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[TowerTalk] remote control of antenna switch via "wired" UHF

To: "" <>
Subject: [TowerTalk] remote control of antenna switch via "wired" UHF
From: Roger Parsons <>
Reply-to: Roger Parsons <>
Date: Mon, 2 Jan 2012 18:05:40 -0800 (PST)
List-post: <">>

I meant to respond to your question earlier, but didn't.

I'm not sure of the specifications of the radio switches, but even if they are 
quite horrible, they should be able to overcome a 60dB path loss. Your cable 
(in the coaxial mode) will have an attenuation of about 10dB at 300MHz. So your 
couplers can stand a 25dB loss at each end - that is, the coupling can be very 
loose indeed.

There are many good ways to do it, but what I would do is to use the coupling 
loop, similar to that used in many SWR bridges, as proposed by Ian GM3SEK. I 
would get a small die-cast box (~3"x1"x1") and put a connector on each end. 
Then join the inners together. Then have another connector on the side of the 
box which goes to the RF switch TX/RX. I would connect onto that a stiff wire 
loop from the inner of the side connector, an inch or so long, and parallel 
with the main line, and then going to a 47R resistor back to the outer of the 
connector. (Or just using the resistor leads, cutting the earthy end short at 
the resistor body.) This will then be a directional coupler (although not a 
very good one), so the resistors should be at the ends connected to the feeder.

The length of transmission line in the box will not be 50 or 75 Ohms, but is 
such a small length on 160m that it doesn't matter. The coupling loop is quite 
short at 300MHz, so terminating it in 47R gives the switch transmitter some 
sort of load - which probably doesn't matter but just might.

You can put the boxes wherever they are convenient in the feedline - as you 
said you are using the differential mode - so common mode choking is 
irrelevant. DC injection points however may matter - depending on their 
circuitry they may greatly attenuate 300MHz.

I (and I strongly suspect others) will be very interested to hear how you get 
on. The idea of doing the same thing appeals here, although it will have to 
wait until the spring. We are promised -35C here tonight with the wind chill, 
so antenna weather has long gone!

73 Roger


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