You got intermod?
Hunting the source of a distortion problem can be facilitated with a reliable portable radio and a little homemade artillery.
Intermodulation, “intermod” for short, has been around for some time. In fact, it’s so well known that most radio users assume it’s a normal operating condition. Intermod has, in effect, crippled the two-way radio industry’s ability to develop the market much further. A trip to any big city with an average portable radio is one way to tell. There may be some sites in service that are relatively free from this curse, but if you’re not one of the lucky ones, the following might be a neat bag of tricks to keep the situation under control. (Radio amateurs may also find this extremely useful for “bunny hunts.”)
A case in point A severe case of intermod happened “overnight,” so to speak, causing interruption to a public safety prime channel. It was determined that this was the classic “A 1 B 2 C 5 D” signal-mixing problem. D, in this case, is the repeater input, associated with: B, a 100W VHF GE base station; A, a 250W continuous-operating VHF transmitter; and C, another 100W VHF repeater. Each system was on a separate tower. All the systems were on the same mountain and within a 50-foot radius. It just so happens that the interference appeared after station A came on the air.
It was simply thought that all we had to do was find the single source of the intermod and fix the problem. (“Must be someone’s transmitter” or, “It’s gotta be in someone’s receiver” or “Maybe it’s a bad antenna.”)
The usual steps were taken, such as investigating who had cavities, duplexers, isolators, sufficient grounding and the like. All the sites were found to have in-service isolators, 10-inch-diameter series cavities and double-shielded coax interconnecting all devices. The antennas and the solid-shield feed lines were verified as acceptable with an Anritsu 113B analyzer. In sum, all parties had the appropriate equipment in service.
Frustration began when a VHF hand-held portable radio was used for the receiver and loop antenna for determining direction. With all transmitters in line, it was (as you might guess) impossible to get a null. RF was getting into the radio, so another approach was needed: radio shielding. (By the way, wrapping the radio in tin foil is a poor approach, at best.)
Figure 1 on page 23 shows a shielding box that was fabricated (from double-sided printed circuit board material) specifically to house a Kenwood hand-held. With the antenna port terminated, the box provides a measured 130dB of shielding-far superior to tin foil.
Fabrication is done by completely soldering the inside joints of all sides and corners. The outside box corners were also soldered with a strip of copper foil to complete the outside electrical connections. The box opening has foil wrapped around the edges to connect the inside foil to the outside foil, as shown in Figure 2 on page 23. The cover is made from sheet copper, and inside the cover is a copper braid around the perimeter that makes the connection from the box to the cover. (The braid is taken from a small piece of coax cable.) The coax connection at the top is a BNC female to male N adapter, with the threaded shell removed, that is soldered to the top plate. The male N side of this connector mates with the Kenwood portable’s antenna connector when the radio is inserted in the box. The top plate also has two small holes drilled immediately above the squelch and volume control knobs. These holes allow the insertion of a small probe to rotate the knobs, if necessary. A few holes are needed for the speaker. (See Figure 3 on page 23.) Keep in mind that fewer holes means better shielding. For the Kenwood’s LCD, an opening is cut to the same size and covered with bronze screen. The sharp edges of the screen are framed by soldering them to an 18-gauge wire.
The four-inch-diameter loop antenna shown in Figure 4 on page 23 was constructed from 1/8-inch copper tubing. Female connectors were used with the loop antennas to allow a double BNC male connector to serve as a rotating joint. It’s best to hold the radio in an attitude that allows you to see the signal-strength meter while rotating the loop.
The small coax loop uses RG 59/U that is for the magnetic field. The small probe favors the E field. The loop antenna performs like any other loop (i.e., a null appears broadside to the plane of the loop). It’s the null that indicates the direction of the signal. Our site test proved the shielding to be acceptable, and the loop antennas were sufficient to indicate the origin of the offending signal.
Going hunting The loop antenna indicated where the strongest RF hot spots on the tower(s) were. The small probes were then used to pinpoint the exact cause. The problem soon became apparent-the enemy was everywhere. Intermod sources included tower bolts, loose tower grounding, chain link fence (30 years old) topped with rusty barbed wire, signs wire-wrapped to the fence matting and, finally, an unauthorized, low-budget repeater without cavities, isolators or equipment grounding.
A simple “homemade” shielded box proved to be a valuable accessory to trace and verify the intermod “culprit(s).” The four tenants have sentenced themselves to an overall cleanup. There was no one “guilty” party. It was more a case of “guilt by association.” N
