In your ham radio adventures, you may have heard the words..choke balun, feedline isolation, common mode choke or a 1:1 balun. In my humble opinion different names for the same device. These devices for the most part are present in a system to control common mode currents. I am not an engineer just a simple ham guy trying to sometimes understand some very complicated and technical explanations. 

I hope to keep the next bunch of lines understandable. Here we go...what are common mode currents! Most ham use some form of 52 ohm coax as it's very easy to route, secure and can within reason be placed anywhere. Ideally, RF will flow out on the OUTER SURFACE of the coax INNER CONDUCTOR and flow back on the INSIDE SURFACE of the coax shield. This is what happens in a perfect world but if a system is unbalanced RF current will return on the OUTSIDE of the outer coax braid. It does not return but it radiates.

Dont forget these BEFORE soldering the PL-259

 I have decided to give my MFJ 1026 noise canceller a call to action and combat this RFI I have been having. In the past, I did own a 1026 and used it with great success to remove RFI our plasma TV was giving me. With the MFJ 1026, you need to have what is called a noise antenna and it has to hear the same RFI your main antenna is hearing. When I used this device to remove the plasma TV issue I had no problem getting the noise antenna to receive the RFI loud and clear. 

This brings me to my head-scratching moment with the RFI issue I am having now. The first noise antenna I tried was my H-800 active antenna. I picked up nothing, I then placed it from vertical to horizontal without success. I then moved it around the property and still I was not able to hear the RFI. I then tried a long wire in the horizontal plane fairly close to my Endfed antenna and I picked up nothing. Again I moved this long wire around the property without success again. 

I do have a spare Endfed that I used for portable use and I ran some coax outside to set up the Endfed close to the main Endfed and low and behold I picked up the RFI BUT it was way too close to my main antenna. The MFJ unit (via an internal light/fuse) was indicating far too much RF was making its way down the noise antenna. 

The other thing I tried was the change the length of coax that feeds my main Endfed antenna but this made no change to the RFI issue.  In July we are getting a new aluminum gutter installed and I may try using that as the noise antenna and see if that works. Up to this point I have not been able to pick up the RFI that the main antenna is picking up.....well other than almost putting a noise antenna ontop of the main antenna that as I said did not work out very well. 

A nice RFI free evening

About a month ago I received an email from a fellow ham who was dealing with some RFI and it seemed to be on a 12 hour on/off cycle. This got me thinking of a timer operated device, the ham had contacted hydro and they came by but really found no issues well none that seemed to connect to a 12 hour on/off cycle. I got to thinking and asked him to check with neighbours if they recently purchased a grow lamp system.  A short time later he emailed me informing me that his neighbour in fact did purchase a grow lamp device. When the device was unplugged his RFI issue went away. He is now filtering up the grow system to remove his RFI issue. 

RFI filled day

Now back to my RFI issue, I have started to log the on/off times for my RFI and it for sure is not on a precise 12 hour on/off cycle BUT it seems to turn off at night. Many times between 23:00 to 23:30. Other times I have seen it off at 21:00 but I can say that it is off each night and the off time my vary. As for the on time it can vary from 07:30 to 10:00. I have also had occasion were it has been off most of the day as well. My understanding is that the device is being turned off/on by the owner and not a timer. Because most of the time the on time is about 12 hours I am thinking it too is a grown lamp. 

I have asked the folks on both sides of me and have been told that neither have a grow system. I do understand they may not be telling me the truth as it really is none of my business. One of my neighbours I have been in his home and I can say that I have not seen a grow system anywhere. I have even wondered close to their homes when I have the RFI and I pick up nothing. 

Sometimes the RFI is not to bad

I do have an MFJ 1026 noise cancelling unit and for this device to work you need to have a listening antenna that hears the same RFI as the main antenna. I have been playing around with noise antennas for a few weeks and my result will be in my next post.....I was very surprised and confused with the results. 

Back in May I posted regarding an RFI issue I was having and at the time I thought is was the neighbors heat pump. A few weeks ago my neighbor was away on business and his heat pump was off but my RFI was still present and more of it! I took my Eaton FRX5-BT put it on AM with the antenna up and took it for a walk about. When I passed by my Hydro smart meter the RFI was present big time but last time there was no issue at the meter. Upon investigation I read that the meter has cycles were it transmits data and thus the off and on issue with the RFI. The other issue is we are in very close proximity to other homes and I am certain my Endfed antenna is picking up not just my smart meter. The good new in all this is my Icom 7610 has great filtering and my NB is able to filter it out and the issues are gone. It's a problem that I can live with along side the rigs filtering.

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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