A few RF Interference measurements

The problem of emitted RF interference came up on Michele Denber's thread about the restoration of her VE301W radio. I decided to start this thread to avoid diverting focus from Michele's restoration effort.

As you can see in the original thread, some discussion ensued about the merits and failings of regeneration. One aspect that was discussed is the problem of emitted RF energy when the detector is allowed to oscillate.

Some regenerative designs can generate a very powerful signal, on the order of 100mW, as Emilio Ciardiello points out in the original thread. This much power could be heard over 100m.

The radio designs that are most likely to emit large amounts of EMI are those that have an oscillating circuit right at the antenna terminals. This is the case with regenerative detectors at the antenna terminals. Some early superheterodyne designs also had poor isolation from the antenna. Another case to whatch out for is early radios without a metal chassis. Some of these may oscillate strongly, if the antenna wire or other external wiring comes near sensitive areas in the radio. Loop antennas can also cause oscillations by picking up feedback from the unshielded coils in the RF and detector stages.

As some of you may have noticed by now, it does not take very much to get me in my lab to make measurements. So I decided to see how my Trav-ler 5 tube portable generated interference to the nearby AA5 Delco Candy Stripe, and to a Radio Shack DX-375 digitally tuned radio.

The Trav-ler is not intended as a regenerative design, but toward the lower end of it's band, it will oscillate if the Volume control is set to maximum. The volume control in the Trav-Ler adjusts the gain of the first RF stage with a positive grid bias that loads the loop antenna, thus reducing it's signal output. This unique method of gain control solves the problem of having a high resistance potentiometer in the signal path, at a time when the common technology was best suited for low cost low resistance rheostats.

I set the Trav-ler to oscillate at it's strongest level and listened for the signal in the Delco and the DX-375 sitting on top of the Delco. Both radios could pickup the signal when arranged as shown in the photos. The surprising part is that the signal disappeared completely when I moved the sensitive DX-375 away by a distance of only 4m. At this distance, the DX-375 was back to picking up a roar of natural static. This is good to know, because all my neighbors are more than 4m away.

I have a microamp meter in series with the detector tube supply in the Trav-ler. You can see it in the photos as the bright rectangle at the top-left of the radio. Under normal operation, I get a 300uA draw. under strong oscillations, this current drops to 50uA from the automatic self bias of the grid leak. This helps to explain why I am not radiating a lot of power.

While I had the sets running, I noticed that I could pickup the local oscillator of the Delco very easily in the Trav-ler and in the DX-375. I noticed this when I swept the tuning knob of the Delco to the low end of the band, such that the local oscillator could be heard by the other radios. I picked up the DX-375 and had to move it away by 2m to loose the local oscillator of the Delco completely to the point that the roar of natural static was back. All these measurements were done around mid-night, so I had plenty of natural static in the AM band.

All these measurements had to be done while the light dimmer in the 70W IKEA floor lamp sitting 2m away was completely turned off, otherwise, the measurements would have been overwhelmed by the dimmer buzz. When I want to listen to AM in my lab I have to turn off this lamp for the weaker stations. This situation is actually improved from the original dimmer noise because I added a good power plug filter.

Summarizing; The oscillating Trav-ler was weaker than suspected and the local oscillator radiation from the Delco was a surprise. And I am not going to disturb my neighbors when I play with my Trav-ler, when I let it oscillate to help me find weak stations.

In both cases, I am under the legal limit in the USA. The general thrust of the US law in this case, is that anyone can transmit small amounts of power in the broadcast bands, with the requirement that no interference be caused to other people. I don't know the exact maximum power limit is. It is probably under 100mW. This is the same law that makes it legal to use a wireless FM transmitter from an portable music player to a nearby FM radio. A couple of years go, passing through London, I noticed that these devices are not allowed on the market. The more flexible US law seems better suited for the USA because the population density is generally substantially lower than in Europe.

It would be interesting to hear from owners of Regenerative sets. Perhaps they can tell us how far their sets radiate interference. Using a modern high gain AM radio is a good way to check interference because natural static noise will set the same noise sensitivity to all radios.

Comments invited,

-Joe

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