T/R switching, or now that it works, how do I get it on the air?

[K8DI]

So in another thread, you'll see I've gotten my first real homebrew transmitter finished/working.  I have a 24 volt line coming out for an antenna relay, which is a Dowkey 310-2301, seen here on page 94:  http://67.225.133.110/~gbpprorg/mil/photoanglo/DowKeyCatalogPart4.pdf

So I have the common connector to the antenna via my LDG tuner, and the NC connected to my Icom IC-7200 as the receiver, and the NO to the transmitter output.  It clicks and such when the PTT is activated/the transmitter is put into transmit. It is supposed to be shorting the NC center pin to ground when activated, and an ohmmeter verifies that it does.  But the receiver also shows S9+40 when in transmit, and I can hear the modulation from it. 

I have two issues:

1) I don't want to damage the Icom (I am willing to open it and modify it, I have zero qualms about affecting its supposed 'value' due to modifications).
2) I don't want to hear echos and feedback from the Icom.

And, there is no available mute line on the Icom's aux connector.  See page 10:  https://www.icomamerica.com/en/downloads/DownloadDocument.aspx?Document=361

What am I missing? How do you all do T/R switching? I thought by getting a vacuum coaxial relay, this would be easy....

Ed

[kb3ouk]

Looks like there isn't an easy way to mute the 7200 like i did with my 718. On that rig you can actually disable transmit by mode so that if i set it to AM and shorted the ptt pin to ground it would mute because the logic of the rig was telling it to go into transmit which had been disabled. Looking at the manual for the 7200, it only allows you to disable a mode entirely, no option for rx only.

[w8khk]

It appears you need to address two issues, one, the protection of the receiver front end by minimizing signal level entering the antenna connector, and two, muting the receiver when transmitting.  I faced a similar situation when I decided to use my SDRPlay RSP2 with my AM transmitter, a Johnson Viking II.

I had added a solid-state sequencer under the chassis of the Viking, which provided external connections for receiver muting (for the Collins 75A3 or 51J4) and a Dow-Key antenna relay.  The RSP2 has USB connection to the computer, but no obvious method for muting, which I will discuss shortly.

Regarding the signal level to the antenna port of the receiver, there are several tests you might perform, short of having a way to measure the actual signal level impressed upon the receiver.

First, try a dummy load vs. an antenna on the transmitter through the antenna relay.  If the overload appears with antenna but not with the dummy load, then the receiver is being impacted by the radiated signal, and probably not by the signal from the antenna relay cable to the receiver.

Second, try disconnecting the receiver cable from the relay, and short that end of the cable, try again.  Next, try shorting the input to the receiver with no cable, comparing the signal level in each case.  You will be able to determine whether the radiated signal is entering the receiver via the antenna port, or through some other path (which you may not be able to alter.)

With my RSP2, I found that it was necessary to add a second relay to eliminate overload.  I had the same relay configuration you have, which shorts the receiver port at the relay, but that was not sufficient.  I ran a length of RG-58 from the main T/R relay to a box with a relay on a perf-board.  This relay coil was driven in parallel with the main T/R relay, but with a switch to disable this relay to allow zero-beat tuning or monitoring.  This relay disconnected and grounded the receiver input, and the overload problem was resolved.  

Because the RSP2 does not have a direct method for muting, I used the other contacts in this additional DPDT relay to either connect the audio output from the computer sound card to the external audio amplifier, or short the input to the audio amplifier on transmit.  You could do the same thing with the speaker output of your receiver, but instead of shorting the output, simply connect to a 4 or 10 ohm resistor during transmit.  You will now be able to mute during transmit, and monitor or zero-beat with this extra relay disabled.  (Note that it is very rare to find that just switching the antenna and shorting the antenna input is sufficient to mute a receiver, due to the high sensitivity level of the receiver.  Some muting methods disable B+ for the entire receiver, some block the audio amplifier, others alter the AGC voltage in the receiver.  With the newer appliance receivers, similar approaches may not prove practical, so external treatment is often the more prudent avenue of choice.)

I used a small Bud Mini-box with BNC connectors for RF (2) and RCA connectors for audio in and out (2)   Since my control relay source was referenced to ground, I used a third RCA connector for relay control.  (On the back of my Viking II, I removed the ceramic crystal socket (originally for the 120 volt T/R relay) and I drilled the center hole to 1/4", and added three RCA jacks to the back...  One for the TR relay, one for receiver muting, and one for line audio in, bypassing the mic preamp stage in the Viking.)

There are probably dozens of ways to solve your muting problem, but this one is simple and does not require modification to your receiver (unless you want to add relay switching to an internal speaker).  I hope this information is helpful to you.  

73, and good luck with the new transmitter!

[WBear2GCR]

Seems simple enough.

FIrst off an S9+40db will not damage the receiver.
It could receive signals off the air stronger than that.

Personally I do not mute my receiver, I use it as a monitor on transmit.
By placing the speaker at some feet from the microphone, feedback is
typically avoided. Sometimes reversing the phase of the speaker will
help, if there is a tendency to feedback.

Some folks do not like to hear themselves.

Assuming you have an extra contact or a way to run power to a
secondary relay, you could just disconnect the line to the speaker
used with the Icom. IF ur still using the little 3" speaker in the rig?
Go with a larger external speaker, much better.

Different folks like different things, but I do not like hifi speakers, I
prefer paper cone, "old school" speakers. Such as those from old
TV consoles, old 50s and 60s hifi consoles, big old radios, and in
some cases old PA stuff and electronic organ speakers. All similar
in design and tonality.

They also play louder for a given power input than do modern hifi speakers.

So, you'd use a relay to open up the line to the speaker.
It's a solid state radio, so it will not hurt the internal audio
amp. IF it was a tube radio, you'd want to put a load to ground
that did NOT get removed when the relay actuated.

In that case you could just put something like a 32ohm (+/- a bunch)
3-20watt resistor permanently across the speaker terminals to keep the
output iron from looking at a no-load condition. You'd lose a fraction of
a watt of power, but so what...

[K8DI]

Did some tests, as suggested...

with transmitter into dummy load, otherwise the same, still S9+40 (or more, that's all the higher it goes).
With cable disconnected from relay but still connected to radio, and end shorted with an alligator clip, and transmitter into antenna, S8.

The other issue with not further isolating the receiver from RF is the long long time the receiver takes to recover from the high level. I tried it on air last night with my buddy across town. I missed half his replies waiting for the radio to un-de-sense...

I'm going to try the second relay approach. It's simple enough and I have the parts on hand already. And I'm pretty sure I'll still be able to monitor myself on it, given a shorted disconnected cable still had S8, and the cable won't be disconnected.

Ed

[w8khk]

Ed, several more things to consider:

If it takes a long time for the receiver to recover when ending your transmission, check the AGC configuration on your 7200.  On page 41 of the manual, it explains how to set the AGC time constant.  Slow AGC is normally used for SSB, but fast AGC is appropriate for AM, where you have a constant carrier to control the AGC.  The slower AGC in sideband mode hangs the AGC for a while between syllables such that the gain is not increased during quiet periods.   Make sure you see F-AGC on the display after changing the configuration.  This might correct the problem you are experiencing...…

Next, you might try shorting the input to the receiver right at the antenna connector, instead of at the end of the cable.  Then try transmitting into the dummy load and then the antenna, and monitor the received signal strength.  If it is less than you noted by shorting the end of the receiver antenna cable, then it might be prudent to use just adaptor/connectors between the back of the receiver and the relay box, thus minimizing the signal pickup as much as possible.   

You also might want to avoid using the same relay to mute the speaker; reason being you do not want the speaker cables to act as antennas during transmissions, which could negate the improvements you have already accomplished.

I looked at the accessory connector pin definitions, and did not see any capability of muting the receiver, or altering the AGC level externally.

If all these things do not resolve the issue, you may have a T/R switch timing problem, indicating the need for some sequencing adjustments.  I do not know how you are doing your transmitter enabling during transmissions, and I admit I have not had time to review your schematic diagrams.  But normally, there is a preferred sequence of steps, with a few millisecond time delay between these steps, for proper T/R transitions.  Usually the antenna relay should be switched from R to T first, along with muting the receiver, either by internal control, speaker disable, etc. Then the transmit function may be enabled once the antenna relay has reached a stable connection to the transmitter.  When going from transmit to receive mode, the transmitter should be disabled a few milliseconds before the antenna is transferred back to the receiver.  There are many methods to accomplish the timing delay, from R/C circuits controlling switching transistors that drive the relay coils, to the simpler method of having one relay's contacts control the coil of another relay.  Cascading relays allows this timing sequence to be accomplished with no other R/C or active components. 

If your transmitter is still providing RF output when the relays switch to receive, even for a few milliseconds, you will have problems with the receiver as you have noted, and you will also possibly arc the antenna relay contacts, damaging the relay over time.  I hope some of this information proves helpful to you.

[K8DI]

....  you may have a T/R switch timing problem, indicating the need for some sequencing adjustments.  I do not know how you are doing your transmitter enabling during transmissions, and I admit I have not had time to review your schematic diagrams.  But normally, there is a preferred sequence of steps, with a few millisecond time delay between these steps, for proper T/R transitions.  Usually the antenna relay should be switched from R to T first, along with muting the receiver, either by internal control, speaker disable, etc. Then the transmit function may be enabled once the antenna relay has reached a stable connection to the transmitter.  When going from transmit to receive mode, the transmitter should be disabled a few milliseconds before the antenna is transferred back to the receiver.  There are many methods to accomplish the timing delay, from R/C circuits controlling switching transistors that drive the relay coils, to the simpler method of having one relay's contacts control the coil of another relay.  Cascading relays allows this timing sequence to be accomplished with no other R/C or active components. 

If your transmitter is still providing RF output when the relays switch to receive, even for a few milliseconds, you will have problems with the receiver as you have noted.

.......  I hope some of this information proves helpful to you.

Uh, yes....it IS proving useful!  Thanks for your detailed insights.

Timing is a problem, a mistake to learn from.  Right now, one relay enables the transmitter and switches power to the antenna relay, meaning the rf is on simultaneously with the coaxial relay starting to switch. Its spec for switch time is 8 mS.  Guess like everything else, it’s never really done....  I’m going to play with some caps and resistors and rework the T/R  wiring so that the coaxial relay is first...and still do the second relay to cut the steady state rf level at the receiver. 

Thanks,
Ed

[K8DI]

How does this look??

Ed

[w8khk]

Ed, that looks similar to what I used in my original Viking II sequencer.  The only thing that appears to be missing is a way to discharge quickly the capacitor that delays the closure of the transmit enable relay.  In my Viking Ii schematic, you will see I added a resistor and normally closed contacts to discharge this relay.  The issue is that if you are transmitting, then release the TR button, then press it again quickly, the 10K resistor may not discharge the capacitor quickly enough, such that successive presses of the TR button enables the transmitter without the requisite delay.

Have a look at my simple Viking II sequencer, the one with no transistors, in the attached schematic.  I also include a later schematic of the sequencer I built for W4BYT and his Viking II, it is a small external box, connected thru a slightly modified accessory connector on the back of the transmitter.  I also share a similar sequencer I built for his TS-830 and Collins 30S1.  It has transistor drive and LED indicator lights, and trim-pots for adjustable delay time.  You probably do not need anything this exotic, but food for thought anyway.

[w8khk]

Here are a few views of the W4BYT Viking II sequencer.  A lot packed into a very small box!

[K8DI]

I've modified the control section to incorporate the simple delays I drew up earlier. I've updated the schematic in the previous thread, too, so if someone like me searches for circuits in the future, they'll see the better variation.

RC timing calculations and my ears listening to clicks hear the relays operating in the right order. The bleeder resistor on the second relay takes under a half second to discharge the cap, so unless I make really short transmissions, it should not be an issue with timing.

The next transmitter will have a complete sequencer. This one wants to be on the air, not upside down on the bench... I will still do something to further isolate/control the receiver, but it will be external to this transmitter. I thank you all for your advice and sharing, especially W8KHK!

Ed

[w8khk]

Ed, it is good that you will have the new homebrew on the air soon.  You are very competent at addressing the issues, and resolve them very quickly.  Your next rig should be even more fun.  The advance planning you did paid great dividends.  Looking forward to working you, perhaps on 40.  Nice to see a new rig with the classic old-style construction, absolutely beautiful.  

Speaking of beautiful, I noticed you live in the "Netherlands" of Michigan.  I recall driving from New Jersey to visit my high-school sweetheart at Hope College just before I entered the Air Force in '65.  I am from Michigan, too.  Born in Mt. Pleasant.  My dad's first call was W8KHK in Mt. Pleasant in 1933, when he was just 14 years old.  If you search and browse a bit on this forum, you will see some of his shacks, all with the black crackle rack construction.  When he was working for the FCC in 1941 (just before entering the Navy) at the monitoring station in Allegan, he was W8VJR.  Later on when he was W2DU, I stole his original call; that is why I am an 8 in 4-land.

[K8DI]

Speaking of beautiful, I noticed you live in the "Netherlands" of Michigan.  I recall driving from New Jersey to visit my high-school sweetheart at Hope College just before I entered the Air Force in '65.  I am from Michigan, too.  Born in Mt. Pleasant.  My dad's first call was W8KHK in Mt. Pleasant in 1933, when he was just 14 years old.  If you search and browse a bit on this forum, you will see some of his shacks, all with the black crackle rack construction.  When he was working for the FCC in 1941 (just before entering the Navy) at the monitoring station in Allegan, he was W8VJR.  Later on when he was W2DU, I stole his original call; that is why I am an 8 in 4-land.

The world is small...

My current wife works at Hope.  My first wife was from Franklin Lake, NJ, and I met her at Hope.

I used to work in Battle Creek (at Fort Custer/civilian contractor), and drove past the Allegan monitoring station every day. Sad to say it is now completely gone. For a few years after it shut down, the remnants could be seen, but no more. Such antennas! A ham's dream.

I have no family ham history, I'm the first. My dad served in the USAF '56-'64, as a computer operator.

Like I said, small world!

I'll keep an ear open on 40 for you!

Ed

[K4RT]

Very nice work by both of you. 

Enjoyable to follow the reasoning and logic of this thread.

Brad

[w8khk]

Brad, thanks for the kind words.  In retirement, I have learned to quit quoting that "time is money".  I try to assist on the forum when I am confident, and otherwise I sit back and learn from everyone else.  That is what makes it great for everyone.

Ed, You are truly correct about the small world.  My mom came with me on the trip to Holland, but we stopped at the Allegan monitoring station.  She was working there at the same time dad was, and you can read about it here:    http://amfone.net/Amforum/index.php?topic=31854.0 under the subheading "The importance of CW".  After reading that story, you can see that my mom and dad met in Allegan, Michigan too!

When we visited, one of dad's FCC colleagues, Alex Polityka, W8FLA (SK since 2008) was still working there.  I had never met him before.  Mom suggested I go in alone and apply for a job.  When I did, Alex said "Where is your dad?"  He knew who I was at first sight!  Mom made the trip to visit relatives, as she hails from Kalamazoo.  On our vacation trips from NJ, and we always loved stopping in Battle Creek for the tour and the free cereal samples!

Like your dad, I was a computer operator before the Air Force, for Johns Manville Corporation.  In the AF I was an electronics maintenance tech for the comm-nav, radar, and weapons release computer systems on the Phantom F4c and F4D aircraft.  Later branched out into PMEL.  Later on, over 30 years with Hewlett Packard and Agilent Technologies before retirement.

By the way, I do have a good deal of ham family history.  My grandfather was W8YNG, but he was licensed later than my dad.  My older brother Bill was WN2ETP, WA2ETP, 5A4TY (USAF, Tripoli, Libya), AG2B, and finally W2WM.  My younger brother John is K4JRM, and My sister Sue was licensed, but let it expire.  Her husband Keith is WD9JCA. If you search on AMfone for "W8YNG", you will see his 1937 AM rig that dad used in the 60's and I recently restored for 40 meter AM.  In use in the same family for three generations, but unfortunately neither my daughter or son show any interest in ham radio.  Perhaps my grandson, or nieces or nephews?

Speaking of coincidences, when I was stationed at DMAFB, Tucson, I had regular skeds with dad in NJ on 15 meters every Saturday morning.  One sked was obliterated by local QRM, non-ham.  I dropped the SB-101 in the car, and headed up I-10 toward Phoenix to keep the engine cool while supporting the power-hungry tube rig.  After a couple hours, we signed, and I was just arriving in Phoenix.  A young lady (Sisley Barnes, K7TLP) called and said I was in her neighborhood, and she invited me for freshly baked cinnamon rolls and hot coffee.  When she met me at the door, she introduced her daughter Joellen.  She was about to introduce her boyfriend, when I said "Hi Jorge".  She wondered how I knew him....   He is the younger brother of one of my HS classmates, Maria, but I had no idea their family moved from NJ to Phoenix and they were close neighbors.  We all became friends and I visited often.  On one visit, Sisley told me we were going for a ride.  We ended up in Scottsdale, at the home of K7UGA, Barry Goldwater.  She said it was her turn to do the phone patch duty for the overseas servicemen to visit with their families stateside.  I learned the ropes, and after that I volunteered once a month when off-duty for the remainder of my assignment at Davis Monthan.  Yes, it truly is a small world, made even smaller by ham radio!