"Fred Abse" wrote in message
news
an.2007.09.16.12.12.19.35340@cerebrumconfus. it...
On Sat, 15 Sep 2007 14:36:05 -0500, amdx wrote:
I was surprised there was no matching section, I thought a folded dipole
would have a higher impedance than 50 ohms.
It does. Around 300 ohms. However, adding parasitic elements reduces the
feedpoint impedance.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
.... is that so?
A folded dipole (thin, half-wave, symmetrical) has the same gain as a
non-folded one but a terminal resistance of about 300 ohms as opposed to
about 73 ohms for a non-folded dipole (two prongs, often written as 75
ohms). Doubling the number of closely-coupled conductors divides the input
current to half in each, and the radiation resistance of each conductor must
be half the radiation resistance of the whole antenna. When a pair of
terminals is applied at the mid point of only one of the conductors the
apparent terminal impedance is that of a non-folded dipole raised to the
power of 2.
The most common form of balun used with folded dipoles is a half-wavelength
of transmission line (of any characteristic impedance!) between the dipole
terminals, one of which is connected to the feeder (and all screens are
connected together if co-ax is used). This also acts as a transformer with
a turns-ratio of 2:1 (it produces the same voltage in antiphase at its far
end), so the 300 ohms of the folded dipole is transformed down to about 75
ohms.
We looked inside the balun of a J-Beam folded dipole from a 3-element Yagi,
for 50 ohms, and couldn't find any component or circuitry other than the
half-wave line between the dipole terminals, and it did look like 50 ohms on
a network analyser.
I've seen a 'genetic algorithm' program for Yagi design that took the
required terminal impedance as an input parameter, along with things like
gain and front/back ratio.
Hope this helps.
Chris