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Topband: Re: 160 Loaded Vertical Antennas (Barry, W9UCW)

To: <topband@contesting.com>
Subject: Topband: Re: 160 Loaded Vertical Antennas (Barry, W9UCW)
From: W9UCW at aol.com (W9UCW@aol.com)
Date: Fri May 16 06:46:35 2003
Hello again, Tom
You responded to my last post with some comments & a question.
 
You said:
>With any monopole antenna radiation resistance carrying uniform current
>radiation resistance is given in textbooks (like Jasik's "Antennas") by:
>1580 * (He/WL)^2

I think you have a syntax problem in that sentence, but I recognize the 
formula.

You said further:
>With wavelength 250-feet and an antenna 9-ft tall we have a maximum possible
>radiation resistance (uniform current case) of 1580*(9/250)^2 or about 2
>ohms. With triangular distribution (base loading only) we have 1/4 that
>value, or 0.5 ohms. In order for such an antenna (between .5 and 2 ohms
>radiation resistance) to have less current than the area 9-ft from the end
>of a monopole, the small system would have to have hundreds of ohms of loss
>resistance because the short radiator is a virtual short-circuit (someplace
>between .5 and 2 ohms).

I think you have that right.

Then you said:
>I= sqrt P/R, so current into the whip to radiate 100 watts of actual
>radiated power as EM energy would be sqrt of 100/2 ohms ranging to
>0.5ohms....or someplace between 7.07 and 14.14 amperes. If you didn't
>measure at least 7-amperes into the whip you were not radiating 100 watts as
>EM radiation, and it would be ~14 amperes if the whip was uniform
>cross-section with base loading only and 100 watts was radiated.

Sounds correct to me, but what's your point?
If you are alluding to measurements we made, I should report that all coil RF 
currents were held to 100 ma. or less. All other types of measurements were 
made using 10 watts supplied to the system under test.

>Even with 1-watt radiated from a short 9-ft whip on 75-meters, current into
>the whip would be between 0.7 and 1.4 amperes.

That sounds right, too.

>Whip feed current with ONE watt radiated would be exceed current 9-ft from
>the end of a 1/4-wl monopole with the 1/4 wl monopole radiating 100 watts!!
>(Current is approximately .4 amperes 9-ft from the open end of a 1/4wl thin
>monopole with 100 watts actually radiated.)

Another syntax problem, but without the word "be", it makes some sense.

>How did you physically manage measuring current in a close-wound 2" coil
>without perturbing the system?

One of the people in our merry band had government connections. He got us a 
dozen small calibrated RF ammeters with very small thermocouples. The meters 
were mounted on the thermocouples and directly on the end turns on the coil. 
I didn't say the 2" Airdux coil was closewound, and it wasn't. When the whole 
study is published, there will be pictures of these devices included. BTW, 
the meters were read using binoculars.

As for the rest of your comments, Tom, I can only say that no large objects 
were "hung off the coils", they were not "physically large compared to the 
wavelength", nor were they helical resonators. The coils were the same types 
and sizes commonly used to load mobile antennas.

All current measurements were made twice because the coil was always reversed 
to see if an anomaly was present in a meter pickup. All other measurments on 
these antennas did not change with or without the "embedded" RF ammeters.

I think your position is clear....that under the condx described, current 
reduction in a loading coil can't, won't, never did, never will happen. My 
position is that it always does, and I've measured it. Niether of these 
hypotheses will  go far to satisfy the real objectives of our study.

Actually, the coil current measurements were a small part of our program, an 
afterthought, if you will, done when the bulk of the work in Fletcher, NC was 
complete. Our focus has always been on real world performance differences for 
various configurations of  mobile antennas, mast lengths, coil positions, 
multiple resonators, mounting angles, matching devices, magnetic mount 
effects, vehicle "footprints", grounding methods, etc. The objectives center 
on getting practical answers about practical antennas. Larry, W7IUV put it 
best in his design criteria  order of importance. 

>"With that in mind, here are my design criteria in order of importance:"

>1) Survivability - Even the most efficient antenna is worthless when broken

>2) Low cost / easy replacement - Sooner or later it will get broken or 
>vandalized (or you will want to "improve" it)

>3) Efficiency

I wish I had said that, Larry.....and probably will when you aren't looking 
:o).


73's Barry, W9UCW
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