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Author Topic: Transmit/Receive Switch -- Build  (Read 1551 times)
W1JKA
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Posts: 1716




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« Reply #15 on: September 16, 2014, 03:37:35 AM »

  As per AXW this reminds me of my first HB Novice set up from the 60s which I still occasionally use. I couldn't afford any type of reed relay so used an old time porcelain based throw switch I found in our barn's old knob/tube wiring system and hoped that I didn't wear out the vol. control on my iffy receiver
  Still using the same switch today as an antenna switch between my K1 and SW20+ home based rigs. I have yet to experience my first mechanical/operating failure with it Wink

 
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KB1WSY
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Posts: 795




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« Reply #16 on: September 16, 2014, 04:11:00 AM »

I think you can rewire the antenna switching to eliminate one set of contacts.

RX common port:  ANT on RX, GND on TX

TX common port:  dummy load on RX, ANT on TX

(Slaps palm on forehead.)

Of course! Many thanks, that's much better. Revised schematic:



I'll get going on building this one soon, should be able to find all of the parts at my local electronic components store.

Martin:  I'm very surprised at the various responses.  Seems that nobody recalls the "good old days" when the RX/TX was done with a simple relay and switch.

The relay was between the transmitter, receiver and antenna.  Switch activated:  Relay pulls in and switches the antenna to the transmitter.  Switch deactivated: Relay drops out and signal goes from antenna to the receiver.

When the switch is activated for TX, the RF gain control on the receiver is backed off and the CW tone is monitored with the receiver.  

That wasn't practical with my set because it gets overloaded -- you cannot get tone, even with the antenna input shorted (it's a regenerative set and doesn't have an RF gain control). Hence, the existence of a sidetone oscillator inside the transmitter -- this was in the original ARRL design, which makes it rather sophisticated for a total beginner's Novice project! In the original ARRL setup, the receiver's headphone output is wired in parallel with the transmitter's sidetone oscillator. Thus, all you have to do is provide a knife switch at the antenna (to switch between receiver and transmitter). Changeover between RX and TX required two maneuvers: change over the knife switch; and turn down the AF gain to zero to avoid getting your ears blasted by an overloaded signal from the RX (if you leave the AF gain above zero, the carrier signal white noise blanks out the sidetone that's fed from the transmitter).

Then, I made things more complicated for myself by adding an extra AF stage, and a speaker, to my receiver. In order to monitor the sidetone on the speaker, it was necessary to feed it from the transmitter's sidetone oscillator to the input of the final AF stage in the receiver. In order for the AF gain control to continue working properly both with the headphones and the speaker, while still "muting" the overloaded signal on transmit, I came up with the above-described proposal for a (now three-pole) TR switch and relay. It will have the additional advantage that T/R switching will now require only a single maneuver: flicking the switch (no need to back off the AF gain any more).

It's a case of getting hoist with my own petard. The only reason I added the speaker was for casual band monitoring, and also to "show off" my station to visitors without their having to wear headphones. But actually operating my station with the speaker switched on requires this rather more complex T/R setup. I think it will be fun anyway, and in the end slightly more efficient than the original ARRL setup: a single maneuver for T/R, plus easy "zero beating" of the receiver with the transmitter.

No one has answered my question about positioning: logically (from an electronics point of view) this switch goes on the extreme right of my desk, near the telegraph key (this enables a simple left-to-right cabling at the back of the desk). But this would mean that the right hand is doing three things: keying, switching over, and writing down the incoming code. Meanwhile the left hand only has a single task: operating the receiver. Is this a bad idea?

Yes I know, these are picayune details. I'm trying to occupy my ham radio time usefully while my next-door neighbors ponder my antenna proposal: finalizing the T/R setup will help me get on the air quickly once those negotiations are completed.

73 de Martin, KB1WSY
« Last Edit: September 16, 2014, 04:52:41 AM by KB1WSY » Logged
KB1WSY
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Posts: 795




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« Reply #17 on: September 25, 2014, 03:20:13 AM »

I've ordered nearly all of the parts. A few observations:

Rotary switches:
--Two-position rotary switches (as oppose to three-positions or more) are a bit hard to track down. Of course I could have used a switch with more than two positions, and only used two of them. Or I could have bought a switch with adjustable stops.
--In this application it was important that the switch be of the "non-shorting" type, to avoid transmitter antenna output being momentarily connected to receiver antenna input each time the switch is flicked. I almost bought a "shorting" switch by mistake!
--In the end the switch I chose was a low-end Alpha phenolic made in Taiwan (I've bought their switches before and been satisfied). New *ceramic* switches are available, but they are incredibly expensive nowadays and I didn't have anything suitable in the junkbox. Correction: There is a company in China that does a 2-position, multiple-pole ceramic rotary for $6 including shipping, but their shipping/delivery estimate was mid-October and I'm a bit wary of the quality (it's one of the companies that also makes ceramic tube sockets; I haven't been impressed by Chinese tube sockets). For my low-power application I don't think there will be any problem with the Alpha phenolic.

For the receiver audio switchover, I bought a 12V reed relay with built-in diode for protection. It's not explicitly rated to function on 9V (which is the receiver voltage) but I think it will probably work. The coil draws only 25mA and will, in any case, only be powered during the "transmit" phase.

I'm mounting the rotary switch on the front of a small Bud chassis case that will match the transmitter.

To save space on my small station desk, I've decided to mount a 25 watt, non-inductive dummy load inside the switch case instead of using the external, "paint can" dummy load that I built. This will be switched in during "Receive" to enable spotting the receiver using the xtal-controlled transmitter.

Finally, to stop this otherwise lightweight little box from wandering all over the desk when the switch is rotated, I ordered a one-pound lead weight (shaped as a 1/2"-thick disk). This will be mounted on the inside of the bottom plate.

This T/R switch is very much customized to my particular rig, at this point in time. I'm fairly sure that a relay-based antenna switchover system makes more sense when the station gets more powerful, and complex. Also, if you use relays you have a lot more freedom in positioning the switch on the station desk.

73 de Martin, KB1WSY
« Last Edit: September 25, 2014, 04:15:15 AM by KB1WSY » Logged
G3RZP
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Posts: 4625




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« Reply #18 on: September 25, 2014, 09:33:59 AM »

Martin,

If you are right handed, then you have the receiver slightly left of centre, and use the right hand for keying and writing. Having the T/R switch convenient to either hand is no big deal, as you aren't needing to write or tune when switching.

My father had a rotary switch for years for T/R switching with no problems, although he had three way - Net, Receive, Transmit.

One would often find marine receivers mounted fairly high off the desk. That's OK where you know where on the dial the station you want is going to be and you need to take traffic from them: for amateur use, where you search the band, it is too tiring.

For perhaps the strangest (and most stupid!) arrangement of a receiver was the R1155 in the Halifax bomber. http://www.qsl.net/g4bxd/lancaster.htm  Halifax Fig 3.
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KB1WSY
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Posts: 795




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« Reply #19 on: September 27, 2014, 04:57:36 PM »

For perhaps the strangest (and most stupid!) arrangement of a receiver was the R1155 in the Halifax bomber. http://www.qsl.net/g4bxd/lancaster.htm  Halifax Fig 3.

That is really stupid! They ran out of room!

The phenolic rotary switch arrived in the mail today. It's small -- just as well I'm only running 4W.



73 de Martin, KB1WSY

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