[BC] Short, Helically-wound Antennas

[Richard Fry]

RichardBJohnson wrote:
>If you have no tower standing, get a 20 ft section of 2-1/2 in 
>plastic sewer pipe
>(two 10-foot sections).... Wind the plastic pole with #10 insulated 
>wire over its
>entire length before you erect it ... This will be the "tower." Find 
>its resonant
>frequency by connecting the end near ground to ground and bringing a real
>portable radio (the kind with a continuously- tuning dial) near it. 
>A sharp peak
>in noise (or distant stations) will occur at resonance. Connect this gigantic
>"rubber duckey" to the coax center and go on the air.  Depending upon
>ground-system losses, it will probably be near 40 ohms which should be
>close enough for low power operation.
___________________

Such a configuration may be self-resonant, but that does not mean 
that it has the same radiation resistance as that of a linear 
monopole at its 1st self-resonance (1/4-lambda).

According to John Kraus in "Antennas," 3rd edition, Chapter 8-22 -- 
the radiation resistance of such a helical antenna is approximately 
the same as that of a linear monopole of the same overall height, and 
not to the length of the wire used to wind the helix.

The radiation resistance of a short monopole is approximately 
(h^2)/312, where h = height in electrical degrees.  A 20-ft monopole 
is about 12-1/2 degrees at 1700 kHz, so the highest radiation 
resistance possible for your configuration for MW broadcast use is 
about half an ohm.

Kraus says in this chapter "The advantage of the helix over a 
straight wire or stub is thats inductance can resonate the antenna."

This means that a separate "loading coil" is not needed, which can 
reduce the fixed resistive losses in the antenna system.  But it 
doesn't mean that a self-resonant, normal-mode helix has the 
radiation resistance and radiation system efficiency of  linear, 
1/4-wave monopole (other things equal).

RF