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Author Topic: Common HF Mobile Frequencies  (Read 4713 times)
ZENKI
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Posts: 934




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« Reply #15 on: June 28, 2011, 02:40:56 PM »

Using a field strength meter.

Anyway the Antenna is 11ft long overall. 6ft base  about 7uh loading then a 4ft whip. Monoband 20 meters. No cap hat. Its not your average hamstick.  Its about 1.5 db down from the full size 1/4 wave antenna

The  antenna is fairly light compared to the extremely heavy screwdrivers.  It uses a very light E-glass fishing rod blank and a high Q coil. I only drive small Volkswagen  golf and I win just about every  shootout that I enter on 20 meters. I compared it directly with a full size quarter wave in the same position when making the measurements. 3inch coil 10# wire.

What efficiency do you calculate  it to be?

I have not heard your signal because of propagation and the times that you are probably mobile. Anyway I am sure we can work mobile to mobile on 20 meters or 40 meters sometime.

Sys signal with that antenna is legendary. However I do respect your comments. Its hard assessing an antennas efficiency from listening to  a band with 10db of fading.  There is no doubt that his installation works, impressive for such a short antenna. I am sure that big car that he drives helps combat some of the ground losses.

Hem. Guess you never worked me.

I'm not impressed with any cap hat design that is mounted close to the coil. I'm also not impressed with the large Hustler coils, as they actually have a lower Q than their smaller counterparts, due to the large end caps.

And if you will, please, explain to me how you came up with a 20 meter efficiency rating of 82%?
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K0BG
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« Reply #16 on: June 28, 2011, 03:24:47 PM »

A few comments. First of all, a field strength meter might be okay (depending on what it is) to compare one set up with another, but not to do an absolute measurement without calibrated equipment. Even with the test gear I have, I can't do absolutes. For comparison sake, I can get down to about .1 dB with the two port VNA. This is a bit better than the resolution I get with the Motorola 2001A. Again, I can't measure absolutes because I don't have a calibrated HF receive antenna. The one I have, is only calibrated down to 50 MHz.

A nominal 10.5 foot, 20 meter mobile antenna (the actual length of the coil doesn't count, but any wire other than what is in the coil does), the requisite coil reactance, mounted as you say (6 feet up) would only need to be about 4 uH depending on the actual Q which is an unknown at this point. If I had to guess, the efficiency is probably closer to 70%, given the overall length, position of the coil, and an assume Q of about 200 or so.

To get an 85% level on a 20 meter mobile antenna, requires a decent sized cap hat, with an effective overall electrical length of about 14 feet, and well mounted with a good metal mass under it.

Even on 10 meters, using a copper resonant element, getting past 85% requires a cap hat (top loading), and good mounting techniques. Do everything correctly, and you might get to 95%. Remember, you have to deal with ground loss (≈2Ω to 5Ω), and mounting loss (dielectric loss, ≈2Ω) to deal with, and you can't fix them!
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ZENKI
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« Reply #17 on: June 29, 2011, 02:23:36 AM »

I used a fairly accurate R&S EMC receiver with a calibrated antenna with a known antenna factor. This setup was in Cal borrowed from work,  its very accurate. Guaranteed to be within a 0.5db. Whats not accurate or controlled is the test range. So my measurements may be meaningless because it was done in a empty car park with power line and metal power poles. So who knows. I was just curious so I did the measurement.

I am happy with the antenna. Eventually I will rebuild it so I dont need any loading inductor just the top hat. However thats  a lot of work.

Since I am dealing with losses that cant really be negated or easily controlled, I will leave well alone. Thanks for the explanation of  the various losses in the typical installation. Mobile operation is always a challenge and the fun makes the challenge worthwhile.
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K0BG
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« Reply #18 on: June 29, 2011, 06:19:41 AM »

Well, at least you didn't settle for a Lil Tarheel or HS Sidekick.

You are very correct about this things you can't measure, and assuming them is a prescription for error.

I wish I was filthy rich like Bill Gates. That would allow me to build a test range like HyGain used to have in Lincoln. The university's field house sits on the site now. All of the old test data is owned by the university, but it just sits there languishing. I'd love to have access to it before someone throws it out.

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