Surviving in the Intermod Jungle

By Mark Bacon, KZ9J 2205 File Dr Decatur, IL 62521

live on top of a gentle rise, a good location for transmitting and receiving signals. I also live in the shadows of two broadcast towers: an AM antenna 300 yards west of my back yard, and a commercial FM/amateur site less than a mile to the east. My station is subject to intermodulation interference (intermod or !M), and ( can quickly tell from my operating position whether a receiver has a strong front end and good spurious rejection. If the receiver is deficient in these areas, a menagerie of squawks, blurps, and groans greet my ears!

Radio frequency interference (RFI) generally falls into two categories: (1) interference to home entertainment equipment from amateur transmitters, and interference to receivers from commercial and household items such as medical equipment and microwave ovens.1 (2) Interference to well-designed receivers from one or more transmitters. Some radio equipment that works correctly and complies with FCC regulations can generate or respond to interference! This RFI often takes the form of IM.2

What the Receiver Hears

Interference present on the same frequency as another signal causes receiver degradation. When strong off-channel interference decreases RF (and possibly IF) gain, the effect is desensitization or desense. The receiver suddenly, but only temporarily, loses its oomph! In severe cases, the receiver may experience both degradation and desense.

To explain how desense works, think of a receiver with a minimum discernible signal (MDS) of -135 dBm, and an overload immunity specification (also known as blocking dynamic range) of -100 dB at the interfering frequency.3 The level of interference that causes receiver desense is greater than -135 dBm - (-lOOdB) = -35 dBm or 3.98 mV across 50 ohms—a rather potent signal! By contrast, any on-frequency interference with a level greater than - 135 dBm has the potential to cause degradation. In other words, on- or near-frequency interference may be apparent at a level up to 100 dB lower than off-frequency interference. Small wonder most interference is of the on-frequency variety.

1 Notes appear on page 14.

Another important receiver specification is the intermodulation distortion (IMD) dynamic range. This is the receiver's ability to avoid generating IM in its front end when subjected to two or more strong, off-frequency signals.4 For a given IMD dynamic range, the greater the sensitivity, the greater the receiver's susceptibility to IMD.

Let's consider two receivers—each has - 85 dB IMD dynamic range. Receiver A has an MDS of -140 dBm while receiver B is 13 dB less sensitive at -127 dBm. Receiver A detects IM when both mixing signals are at a level of -140 dBm -(- 85 dB) = -55 dBm. On the other hand, the threshold mixing signal level in receiver B is -127 dBm - (- 85 dB) = -42 dBm. Although receiver B is 13 dB less sensitive than receiver A, it has a 13 dB better immunity to IM. This illustrates a point often overlooked—receivers with identical IMD dynamic range do not generally have the same susceptibility to IMD! Adding a preamplifier to improve the sensitivity of a receiver also increases its IMD susceptibility—usually by more than the gain of the preamplifier!

Sources of IM

Most IM is generated either in the RF amplifier and mixer of the receiver, or in the final amplifier of a nearby transmitter. IM, however, can also be produced in the environment—diode or nonlinear action in rusty tower joints, weathered feed-line connections, corroded dissimilar metal junctions, and saturated ferrite cores, for instance.

The most common IM is of the third-order, 2A-B variety. Here, the second harmonic of transmitter A mixes with the fundamental of transmitter B to generate a product near the fundamental frequencies of both transmitters, assuming both fundamentals are within about 5% of each other. For example, transmitters A on 444.525 MHz and B on 444.550 MHz can mix to radiate IM products on 444.500 MHz (2A-B) and 444.575 MHz (2B-A). IM of the 2A+B type can also occur, but is generally less important than 2A-B.

This phenomenon can be demonstrated convincingly with two hand-held transceivers operating on the same band and by use of a spectrum analyzer. Set the two transceivers on unused adjacent or alternate-channel simplex frequencies.

Tune the spectrum analyzer to a frequency midway between the two transmitters. Set the sweep width to 0.1 MHz per division. Place the transceivers back-to-back (for tight antenna coupling) and key both units. Note the strong third-order IM products on either side of the two fundamentals. (You'll probably see a "picket fence" of odd-order products through at least the ninth order!) Now, while keeping both radios keyed, move them away from each other. Watch the higher-order IM products drop out and the intensity of the 2A-B products decrease as the antenna coupling decreases.

Another frequently encountered form of intermodulation is second-order, A±B IM. Signals from a cable channel AA (on 301.25 MHz) can mix with a 154.310-MHz commercial high-band signal to produce interference on 146.94 MHz (A-B), and on 464.560 MHz (A + B) in the UHF commercial segment. (By the same token, emissions on 301.25 MHz and 146.94 MHz can mix to produce interference on 154.310 MHz and 448.190 MHz!)

In dense RF areas, third-order IM of the A + B-C variety, involving three transmitters, is possible. Fifth-order IM, 3A-2B where frequencies A and B are fairly close, is known to cause trouble as well.

Pinpointing the Problem

Is the IM being generated in the receiver or external to it? Insert a 6-dB attenuator pad in thes antenna line to the receiver (Fig 1), If the IM strength decreases by 6 dB with the pad switched in, the IM is being generated outside the

Fig 1—A 6-dB switchable attenuator suitable for receiver IM tests. The attenuator can be housed in a small, die-cast box. Resistors are Vi-W, 5% metal-film or carbon composition. Keep component leads as short as possible. S1 is a miniature double pole, double throw switch.

receiver. If the IM is reduced more than 6 dB, the mixing occurs in the receiver's front end. Second order, receiver-generated IM drops 6 x 2 = 12 dB with a 6-dB pad. Third order or 2A-B IM decreases 18 dB, and so forth.

IM that is produced outside the receiver and conducted down the feed line is more difficult to track. Before the National Guard is called out, some evidence gathering is in order. Is the interference more audible at your location than at those of neighboring hams? If so, chances are you're dealing with external rectification on or near your premises. Detective work with a portable receiver helps. When you get close to the source, replace the whip antenna with a "sniffer" loop or RF current probe.5 Look for culprits like rusty chain link fences, loose or corroded tower bolts, and antennas overdue for maintenance. Environmental IM is sometimes traced to another receiver in the shack that generates the IM in its front end and radiates it.

If you and other amateurs within a several mile radius are experiencing the intermodulation at about the same signal strength, the IM is likely to be 2A-B IM. This form of IM is generated in the final stage of a local transmitter. A 100-W transmitter can easily radiate 10 mW of IM by signal mixing with a nearby transmitter of similar power. (If 10 mW sounds insignificant, I've used that power level several times over a distance of 2 miles to check into a 2-m net at nearly full quieting!)

Intermodulation usually has a characteristic signature that distinguishes it from other signals. A signal that comes and goes erratically, with a conversation, is most likely IM. If two voices are heard simultaneously at normal levels, you're probably hearing second-order IM. If at least one voice sounds loud and distorted, the IM is likely to be 2A-B third order (or possibly even fifth order). In 2A-B IM, the deviation of A is doubled along with the carrier frequency, whereas the deviation of B stays the same. Sometimes signaling tones or data are heard from a paging or mobile telephone service.

Commercial services, like most amateur repeaters, have automatic CW identifiers. Diligent monitoring may reward the listener with a call sign, the key to identification. Several years ago, a mysterious, but persistent IM at my QTH finally yielded the call of a land mobile service. A check of a computer listing of regional commercial services gave me positive identification and the service frequency.

If you can get one call sign of a commercial service, call the shop manager of a large, local land-mobile service facility. Most service shops are interested in community relations, es-

(Dt 1 MILE

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Fig 2—Estimating the IM level in the transmitter of a repeater (see text).

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