Tuesday, March 3, 2009

Amateur Receivers circa 1950s

I recently read with some interest K2TQN's column about the HBR Receiver in the February issue of QST. I had never heard about this receiver until this column -- this design likely pre-dates my involvement in amateur radio by 10-15 years. 

When I got into ham radio in the early 1970s, the transceiver was beginning to come of age. In the 1960s, SSB had replaced AM as the primary mode for voice operation. Transmitters and receivers for SSB have many of the same circuits -- so it made sense to share these in a single box. Before then, most hams had separate transmitters and receivers.

Looking at the HBR receiver stage descriptions, the basic design seems foreign to our modern designs. Selectivity is provided by high-Q LC circuits in the low-frequency IF near 100 kHz. But such a low IF frequency wouldn't result in very good image rejection, so a first IF near 1700 kHz comes first. 

Now, in the 1950s and 1960s, the amateur bands were simpler. 160m was covered up with strong LORAN A signals and was virtually unusable. The 30, 17 and 12m bands hadn't been invented yet. So, most radios only needed to cover five bands. The HBR accomplishes this through plug-in coils. 

From a modern view, the lack of bandswitching and sharp IF filtering stand out. It's hard to imagine changing bands by opening up the top cover and swapping out coils -- particularly when a couple of them have potentially dangerous plate voltages on them. Perhaps a band switch was something of a luxury.

The IF filtering confuses me. My little 40m receiver has excellent IF selectivity from just four crystals. Hams in the 1950s could have built ladder filters in their receivers using crystals from 2-10 MHz. Although, I'm not sure the ladder filter design was invented until the 1970s. Perhaps the cost of the crystals was prohibitive -- crystals are often expensive, even today.

In the same issue of QST, there was a reference to an all-transistor receiver design by W2TGP. I looked up the article in the ARRL archive. It was very odd to see a rig using all PNP transistors, especially with a +12 volt supply. (The power goes to the emitters, and the collectors connect to ground -- somewhat opposite of today's convention)

Other than the use of transistors, the design is a curious mix of older and modern elements. The rig is small, so there's plenty of room for a bandswitch for five bands. Selectivity comes from a 455 kHz mechanical filter --  455 kHz is a bit low for good image rejection, so the first IF is around 2000 kHz. 

This transistor receiver likely outperformed the HBR in terms of selectivity, but the HBR likely had better dynamic range. The single-ended mixers of the 1950s can't hold a candle to even simple Gilbert-cell mixers today.


  1. I'm another ham. Have been for 30yrs. You seem a bit arrogant, and have no appreciation for things built and designed around Vacuum Tubes. Try working on a 1945 transmitter that holds a 400W carrier w/no modulation w/o any fans! Sure beats the "rice-boxes"! Try an R-390 receiver - no filters - all by RLC circuits! No current rice-box can match it today! N0DGN

  2. No appreciation? Well, maybe it didn't come out in this posting.

    I"ve been an extra-class ham for 30 years. When I got into the hobby, tubes were just starting to be phased out in favor of transistors. I've worked with some tube gear early, but I've sold all of it in favor of solid state gear -- the exceptions being my AL-80A amp, my "Novice" transmitter, a piece of old test equipment or two, and my first receiver, a Heathkit GR-81.

    Since I wrote this, I did some research into modern ladder filters. I don't think they came about until around 1970 -- but I couldn't find the seminal article. That would explain why in the 1960s receivers were using the LF IF strips for selectivity instead.