> unproper linear loading arrangements. Back to the matching circuits,
> they may introduce losses when some of their elements have losses,
> tipically a gamma capacitor with a lossy dielectric @ 28 MHz is one of
> the the worst cases for HF, or when the mechanical construction (i.e.
I'm not aware of any gamma capacitors that employ lossy
dielectrics, mostly because lossy systems would be self-
eliminating at the power levels we run. Most dielectrics are
polyethylene, which is one of the lowest loss materials available.
As in a transmission line at HF, dielectric losses are not a factor.
The gamma match does not carry significant current, no matter
what impedance element it is matching. It does not matter if the
element's loop radiation resistance is 5 ohms or 45 ohms, the
gamma rod sees 50 ohms.
Current in the gamma capacitor is never more than the current in
the 50 ohm line feeding the antenna, and voltage across the
gamma capacitor is the reactance times the 50 ohm transmission
line current...which has nothing to do with loop radiation resistance
of the element.
> with an hairpin) involves a number of different parts with several
> joints and the impedances in those points is already rather low.
> Although nothing fails and noone notices the heath, or neither a
> signal decrease is roughly appreciable, when discussing about marginal
> gains like 0.25 or 0.5 dB (a 5% or 10 % of the power) already a simple
> 0.5 Ohm series resistance in a 20 OHm radiator adds an extra .22 dB
> loss.
While it is true the current is maximum in the hairpin, if we lost 5 or
10% of the power in a joint or a few joints in the antenna we would
certainly have problems. Imagine 150 watts of heat distributed in
perhaps two connections, and picture how hot the tiny surface area
of those connections would become.
My L-network matching the common point of a phased array has a
feed impedance of approximately 20 ohms, and it uses only a ten
turn #8 AWG inductor bolted to a capacitor. The losses in that L
network, which are substantially more than I expect from a
reasonable size hairpin conductor, produce less than 30 watts of
heat out of 1500 watts applied.
Most of the joints in a traditional hairpin are at lower current areas
of the a hairpin, at the 50 ohm point of the system. Most hairpins
of good design do NOT interrupt the conductor at the highest
current area.
It makes no sense to redesign an antenna to raise the feedpoint
impedance to 50 ohms and give up performance, when any small
losses in matching could easily be handled through simple proper
construction techniques.
I'm reminded of Lew McCoy's silly article a few years ago in CQ
Magazine about terrible loss in yagi elements, and his suggestion
that a #16 wire element has less loss than 1"diameter aluminum
tubing because the wire "has no lossy joints".
With ten ohms, we only deal with 12.5 amperes. These are not
small loop antennas with radiation resistances of .05 ohms and
hundreds of amperes of current, where pressure joints do become
a mensurable problem.
73, Tom W8JI
W8JI@contesting.com
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