What is Intermod?

Intermod is the common name for the interference we hear from other transmitters such as pagers, police and taxis. It stands for Intermodulation Distortion and occurs when the RF amplifier in your receiver is driven non-linear by a strong signal. Two or more signals then mix in your receiver's RF amplifier and create a signal on your listening frequency.

At DCI, we have answered hundreds of phone calls about intermod. From this experience, we have come up with the following general statements about intermod:

• If you put a 10 dB attenuator between your antenna and receiver, and your intermod goes away, then it is being created in your receiver and filtering will help. If the intermod does not go away, then it is probably being created elsewhere and requires a different fix. Filtering at your site in this case will probably not solve the problem.
• The signal you hear as intermod may not be the one causing the problem. When using a preamp, the intermod is almost always created in the receiver following the preamp, and not the preamp. Good filtering and a good preamp - when both are used together - will almost always improve your receiver sensitivity.
• If you reduce the strength of signals causing intermod, then the intermod products (mixing products) will be reduced by the 3rd power of the reduction. e.g. If you attenuate all signals with a 10 dB pad, then any intermod products will go down by 30 dB.

What will a bandpass filter do?

DCI bandpass filters will reduce the strength of interfering signals while allowing the signals you want to hear to pass through at virtually full strength.

Benefits of a DCI Bandpass Filter:

• Eliminate the random squeals, squawks, beeps, and strange voices from your VHF or UHF radio.
• Improve receiver sensitivity.
• Allow you to hear the signals you want to hear, even in crowded urban centers with strong police and paging systems.

Features of a DCI Bandpass Filter:

• Filters are totally passive and do not require DC power.
• Filters accept full transmitter power and reduce spurious emissions.
• Filters come with high quality silver-Teflon® connectors.
• Filters are made of extruded aluminum, copper and brass. There are no components inside to burn out or fail.

Teflon® is a registered trademark of DuPont.

How does intermod differ with different radios?

Handhelds

A handheld radio usually operates properly with a rubber duck antenna, but when it is connected to a mobile or base antenna, the HT becomes susceptible to intermod. This happens because some of the signals become strong enough to drive the RF amplifier non-linear. DCI filters allow you to benefit from the gain of a high performance antenna without creating an intermod problem.

Mobiles

Mobile radios have better intermod immunity than handhelds, but if the signals are strong enough, mobiles too will be susceptible to intermod. Whether or not this happens depends on the radio, and the strength and frequency of the signals. Some metropolitan areas have so many strong signals that most VHF/UHF amateur transceivers experience intermod. The DCI filters are very effective in reducing intermod in high RF environments.

Base Stations or Repeaters

In many situations, a DCI filter will increase the sensitivity of a base station or repeater, particularly when using a low-noise preamp. The increase occurs because the DCI filter reduces the strength of both background noise and out-of-band signals. The result is less intermod-generated noise on your active frequency.

Should I use a preamp?

A low noise preamp can be installed between your antenna and receiver and will usually improve the sensitivity of your receiver. However, there are several things to consider before installing a preamp.

First, the amount of the improvement will depend on how good your receiver is to start with, and how much loss there is between the receiver and the antenna.

If the receiver has a very low noise figure, and there is no loss between the receiver and the antenna, then a preamp will be of little help but you may still notice some improvement. This would be the case with most modern handhelds when used with an antenna that is close (short coax) to the handheld.

Second, if the receiver has a less than perfect noise figure then a preamp will help. This is especially true if your antenna is at the top of a tower with a long run of coax, and you are able to put the preamp at the antenna. Putting the preamp at the antenna is like putting the receiver at the antenna. You get the signal before it is attenuated by the coax, thus your signal to noise ratio is improved.

Third, many times when you add a preamp, the sensitivity of the receiver gets worse, and intermod pops up everywhere. What is the reason for this?

When you add a preamp, especially a broadband one, all signals from police, firefighters, etc. get amplified at the same time. If your receiver created intermod with out-of-band signals stronger than -50 dBm then after adding the preamp with 20 dB of gain, all signals of -70 dBm will now be amplified to -50 dBm. You may have signals with strengths of -30 dBm before adding the preamp. These will now be -10 dBm; very strong signals indeed.

All these new strong signals will now start mixing in your receiver and creating intermod that never existed before the preamp. As well, if signals are strong enough they will desense your receiver without causing intermod. The only answer in a case where adding a preamp makes things worse is to reduce the strength of the strong, out-of-band, signals with filtering.

In summary, a good gallium arsenide (GASFET) preamp, if used with good low loss RF filters will almost always increase your sensitivity.

What if I want a very narrow filter?

It would be nice to have a filter to pass just the one signal that you want to hear, but the physics of filters doesn't allow it. The narrower a filter is made, the higher the insertion loss. For example, at 440 MHz our 4 pole filter with a 10 MHz wide passband has a loss of about 0.4 dB. If we narrow the filter up to a 5 MHz passband the loss will go up to about .8 dB. At a 2 MHz passband, the loss will be around 1.5 dB and so on.

In practice, you want the widest filter that will eliminate your interference so that you get the lowest insertion loss possible.