Technical Topics, May 1999
Last Updated on October 14, 2005 by Ralph Brandi
The Radio Boys Search For The Mystery Phantom Signal
NASWA member, Journal compositor, and NASWA web site guru, Ralph Brandi, ran into an interesting problem recently which I would like to share with you this month.
One of the problems we encounter when moving into a new location is not knowing what to expect in the way of radio interference. The prudent person will take a portable radio and listen for the usual power line leakage, light dimmer buzzes, etc. But some of the more subtle problems are sometimes missed. That was Ralph’s problem. Ralph wrote to me to see if I had any suggestions. Here is what he said.
“I live about a mile south of 500 watt WHTG on 1410 kHz and a mile and a half north of 2500/1000 watt WADB on 1310 KHz. I’m finding them breaking through on 60 meters, particularly the 1310 station. But it’s not third-order intermod, because it doesn’t follow the formulas. The formula is much simpler. I get the station mixing with a strong station on 49 meters at minus 1310. So for example, If BBC is broadcasting in the morning on 5965, I’ll receive WADB with a little BBC at 4655. With all the strong signals on 49m, I’m getting WADB almost up to 4900 kHz, and it’s driving me nuts.
“I have a MW filter from ICE that notches out 500-1700 kHz, but putting it in line hasn’t helped at all. I can tell it’s doing what it’s supposed to because MW is pretty dead on my long wire, locals only.
“The radio is an R8, and my antenna is 70′ running directly from the radio out the window, around the corner, and tied to a pile of PVC lawn chairs in the back yard. (This is referred to as the ‘Inverted Lawn Chair’ antenna.) It’s a stopgap measure until I figure how to get coax through the window and out to the woods in back of the house. The house has vinyl siding.”
Ralph had already done two good things to narrow down the problem.
- He calculated the mathematical relationship that described how the broadcast band signal was showing up on the 60 meter band. He figured out that the frequencies he heard were the difference between the strong 49 meter band signals and his local broadcasters.
- He inserted a filter in his antenna lead to reduce the amplitude of one of the interfering signal components. The fact that this filter did no good gives us another clue on where the mixing product is coming from. More on this later.
I suggested Ralph run a simple test to show whether this mixing product was occurring in his receiver or somewhere else. I asked him to insert the input attenuator on the Drake R8 and let me know how much the interference dropped on his S-meter. He did that and reported the 20dB attenuator caused a 3 S-unit drop in level on the 60 meter band mixing product. This test combined with the fact that the ICE filter had no effect allowed me to deduce that the mixing was not occurring in his receiver. Here’s why.
Mixers are non-linear devices. If you reduce the power level of the strongest component going into a mixer, the power level out of the mixer will drop by more than the original reduction. Most S-meters have about 6dB difference between each S-unit. Ralph measured 3 S-units or about 18 dB. That’s close enough to draw the conclusion that things downstream of the attenuator are operating in a linear manner. This is confirmed by the fact that greatly reducing the level of the local broadcaster with the ICE filter had no measurable effect on the level of the mixing product.
The conclusion is that the mixing is occurring before the receiver and is being re-radiated and received on the antenna. It is something called a “passive intermod product”. Mixers are essentially diodes. Some mixers use more than one diode for greater efficiency, but in essence all mixers use diodes to provide the necessary non-linear element.
Any metal object touching another metal object can set up a diode through point contact rectification. Remember the old crystal detectors with their “cats-whisker” wires that had to be adjusted for a good spot on the galena metal at just the right pressure? It was an early example of a diode made by point contact rectification.
I once worked in the field of space and satellite communications. Antennas designed for this service are usually used for simultaneous transmitting and receiving. The antenna designers had to be wary of the passive intermod phenomenon because the antenna surface itself could generate these products when many kilowatts of RF were involved on the transmit side. Segments of the big dish antennas had to be electrically bonded or insulated from one another to avoid weak metal to metal contact. Failure to observe this precaution could result in passive intermod products of the transmitted frequencies showing up on the receive frequencies. With the extremely weak signals associated with satellite and deep space links, it did not take much of an intermod signal to wipe out the received signal.
In Ralph’s case there is something in his yard or home that is acting as a point contact rectifier. It is probably connected to a large physical structure that is acting as an antenna on the short-wave bands. It could be a drain spout for collecting and diverting rain water off the roof. These are usually made of thin-walled aluminum tubes which are force fitted together but not electrically bonded. The poor contact becomes a point contact diode mixer and the rain gutter acts as a pretty efficient antenna.
Other common point contact rectification source are the power lines and guy wires running through your backyard. The guy wires are often tied to the neutral bus on the pole. Where the guy wires attach to the ground anchor the electrical connection is usually less than a solid electrical bond. After years of flexing and wear of the guy wire, rust and corrosion will build up on the mating surfaces. The joint becomes a point contact rectifier and the guy wire acts as an antenna.
The same thing can happen from loose hardware on the overhead electrical lines. The power company will often whack their poles with a big sledge hammer while listening to the effect on the radio. The hammer sets up a vibration in the wires that will quickly reveal a loose connection.
The easiest way to cure these kinds of problems is to snoop around with a portable radio tuned to the intermod frequency. When you find the structure re-radiating the signal, you will then be able to physically examine it for loose connections between elements. WARNING: Leave correction of electrical service problems to the utility.
Another approach is to get your antenna as far as possible from any other metal conductors. Use a coax cable shielded-lead-in feed line. The RG-6 cable, used for backyard satellite dishes, can be buried in a narrow slit in the lawn providing further isolation.
If you have technical problems or questions, drop me a line, preferably by e-mail. I’ll do my best to help and will share what we learn with other Journal readers. Until next time, stay tuned.