[R-390] Small Voodoo Antennas

[Bob Camp]

Hi

The wire wound arrow shaft antenna working well  is actually a proof of 
Maxwell's fifth equation which more or less states "the antenna you 
count on the least will always work the best".

Most of us go (myself *very* much included) go overboard on  receiving 
antennas most of the time.

Transmitting antennas are a bit of a different story. A lot of the 
theory we are taught applies only to a transmitting antenna. Which 
parts of the rule book apply to which kinds of antennas is rarely made 
clear either in the text books or in class. Often the reaction of 
anybody who has used an antenna is "electromagnetic theory is bunk". 
It's not bunk, it's just so complex that they rarely teach it in a 
comprehensible fashion.

If you sprinkle a bit of salt (maybe a bit more than most people would 
....)  on a  damp piece of yarn (say very damp) looped over the back of 
a chair (a large chair)  and hook it to your R390 whip antenna input. 
It is acting as an adequate antenna *if* the radio noise level (front 
end noise + antenna noise) rises when it is connected to the radio.  It 
may not be the best antenna you could have but it is doing a job for 
you. For the full recipe and other exciting details on this dish please 
visit our web site ....

A lot of modern antennas are amazingly small by the standards of days 
gone by. One excellent example of this is an amplified whip antenna 
that a bunch of the guys from this list came up with. It uses a very 
expensive FET running hot enough to cook an egg. Very good performance 
from sub 100 KC up through 30 MHz (it even does that little trick as 
well ...). Most loran-C antennas are *very* small when you consider the 
wavelength of a 100KHz signal.

Most of us have more trouble from local noise than we used to. A small 
antenna located far from, or maybe at right angles to a noise source 
may be a better bet than a nice big one that runs right into the noise.

Soon by virtue of broadband over power lines we all will be able to get 
a *lot* more experience with this sort of thing. Or I suppose we could 
just stop playing with radios. Then we could turn this into a full time 
humor list ....

	Take Care!

		Bob Camp
		KB8TQ



On Jul 10, 2004, at 5:07 PM, Rbethman wrote:

> Hey Gang!
>
> Take this in another direction.  Using one of those 50 ohm antenna 
> connectors, I attacged two fiberglass arrow shafts.  I joined them 
> together with a 1/4" dowel.  I wound its length with #26 enameled wire 
> VERY tightly and closely wound.
>
> I have NO idea as to its impedance.  BUT - my den is mostly below 
> ground level.  The recption ia AS GOOD as an outdoor antenna of either 
> dipole or ant other I've strung!
>
> The R-390A loves it and performs flawlessly.  YMMV.  But it sure works 
> for me!
>
> Bob - N9DGN
>
> Bob Camp wrote:
>
>> Hi
>>
>> If Maxwell is right (and in better than a century nobody's proved 
>> different) then you can make some pretty neat smaller antennas. The 
>> problem is that nobody has ever figured out how to do a good enough 
>> job of it.
>>
>> This actually gets back to another R390 issue I keep fishing around 
>> on.
>>
>> One of the ways to do a "compact dipole" is with a small resonant 
>> loop. The output impedance of these things can be almost anything 
>> depending on their design. They are inherently a low noise balanced 
>> antenna and you mess them up a bit when you convert them to a 
>> balanced line. The lowest noise configurations are generally shielded 
>> and balanced.
>>
>> Now let's see, shielded balanced antenna that may have an unusual 
>> impedance. What radio's antenna input does that bring to mind?
>>
>> So far I have found absolutely no evidence that such an antenna ever 
>> figured into the design of the 390, but I keep fishing.
>>
>>     Enjoy!
>>
>>         Bob Camp
>>         KB8TQ
>>