[TowerTalk] Voltage at ends of yogi parasitic elements?

[jimlux]

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.