Duplexers

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Posted by w0ep on April 26, 2011 at 8:00 pm

[Today,  no running]

I have been reading up on duplexers in support of my recent acquisition of a large set of band-pass VHF filters.  At one time  in the past I was interested in building a 2-meter repeater.  But I have kind of left that idea behind.  There are already a lot of repeaters available to me and most of them go unused.

But, now that I have some cavity filters I would at least like to play with them a bit.  Unfortunately I don’t have a signal generator in the 100-180 Mhz range.  All I have is CW/AM down to 144 Mhz and FM up to 148 Mhz.  I guess I could work with that a bit and see if I can find the passband on one of these filters.  It would be comforting to know they will work in the right frequency range after all.

The www.repeater-builder.com website is a great resource for anyone interested in building a repeater.  And there is a Yahoo group for aspiring repeater builders.

I still don’t think I am interested in building a repeater.

For anyone who doesn’t know, a repeater is a radio receiver and transmitter that work at the same time.  Stuff coming in on the receiver is piped out through the transmitter.  It is common to have both the receiver and the transmitter share the same antenna.  Since the receiver is listening for faint signals and the transmitter is generating a loud signal on the same antenna, the repeater has to have a mechanism to keep the signals separate.  First the transmitter and receiver don’t operate on the same frequency.  Second, a filtering system is used to keep unwanted signals from going into the wrong places.  High Q cavity filters are used so that the two signals can be relatively close together.  And the filtering system is called a duplexer.  On the 2-meter amateur band the traditional frequency separation is 600 kHz.

The purpose of a repeater is to extend the range of small systems.  Generally you put the repeater at a high, prominent point where it can hear signals from small hand-held transmitters.  And the repeater transmitter likewise covers the range back to the hand-held units.  Instead of having then a walkie-talkie range of a mile or two you can get extended range of the elevation and higher transmit power.  But the users still just use walkie-talkie sized equipment.

 

Filed under Ham Radio, Operating
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