[BC] Isotron AM Antennas

[RichardBJohnson at comcast.net]

In the "olden days," we had a "grid dipper." These
would find the resonances. The problems with computer programs
are the models. What is the capacity to your counterpoise from
the end of the helix? Damned if I know either. After the effective
length of the contraption exceeds 90 degrees, it starts to act
like a capacitor hanging on the far end. What value, and what
length to get that effective length? Damned if I know, either.

That's why one needs to use the "scientific method," meaning
experiment, rather than an analytical method. You need to know
just enough to be dangerous, then experiment.

In UHF design in the olden days, we used a helical resonator to
form a high Q trap. It was an open-ended silver-plated copper
coil inside a drawn metallic container with no holes. At about
400 MHz (Megacycles --to get in the frame of mind), such a
circuit had a Q of about 400. If you were to remove its cover,
the Q would go down to about 60. The difference was caused by
radiation resistance! When it had a cover, nothing could leak
out so the Q was determined by the effective resistivity of
the silver-plated copper coil. Once this was exposed to the
environment, it could radiate and that radiation resistance
lowered the Q.

With a "giant duck" antenna, as opposed to a "rubber ducky,"
the idea is much the same. You build a series-tuned circuit
in which the resonating capacity is whatever exists at its
end, and whatever inductance is necessary to cause resonance.

If there was zero loss and radiation resistance, the giant
duck would have zero impedance. Real-world components, in
the absence of radiation resistance, would result in perhaps
5 ohms of loss resistance at RF frequencies, much greater
than the DC or 60 Hz losses. If the radiation resistance was
45 ohms, determined by the device geometry, you would have a
good match, 45 + 5 = 50 ohms, and the efficiency would be
45/50 = 90%. That's about all the math guestimations one
can make. 

You could add capacity at the top, effectively creating a
base-loaded vertical, but this increases the Q, lowering
the bandwidth. Basically, you do this if you have cut off
too much wire when you were tuning the device length.
 

Cheers,
Richard B. Johnson
Website http://AbominableFirebug.com

----- Original Message -----
From: "Tom" <Radiofreetom at gmail.com>

So, a six-inch by 10 ft PVC pipe wound with that same 100 ft of #10 (I 
have a 500' spool of #14 available) insulated wire will self-resonate 
higher in the band than the 2 inch x 20 ft. 

Then adding in a whip antenna at the far end of the coil will add 
capacity, right?  Moves the resonance DOWN, if I'm remembering correctly.