Topband: Antenna Switch

[Milt, N5IA]

----- Original Message ----- 
From: "Victor A. Kean, Jr." <vkean at k1lt.com>
To: <topband at contesting.com>
Sent: Monday, August 03, 2009 6:58 PM
Subject: Re: Topband: Antenna Switch


> 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.
>
> Victor, K1LT
---------------------------------------------------------------------------------------

Essentially the same design was done in 2000, in triplicate, and utilized at 
XZ0A.  All Beverages were band split and filtered, and then fed to the three 
remote switches for independent control of the antennas at each of the three 
low band stations.

These models have 17 selections for 17 antenna inputs and are currently in 
use at the N5IA, N5BG and NI5T operations.

I echo the current situation with the availability of the discreet DTMF 
encoder and decoder chips.

Another model was constructed in 1996 based upon a digital stream encoder 
that was available at that time.  It is still in use also.

The DTMF audio works very reliably over 2,000' plus of 1/2" 75 Ohm CATV 
hardline.

73 de Milt, N5IA