[TowerTalk] Voltage at ends of yogi parasitic elements?

[Jim Thomson]

Date: Mon, 17 Oct 2016 11:47:49 -0700
From: "Richard (Rick) Karlquist" <richard at karlquist.com>
To: "john at kk9a.com" <john at kk9a.com>, towertalk at contesting.com
Subject: Re: [TowerTalk] Voltage at ends of yogi parasitic elements?
On 10/17/2016 9:44 AM, john at kk9a.com wrote:
> That is a good question.  Years ago I thought of building an 80m vertical
> with a relay on top to make it a 160m inverted L at my P40A home but I did
> not know if even a vacuum relay would hold up and I knew that there would
> be RF issues with the control wires running down the vertical. I had never
> seen it done and did not want something likely to fail. If you are just
> changing band segments most manufactures are using switchable loading
> coils at the feed point.
>
> GL
> John KK9A
>

For many years, I have had an insulator with a relay across it at
the 60 foot point of my 90 foot vertical.  At first I tried vacuum
relays, with ratings up to 8000V.  All worked for 6 months or so
and then failed.  Then I figured out a simple way to modify a
big contactor to increase the contact spacing to 1/2 inch or so
as described here:

QST, May 2009, page 66

This relay has given many years of trouble free service.
It is likely that it has many 1000's of volts across it on
80 meters at 1500W.  It just works.

I also use MJN series relays at the center of my 80 meter
inverted vee to switch in various mica capacitors to tune it
across the band.  Again, it works perfectly.  In that case
I didn't need to modify the MJN relays because the voltage
was within their 1000V rating.

Rick N6RK

##  I use a modified HV rated  mechanical contactor to do the 15M tap on my
20m tank coil on a hb amp.  A contactor is just a SPST-DM  set of
contacts. IE:   2 x sets of contacts in series.  In order to arc, the hv has
to arc both sets of contacts simultaneously.   I added one small SS flatwasher
to the solenoid assy, to increase the air gap between both sets of contacts.
This increases the V rating by a whole bunch, and with 2 x contacts in series,
the total V stand off is huge. 

## another method that works is to use a vac relay with a  greater V stand off.
Instead of 8 kv rated contacts, use  15 or 25 kv rated contacts.  Another trick that
works is to use 2 x vac relays, with contacts in series, to double the total peak V ratings. 

##  If you really want to go crazy, use a vac contactor, then you can get 50-200 kv standoff. 
A vac contactor is typ about the size of an 813 tube.  

Jim  VE7RF