A problem which often plagues T-hunters is Intermodulation Distortion, or "intermod". This is caused when signals from unwanted stations mix in your receiver to produce interference on the frequency to which you are tuned. What you often hear when this happens are awful squawks, buzzing noises, and squeals that drown out the signal of the hidden transmitter.
Today's receivers are often built with "wide" front ends, to allow scanning of air bands, public service bands, etc., and this very feature makes them susceptible to overload from strong stations such as Pagers and FM Broadcast stations.
Here in Albuquerque, Many of the offending signals come from Pagers, which operate in 152-153 Mhz band.
Described below are some things we have tried, in an effort to cut down this interference.
Adding attenuation can certainly prevent strong interfering signals from overloading your receiver, but will not usually help to eliminate intermod problems, and may prevent you from hearing the hidden transmitter. However, attenuation is your first line of defense, so if the strength of the signal from the hidden transmitter permits, you should use it.
One of my favorite T-hunting radios was an old ICOM IC-22S. This radio not only had a real S-meter, but was built with helical resonators in the front end, tuned to cover 2 Mhz of the 2-meter ham band. This radio NEVER had problems from pager intermod. Alas, modern radios don't have this feature, mainly due to cost, but also because consumers want radios that can cover all the public service and aircraft bands too, and helical resonators are very narrow banded.
Helical resonator filters can be obtained commerically, however, or can be home-built, if you are so inclined. For those who want to build their own, check out the construction tips in the TRANSMITTER HUNTING book, page 179, or recent editions of the ARRL HANDBOOK. Homebuilt resonators can give quite good performance, but most I've seen have a fair amount of insertion loss, too, which could be a problem when tracking weak signals. To give best performance, shields and coils should be silver plated, seams should be totally tight, and high-Q coils and capacitors must be used.
On the commercial side, our group has had good results using the PAR Electronics 2-meter Intermod Filters. Their VHFDN152 filter is a NOTCH-filter, designed to eliminate pager signals from 152 to 153 Mhz. They have several models available. Their smallest, the VHFDN152HT, is fitted with BNC connectors, will handle 20 watts of power, has about 45db of attenuation at 152.4 Mhz, and less than 1db of loss in the 2-meter ham bands (by my own measurements).
A review of the DCI Helical Resonator filters appeared in a recent issue of 73 magazine (January, 1995, pg 38), and they would appear to have similar performance characteristics to those of the PAR filters. These filters are PASSBAND-filters, designed to pass only a limited range of frequencies, such as the entire 2-meter band, or the 70-cm band.
The GaAsFET-preamplifier/helical-resonator unit built by Hamtronics does a fair job of cutting down intermod. However, the noise figure of this unit is about 6db, making it not too great for weak signals. Here is a quote from the Instruction Manual: Note that the 7 to 8 dB insertion loss of the helical resonator ahead of the amplifier does not allow the full potential of the low noise amplifier to be realized. While the preamplifier itself is a low-noise amplifier, the insertion loss of the helical resonator ahead of the amplifier adds to the noise figure; so the overall effect is not a low-noise preamp in the ordinary sense, but rather a filter for reducing intermodulation products. The amplifier is used merely to make up for the loss in gain. When hunting hidden transmitters which are putting out a fairly good signal, this unit can cut down the intermod quite well.
A simple piece of coaxial cable cut to a quarter-wavelength can be used as a filter. You simply install a "tee" connector in your feedline, with the radio connected to one port, your antenna connected to the second port, and the coaxial cable "filter" (or "stub") connected to the third port. The trick is to cut the "stub" to exactly the right length. What you want to do is tune it to "short out" a specific frequency, such as that of an offending pager.
I tuned one using an ICOM R7000 receiver, which has an excellent S-meter. First, I cut the intended filter somewhat longer than necessary. When calculating how long a piece to start with, simply divide the frequency (in Mhz) you want to "kill" into 2952.72, then multiply by the velocity factor of the coaxial cable you are using. (Most coax will be in the range of, say, 0.66 to 0.75). Then ADD a few inches, to be safe. For example, if I want to "kill" a pager which operates at 152.2 Mhz, I divide 2952.72 by 152.2 to get 19.4 inches. I multiply this by the velocity factor of 0.66 to get 12.8 inches. The final filter will probably end up near this length once it has been correctly cut to size. Then, I ADD about two inches for safe measure.
Next, I tune the R7000 to 152.2 Mhz, where a pager is putting out a 30db over S9 signal. I place a "tee" connector in the antenna line at the back of the receiver, and attach the stub coax to one port. Then, watching the S-meter, I begin to cut the coax stub down. As the stub gets shorter, the S-meter will begin to drop, showing that we are indeed notching out the pager frequency. As you get nearer the exact frequency you will find that even a 1/16th of an inch will make a difference in signal strength, so be very careful and work slowly. If the S-meter starts going back up, you went too far. (OK, I did trim too much the first time I tried this, but the second one I cut was better). The final length came out to be 12.5 inches.
What you will end up with is a filter that will knock about 35 db off that pager signal. Back in the 2-meter ham band, you will also see some attenuation, perhaps as much as 20 db, depending on the quality of the coax you use, but you will also have cut down the intermod from the pagers significantly.
Besides, it doesn't cost much, and it's a start at eliminating pager intermod. You can also use this type of filter to cut down other problem stations, such as major FM Broadcast stations.
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