The AM Forum

As I put together my "authentic" 1930s station, one question which keeps coming up and I've kicked around
with other fellows more knowledge than I, is how did the early stations handle transmit/receive switching?

Reading the early ARRL Handbooks, I've seen various combinations of large knife switches, but never see a T/R relay.
I do use a T/R relay with the 1947 Collins 30K/75A1 station, but this was supplied by Collins,
don't think that is generally the case for the 1930s stations, so please correct me if I am wrong.
As cool as the knife switch is, wondering what thoughts are about what is and is not correct for this period.

I should note that I am using balanced line, and both the TX and RX here are set up for balanced input/output,
no coax, just wire.

I'm trying to construct this as accurately as possible, even down to the old QSL cards on the wall, so
I'm looking for your feedback, suggestions, thoughts, comments, even photos on this question.
Ideally if you've got a photo of an old station which clearly shows the T/R wiring layout, please send it over.

Thanks!


Cordially,
Bruce W1UJR

most of them used totally separate TX an RX antennas until after WW2. the rx antenna was not considered nearly as important as the tx antenna. those that did employ some type of switching used big open frame relays if they were rich, or standard knife switches if not. coax for the most part did not exist in ham circles. it was all either open wire or single wire feed. SWR was not a consideration either. what we would call a dipole today was then known as a double zepp.

If you were rich you could afford a hot wire rf ammeter, if you were poor you use some type of bulb indicator and tuned for maximum on the meter or max brilliance on the bulb in your feeder wire.

Yes, big knife switches or separate antennas.  I have also seen home brew "ganged switches" with each gang consisting of a metal bar with a knob on one end and a screw in the other end, all mounted on a panel (probably wood). The bars rotate around the pivot screws and press against fixed contact points which consisted of what appeared to be flat filed-off brass bolt heads.  Another bar made of bakelite or similar material linked the individual metal bars.  These could be configured as a DPDT  (or bigger) switch.  I saw one of these one time at a hamfest.  It had the bars supported by bolts on the end of porcelain standoff insulators and the fixed contacts were also mounted on porcelain standoffs.  Everything was jumpered with old buss bar type square wiring.  I asked the seller what it was.  He said he thought it was an "antenna transfer switch".  

Home brew or something made out of old scraps would have been the rule in the 20's and 30's.

73,  Jack, W9GT

Sounds about right. From what I have seen reading old QST's, separate antennas and switching the rcvr to standby before hitting the plate switch was the norm. I guess a lot of hams did it that way at one time. Because, as I recall, the ARRL Handbook of the 60s scolded that "any self respecting ham should not have to throw more than one switch to T/R".  

 What I want to know is how the old buzzards of the 30s used "twisted pair" feedline.

Twisted pair feedline presented close to 70 ohms or so impedance.  Since most of the rigs used link coupling it was a natural.  Very simple indeed!  In my collection of stuff, I have one of the earliest pieces of "RG-8" type of coax and a letter from Belden talking about it.  My impression was that open wire line and end fed wire antennas were the earliest type of feeds with zip cord twisted pair somewhere between the middle before coax became available.  Its amazing how things have come full circle.  Many Ham's today use open wire line which I switched to about 35 years ago and never looked back!!   
Joe, W3GMS   

Antenna relays and TR switching were advertized and written about in the early 1930's in QST. A 1926 article discussed building a break-in relay. Proper sequencing of transmitter relays was discussed as early as 1929. VOX was covered in 1936.

C'mon you guys.
I'm repeating others here, but...

Everybody knows (particularly 'VYZ, 'cause I think it was he that told me) that AM QSO switching was by the trusty, solid contact solder pot.  For transmit, you plunged the fired iron into the pot, inserted the key line contacts and let cool.  For receive you plunged again and pulled the wires out.

This method using melt theory lag guaranteed decently buzzardly transmit times and allowed the convienent pause for polite listening between 'paragraphs' so that others might join the qso.
 

I guess thats how those hams got "thick skin" Rick!! 
Joe, W3GMS

I am presently using an open frame DPDT antenna relay that was made in the 30's, apparently designed to directly switch open wire line.  I presently use only one pole of mine, to substitute for the Dow-key I crapped out with the Gates on 160.  I found it unused, new old stock, n.i.b. at a hamfest for a few bux, years ago.  The coil is 110 v.a.c., with a contactor type plunger that resembles a transformer.  The coil is wound on the E-laminations, and the armature is attached to the I-laminations, and the whole thing is spring loaded.  The relay contacts are very large, about a cm. in diameter, and the relay travel is enough to provide about a 1" gap between open contacts, which are mounted on ceramic pillars.

The base of the thing looks like it is made with Masonite with black wrinkle finish.  It had a design flaw, however.  The movable contacts were connected to the rest of the relay using "flexible" stranded copper wire.  That word is in quotes for a  reason - it took only a couple of months for metal fatigue to cause the copper wires to break, and that failure cost me a 5-amp thermocouple rf ammeter when the transmitter feedline matching network unexpectedly lost its load.  I repaired the relay, substituting the original stranded copper with a stranded bundle of extremely fine copper wire, unwound from a piece of copper wire rope.  I don't know what that stuff was used for; it was another hamfest find. But that quarter-inch+ rope is about as flexible as a piece of cotton string, and has never failed in the relay now after more than 6 years of use.  I still check it regularly for signs of fraying, but so far, nothing.

I know the relay dates back to the 30's, because I ran across an ad for it in a '37 or '38 issue of QST or RADIO.

Another problem was that it is very noisy.  The a.c. coil buzzes, and the contacts make a loud "clank" whenever the relay engages, and I found that very annoying.  I fixed it by mounting the whole thing inside a small wooden enclosure with a hinged door.  The relay is mounted on springs, similar to the configuration of old style double-button carbon microphones mounted inside a ring. The springs are fixed to the sides of the box with eyelets and rubber grommets.  When I close the door, the sounds of the relay are barely audible.  I use a small 24-volt DC pilot relay to energise the big one.

After I charge up the battery in my camera, I'll take some photos.  This thing was probably intended for top of the line de luxe ham stations at the time, but I suspect many were thrown away after a few weeks of use after that stranded copper pigtail wire crapped out.

Coax works okay, but homebrew twin lead just looks so darn cool.

hams in the middle of the real Great Depression just didnt have the money to adopt those things on a wide scale. They existed, but not in the vast majority of ham's shacks at that time.

Bruce will have to judge how authentic he wants the station to be I guess. I'd place money on that not one ham in 100 in 1933 had some type of automated T/R switch. Some did, but that same person would have also had the best of everything, maybe even a Collins TX or a Hammarlund Comet Pro or a silver dial HRO.

Bruce, what exactly is the gear you are putting this repilca station together with? If tilted towards the uppercrust ( at that time ) then by all means use a T/R switch of the era if you can find one or make one thats accurate. Thats what they would have done, but the great majority of stations at that time didnt use one.

most hams were lucky to have a type 10 single tube TX and a HB regen. A '10 sold for a 1.85 from Uncle Dave, which would be around $ 35 bucks today. The average weekly wage of a day laborer was 20 bucks. It was tough to be a hammin in the middle 30's.  :P

Another commonly used feedline was plain electrical zip cord.  It is near 120 ohms and since most hams had 100 watts or less it was perfect.  I used the knife switch method myself since I didn't have room for two antennas.  The antenna was a 120 ft. end fed long wire which I ran every which way to get it on the lot.

Now today if I had open wire line, I have an open frame relay to switch it.  Not sure what it was in orgianally, but it is a nice relay with a ceramic base.  

Here is the ganged switch I was talking about:

73,  Jack, W9GT

Bruce,

Did not hear you in the AWA Linc Cundall contest last week. Too busy or skipping over me?

I like the separate antenna idea with vintage gear if you have room and it works pretty good for monitoring your own signal as well. I used two command set transmitters, one on 80M and one on 40M, in the test, both connected to separate dipoles. Then I had a Regen and the BC-348 on a switch connected to the beverage. No switching.

The TCS gear ( I made one 160M contact) has a great TR switch built in and it is so fast and the AGC is so weak, that you can hear right through the dits. That rig is true break in.

Mike WU2D

I imagine there was quite a delay when it was time to turn it over to the next person in a round table.
Swtiching from TX to RX and back in the proper sequence.
Someone's going to say they had sequencers back then........................


Phred