N8GMZ’s RFI Hunting Rig

A compact, easy to build, sub-$100 direction finding setup

Utilizing a small 70cm Yagi and a Quansheng UV-K5 radio running modified firmware you can hunt and locate signals and RFI sources quickly and easily, without having to fabricate parts or empty your wallet.

In this article I am going to talk about my experience with RFI, the equipment I put together to hunt it, and how I was able to effectively find it without needing to purchase expensive specialty equipment. The direction finding rig that I have developed costs under $100, is made of commonly available parts that can be sourced from someplace like Amazon. It has proven effective in finding multiple sources of power line RFI that have then also been verified by a power company engineer.

Before Hunting RFI: Determine if it is Local or External

External RFI can be bad, as mine was, but it’s not always the primary issue. You can have RFI issues from local sources in your QTH that can be heard through the radio. Before starting to hunt RFI outside of your house you need to find out if the RFI is actually being caused by something in your house. It is the first step in effectively hunting RFI. To do this takes a little planning and some effort but costs you nothing.

You will want to run your HF radio from a battery, and turn power off to the rest of your house. To do this you will want to tune your radio to where the noise is particularly bad and note the s-meter reading. There may be multiple frequencies you’ll want to look at. Note them and review them with the power off. You’ll do this in AM mode if possible.

In my case I found it to be particularly bad around 14.820MHz with an s-meter reading over s9. Having a waterfall display is key, and if your radio doesn’t have one, consider using a RTL-SDR v4 SDR dongle and a laptop that can run on battery power. The RTL-SDR v4 is a powerful, inexpensive SDR that covers the ham bands 1-30MHz and beyond, without the need for a converter. At the time of this article it is $30 on Amazon. The software is free. I use SDR# or SDR++ for this. You just hook it into your HF antenna as you would a radio. Using your radio waterfall or a SDR will help you determine the type of RFI you are hearing, as well as if it is local or external.

With my radio on battery I turned the power off to my house at the main panel. No change in intensity of the RFI was registered on the radio or SDR. The noise was still present. I waited a while, in case there was some sort of capacitor somewhere that needed to discharge, but it stayed over S9. This was a clear sign that the noise was coming from outside my home. The signature from the RFI on the waterfall was representative of power line noise, and not a solar controller or battery charger. I was fairly sure I had power line RFI at this point. We will get back to this later, but depending on your findings from this essential first test, you may see the noise disappear or lessen when power to your home is shut off. In that case, you will want to address the local RFI first.

Identify and Eliminate Local Sources of RFI

Based on your findings from turning your main breaker off you may want to investigate local RFI inside your house. The goal being to seek out and remove interfering local sources of noise before pointing a finger at the external RFI that may also exist. Local RFI is RFI that is in your own house: cell phone chargers, appliances, computers, wall warts, transformers, EV chargers, LED lights, etc. They are all prime suspects for local RFI. After reading a few articles on RFI hunting, and doing some research on the Internet, I started looking for things in my house using an inexpensive AM/FM radio from Amazon set to AM on a frequency where no radio station could be heard. Holding it close to something, it would start to buzz loudly when RFI was radiating into the radio.

I quickly found the LED lighting strip I had installed on the back of the flatscreen TV to be a source of RFI and unplugged it. I also discovered a landline telephone box, installed on the outside of my house in close proximity to my antenna coax, to be making noise.

After a call to the telephone company listed on the long unused box, I was told that it was my property, and I could remove it, which I did. I first tested the wire terminals in the box for live power, and finding none, cut the telephone cables back away from the house with some insulated wire cutters. I pulled the wire as far back underground as I could. Removing the box from the side of the house I found a transformer of some kind that had been waterlogged and rusted. Removing the box and wires seemed to quell the noise from that area.

During some later searches it was found, almost randomly, that the LED display on the air fryer was very loud, and so I unplugged it. We only plug it in to use it now. The stove display also had a low level of buzz to it. My wife vetoed a request to unplug it. At some point you will accept that you won’t be able to completely eliminate every source of RFI, and will need to pick your battles.

A while later I discovered that the battery charger I used to charge my POTA batteries would put out quite horrible RFI across the band, shown below. The charger stays off unless in use now.

There were multiple other small sources of RFI throughout the house that were addressed, however no real overall improvement to the noise floor on my radio in the HF bands was noticed. It was still poor and I was continuing to have problems hearing stations that could hear me fine.

I attempted to filter the noise, first with lowpass and highpass filters, then bandpass filters, and finally with a coax noise filter from Palomar Engineering. Nothing worked. The noise was not coming from out of band. It was widespread and was rapidly getting worse. The filters on my radio did a good job of masking it, but the signals I wanted to hear were also lost for the most part. It progressed to a point where I was having trouble hearing stations in the US that I had previously been able to hear many times over. At the peak of the issue most HF bands were unusable, and I was only able to do digital weak signal modes or use a remote SDR to listen.

When it rained the noise would get worse, a classic symptom of power line noise. At this point I was convinced that the growing RFI problem needed to be dealt with, that it wasn’t going to go away on its own, and that I would need to hunt it down myself to improve my situation.

You Are Not Alone

I certainly didn’t do this all by myself. My journey that eventually lead to successfully hunting RFI would not have been possible without the help of others, especially Jeff Stuparits, W4DD, the creator of the RFI hunting software “RFI Mapper”.

Jeff was on the receiving end of many of my questions, and even made himself available to me via a Zoom call to review the RFI issues I was having. Much of my knowledge in this area is based off the work he has done, and the methodology that he uses. Jeff has worked with many hams, and the ARRL, on numerous RFI cases, and is very knowledgeable with how to successfully find and eliminate RFI interference from external sources.

You can find some of Jeff’s seminars and RFI hunting experiences here:
AWA Communication Technologies Museum – Hunting Down Power Line Noise and Other RFI
East Coast Reflector – Jeff W4DD Presentation of RFI – Presentation View
GarsTube – Locating Power Line and other RF Noise Jeff Stuparits, W4DD

Don’t feel like your problem is yours alone. Seek out help with the issue. Ask questions if you’re unsure. Trying to tackle something like this takes more than just putting a radio together and waving an antenna around. It’s rough, and there are people out there that are willing to help if you ask.

RFI Hunting Equipment

After speaking with Jeff multiple times, and being told that the noise I was hearing was almost certainly power line noise, he recommended that I hunt it myself with my own equipment. The power companies historically are, for the most part, ill-equipped and untrained in RFI hunting. Isolating the pole(s) the RFI was coming from would give me the best chance for getting it resolved.

Jeff recommended using a 2m Yagi antenna, a multi-level attenuator, and a radio that does AM air band (136MHz). Without getting into the technical side of what a power line RFI signal is, just know it is very broadband, and you will be able to find it using almost any band. While you may not hear the crackle of power line RFI in SSB mode, it can be very noticeable in AM mode. Being able to do AM mode is key in hunting power line RFI, and will provide you the best results.

However, I did not have an AM air band radio, an attenuator, or a 2m Yagi antenna… This is where my Dutch-blooded frugalness kicked in, and why I am writing this article instead of just pointing you to previous proven methods for hunting RFI.

How could I hunt this noise cheaply without buying hundreds of dollars worth of equipment? Jeff suggested building the 2m Yagi instead of buying one, and provided me with his personal and proven design for it. That was a low cost option, absolutely. But when I mentioned that I had a cheap $40 3 element 400-470MHz Yagi that I had purchased previously to do foxhunting with, Jeff responded, saying that a fellow RFI hunter he knew used 70cm to hunt RFI with good success, and it could be worth a try. To do it with the existing antenna I would still need a radio that can do AM on 70cm (most portable radios only do AM on 2m air band) and an attenuator to hone in on the signal where it was at its strongest, so it couldn’t overpower the directionality of the antenna. Both of these items were looking like they would need to be purchased to hunt RFI. The last thing I wanted to do was tell my wife I had bought another radio, right on the heels of getting my IC-7300.

However, I already had a Quansheng UV-K5(8), also called a UV-K6. This is a $20-30 VHF/UHF FM handheld 5w radio that is hackable. I love these little radios. They are great fun to play around with but many agree most of the modifications that you can do don’t have much practicality outside of being experimental toys. I had gotten mine on a sale for $18 and it had sat around after the initial novelty wore off.

A couple of the modifications I knew about, being able to do AM, FM, and USB modes on any band, as well as a spectrum scope mode for scanning a range of frequencies, caught my interest. This would give me AM mode on 70cm band if true. Transmitting out of band on these radios without additional hardware modification is not great, or approved by the FCC, but using it for RFI reception on AM seemed promising to me, especially with the spectrum scope feature. As I looked into it more, I found you can also change the amplifier gain, providing a software based attenuation. It seemed like it could work.

I couldn’t find where anyone else had done this before but was willing to try it. I loaded modified firmware, called EGZUMER on to my radio for the experiement. You can load it via a browser and a programming cable. There are many write-ups on how to load EGZUMER so I won’t get into it here.

I then hooked it up to my super cheap 70cm Yagi and went hunting.

My first attempts to find the power line noise with this sub $100 rig were initially unsuccessful. I was still learning to navigate the EGZUMER software, and dial in the settings to hone in on the RFI. During these trials with the RFI rig, I ended up finding a multi-step attenuator on eBay for a decent price and added it to the rig. However, I discovered that since I was using a Yagi antenna with a relatively low gain, adding the attenuator would quickly reduce the signal to where it was difficult to detect from any distance. It may be more useful if the antenna has more gain off the end though. While ending up not needing it and removing it from the final version, this early setup, shown below, is typical of RFI hunting rigs where attenuation can not be done on the radio via software.

It took some more experimenting with the rig and a lot of trial and error, but eventually, around the forth or fifth expedition out into the neighborhood, I was able to locate the power line RFI to a single pole, call it in to the power company, and have an engineer come out and verify it. This then generated a service order to be created to repair the pole.

Using the same rig, I have found other sources of external RFI around my QTH and made requests for repairs. One was a streetlight ballast that would only create RFI when it was dark. Another was an EV charging station that the local government had installed. I have also used the rig to find RFI inside the house, and found it to be very effective for that, being very portable and small.

RFI Rig Details

Here, shown below, is the final version of my rig. It consists of a 3 element 400-470MHZ Yagi, a Quansheng UV-K5(8) radio, a male to male SO-239 coupler, and a female SO-239 to female SMA adapter. The handle is a piece of hot water pipe insulation. Much of this I already had, but if it were to be purchased, the cost would be under $100, including the programming cable needed to load the modified firmware. You could use a male SO-239 to female SMA coupler instead of the two connectors I used. This rig is small, and can be easily used inside houses, or kept on the passenger seat of your vehicle during RFI hunting expeditions.

I named it the N8GMZ RFI Rig, because that’s what hams do, name stuff after their callsign. Very self-serving I suppose, but hey, I haven’t heard of anyone else doing this already, so I might as well plant the flag first, hihi! In all seriousness, it’s simply a very inexpensive, purpose-built, UHF portable radio for direction finding that I hope can help other people with their RFI issues.

Using the RFI Rig to Hunt

Now we’re going to get into the meat and potatoes of this. Because this is where you will find success or failure in your RFI hunting endeavors. We’re going to assume you have a directional antenna with gain off one end – The 3 element Yagi has a rated gain of 7dBi, providing directionality in pinpointing the RFI. More gain is probably better, but the size of the 3 element Yagi is convenient. We’ll also assume you have a UV-K5 or UV-K5(8) set up with the modified EGZUMER firmware. I am using version 0.22 of EGZUMER at the time of this article. With your radio and antenna operational you can begin programming it for effective RFI hunting.

Step 1 – Initial setup:

Power on the radio and put it in VFO mode, if it not already, by pressing “F” then “3” on the keypad. Enter 445.500 for the frequency. This will be your base frequency. You may need to try others later if the RFI can’t be locked on in the range provided. 445.500 worked in my case and got me close enough to the RFI signature to find it.

Step 2 – Enter Spectrum Scan mode:

Press “F” and then “5” to enter Spectrum Scan mode. The screen should look like this:

Press “0” to toggle to AM mode and then use the “*” and “F” keys to adjust the dotted bar on the screen up or down. This is your squelch level. Only peaks on the spectrum scope that break the dotted line will produce audio. Lower it until the highest peak breaks squelch. Listen and hear if it is the RFI you are hunting for or something else. If it is not the RFI, press “Side Button I” to exclude it and keep adjusting the squelch while directing the antenna to where you think the RFI is, in an attempt to lock on the RFI signal. You may need to move frequencies multiple times before locking on to the RFI signal.

The Spectrum Scan mode button functions are as follows (taken from the EGZUMER wiki page):
1 / 7 – increases/decreases frequency step between consecutive bars
4 – toggles the number of bars (channels) in the graph
2 / 8 – increases/decreases frequency step size by which the graph is scrolled with UP/DOWN buttons
5 – shows a frequency input box for lower sweep frequency (value in MHz, * – decimal point)
3 / 9 – increases/decreases maximum dB value (vertical scale)
6 – toggles receiver bandwidth
* / F – increases/decreases squelch level
0 – toggles modulation type (FM/AM/USB)
Side Button I – excludes current frequency from the spectrum scan
Side Button II – toggles backlight
EXIT – exits to a previous screen/function
PTT – switches screen to detail monitoring of last received frequency

Step 3 – Lock in the RFI Signal

Once you have the RFI breaking the squelch of the radio and you can hear it, press the PTT button to enter the Detail Monitoring screen. This will lock the frequency and is where you’ll attenuate the signal so that you can determine it’s origin. The rest of the RFI hunting is done from this screen.

Step 4 – Detail Monitoring Mode

Once in the Detail Monitoring screen, you will see the locked frequency, the signal strength bar, represented by the large black dots, and the S-meter with dBm readings. The vertical line on the signal strength bar scale is the squelch level, this is controlled with the “*” and “F” keys as it was on the Spectrum Scan mode screen. Set the squelch level so that the RFI signal just breaks squelch, then move the antenna from one point of interest to the other. Hopefully it will break squelch in one direction and not any other. This is what you want.

Step 5 – Pinpointing the Source of the RFI

Having locked in on the RFI signature frequency by first using the Spectrum Scan mode, and then the Detail Monitoring mode. You’ll find that as you near the source of the RFI it will most likely become too strong to determine the direction, and you will pick it up off the sides and the back of the antenna. This is when you will begin adjusting the PGA (Programmable Gain Amplifier) setting on the Detail Monitoring Screen, effectively attenuating the signal. This is done by pressing the “M” button until the PGA setting is highlighted, and then using the Up and Down button to change the value.

You will want to lower the PGA setting and then also lower your squelch level (using the “*” and “F” buttons). This will provide the necessary attenuation required to give the signal directionality in strength while still breaking the squelch of the radio when pointed at the strongest direction. It will also decrease your range, so you’ll want to do this when you think you are close to the source. I have played around with the LMAs and LMA settings but found the PGA makes the biggest impact on attenuation. I leave the IF setting alone.

Finding the Bad Power Pole & What to do After

Eventually (hopefully) you will narrow the RFI noise down to 2 or maybe 3 poles in a small area. Even with attenuation you may have some trouble determining which pole it is coming from. You may hear it on multiple poles if it is loud enough. This is where you will employ a “Conquer by Dividing” method. Stand equidistant between two poles and point your antenna at each. One should be louder. Go to that pole and then past it until you are equidistant from the loud pole and the next pole. Point the antenna at each pole. If the pole you passed is still the loudest, then that is your pole.

Jeff, W4DD, gave me some great advice. He told me don’t trust your ears, use the S-meter and watch the dBm on the screen. This is good advice when you are unsure if what you are hearing is getting stronger or not.

In my case it was very obvious what pole it was. The RFI noise was very pronounced and was even radiating out into the guy wires for the pole that went out and down into the ground. The pole to the east was quiet, and the pole west of the problem pole had some noise, but less than the pole identified as the primary problem pole. Using just the small Yagi and my radio’s software controlled attenuation, I was able to determine the source of the power line RFI down to a single pole.

Determining which power line pole is causing the RFI is as accurate as you will be able to get with the N8GMZ RFI Rig or any Yagi/radio setup. An ultrasonic dish is needed to isolate the exact component on the pole that is noisy. It could be a lighting arrestor, a cracked insulator, a fused switch, or even the staples used to secure wire to the pole. At some point I may build a dish, but for now, the Yagi has worked well enough to determine the pole with power line RFI and phone in a repair request.

After finding the pole you will want to note the pole number and the street address or cross street. If you aren’t able to narrow it down to just one pole, record both pole numbers you suspect. Then it is up to you to contact your local power company and provide them with this information.

I was lucky enough to be put in touch directly with the local field engineer, and he even let me know when he would be in the area to verify the noise. I was able to meet him and we both were able to identify the noise with our own equipment. He later emailed me and let me know that a work order had been issued for the repair to the primary suspect pole, as well as the pole next to it that was also fairly loud, but not as loud as the first.

I also reported to him a streetlight that I had identified in my searches but could not verify when the engineer was on-site. It was daytime and the pole was only noisy at night. But having seen that I was able to find RFI with my gear, he took my word that the streetlight was causing an issue and put in a repair order for it.

At the time of this article, the repair work window is still open, and I am still waiting for the repairs to be made. Some violent summer storms have complicated things, and tied up work crews in the area. I will post an update when work is complete. With my trusty little RFI rig that I keep in the car I regularly have been checking the pole to see if the issue has been resolved.

Jeff, W4DD, after hearing recordings of the noise I was experiencing, said it was so bad, in his opinion, that it could be from multiple sources. Even knowing this beforehand I have chosen to only address the closest sources of RFI found, when those are cleared up I will revisit the issue again. There may be more bad power poles further away from my QTH adding to my RFI issues, but I sleep well at night knowing that I would be able to find them if they exist.

Conclusion

The equipment I chose to use was financially driven but I have had repeatable success with it and plan to continue to use it to hunt RFI sources for myself and others. The low cost and effectiveness of the equipment, and the fairly straightforward operation of the radio software is appealing to me.

There are many different ways to hunt RFI using various radio equipment, like directional coax loop antennas, Kraken SDR direction finding arrays, or RFI mapping with GPS assisted software. Looking into different ways of doing it and choosing your own path can be just as rewarding and effective. I encourage you to look at what has been done, and what you can do, and make the attempt on your own. Keeping RFI out of radios is a constant struggle for many, especially with all the new electronics and gizmos in today’s world.

Footnote

After being asked about my journey and what equipment I used multiple times by various people interested in it, I decided to spend a rainy weekend afternoon typing this all out, in the hopes that it helps someone else out there who is going through what I have been going through.

I can be reached at CQ.N8GMZ@gmail.com

73, N8GMZ

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