I have been wondering the same. Is it possible to use commercial traps (or
homemade for that matter) to disconnect parts of the antenna that are not
needed for the other bands?
For example, build a T, where the vertical part is 1/4 for 40 m. and the
horizontal part, which acts as a hat on 80 m, is connected to the vertical via
a trap for 40 m. On 40 m, the trap will "disconnected" the hat and the antenna
will act as a normal vertical, while on 80 m the trap will "connect" the
horizontal part and load the vertical properly for 80 m.
Rudy N2WQ
Alex Malyava <alex.k2bb@gmail.com> wrote: 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 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?
>
> 73
> 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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