antenna patterns. While shootout personnel may know about them, they ignore them.

If there are any commonalities to antenna shootouts they're easy to spot. The most important one is mounting the antenna with as much metal mass directly under the antenna as possible. Thus, pickup bed mounted antenna seem to reign. Higher mounting always outshines trailer hitch mounts. Antennas with large cap hats properly mounted at the very top of the antenna also score high. To a lessor degree, properly choked coax and control lines typically place higher than those without. Rear mounting on vans, SUVs, and similar vehicles always score low. If I had to describe the issue, it would be free and clear always reigns. I could go on, but the picture is evident.



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W5DXP
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« Reply #3 on: October 09, 2012, 05:34:23 AM »

They're not scientifically normalized, the measuring methodology is poor at best, so the resulting data isn't much more than a brag tape!

The 80's-90's CA 75m mobile shootouts were well run by RF engineers. Power delivered to the antenna was measured and normalized, i.e. factored into the results. A ferrite rod receiving antenna on a pole was used to minimize the effects of human bodies. The RF measuring device was borrowed from Lockheed. The compiled results of three of those shootouts are at:

http://www.w5dxp.com/shootout.htm
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W8JI
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« Reply #4 on: October 09, 2012, 07:38:44 AM »

I think, other than the issue with vehicles and (in particular) mounting, most shootout data is better than people expect.


The 80's-90's CA 75m mobile shootouts were well run by RF engineers. Power delivered to the antenna was measured and normalized, i.e. factored into the results. A ferrite rod receiving antenna on a pole was used to minimize the effects of human bodies.


An Italian will also work.


Seriously though, a properly constructed small loop is more stable than a rod antenna. I think either is OK, when properly constructed and used, and when properly constructed they can be right next to someone and no be affected when on lower bands. There is no reason to get them far away from people, unless there is a design problem.


Quote
The RF measuring device was borrowed from Lockheed.


That must mean it is more accurate than one owned by Northrop-Grumman, or even worse some Ham. :-)

Seriously, with almost no equipment at all it is possible to have very accurate comparisons. While the vehicle has some effect, the antenna almost clearly must dominate some of the extreme difference. Look at some of the systems that use large metal coil end caps and huge capacitance hats just above the coil. 

73 Tom
« Last Edit: October 09, 2012, 07:42:06 AM by W8JI » Logged
W8JI
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« Reply #5 on: October 09, 2012, 07:49:09 AM »

If there are any commonalities to antenna shootouts they're easy to spot. The most important one is mounting the antenna with as much metal mass directly under the antenna as possible. Thus, pickup bed mounted antenna seem to reign. Higher mounting always outshines trailer hitch mounts.

So....


Has someone ever moved an antenna from one location to another on the same vehicle and measured the change with ONLY a mounting position change?

Has anyone ever changed antennas on the same vehicle without a mounting position change?

Has anyone ever mounted the same antenna in the same position on two different size or height vehicles?

If more than one physical or electrical thing changes, how can we tell what makes the FS change?

73 Tom
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K0BG
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« Reply #6 on: October 09, 2012, 11:17:33 AM »

Using a 12 inch loop, 12 feet above the surface for receiving, and a miniVNA to measure the difference....

Yes to question one. Bumper vs. top of the quarter panel. It depends on how close (distance wise) you measure the difference. About one wave length, you almost can't tell the difference on 80 and 40. Six to 7 wave lengths, about 2-3 dB.

Yes to question two. HiQ 580 vs. Scorpion 680. Eighty meters about 2-3 dB. Twenty meters where the HiQ plunger is very close to the top of the coil, an easy 10 dB difference. Both cases with a 5 foot diameter cap hat mounted 4 feet above the coil.

No to question three.

Answer to question four. I don't think you can with reliability. One change that is always very noticeable (≈6 dB average depending on the band), and that is replacing a 102 inch whip with the aforementioned cap hap. Retuned of course.

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W8JI
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« Reply #7 on: October 09, 2012, 01:30:44 PM »

Has anyone ever changed antennas on the same vehicle without a mounting position change?

Yes to question two. HiQ 580 vs. Scorpion 680. Eighty meters about 2-3 dB. Twenty meters where the HiQ plunger is very close to the top of the coil, an easy 10 dB difference. Both cases with a 5 foot diameter cap hat mounted 4 feet above the coil.
   

That's a huge difference for a different antenna on the same vehicle.

Which antenna was better?
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K0BG
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« Reply #8 on: October 09, 2012, 03:21:44 PM »

The Scorpion.

The HiQ plunger is aluminum, about 1/2 inch thick. The peripheral has contact material embedded in it. The end caps are aluminum too. The top one is about 2 inches thick. With the large cap hat on (there are photos of the cap hat on my site), and the plunger clear to the top, the antenna is resonant just below 17 meters. At that point, there are about 5 turns of coil unshorted due to the basic design. You can operate the antenna on 15 meters, and the SWR will be almost as good as it is on 17 (<1.5:1). On 12 it is a bit higher (2:1??). On 10 it is about 3:1, and on 6 meters 4.5:1. The only way that can happen is for the coil to be operating above its self-resonant frequency.

One of the salient points which started some of the malicious palaver directed towards me personally, was my attempt to persuade the owner to switch over to a self-reinforcing polyphenylene like TecaMax®. That, and about the same time the HiQ 580 came in almost last in a 3905 Group shootout, and the rest is history!
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W8JI
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« Reply #9 on: October 09, 2012, 04:56:37 PM »

That's unfortunate. I especially think it is unfortunate because of the home page tone about you.

I'm not sure where we are heading in this country, but downright nastiness is spreading like a disease.
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K0BG
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« Reply #10 on: October 09, 2012, 05:50:14 PM »

My mother always taught me that what goes around, comes around. Thus, I would like to think, that in the end, we all pay for our transgressions. I trust I'll live long enough to see that truth.
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W2RI
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« Reply #11 on: October 10, 2012, 06:52:38 AM »

If anyone believes those shootout results are accurate - well, it doesn't say much about their judgement.
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WB6BYU
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« Reply #12 on: October 10, 2012, 09:15:02 AM »

I think you have to distinguish between the accuracy of the measurements and the
accuracy of the conclusions drawn from them.

From what I've seen, the tests give a reasonable representation of the performance
of specific mobile antenna installations - that includes the antenna, ground system,
matching network, vehicle, mounting method, etc.  Sure, there are some sources
of errors, such as whether the distance to the receiving antenna is measured from
the transmitting or the nearest corner of the vehicle, but those can be corrected
for.  Except for very poor installations - for example, where the antenna is shielded
by part of the vehicle - I'd expect that relative results should be good within a dB
or two.  Repeatability can be assessed by measuring each installation multiple
times.

What it doesn't tell you, however, is the relative qualities of the individual antennas
in other installations, or how they perform on other bands, or where the major
sources of loss are in each case.  You can do some further testing, such as comparing
several antennas on a common mount, or the same antenna on different vehicles,
to get a sense of this, but that really isn't the purpose of the shootouts.


So if one mobile installation measures 3dB better than another after correcting for
as many variables as possible, it is likely to be because it is, indeed, a more efficient
installation, especially if that measurement is repeatable.  But that doesn't imply that
one antenna (or vehicle, or mounting method, etc.) is that much better than the
other.
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W8JI
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« Reply #13 on: October 10, 2012, 09:18:34 AM »

If anyone believes those shootout results are accurate - well, it doesn't say much about their judgement.

Can you give an example of the problems you see?

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W5DXP
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« Reply #14 on: October 10, 2012, 02:30:50 PM »

If anyone believes those shootout results are accurate - well, it doesn't say much about their judgement.

This chart at the bottom of this web page is based on the distilled results of three 75m CA mobile shootouts.

http://www.w5dxp.com/shootout.htm

In your opinion, how should these rankings be rearranged for greater accuracy? For instance, should the Outbacker be ranked higher than a Bugcatcher?
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