On Mon, 27 Sep 2004 21:18:45 GMT, "John Smith"
wrote:
Shown in "Antennas for All Applications" on page 820, figure 23-17 (a) is
a folded dipole which looks as if it could be easily constructed from
RG58.

Um, actually it quite explicitly states it uses "Cool Cable." You
might try some Hip Cable as a substitute.

This antenna would be appealing because of the natural balun.

The "natural" of the BalUn is probably in the balance and the
orthogonal geometry.

However, they make no mention of the required coax velocity factor nor of
the terminal impedance.

Reckon on the outer surface velocity factor (it is really only a thick
radiator) which, if you are using solid coax is 1 (lower if it has
insulation).

I tried constructing one and measuring with a
vector voltmeter but the data were inconclusive. That is, I don't think I
know how to make measurements like this at 440 MHz yet. It is much
trickier than I thought.

Please share your data.


In any case, can someone tell me:

- Can RG58 be used, or will the velocity factor mess up the antenna?

already answered.

- What will be the terminal impedance?

Depends on dimension, folded dipoles multiply the load on the basis of
separation and ratios of thickness of each side of the fold (in this
case I presume it is identical - so the relation, if I recall
correctly will be roughly 4 fold for halfwave dimension or 300 Ohms).

I concentrate mostly on HF design, so someone will probably (now why
would I say probably?) correct any mistakes offered here.

- Will the terminal impedance change as the gap between the elements vary?

Only if it is a remarkably variation. Bench top precision in
construction at 440 MHz is not going to upset anything radically.

(I consider the "terminal" at the midpoint of the bottom element)

It is actually at the gap. The midpoint of the bottom element is
electrically neutral which makes mounting simpler.

The picture of this antenna can be found at

http://www.sophisticatedsolutions.us...d%20Dipole.jpg

73's
Richard Clark, KB7QHC