28 years of hilltop operation on 2m
In my city (Palo Alto, CA), the fire department is in the 150-160 band. They run 100 W in the trucks and on the two repeaters. County fire is also in that band. Cal Fire (state), too.
We’ve had some weird intermod between the MURS radios (unlicensed service, for neighborhood emergency response) and the 2 m ham frequencies. Cavity filters start at $350 bucks, so we’ll probably change frequencies.
Pagers are often in that band, too. It is a pretty crowded bit of spectrum.
In the UK (probably most of mainland Europe) it’s pagers that are the main problem.
I’d suggest some experimentation is worth while.
In this situation you don’t need repeater-diplexor levels of attenuation (70-90db) of the unwanted signal. You only need to get the signal level below the level where it is causing either the RF amplifier or the mixer to flat-top due to the high level signal. Once the amplifier is overdriven in this way its gain is reduced as during the period of a 145 mhz wave, the amplifier is running at a no-gain part of its response. (Once you reach a flat area of the response, the gain represented by the slope of the curve is zero).
So how far do you need to reduce the interfering off-frequency signal? No-one can tell you without measuring it. If you use an attenuator and find how much is required to restore the amplifier to normal operation, you may find it is quite modest. Possibly 10 db would be enough, but each situation is different. Up to 30 db would be needed if the local interference is strong enough and the receiver design weak enough. But cavity filters (which are large and heavy, usually) are overkill for this situation.
If intermodulation is the problem, that does justify better filtering. But physical separation in both vertical and horizontal directions also provides attenuation so make full use of the 25m high activation zone.
The low pass filter is suggested as it has lower loss than a band pass filter due to having fewer elements. And it is a proven solution, my friend and occasional field day collaborator Dale VK1DSH had this problem near the TV transmitter tower in Canberra. His simple filter reduced the interference enough to allow an icom V85 to operate happily near the TV towers.
The design was published in AR magazine (WIA journal) a few years back. I could look into providing a copy of it if there was interest.
73 Andrew VK1DA/VK2UH
This topic caught my interest because I was just on Black Mountain, W6/NC-150, and having trouble on 2 meters. Unsurprisingly, all the summits that are great for VHF seem to already have lots of towers on them.
People still use pagers? I thought they died away when cell phones got popular.
By permission of the author and the editor of AR, the article has been placed on my website. About half a meg, PDF, 4 pages.
There are three filters described. The first is a bandpass filter using coupled tuned circuits. The second and third are low pass filters. The second filter in the small box with BNC at either end is a low pass and is the one that was used for an Icom V85 HT. The third is another low pass filter suitable for a higher power level. Response curves are included.
Although BNC and N type connectors were shown, SMA or other connectors may be required for different HTs and no difference in performance would be expected at that frequency. In recent reviews of the Amidon catalog I found that material type 12 has been replaced by another, so the specific cores used by Dale would be replaced by the newer versions without degradation.
Hope this is useful.
Appreciate all the responses, certainly a lot to digest. From what I can tell most of the Public Service and DNR stuff appears to be 150Mhz+ or up in UHF so low pass filter seems like the safest bet. Until I can put one together going to definitely keep the attenuation in mind.
Thanks, been here 10+ years but moved away briefly to southwest WA, happy to be back up by the mountains.
Thanks Andrew, those look great!
Wow - I might actually have to build one or two of these. Filter #2 looks quite convenient.
The reason for this can be a completely different than repeater overloading. This summer I visited Livermore Amateur Radio Klub’s meeting, and members complained about poor signal quality from the local repeater. Later they discovered that it happened because repeater’s battery did not get sufficient charge from the solar panel.
Pagers used to be an issue in the US, but it seems that they have fallen out of favor. I rarely hear anyone complaining about pager interference anymore. Other regions may be different, of course.
Multiple times now, I’ve had the experience of having a crummier VHF antenna produce better results because it attenuated some local interfering signals. One time I ended up using a pair of HTs: one with the “good” 1/2-wave antenna to transmit a stronger signal and the other with just the rubber duck on receive. This combination worked out to be significantly better than either antenna alone.
Whenever, I see there is a “radio site” on the map of a SOTA summit, I expect trouble on VHF. The first thing I try to do is get some distance between me and the radio site but that’s not always possible. A directional antenna usually helps, too.