On Apr 7, 8:36 am, "Antonio Vernucci" wrote:
I wonder what you consider narrow bandwidth. 500 KHz seems to be about par
for a decent 6 m beam.

This antenna shows an SWR of 1.7 at 150 kHz below the resonant frequency, where
the SWR is just 1, so I consider it narrow. Another antenna I have, also using
an hairpin match but at 50 ohm instead of 200 ohm, is by far broader. On both
antennas I have about 100 feet of low-loss 1/4" Andrew hardline, so the SWR is
only slightly influenced by cable loss.

Talking with the manfacturer, he told me that he preferred rasing the antenna
impedance up to 200 ohm, instead than to 50 ohm, because a 4:1 cable balun can
me more easily realized than a 1:1 balun. But doing so the feed system Q factor
increases quite a lot, causing a significant bandwidth reduction. Moreover ohmic
losses increase, due to the higher circulating currents (for a given RF power),
but I am not able to predict whether such extra losses are significant or can be
disregarded for all practical purposes.

73

Tony I0JX

I'm trying to find a good way to wrap my mind around this "general"
problem. I'm not at a point I'm really comfortable with yet, but I
offer the following ideas as "food for thought."

First, the matching is being done essentially with an "L" network (or
rather the balanced version of an "L" network), where there is a load
resistance (the resistive part of the feedpoint impedance, which
includes radiation resistance and element loss resistance reflected to
the feedpoint), the series capacitive reactance of the feedpoing
element, and a shunt inductive reactance, provided by the hairpin.

Because shortening the driven element causes a decrease in resistance
and an increase in capacitive reactance, it's possible to find a
length that allows matching to any of a wide range of resistances.
But the higher the resistance to which you match, the shorter you need
to make the element and the lower the feedpoint resistance. The ratio
of feedpoint resistance to matched resistance determines the loaded Q
of the matching network; as you make the matched resistance higher,
the loaded Q goes up rather quickly. If you know the loaded Q and the
unloaded Q of the hairpin, you have a good handle on the amount lost
to heat in the hairpin: if the hairpin Q is two times the loaded Q,
half the power is dissipated in the hairpin, for example.

However, unless you build an antenna with a very low feedpoint
resistance at resonance, there almost certainly won't be an efficiency
problem: the reactance changes quickly enough with changes in driven
element length that the resistance won't drop much by the time you
reach enough reactance to get a match to 200 ohms. It appears that
the loaded Q of the match to 200 ohms for your case will be less than
4. I would think unless you really messed up badly, the hairpin
unloaded Q should be well in excess of 100, and if that's the case,
the power lost in the hairpin would correspond to well under 0.1dB
signal level change.

On the other hand, I'm surprised by the comment from the manufacturer
about difficulties making a 1:1 balun. I have had good luck using
ferrites and/or self-resonant coils of feedline and/or coils of
feedline specifically resonated with additional capacitance. A 4:1
balun from a "hairpin" of 1/2 wave of coax, arranged symmetrically, is
easy enough to make, but I would not rule out using a 1:1, if it has
advantages for you.

Cheers,
Tom