Since E-mail to VK6VZ bounces from contesting.com with a
message his server does not accept junk mail, I'll post my
response here.
Hi Steve,
If you use a top capacitance that greatly exceeds the distributed
capacitance of the vertical element, the bulk of current flows into
the capacitance regardless of where you place the inductance.
I use that to advantage in my mobile.
> What I am wondering about trying instead is a 50 - 55' top loaded vertical
> with a large square section coil wound on a big 3 litre soft drink bottle
> or plastic bucket with 3mm or so diameter wire, working against a small
> top hat consisting of two wires about 10 - 15' long.
I doubt you'll get very high Q in the inductor using that scheme. I'd
opt for a larger hat and moving the coil down low where you can
use a good one, and have access to it.
There is a less cancellation than you might expect with a drooped
hat, because the hat distributes the current rather than having a
concentrated flow that folds back against the radiator close to the
radiator. I'd pull the hat out as much as possible, and would not
worry about getting the entire system all the way to 160 without a
coil. There would be very little difference if you hit 3 MHz, and just
made up the difference with inductance or added so much hat the
thing hit 160 on its own.
Especially if the ground system is fairly decent.
> I have also considered trying a simple folded radiator of around 55 - 60'
> long and feeding it with an omega (two capacitor match, just a like a
> shunt feed tower. However, I am concerned that with such a short radiator
> the efficiency would be very low.
It sure would, it would have the radiation resistance and ground
current of a short base loaded monopole...about
1/4 the radiation resistance and twice the ground current of the
same height antenna with a big hat.
I was quite successful using four wire hats on 60-70 ft verticals in
the early years. I never really could see any difference going to full
size verticals.
73, Tom W8JI
w8ji@contesting.com
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