The AM Forum

Guys

The P.S. for my Multi Elmac is ready, and the RX and TX are working now, so just need to
make some mod. for the TR circuit, any idea will be appreciated,

Gracias

Pedro XE1YZY

Pedro,

Do you have a TR switch like a DOW KEY or a Johnson? The Elmac pair is set up to be able to operate transceive easily. I think from a switch on the front panel, you can set it up to drive any TR relay AC or DC.

I am headed on a trip this weekend so other can chime in.

Mike WU2D

For an electronic T/R unit, check the older versions of the William Orr W6SAI handbook.
(You specified "electronic" - not electro-mechanical.)

Also Dow Key made a tube based electronic T/R "switch".

These are suitable for break-in operation.
The transmitter is always connected to the antenna, the receiver's signal is sent through two sections of tube, which during transmit is auto biased off by sampling the RF, and in no case can the signal to the receiver exceed the amount that a small receiving tube can deliver, no matter what. Muting, if desired must be acheived via other means.

If you just want a T/R Relay, then shop ebay for such... the Ruskies are selling vacuum relays all over ebay, and other sources have them rather cheap these days...fwiw.

These days, one can bias some big PIN diodes for the same sort of all electronic T/R switching, I do believe.

              _-_-WBear2GCR

Hello Mike and Bear,


          Well yes, Im looking for an electronic T/R relay because I want to have QSK with the AF68, the original TX don´t have any provision for stand by in the front panel, neither the PMR7, only with the Microphones PTT  you can activate the relay circuit .

Im going to take a look into the William Orr Handbok, Next month we have a field day, and I want to see the face of my Yaesu 817 fellows when I have better report in a Portable operation   ;D, we can also boatanchoring portables isn´t? ( even if my car's battery dies in the effort )

Pedro,
           You might also look into some of the early 1960s ARRL handbooks. they have a pretty simple cathode follower circuit project for a QSK / break-in T/R switch.  although i still prefer the old fashioned electro-mechanical relay.
                                                                                    The Slab Bacon

Attached below is the Johnson TR switch .pdf file from the BAMA site:

HI Slab Bacon,

Yes the old fashion T/R its also ok for me in Phone, but when Im using CW, have a QSK have some advantages,

Tom,

Thanks a lot for the Information!

Best regards

XE1YZY

I thought those tube type TR switches were TVI generators. Also remember some tube brands would flash over.

Another problem is that the transmitter is connected to the circuit at all times.  Depending on the transmitter, it may generate noise, which is fed directly into the front end.  Another problem is that the rf goes through one or more additional amplifier stages before the receiver.  This introduces additional noise, and reduces dynamic range and resistance to intermodulation between strong signals.  For the cleanest, quietest receive, you want as few active stages between the antenna and the primary selectivity filter as possible.

I would opt for a separate receiving antenna and a PIN diode circuit to isolate the antenna from receiver during key-down, to avoid overdriving the receiver with rf.

actually a pin diode across the RX input will act as a limiter as long as it can handle the RX power picked up by the TX antenna.

Hi Don, & Francis...

          I really like this idea about the PIN DIODE, let me tell you why,  a couple of weeks ago I found an outstanding receiver, its a Rhode & Schwarz EK 56/4, this rig have 20 FI steps, and a really nice front end, works very good , gold plated conectors and silver shields all over the circuit, a bunch of connectors in the rear, but dont have a "Mute" or " stand by switch",

I guess is more a Monitor receiver or Lab rig,  Than a comunication receiver, I looking for the manual in the net,  and found a couple manual vendors in Germany, the cheaper manual price  I could have for the manual was 150 Euros plus shipping  :o, a lot of money for a photocopies of a  1974 rig,  tampering the circuit without a manual is impossible,

Ok, tampering  for what? for install a Mute in the RX, but maybe an external PIN DIODE circuit could be the solution for use this RX as a Comunication rig, with no more of 100 watts in the TX...

You follow mi Idea?, a SFH203 or BPW43  could works?

TNX Guys!

Hey Pedro,

Here is a series diode switch circuit which could provide protection for the receiver. This is only a conceptual circuit. It is untested!

The path from antenna to receiver is ON when the diodes are biased ON by the open collector logic gate pulling them to ground through the choke. When this happens 5 mA flows through the diodes which is enough to turn them on and RF can pass. When the gate is open (High) the diodes are biased off and you will get around 40-50 dB of protection. This is enough to make 100 Watts look like a couple of mW or so. The shunt diodes provide more protection.

Since the logic gates which control the diodes must have correct input voltage to switch you will have to fool around with the attenuator which feeds the diode detector to get the right pulse voltage level during keying. The time constants should be fast.

The extra gates can be used as an wired OR circuit as shown. This shows another  novel way to utilize a series antenna switch. The idea is to pick up amplify and detect the vehicles engine noise and shape the pulses and allow them to blank the receiver. Not a bad idea for AM mobile. That is the second input shown.

Mike WU2D

The circuit has problems. Did anybody catch the big one? The open collector OR connection is supposed to protect whether you go to transmit OR you have a blanking pulse. The problem is my clever connection actually formed an AND gate! See below..

Mike WU2D

I would never put the front end or a good RX a pin diode away from death. Get a good relay. I bet an R&S first mixer costs more than a fast relay.
BTW Mike A pin diode antenna switch will only work is the diode turned off is reverse biased higher then the RF voltage peak swing.
So if you are running 300 volts peak to peak (about 100 rms) you need to reverse bias the RX diode at least 300 volts or the RF will turn it on and leak over part of the cycle. A real slow Trr will help but....5 volts is now enough reverse bias for 100 watts.
I would put a strong active limiter ahead of the RX to protect it if you really want to use a pin diode TR switch

That is definately a scarey thought when you really sit down and think about it! Especially if you are running a really high powered rig. I would also feel better with some kind of redundant safety built in. Crap out a mega buck receiver for a $.10 pin diode. Kinda scarey. I would prolly be comfortable with it for some QRP rig, but not a real (especially strapping high powa) transmittah.
                                                The Slab Bacon
                                           

Ya Frank - Don't test this circuit with a WJ or even an HRO-500!

I thought we were trying to switch an Multi Elmac receiver? Is that on the premium RX list?

Actually the military uses pin diodes to switch all kinds of power in electronic warfare gear where they need fast pulses of RF. Of course they have plenty of taxpayer cash to test their circuits.

Ask Mark WA1QHQ how much fun he is having with diodes in his ricebox tomorrow at Deerfield.

Mike WU2D

Hi Mike,
Thanks a lot for your  ideas,I'm studding your circuit, and sorry for the confusion, I told to Frank and Don about using this ideas also in my Rhode & Schwarz receiver, because this receiver don't have any "mute" or "stand by " position, and I would like to use this RX as a communication receiver in a CW operation

I let you know about my progress!

Thanks again

Pedro XE1YZY

Mike,
I've been using PINs for years and played with RADAR a bit IFF and TACAN
It is all about Trr and bias. 4000 series diodes are fairly good pins above 5 MHz.
fc