On Jun 2, 2008, at 1:32 PM, Bob Tellefsen wrote:
Brian
What are your expectations for this antenna?
Used as a halfwave, say on 40m, its pattern will be that
of a regular dipole. However, if you feed it at the end,
it becomes an endfed full wave on 20m. It will not
have a nice major lobe at right angles to the wire, like
the dipole. Instead, it will be a four-leafed clover type
of pattern.
Yes, I am aware of the various patterns that occur with different
lengths of radiator. That wasn't the question. The questions were
(restated for clarity):
1. What is the matching range of the KAT100, i.e. will it match an
antenna fed at a voltage node (multiple of a 1/2 wave) rather than a
current node (odd multiple of a 1/4 wave)? Yes, I know that the
ability to achieve a conjugate match depends on the capacitance and
inductance range of the tuner. Also this affects the range of the real
component (resistance) that can be matched and what frequency you are
trying to match it on.
2. Does the BL2 have a problem when looking at a really high
impedance, i.e. when center feeding a full-wave doublet? And yes, I am
aware that I really wouldn't want to feed the balun with coax from the
tuner but rather should put the balun as close to the tuner as
possible and then run a low-loss balanced line from the balun to the
doublet and that the feedline will act as a transformer to change the
impedance (resistance AND reactance) as seen by the balun and,
therefore, the tuner.
I guess the real question is whether I want to get a KAT100 and a BL2
or just stick with my SGC-231. The nice thing about the KAT100 is that
it doesn't draw power unless it is tuning. My SGC-231 draws power all
the time to hold in the relays. That sucks up more battery. Also it is
physically bigger. Looking at the schematics for both I can see that
the SGC-231 has a greater range of C and L than the KAT100 does but
maybe the KAT100 is enough, especially when ganged up with the BL2 to
provide a 4:1 impedance ration to start with. I know the theory but
was looking for a quick answer from the field of people here who have
more experience with this than I do.
If you want the pattern on the harmonic band to be at
right angles to the wire, then you need to center feed it
with the 450 ohm line. That will give you what is called
two half waves in phase, with a nice major lobe at right
angles to the wire and about 2 dB gain over a simple
dipole.
Right. I was aware of that. Right now I am primarily interested in
what the limitations of the KAT100 and BL2 are.
But I appreciate where you are going with this. You are thinking along
the same lines I am but I was still back at the basic mechanics of
matching using the KAT100 and then using the balun to feed the
balanced line irrespective of the radiation pattern. And, yes, end
feeding a 1-wave wire or greater gets you four major lobes. OTOH, take
four of those and put them into a rhombus ... :-)
--
73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com
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