Topband: Antenna Switch
Victor A. Kean, Jr.
vkean at k1lt.com
Mon Aug 3 18:58:10 PDT 2009
On Monday 03 August 2009 5:55:54 pm KB8NNU wrote:
> I am looking for ideas on a good receive antenna switch that has good isolation for beverages, loops, etc.? Commercial or plans....both would be great!
I home brewed a switch consisting of 14 relays powered and signalled over the coax line. I put two of these switches in a box outside in the area where the 6 center fed, 2-wire Beverages all cross. The first switch passes the 13 "unselected" signals to the second switch, which passes its unselected signals to 75 ohm 1/2 watt carbon comp resistors. Obviously, the "selected" signal from each switch goes back to the radios.
The relays are NAIS JW1FSN-DC12V which has 10 amp @ 250 volt SPDT contacts. The wiper goes to the antenna, one side to the bus that collects for the receiver, and the other side is passed on to the next switch or the dummy load.
I used a DTMF decoder chip and a 4 to 16 latching demultiplexor to drive the relays. Unfortunately, DTMF is an obsolete technology, and the chips are no longer available except as a diminishing stock at hobby electronics suppliers. Maybe a newer technology could use 1200 baud modem chips if those are still available. Or, DTMF signalling with DSP and PIC processors. Anyway, audio frequency signalling makes separation of RF, AF, and DC simple.
I fell into the relays conveniently (company surplus), and the rest was circuit boards from Express PCB, a $40 plastic NEMA box, and some LEDs, 2n3904s, and lots of F sockets. I also wound 16 1:1 transformers on the ever popular binocular cores for isolation on the antenna side. The expensive parts were some DTMF keypads still available from PIPO. The whole setup cost about $400. But I have pushbutton switching to any receive antenna from either of 2 radios.
Just an idea if you are inclined to build.
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