[TowerTalk] Coax Lengths for Stacked Yagis
jim.thom at telus.net
Sat May 5 10:28:29 EDT 2018
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
<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.
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