Date: Mon, 17 Oct 2016 08:00:52 -0700
From: jimlux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Voltage at ends of yogi parasitic elements?
On 10/16/16 8:09 PM, Rob Katz wrote:
> I?m playing with some wire yagis, and I?d like to put relays at the ends of
> the parasitic wires to switch the director to a reflector (and vice versa).
> If I?m driving the antennas with a full 1.5 KW, what would be the maximum
> voltage that I can expect at the ends of the PARASITIC elements? Even a
> rough estimate would be useful, I just need to get a sense of what kind of a
> relay I would need to use.
>
The voltage will be "high".. Measuring or modeling the voltage is
challenging, but it goes in proportion to the current in the element,
which *is* modelable and measurable.
Moral of the story: switch an inductor or capacitor at the feedpoint:
it's easier to get a high current relay than a high voltage one, it's
easier to switch something at the boom than the tip of the element. The
antenna doesn't really care about the physical length of the elements,
it's all about the electrical length.
### the issue with relays at the tips is... you will have a helluva time
keeping RF out of the
control cabling that feeds the relay at each tip. EACH control cable would
have to run the
entire length of the wire ele.
## switch a coil or stub or cap at the center of each parasitic ele.
(You dont even want to know how much peak V is at the tips of the DE, its enough
to easily light up a 4 ft long FL tube).
Jim VE7RF
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