One thing to watch out for when doing SOTA activations is the presence of strong Radio Frequency Interference (RFI) on some peaks. The typical scenario is that the summit is also an established radio site with transmitters that interfere with your ham radio operations.

I use the VHF/UHF bands for SOTA, so I am writing from that perspective. My impression is that HF interference is much less likely because these radio sites don’t usually have any HF transmitters. However, they may have broadband noise sources such as networking equipment, power line arcing or switching power supplies that can create problems on HF. Anyway, this post is focused on 2 meters and higher bands.

In my experience, the transmitters at radio sites may include land mobile repeaters (VHF or UHF), NOAA Weather (162.xx MHz), TV/FM broadcast stations and mobile wireless (cellular) systems. The TV/FM broad stations are really bad news because they run a crapton of RF power.

The worst summit I have encountered is Sandia Crest (W5N/SI-001) near Albuquerque, NM. See trip report here. They even have a sign in the parking lot to warn you that the RFI may wipe out your car’s keyless remote.

Some other notable RFI-heavy SOTA summits are Monarch Ridge South (W0C/SP-058), Squaw Mountain (W0C/PR-082), Terry Peak (W0D/NW-002), and Greentop (W4T/SU-076).

In a high RFI environment, your radio receiver gets overloaded such that you can’t hear stations calling you but they can hear you just fine. This results in the SOTA activator calling and calling while the chasers get frustrated that the activator never hears their call. Not good. It may not be obvious that this is happening. This blocking of the receiver may come and go, depending on which transmitters happen to be active.

Mitigation Strategies

There are a few things that you can do to deal with the RFI:

Move Away From The Source

Probably the first thing to try is just moving away from the source of interference. This may mean moving away from the highest point on the summit but it may be better overall to give up a few feet of elevation to not have the interference. You’ll need to stay in the activation zone to be a legitimate SOTA activation.

Use A Better Radio

Some radios are better than others when it comes to receiver performance including the ability to reject unwanted signals. The low cost radios from China (Baofeng or similar) generally have lousy receivers so they are a poor choice for operating from an RFI-intense summit. Many people report better results with the Yaesu FT-60, a solid performer. Commerical radios from Motorola are even more robust. I’ve been using a small mobile radio for SOTA (Yaesu FT-90) which outperforms most handheld radios.

Change Your Antenna

Using a directional antenna can help…just point it away from the source of the interference.  Oddly enough, using a worse performing antenna can help improve your ability to communicate. For example, a rubber duck antenna on a handheld radio will allow less of the interfering signal to get into your receiver which may improve your ability to receive. As long as the antenna is “good enough” to complete the radio contact, it may be the way to go. One trick I’ve used is to deploy two radios, one for receive with a crummy antenna and the other for transmit with a better antenna. That way, you still radiate a stronger signal while reducing the interference into the receiver.

Use A Bandpass Filter

You can insert a filter into your antenna feedline to reduce the interfering signal. The best approach is to use a bandpass filter that passes the frequency you are operating on but attenuates other signals.  SOTABeams offers a compact bandpass filter for the 2m band. (Note that it has a 5W power rating which is fine for handheld radios but not more powerful transceivers.) DCI Digital Communications offers higher power filters but they are much larger in size.

Change Frequency

Sometimes a small change in frequency might help a bit if the interference is limited to certain frequencies. Another tip is to try another band. That is, if you are getting interference on 2 meters, you may find that the 70 cm band is better. Or vice versa. It all depends on the transmitters at the site.

73 Bob K0NR

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