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Author Topic: T/R Switch  (Read 7498 times)
KC2YQY
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Posts: 40




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« on: August 24, 2010, 04:21:54 PM »

What do QRPers use for a transmit/receive switch when using a a separate QRP transmitter and a receiver? Any recommendations? Thank you in advance. -Bill-
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W5FYI
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Posts: 1054




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« Reply #1 on: August 24, 2010, 08:17:20 PM »

At QRP levels, it's easy to use a pair of diodes, an inductor, and a capacitor configured as an "automatic" TR switch. The way it is set up is the transmitter feeds the antenna directly. However, the receiver taps into the antenna line with a series inductor-capacitor filter with parallel front-to-back diodes between their junctions and ground. The inductor-capacitor pair are selected to pass the receiver's input frequency.

When transmitting, any energy that gets to the receiver inductor will probably have enough voltage to cause the diodes to conduct; the RF essentially sees a grounded choke. Any RF that makes it through the inductor goes directly to ground, thus completely bypassing the receiver input circuitry. However, when the transmitter stops feeding the antenna, the only voltage on the diodes is from the received signals on the antenna, which are usually in the microvolt range (way below the diodes' conduction voltage). These low-voltage signals will pass through the series inductor-capacitor filter directly to the receiver input.

Other schemes use transistor or JFET muting circuits 9the receiver line is turned off while transmitting), ot CD4066 IC switches (super fast solid-state relays), and just plain old DPDT switches.
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KE3WD
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« Reply #2 on: August 24, 2010, 08:19:16 PM »

You may also be able to simply monitor your own transmission, no need for a sidetone or the likes...
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WB8YYY
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« Reply #3 on: September 09, 2010, 09:10:14 AM »

Bill

If separates are your thing, something like this would be a good investment to mix and match Rx and Tx widgets --

http://www.wa0itp.com/mbmagicbox.html

Its a current design and full featured.  Its a little more than a dow key relay (if you can find one).  This is from a QRP club, and the designer does excellent work also.  Note these kits are short run -- meaning if you wait too long there may not be any. 

On your TT1340 looks like you have a concensus!  After building your one tube Xmtr you should be ready to take on the black box rig maybe this winter as the snow flies.  Hardest part is aligning the BFO/Xmtr and its pain to have to pull the board out of the chassis.  I would be glad to email with you during the build - and even open my unit for comparison measurements if needed.  With your building/buying binge maybe you have returned to ham radio - if so enjoy and hang in there (and be careful with high voltage!). 

73 Curt
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