Cellular VHF Interference

Question

I just got my license a week ago. Installed a new VHF/UHF radio in my car and everything seems to be working great. I've been heard at distant locations and picked up signals from repeaters that are way beyond where I expected to reach. However after programming my radio, I noticed that one VHF frequency in particular, 147.000 MHz, got quite frequent interference (by "interference" what I really mean is that when I leave it in memory scan mode, it often stops on that frequency, picking up a signal with no information in it for a few seconds, and then moves on). I've tried to adjust squelch settings and that doesn't help at all, so it really seems to be detecting something. (Unfortunately avoiding this frequency is not an option, as it is a local repeater frequency.)

After about a week I realized a pattern - this happened either whenever my cell phone received a message or whenever the lights lit up on the phone (due to me pressing a button, for example). So it definitely seems like the cell phone is at fault here.

So now I'm looking at ways of mitigating the interference from the phone. Granted, I'm not even sure what should be causing the interference. My first instinct is to add a low-pass filter at the location where the antenna wire hooks into the radio, but it seems like what I'd be looking for is something on the order of a 600MHz cutoff low-pass filter and it doesn't seem like ferrite chokes exist at that high frequency (without attenuating the UHF or VHF signal as well).

Alternatively, I guess it could be some other signal from the phone causing interference, but the pattern seems to suggest it is the RF from the phone (which I believe will either be on the order of 800MHz or 2GHz).

I imagine I can't be the first person to experience this problem - any advice for a new ham? I'm really just not sure where to start trying to filter this signal - I've looked through catalogs but come up empty on any kind of ferrite core that meets my need.

Answer 1

I wouldn't be surprised to hear such interference to an HT laying by the phone; neither would I be surprised at interference from outside sources (e.g. electronic pumps at gas station). That the phone is strong enough to be picked up by a radio with an antenna on the roof (or at least outside the cabin) is more surprising to me.

While there are many things that can affect this kind of interference, consider installation details: where are the radio & antenna mounted, how do you have power run, is there any other chassis grounding (is the radio bonded to metal on the car, or screwed into plastic), length & placement of extra coax, etc. Also, where do you usually lay your phone, and what effect does putting it different places have -- that might clue you into the ingress point for the RFI.

I'm thinking that a low-pass filter may not do much regardless. Whatever the nature of the interference, it's probably not enough to downright overload the receiver, but rather is actually carrying a pattern that includes a frequency in the neighborhood of what you're trying to receive. Or, perhaps it's a frequency that's skipping the front-end, maybe going right through the case of the radio, and hitting an IF stage; in that case, you'd have a hard time keeping it out unless you encased the entire radio...so I'm sure there's a better way.