there are a few things you can do.
1 - use 2-4 wires top hat which is connected through 7MHz LC-filter
2 - put a top hat and use remotely controlled matching network to
switch the bands and match antenna's impedance on each band to 1:1
3 - same as #2, more complex matching network which needs no remote
switching, but you spend some time fine tuning it.
4 - same as #2, but choosing properly height of the mast and
number/length of hat wires you may get an antenna which you can feed
through simple serial LC network.
I am building 160-80-40 60 feet high vertical using the option #2.
#2 can be found at http://dl2kq.de/ant/3-1.htm. that page is in
russian, am not sure whether DL2KQ has english version of it, but you
can look at pictures/schematics.
#3 - bottom part of http://dl2kq.de/ant/kniga/5151.htm page, has 80-40 antenna.
#4 - http://dl2kq.de/ant/3-29.htm
On 3/17/07, Lee Buller <email@example.com> wrote:
> Ladies and Gents,,,
> Last year, I build a homebrew 1/4 wave verticle and mounted it on the
> ground with 16 radials underneath it. I am in the process of adding 16 more
> radials. The antenna is made out of aluminum tubing starting with and 1" 1/4
> and going down to 3/4". The length is 32 feet and with 16 radials under
> it...it resonates at 7.100...at least it is flat there...and the MFJ 259 says
> it has imdedance of 55 ohms...R=53-55, X=3-6, Theta is 10 degress, Works
> good too.
> What I want to figure out is how to had a "top-hat" to the antenna to bring
> it down to 80 meters. I know that the MFJ brand of verticals uses a top hat
> loading scheme and I wonder how you calculate or figure out the capacitance
> one would need to build something like this? Or, is it trial and error?
> In addition, the top of the antenna is fairly small tubing (I am afraid I
> will bend it with to much weight or load) and I need to come down on the
> antenna about 8 feet to install the top hat. I guess one would have to use
> trial and error to find the right combination of "spokes and Lenghts" to make
> the capacitance hat. Looking at the Handbook, it does not really have a
> "formula" for such things.
> Anyone have any insite? Give me a starting point?
> Lee - K0WA
> In our day and age it seems that Common Sense is in short supply. If you
> don't have any Common Sense - get some Common Sense and use it. If you can't
> find any Common Sense, ask for help from somebody who has some Common Sense.
> Is Common Sense devine?
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