[TowerTalk] Coax Lengths for Stacked Yagis

[Jim Thomson]

Date: Fri, 4 May 2018 16:15:01 +0000 (UTC)
From: Richard Smith <n6kt1 at sbcglobal.net>
To: "towertalk at contesting.com" <towertalk at contesting.com>
Subject: Re: [TowerTalk] Coax Lengths for Stacked Yagis

<Hi Jim,
<Thanks for your suggestions. Regarding the ceramic vac relays, do you use Jennings, Kilovac, or another manufacturer's relays?
<Are the HV DPDT mechanical relays open-frame relays?? Is there a manufacturer that you like?
<73, Rich, N6KT

##  In the past I have used Jennings,  Kilovac, and also Gigavac.   The kiolvac has more robust  innards vs the  Jennings.  The gigavac brand
is better than all of em.  The gigavac GH-1,  rated for  5 kv test, will easily  hi pot test  to >  7.5 kv.  I use  gigavac  G2 relays.  These are bigger, and are 
rated for 17 kv test.  They too,  hi pot test well above that.  You can get either of these relays relays   in either a 12 vdc  coil version, or a  26.5 vdc version.
The G2 can be purchased with either a flange base...or a threaded base, with the huge mating locknut.   The G2 can be ordered with either the hollow tubes
for the contacts.... that will handle up to 8 gauge wire.... or  the version that has embedded threads inside the  ends of  the hollow  tubes,  so it makes it easier to terminate 
stuff like silver plated  copper strap, or crimped and soldered lug typ connections, etc. 

##  For  your application,  the GH-1 would be ample.  I have seen  5 kw  stuffed through em on a regular basis on 6M band. 
I forget the brand name of the  DPDT open frame mech relay used in my  1 in,  6 out remote  switch box, but they came from array solutions
years ago.  Red HV micarta used for the insulation, all contacts in parallel.   I believe they are  20A rated contacts.   You can also get them
with 30A rated contacts.   I believe they were 10-15 kv rated 12 vdc relays.   Paralleled  contacts means redundant contacts.  It also means you just doubled the current 
ratings...which in turn now quadruples the power rating.   That remote box uses  strip line techniques, with  two, silver plated 
copper straps, aprx  .75 inches wide each.   The spacing between em is aprx .3125 inches.   The Z between em is  50 ohms.   DPDT relays used
in this config, so that the UN used ports have their center conductor grounded.  This setup  used  7-16 din connectors for both the input..and also
the 6 x ports.  Its  20 kw rated.    A  buddy built a HB version of this..but used  12  ports.  One relay per port.   Whichever relay is energized, that port gets its center conductor 
connected... via its  relay, to the hot buss..that has the 1.5 kw on it.   The same box can also be built using a mess of  SPDT ceramic vac relays. 

##  For a 2  or  3  or  4 stack of MONO band yagis,   I designed a system for a buddy, that used a simple  L  step down network in a box.  Just a mess of  paralleled  5 kv rated 
NPO ceramic doorknob caps, like HEC brand, or centralab.   Centralab is now  ITT  Jennings.   Bought surplus. Caps are wired from hot side of input connector to chassis.
.25  or  .375  or  .5 inch OD silver plated  copper tubing coil,  is wired between center conductor of input connector.... to  ALL the center conductors of the  2 to 3...or 4  paralleled  output connectors. 
The simple step down  L net network  just drops the Z from 50 ohms...  down to 25 ohms... for a 2 stack.... or   down to 16.66 ohms  for a 3 stack..... or 12.5 ohms  for a 4 stack.
In the above case, ALL the yagis are driven in phase, all the time, with equal length 50 ohm feedlines to each yagi.   2 yagis  =  25 ohms.   3 yagis  =  16.66 ohms etc.  Simple cu strap bonds all
the center conductors  together of the 1 to  4 output connectors. 

##  IF the 2 or  3  stack yagi   needs to be switched such that individual yagis can be fed, one at a time,  relays used in a config, such that  individual yagis  are routed to main feedline coming up the tower,
bypassing the L network.    
In the case of the 3 stack,  if you want, more relays used, and configured, such that any 2 of the yagis can be  be used simultaneously.  In this case, the  L network is configured for 20.8 ohms..  which is the mid point
between  the required  16.66  ohms for driving all 3 yagis  in phase.... or the 25 ohms required for driving any 2  yagis in phase.   IE,  top and middle,  top and bottom,  middle and bottom.   Another relay used
IF  you want to drive any one of the  pairs of yagis out of phase..aka BOP.   No 75 ohm coax used anywhere.    

##  The simple L network has been used on  40, 20, 15, and 6M.   It can be used on any band.   The box is tested by installing 50 ohm  resistors on the output ports.   For a simple L network, where  no switching is used,
and all yagis  are driven in phase, the box is tested with a  25 ohm resistor   across either of the 2 ports on a  2 stack array.  On a 3 stack array, a 16.66 ohm resistor is connected to any of the 3 ports.   Or a 12.5 ohm
resistor used on any of the  4 ports on a 4 stack array. 

http://www.gigavac.com/sites/default/files/catalog/spec_sheet/g2.pdf     http://www.gigavac.com/sites/default/files/catalog/spec_sheet/gh1.pdf

Jim   VE7RF