[TowerTalk] Voltage at ends of yogi parasitic elements?

[Jeff AC0C]

I've done element loading and reversing using center switched loads at the 
end of a coax line just as K1TTT mentions.  Works FB.

Plus you have to contain the voltage pickup in the control wires heading out 
to that dream relay on the end of the elements.  I have a vac relay up at 
the top of my elevated 80/160m vertical (full size 80m so the relay is up 
75') and that's manageable.  But I can lay that down for repairs if needed.

73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie

-----Original Message----- 
From: jimlux
Sent: Monday, October 17, 2016 9:24 PM
To: towertalk at 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.

In a well designed 3 element Yagi, the currents in all the elements (at
the center) are fairly comparable. That is, you're not going to see 1
amp in the driven element and 0.01 amps in the other 2 elements: more
like a 1:2:1 kind of ratio.  "Superdirective" Yagis (which is basically
any Yagi with gain > 6dB) will have higher currents in the parasitic
elements.  Think of a directive antenna as a kind of multiple resonator
circuit with lots of energy circulating around among the elements.

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.
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