RFI and DH+


Thread Starter

Mark Erdle

I've got a customer with 2 SLC 5/04s in separate buildings and a KT card in a PC based SCADA system all connected by DH+. The customer, a local village public works department, uses fixed and mobile radios communicating on 39.10 mHz. The DH+ seems to generate considerable RFI on this frequency, even with the A-B supplied ferrites installed on the blue hose. Anyone seen this before and found a solution, other than to get another radio frequency?

Jay S. David

You can try using fiber optic cables to transmit the DH+ signals. A bit expensive but it will totally eliminate your noise problems.

There are available DH+ Fiber Optic Modules that you can purchase: e.g. 1771-AF1

Hope this helps.

Gerald Beaudoin

Before changing to another frequency, you should definitely get someone with a good spectrum analyser (any reputable two way radio shop will
have one) to take a look at just what is being generated and where it's coming from. With this kind of information you will be much more likely
to be able to go after the real culprit and such an instrument will allow you to see if you are effectively dealing with the problem or just
adding a whole bunch of stuff that doesn't do anything for you.

Gerald Beaudoin
Leahy Orchards Inc.
[email protected]


I have seen the effects of plant radios on AB SLC analog input modules. The solution change to digital communications. The process has too many
analog devices to try to isolate the effects of the RFI.
Changing to another frequency is a shot in the dark and not very likely to improve anything. Try a site survey first, see the nature, source and
spectrum of the interference signal and decide what to do afterwards. The fact that a radio operating on a certain channel is swamped with noise when a digital device is on doesn't mean the digital device is generating in-band noise. The jamming signal might be at a totally different frequency and be converted to your channel frequency due to poor receiver front end IP3 (poor linearity) . Analyzing all possible signal harmonics is hard, since multiple scenarios are possible, like the DH+ 7th harmonic might fall on the image frequency of your receiver , or the 5th harmonic might be in band, or a local tv station might bring the receiver front end to compression AND the sum or difference between any integer multiple of the tv station channel and any integer multiple of the DH+ might fall either in band, on the image
frequency, leak straight into the receiver's IF , etc . Not at all straightforward without tools to see what's going on.