I just threw together a quick model, and it looks like my estimate was
too high on the feedpoint impedance. It should be close enough to 50
ohms that a single cap should do it. Resonant frequency was 1.5 MHz.
Jerry, K4SAV
K4SAV wrote:
> I would expect that 120 of Rohn 45 with a three element 40 meter Yagi on
> top to be resonant somewhere around 1.6 MHz. Since it is so tall, if
> you match it with a series cap, the feedpoint impedance will probably be
> too high, maybe about 80 ohms. I suspect that you will need a two
> element matching network.
>
> Also if that 80 meter four square is made from bent vertical dipoles
> with the feedlines going over to the tower, you will need to add some
> good chokes on the feedlines to keep the dipoles from loading the tower
> and moving the resonant frequency even lower.
>
> I would recommend modeling this to make sure the tower isn't going to
> mess up the four square. You do have a lot of freedom in changing the
> tower's resonant frequency since it is floating, but a model would alert
> you as to what to expect. I think this should work well if done correctly.
>
> Jerry, K4SAV
>
> Mark Robinson wrote:
>
>> Thanks for all the help on the series rf caps. I am thinking of putting up a
>> 120 foot Rohn 45 tower which is insulated at the bottom, so that it becomes
>> a 1/4 wave vertical. I would like to feed this tower with a series cap for
>> 160m operation provided I have enough top hat effect the beam mentioned in
>> the next sentence. I would like to put a 40m beam on the top and hang 4 80m
>> four dipoles off the tower configured as a Comtek 4 square . Will all this
>> play together or am I missing something. I am not sure how the 40m beam will
>> play on a "hot" tower.
>>
>>
>> thanks Mark N1UK
>>
>>
>>
>>
>> _
>>
>
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