Eliminating Noise With The 1:1 Voltage Balun

This little gadget might save your DXing career, the 1:1 isolation voltage balun. Follow along as we talk about bothersome electrical noise, weak signals, and how we might use this balun to dramatically lessen your noise problem.

The 1:1 Isolation Voltage Balun

I've been fighting electrical and digitally generated noise here for a few years now. It's been a tough go. New battles rise up often. Most recently, the next door neighbor bought a new appliance which is throwing even more hash across the AM broadcast and shortwave broadcast bands. What to do? Give up radio? I entertained that idea for awhile.

Over these years, I've tried it all - impedance matching, ferrite sleeves, choke baluns, taping up coils of coax, strategic-positioning of antennas, probably more. Moderate success in noise elimination was sometimes gained but total success has eluded me. 

Noise levels here, even with on-ground antennas, can run as high as S9 or more on a traditional signal strength meter. Welcome to the modern era, dear radio DXer. S9 is your new "atmospheric" noise floor in a manner of speaking. If your desired signal isn't a little or a lot above that level (we'll call that the signal-to-noise ratio, SNR), you're not going to copy anything intelligible from it. If below S9, forget it. You will hear nothing because the signal is buried in the noise.

Most modern receivers, and that includes SDRs, will produce meaningful, copyable intelligence from a 1 microvolt signal if the noise floor is low enough. A 1 microvolt signal will show S3 on our strength meter. That's pretty low, but common when dialing across the bands. In order to hear that signal our noise floor must be lower than that.

Let's calculate this out. The 1 microvolt signal coming into our receiver is -107 dBm on the power scale (-107 dB less than 1 milliwatt into 50 ohms). Let me illustrate how infinitesimal a signal that is. The decibel or dB scale is a logarithmic scale. Every increase (or decrease) of 10 dB multiplies (or divides) our signal's power by a factor of x10. In ratio form, our 1 microvolt signal at S3 is only 0.000398 the voltage strength of our S9 signal we talked about earlier.

Depending on the signal and the receiver, many receivers can produce intelligence from a signal if it's at least 10 dB stronger than the background noise. Thusly, to receive our 1 microvolt signal our noise floor must be at least -10 dB below, or not more than -117 dBm. With the proper receiver, hardware, antenna, and some luck, we might be able to accomplish this.

One of the tricks of lessening noise pickup is to get your antenna as far as you can from any noise source. Whether on-ground or in the air, put it out in the far corner of the yard as far away as possible. I've found results usually start to improve at about the 100 ft. distance. In the extreme out on the farm, I've run the coax feeder out to on-ground wire antennas which were 200 ft. away. Great results in noise-abatement can be had at that distance. Don't worry about long runs of coax. In the bands we are concerned with, attenuation is still borderline microscopic with RG-6 common cable coax. RG-6 is available and cheap, at 75 ohms, which makes nary a difference when receiving at MW and SW frequencies.

Seriously consider the on-ground antenna, that is, an antenna which lies on the ground. Yes, it goes against common sense, doesn't it. Old hams and SWLers pre-1980s are of a mindset to hoist as much wire into the air, as high as possible. Today, often what you get is S9++ noise on your receiver. I've experimented with many on-ground antenna types, among them the LoG (loop-on-ground), the DoG (dipole-on-ground), and what I'll call the VDoG (vee-shaped dipole-on-ground). After a lot of experimentation, I became a convert to the on-ground antenna school of thought. You must start thinking in SNR (signal-to-noise ratio), not brute S-meter readings.

My current antenna is what I call a VDoG, the vee-shaped dipole-on-ground. It's positioned as far out in the yard as I'm able to get it. It's fed with 80 ft. of RG-6 coax up to a second-floor window. The antenna itself is two sections of 22 ft. #18 stranded insulated wire fed at the middle, arranged to form an angle of about 80 degrees with the open end facing south. It shows a little gain towards the open end, about unity gain on the sides, and a bit of a null at the back, or north. From here in western New York, it's good for mediumwave DXing, southerly along the eastern seaboard all the way to Cuba.

Even with this low-noise antenna, in my eco-system this antenna hears a lot of noise due to all the electrical buzz flying around, amounting to about a steady S9+ across the AM broadcast band. The noise is essentially electrical hash coming from mine and the two adjacent houses, the power lines which transit across the back edge of the yard, and the ones feeding the properties.

The VDoG, the LoG, and the DoG are balanced antennas. If you want to lower your noise problem, stick with a balanced antenna. Avoid end-fed wires, even if layed on the ground.

Common noise-abatement thinking would be to place a choke balun at the antenna feed point and then perhaps clip a few ferrite snap cores to the coax end coming through the window, just before it connects to the receiver. I installed an MFJ-911H 4:1 current balun I had at the feed point and snapped some cores on at a strategic point. It produced little improvement, helping a little more at the higher end of the MW band than the lower.

The 1:1 Voltage Balun

A single item I accidentally stumbled upon about a year ago was what killed the noise dead in its tracks. That was the 1:1 voltage balun, which will isolate the entire feed system, including the coax shield, from the receiver. These are sometimes called galvanic baluns.

Of course, this type of balun can be home-constructed, and in the past, I've wound these myself on ferrite cores and used them with varying success. On larger cores they become an unwieldy mess, and the connecting wires are a noise magnet. Size matters. The extra exposure of a larger object to RF hash matters. Searching eBay one day I found compact, miniature versions of these from China already constructed on a very small circuit board, complete with SMA connector on each end. The entire board is only about an inch long. They are 1:1 ratio, 50 ohm impedance in/out. More importantly, they have no physical connection between input and output, and no grounding. The board uses a Chinese clone of the Mini Circuits 1:1 balun and costs about $8 apiece with a little shipping added to that. I ordered one. It worked so well I ordered two more.

The 1:1 Balun in use with the HF+ Discovery

Positioned correctly in the coax feed to the receiver, the resulting reduction in noise was miraculous. Positioning is important. Inserted right at the receiver input (SDR here) using a double-ended male union produced the best results. Looking at the schematic diagram this little balun should be bi-directional, meaning either connector should be able to be used for input or output. It didn't seem the case. Connecting it one way resulted in a little better noise reduction than when reversed.

Here are graphics of the results, the before balun and after. Graphic shots were taken with an Airspy HF+ Discovery tuned to 860 KHz at 4 AM local time, lots of skywave arriving, showing the MW band span from 500 KHz to 1000 KHz. The noise floor goes from an unmanageable average -70 to -80 dBm (~S9) to an astoundingly low -110 dBm (S2) across the band and holds close to that figure across the shortwave bands to 30 MHz too! Look particularly at the frequency range 500 - 620 KHz. The crazy electrical hash bubble there has even been completely removed.

Before application of 1:1 isolation balun

After application of 1:1 isolation balun

Most signals run 25-35 dB above the -110 dBm noise floor, peaking in the -70 to -80 dBm range, equivalent to S8-S9 strength. But S8-S9 means something here, as our noise floor is at S2! Signals are strong and well out of the noise. DXing is possible again.

I won't suggest that the 1:1 voltage balun will work for total noise abatement in all cases. It works for me in my environment, as you can see. If you have severe noise problems, consider trying a low-noise antenna and this low-cost, miniature isolation balun.

Here is the eBay link to the 1:1 isolation balun:

https://www.ebay.com/itm/166955230066

If the eBay link is dead, try searching eBay for "0.1M-550M 1:1 Isolation Transformer High Frequency Transformer SMA Connector".

I hope this article helps with your noise abatement.