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Author Topic: Antenna shootout results  (Read 2462 times)
K0BG
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« Reply #15 on: November 29, 2009, 01:10:28 PM »

As for plungers. One made in California, one that looks more like a screwdriver made down in S. Carolina, and another made in Ohio, use a fixed coil. The bottom of the coil is electrically connected to a plunger which moves up and down inside the coil, although the one from Ohio is on the outside of the coil. As the plunger moves up, more and more of the coil is short tapped. Short tapping isn't ideal as doing so reduces Q. Whether this is a detriment, depends on a lot of factors. In one case, the top end of the coil is a large chunk of aluminum. Even when the plunger is all the way up, some coil is still active, and caught between the metal plunger and the metal end cap. Under the right circumstances, the coil operates above self resonance, and the losses are almost massive.

In the traditional screwdriver, the coil is swallowed by the mast. At the top of the mast are contacts which circle the coil. The top of the coil is connected to the whip, obviously. The lower end of the coil is inside the mast. It doesn't actually contact the mast, but in some models it does.

Cheaply made screwdrivers often have enough slop that the bottom of the coil that it is possible that it contacts the mast, and when it does the coil's Q and resonant frequency changes. I know of two that do this, and it is very evident when you're traveling down the highway. At first, I didn't know what was causing the problem, but did manage to figure it out in due course.

I've put a lot of thought into this, and I can't see an easy way to measure the current in the unused portion of the coil. About the only thing you can 'see', is when the gap between the coil and the mast gets too large. There can be arcing between the lower sections of the coil, and the interior of the mast. I personally have seen the results of this happening when playing with inexpensive screwdrivers. Based on nothing other than that, one has to draw a conclusion that circulating currents can be significant.

As Belrose points out in many of his articles, the best solution is to have a coil of the exact size needed with no shorted turns. I'll buy that. But the rest of us like the idea of changing bands while under way. The least wind loading is offered by one of the two types mentioned. However, when compared to a fixed coil, both methods are a compromise Q wise, all else being equal. Since so many factors are at play, it's difficult to calculate and/or measure reductions in Q. About the only thing you can do fairly reliably, is measure field strength, and compare that to the same antenna modeled with EZNEC or NEC4. Belrose wrote about this methodology in an article which appeared in the ARRL Compendium #4. I think I've read that article 10 times.

I am not encumbered with massive amounts of test gear, and I don't have the real estate either. This sort of leaves me on the short end, so to speak. In any case, if one really had the time, inclination, and equipment, testing a few models of each over their operating range, would certainly be enlightening.

Alan, KØBG
www.k0bg.com

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WX7G
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« Reply #16 on: October 05, 2012, 07:50:08 PM »

I ran across a reference to this antenna shootout and the Hi-Q company again today. Looking at the test conditions it is VERY obvious why the Hi-Q antenna did so poorly. It was mounted on the rear bumper of a van! Mounted on the bed of a truck as the "winning" Scorpion antenna was the Hi-Q might have been the winner.
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W8JI
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« Reply #17 on: October 08, 2012, 08:00:47 AM »

You'd be surprised at how little loss of Q often occurs from shorting turns under the right application.

On the other hand, there can be a huge problem from not having shorts at multiple places in unused coil.

Look at a Smitch chart of a roller inductor near the bottom of this article ("The Roller Alone" section):

http://www.w8ji.com/tuner_baluns.htm

Loading coils for antennas can have the same problem.

73 Tom
     
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W8JI
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« Reply #18 on: October 08, 2012, 08:48:29 AM »

One question (among others) I have about the shootouts is the method of measurement.

If I understand correctly, all the remote measurements are taken at ground level "several wavelengths away." If that is correct, then the data isn't worth a lot (which everyone seems to agree upon anyway.)

How about measuring at 10 degrees elevation from 20 or more wavelengths away? It wouldn't be easy to do this, but it could be done.


I might be wrong, but I'm guessing you (like "Woody") do not understand antenna patterns in commonly used Ham programs.  The patterns we see in EZNEC  are based on calculations at a very long distance from the antenna over a flat ground media. The calculation distance is so large that any low angle signals are forced to near zero by earth attenuation.

At casual glance, without thinking about how pattern is calculated, this gives us the false impression peak radiation is at some angle higher than zero degrees. While the model is accurate at distances of hundreds of wavelengths over a flat homogeneous earth, it doesn't represent the elevation pattern at reasonable distances at all.

Misunderstanding  pattern calculation methods causes us to accept TOA as being accurate for verticals, and come up with all sorts of weird theories about vehicle chassis radiation. It makes us *think* we should measure at some higher angle to be in the main lobe. This would indeed be true if we were measuring ten or twenty miles from the antenna, but the simple solution is to not measure twenty miles away where groundwave has deteriorated to zero.

Let's be clear about this. TOA and low-angle elevation pattern does NOT apply to vertical antennas unless we are at infinite distance. We can certainly use model pattern to compare efficiency between verticals of similar height, or to see if some ground systems are radiating at high angles, but it is a terrible mistake to think the pattern at some huge distance over flat earth in away way represents things at modest distances.

There is nothing wrong with comparing short vertical elements on groundwave at distances of a wavelength or two.

Now if an antenna is mounted out on a vehicle's end there certainly can be pattern distortion, and there might be a little radiation from the chassis, but radiation is minimal. The earth tends to cancel any high angle chassis radiation, and the chassis is normally fairly wide and tall compared to length.

I don't see anything wrong with a 1-2 wavelength distance groundwave measurement of short vertical antennas.
 
73 Tom
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W5DXP
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« Reply #19 on: October 08, 2012, 03:08:43 PM »

While the model is accurate at distances of hundreds of wavelengths over a flat homogeneous earth, ...

I wonder how many hams are members of the flat earth society? Smiley
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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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