Topband: VK6VZ

[Tom Rauch]

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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