Thank you Hardy and everybody for your responses. I’ve built many wire yagis
that are loaded in the middle. I’m looking to put up a new 3-el on 40m with a
rope catenary as the “boom,” so I want to eliminate the weight in the middle to
reduce the sag. It’s also over my driveway, so I don’t know if dropping
open-wire line would pass the XYL test :-) It looks like the end voltage would
be too high for relays, but I’m still curious to have a number on that voltage
if someone can figure it out.
I appreciate all the generous folks in this group!
73,
Rob K4OV
> On Oct 17, 2016, at 06:46, Hardy Landskov <n7rt@cox.net> wrote:
>
> Rob,
> I would switch in an inductor for a reflector and a capacitor for a director
> at the CENTER of the element. Use a shorted stub for the reflector and an
> open wire stub for the director.
> The voltage at the ends is very high even in parasitic elements and you would
> have the problem of RF getting into the switching wires. Not many relays will
> survive that environment and I don't know anyone who has tried that.
> I would try running 1/2 wave open wire line down to the ground and switch in
> the lumped elements there. Then put lumped elements in at the antenna. Of
> course you need split parasitic elements.
> I have built 2 el 80 meter Yagis with a DE and Reflector using open wire
> line(1/2 inch Al tubing) back to the center of the antenna so I could reach
> the stub and adjust the shorting coil for F/B ratio. Then used a latching
> relay to short out a portion of the line to go to the fone band. Of course a
> matching network would have to be switched in going from CW to FONE.
> But if you are working with wires, a spring loaded knife switch will work.
> But you would have to go out to the tower to change directions.
> I think I remember one guy who used retractable radio antennas from cars in a
> junk yard.
> And there is SteppIR if your VISA card can stand the hit.
>
> I would be interested in other solutions myself.
> Hardy N7RT/4
>
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Rob
> Katz
> Sent: Sunday, October 16, 2016 11:09 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Voltage at ends of yogi parasitic elements?
>
> 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.
>
> 73,
> Rob K4OV
>
> --
>
> Rob Katz, Senior Partner
> 919-219-5324
>
>
>
>
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