Re: hairpin matching, K4SAV writes;
>Maybe it's because of one of the disadvantages (which you didn't list).
>To change the resonant frequency of the antenna when using the hairpin
>approach, you have to prune the antenna
Yeah, quite possible. I do like motor-drive tuning. But you don't
*have* to go out to the 50+jWhatever point to do that... though you do
tend to need a pretty huge capacitance if you want to tune an antenna
without much inductive reactance. My tuning cap is 2000pF max, just
44 ohms ;-)
N7RT writes:
>Another advantage to the capacitor approach is the current maximum is
>farther away from the ground/radial system. I think one would see a definate
>field strength improvement
Well, I dunno. Top loading certainly gives an improvement If we're
talking about a 65 foot vertical antenna on 160m... but once you've
top loaded it to resonance, there's a lot of current all up the 65
foot section. You have to get 60% up the vertical section before the
current drops below 90% of the base value. At the 65 foot level, you
still have 75% of the current you had at the base.
A small amount of lengthening would put the current max in the middle
of the vertical section, but enough to get you to 50 ohms gets a lot
of current flowing in the horizontal section. That might make it a
better all around DX/domestic antenna, but it seems low angle field
strength starts to go down a bit.
It'd be different if we were talking about going *straight UP* but
except for the balloon guys and guys with much larger towers than I'll
have for a long time, it's hard to go much over 1/4 wave on 160 ;-) I
suspect that it doesn't hurt me on 40m to have a current max up 30
feet above ground... but on 160m most people will just hope to get
mostly constant current flowing in the vertical section, and going too
long on the top loading is just going to "pull" lots of current out
onto the horizontal.
- - - - - -
Anyway, top loading is great and all, but something to remember with
all this talk of top loading is that the Hy-Gain vertical N9WX was
wondering about in the original post is kind of flimsy. I'm not
disparaging the product, I bet it works great as a VERTICAL vertical,
but the top section is 5/8" thin wall aluminum tubing.
I'm in the same boat here with the Spiderbeam vertical pole and a
wire. Mechanically, the top couple sections of these things aren't
good to go for a single horizontal or sloping wire. You might do
better mechanically with a three-direction hat.
But in the real world, at some point you start having to consider
reducing the vertical height six or twelve feet , and then you run
some EZNEC models considering that you're going to have to slope your
top loading hat wires down to 15' above ground level because you don't
have any other tall support, you get some radiation cancellation from
such a steep fold-back , and all of a sudden, all the difficulty of
top loading buys you is 1.5dB and an antenna that will come down in a
light ice storm . (Can you tell I weighed my options and decided
against going with top loading here?? ;-) )
Top loading = usually good thing, but there are some instances in
which you probably can't get much advantage out of it. I stand to
gain a dB, maybe two in my estimation and I'd lose the ability to
usefully work 40/30 with the same antenna. I'd do a lot better if I
just gave myself a 12dB shot in the arm and used an amp on Topband ;-)
A 65 foot base loaded vertical is not the ideal Topband antenna, but
it is a MUCH better proposition than an *UNLOADED* 65 foot vertical!
73,
Dan
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