[TowerTalk] 120' 160m vertical

[Mark Robinson]

Thanks Jerry,

I need to improve my modeling skills. Thanks for checking it out.

Mark N1UK

----- Original Message ----- 
From: "K4SAV" <RadioIR at charter.net>
To: <towertalk at contesting.com>
Sent: Wednesday, 18 February, 2009 7:00 PM
Subject: Re: [TowerTalk] 120' 160m vertical


>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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>
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