Radio frequency filtering is an often overlooked aspect of radio feeds that can greatly enhance the listening experience. Although not always necessary, situations in which there are strong transmissions on adjacent frequencies can lead to some less than desirable situations:
- Desense: In order to protect its sensitive circuits, a receiver will purposely attenuate (reduce the signal strength) of all incoming transmissions in the presence of an excessively strong transmission. You will not hear any bleeding over of any adjacent channels, but the reception of your target frequency will suffer as your receiver’s effective sensitivity becomes greatly reduced.
- Third Order Intermodulation: A common form of “bleed over,” this occurs when two strong nearby frequencies mix within the receiver and create a product (unintended transmission) on the frequency you’re trying to listen to. For more information on how this works, check out this YouTube video.
- Overload: When close enough to an extremely strong transmission on a nearby frequency, some receivers will be unable to deal with the signal strengths and simply become overloaded on all nearby frequencies.
Some common sources of strong adjacent transmissions to railroad voice communications are NOAA weather transmitters, paging transmitters, and FM broadcast transmitters. While this may not affect a radio feed located in a rural setting, more densely populated areas can pose a challenge. A spectrum analyzer or even an SDR can be helpful in locating strong nearby frequencies; however, if you don’t have either, you can always look to resources like the RadioReference Database or the FCC’s Universal Licensing System to find possibly culprits.
There are numerous types of filters available, but here are the most common ones that would be used in conjunction with a radio feed:
- Bandpass Filter: Also known as a Preselector or Window Filter, these filters pass only a specific range of frequencies while rejecting everything outside of that range. Bandpass Filters are helpful in situations where there are many strong adjacent frequencies affecting the receiver.
- Notch Filter: Also known as a Trap Filter, these filters reject a specific frequency or range of frequencies, and are typically used where one specific strong adjacent frequency is causing issues for the receiver. FM Broadcast Notch Filters are one common example.
- High Pass/Low Pass Filter: These filters pass all frequencies above a specific frequency (high pass) or below a specific frequency (low pass). These tend to be used more with higher frequency receivers, but could be used for railroad listening in certain situations where the offending frequencies are all located above or below the desired listening frequencies.
Filters vary in strength, performance, and of course, price. Cheap, Chinese filters designed for SDRs are are virtually useless and serve only as a waste of money. Stick to good quality filters from brands such as PAR Electronics or Mini-Circuits for consumer-grade; or Sinclair, Telewave, Comprod, or Pyramid for commercial-grade. Situations in which the offending frequency is very close – such as 1 MHz away – from the desired frequency is where the commercial-grade filtering products truly shine, but they come at a premium price. Fortunately, used commercial-grade filters that are still in good, working condition can be found for pennies on the dollar on eBay or Facebook Marketplace.
You might be wondering, if filtering is good, why not just always include filtering in every radio feed? Because, unfortunately, filtering isn’t free – both monetarily and in term of received signal strength. Bandpass Filters, Notch Filters, etc. all have insertion loss, which will attenuate (reduce) the received signal slightly. If you are already using a high quality commercial-grade receiver, its selectivity may already be enough to stave off any adjacent frequencies without degrading its sensitivity – in which case adding filtering will do more harm than good.
Where filtering becomes important is when the receiver cannot cope with those strong adjacent frequencies and starts suffering from those things like desense, intermodulation distortion, or overload. In this case, filtering out the offending signals will get your receiver back to performing the way it should. This is why filtering is regularly found at tower sites and base stations in particular, while it is rather uncommon in mobile installations.
Note that it doesn’t take much to silence these offending signals, as their strength rises and falls at a much higher rate than the desired signal: typically for every 1 dB increase in the strength of the desired signal, the undesired signal increases by 3 dB, with the opposite also being true. Therefore, we can reduce the strength of the unwanted signal at a much higher rate than which we would our target frequency. The video above about Third Order Intermodulation goes into this concept in greater detail.
A perfect example of this is the attenuator function in many consumer-grade scanners: often times the user will be unable to receive a distant signal due to nearby strong signal sources causing desense. Perhaps counterintuitively, enabling the scanner’s attenuator, thereby reducing its sensitivity, suddenly allows that distant signal to be received. As the noise floor around your target frequency falls much faster than the strength of the desired signal, the desired signal can emerge from the noise and suddenly become audible.
In conclusion, while filtering should not automatically be included in every installation, it is a valuable tool to be used when combatting strong adjacent frequencies that are more than your receiver can handle. If you suspect your radio feed is suffering from one of the problems listed above, research other frequencies in use in the area and get a visual on the radio spectrum at the site, be it through a Spectrum Analyzer or an SDR. Knowing what you are up against is critical in determining whether filtering is needed, and if so what type of filter.