Can you build a HF SSB radio transciever in one weekend, while on the road, at parts from a swap meet? I can, but apparently not without setting something on fire.
Of course the swap meet I’m referring to is Hamvention, and Hamvention 2016 is coming up fast. In a previous trip to Hamvention, Scott Pastor (KC8KBK) and I challenged ourselves to restore tube radio gear in a dodgy Dayton-area hotel room where we repaired a WW2 era BC-224 and a Halicrafters receiver, scrounging parts from the Hamfest.
Our 2014 adventures were so much fun that it drove us to create our own hacking challenge in 2015 to cobble together a <$100 HF SSB transceiver (made in the USA for extra budget pressure), an ad-hoc antenna system, put this on the air, and make an out-of-state contact before the end of Hamvention using only parts and gear found at Hamvention. There’s no time to study manuals, antennas, EM theory, or vacuum tube circuitry. All you have are your whits, some basic tools, and all the Waffle House you can eat. But you have one thing on your side, the world’s largest collection of surplus electronics and radio junk in one place at one time. Can it be done?
Options for HF transceivers are very poor at the sub-$100 range especially those made in the US which are often collectible and valued much higher if there is a chance of functionality (e.g. Heathkit, Drake, and others). We would likely end up with some very early SSB gear or possibly a late 70’s or early 80’s solid state rig. The pickings are slim.
Choosing the ‘Donor’ Radio
Early Friday morning we found a Ten Tech Omni A with a jammed-up PTO (permeability tuned oscillator) for $80. This radio did not have the optional frequency counter, instead it had a broken slide-rule tuning indicator. I liked its modularity, you can clearly see the signal flow by looking under the top cover.
Early on Friday morning Scott located an old vacuum tube powered Hallicrafters SR-160 project radio for $60. Unfortunately 53 years of age resulted in a few dried electrolytics and drifting resistor values. It did have a few things going for it, it came with a power supply, microphone and I brought my stock of high voltage caps so we could re-cap it. Re-capping and a possible re-alignment would be time consuming.
Friday evening the Omni-A powered up and seemed eager to run. After several quick-fix attempts with the PTO we tried a Google search and realized that the mechanical jam was a very common problem. The best way to fix it was to completely disassemble the mechanism, soak it with isopropyl alcohol, then lubricate with lithium grease. There was no time to completely disassemble and clean the mechanism (it’s akin to a fine pocket watch) and the only lubricant we could find was a small can of WD40 from the gas station next door. After spraying the PTO mechanism and working the threaded brass pieces with a pair of channel-locks it came loose. This radio was able to tune in numerous SSB stations with a 20′ length of wire hanging out of its antenna jack.
Next we tried to re-string the tuning indicator, this didn’t work out too well. So we ended up with a working transceiver but no idea what frequency it was on. We did consider boot-strapping the Halicrafters VFO into the Omni-A, both are 5 MHz VFOs, but this was abandoned in favor of a faster solution.
To determine what frequency we were on we used an old Hammarlund HQ-170 receiver that had been procured separately and not part of the competition (when I see a nice boat anchor it’s hard to resist the buy). This radio worked without need for repair thanks to Hammarlund’s use of ceramic disk capacitors. We used the HQ-170 to zero-beat the bottom range of our frequency privileges then decided to only reply to CQ’s or jump into another ongoing QSO. Frequency awareness problem solved.
Antenna: Outdoor Wasn’t an Option
For almost zero cost we bought a couple of spools of cold wire, some random lengths of coax, and a few WW2 surplus variable capacitors knowing we could easily wind inductors with the scrap wire. Our raw material stash was complete when someone gave me an old VSWR meter for free. It seemed like the dipole antenna or random wire option was the best bet. Unfortunately the window of the hotel room would not open. With nowhere to string the random wire antenna it was back to the drawing board.
How do you make a decent indoor antenna that would work within a concrete structure? The magnetic loop seemed like a good option. A mag loop works on the magnetic field component of the electromagnetic field. Mag loops are used in almost all AM broadcast radios and often used by radio amateurs in Western Europe where space is at a premium. Unfortunately they’re not too efficient.
Efficiency depends on the ohmic losses in the mag loop conductors at your operational wavelength. Furthermore, the directivity (antenna gain is the product of directivity and efficiency) of a mag loop is usually poor due to the small physical aperture of the loop. For best performance we want to make the loop as big as we could reasonably fit in the hotel room and also use the thickest-available cable or wire for the loop itself.
Locating stuff at Hamvention is not like shopping the Digi Key catalog. It is a hunting and gathering exercise. In my experience it is difficult to seek out one specific item, the mode of shopping at hamventions is more akin to ‘you will know what you want when you see it,’ rather than searching through your favorite parts catalog.
After failing to get a 4′ loop of RG58 working on Friday night I was on the lookout Saturday for a replacement. For $10 a length of ¾” hardline or heliax coax cable (a very large industrial coax), larger gauge hook-up wire, and some terminal lugs were procured. Now we had everything needed to make a serious mag loop.
Rzather than an inductive coupling we decided to use capacitive coupling. One variable cap for resonating the loop and a second in series for coupling. We used short lengths of #14 wire and terminal lugs back-filled with solder to wire it up.
After peaking both caps for maximum receiver noise the loop tuned up well at low transmit power, easily achieving less than 1.5:1 SWR. I did notice that the loop de-tuned when I touched the feed coax. Because of this I rolled a shunt balun with 10 turns of RG58 directly in front of the coupling capacitor. Hand capacitance problem solved, SWR was well behaved, the loop was now being fed with a balanced line.
Time to get on the air. We turned up the power on the Omni-A and keyed the radio. Unfortunately it did not seem like we were transmitting 100 watts. The lights were not flickering in the hotel room or even on the radio itself. Nothing in this radio/antenna system seemed like it was trying to transmit. The output power meter on the radio read flat, one or two watts.
We did confirm good SSB transmit audio with the Hammurland, so something was getting out but not much. After goofing around for a while with the OmniA is seemed like it was a transmitter problem within the radio. As a last ditch effort the Omni-A’s manual was consulted (isn’t that alwasy the way?).
Automatic Level Control (ALC) is kind of like an automatic gain control (AGC) for a receiver except that it is used for transmit. With this you can achieve a higher average power than transmitting a simple linear voice SSB signal. ALC is something I have not incorporated into my own radios that I’ve built. Stupidly enough and not being at all familiar with the Omni-A we had left the ALC control at 0. Who has time to read the manual anyway?
We turned it up to about 3/4 clockwise, whatever that meant. The transmitter kicked on strong. Lights in the hotel lights dimmed to the SSB carrier, power meter on the radio read a solid 75+ watts. The Omni A worked after being neglected for decades, what a tough radio!
Time to get on the air. Scott was performing some ‘Check 1 2 3 4 tests’ at full power when I observed a very bright arc near the tuning network. To be sure I asked Scott to try again, I observed something near the mag loop’s coupling capacitor that appeared to be breathing fire in proportion to his voice.
The tuning network was made up of two very large and old variable caps, these were large WW2 surplus variable caps, one of which used ceramic insulators and the other was made of slate. These should be able to handle the RF current and voltages. These caps and the associated wiring were laying on the carpeted floor at the base of the loop which was propped up against the hotel bed.
I had used very short lengths of #14 wire to tie the network together. The insulation break-down voltage was rated for 600V. This is standard stuff you could get at the local hardware store. I asked Scott to key the radio again, while I observed the tuning network and there it was! A solid arc. Wow it was a sight. Do it again! Another arc. On the third try something lit on fire. Smoke filled the room.
It was easily put out but the smoke was nasty. We cleared it out of the room and luckily did not set off the smoke alarms. Upon inspecting the tuning network wiring it seems that the insulation had broken down and burned on the cheap #14 wiring. To fix this, I re-adjusted the wiring to put more space between the wires and placed a hotel ice cube tray under the network for added insulation. We were back on the air without further trouble.
We made contact with a station in Florida, received a signal report of 5 by 7. Good enough.
Hamvention is made all the more interesting with improvised radio sport. Previously antique radio receivers were brought to life, and now a parts radio was put back on the air and out-of-state contact made. What should be the next challenge? Who else wants to join the fun? Let us know in the comments below.
Gregory L. Charvat, Ph.D, likes to put old stuff back into service in short periods of time, is CTO of Humatics Corp., author of Small and Short-Range Radar Systems, co-founder of Hyperfine Research Inc. and Butterfly Network Inc. (both at 4catalyzer), editor of the book series Modern and Practical Approaches to Electrical Engineering, guest commentator on CNN, CBS, Sky News, and others. As visiting research scientist at MIT Media Lab he created the Time of Flight Microwave Camera. As technical staff at MIT Lincoln Laboratory he created a through-wall radar imaging system that won best paper at the 2010 MSS Tri-Services Radar Symposium and is an MIT Office of the Provost 2011 research highlight. Greg has taught short radar courses at MIT, where his Build a Small Radar course was the top-ranked MIT professional education course in 2011 and has become widely adopted by other universities, laboratories, and private organizations.