Cut the Noise: Listening on Antennas for Other Bands

(Royalty-free image courtesy of Dreamstime.com)



Some Good Advice

I am still a young ham, and learning new ways to improve the quality of DX signals. Last summer, I received some help from a knowledgeable ham who taught me the trick of using off-tuned antennas to hear signals buried down in the noise. By “off-tuned,” I mean receiving on antennas cut for a different frequency than the band one is trying to work, or purposefully adjusting your antenna tuner to an off-tuned setting.


Do not be “That Guy”

Some of us live at a noisy QTH. We can work to reduce that noise, but in a big city, there is sometimes not a lot you can do. As for myself, I was having a lot of difficulty hearing over the 40m static at my QTH. For weeks, I tried to work the California coast, from South Korea, but could only hear very strong stations on very good propagation days -- and it was painful. I studied my radio’s DSP, and gave it a real workout. I tried several different configurations on a variety of basic antenna designs. Basic theory helped, and I knew I was getting out, and even receiving 57 to 59 reports once and a while, but I could barely hear the powerful stations coming back to me. Nobody wants to be “that guy” who gets out, but “doesn’t have good ears,” but I have to admit, that is exactly who I was.


Solution to a Problem

I continued working and listening for 40m DX every night, and was going deaf trying. After mentioning my predicament in a QSO, the more experienced ham suggested I try listening on my 20m monobander, but with a little added AF gain. It sounded like something that would not work, but understanding that he knew better, I tried it, and whaddya know? I could suddenly hear signals that were not there before! Of course, I was not able to transmit well on that antenna, and tuning it did not help it approach my 40m monobander’s signal power. Amazingly, however, I found I could listen on that off-banded wire, then flip a switch back to the resonant antenna for transmit. With an ear-to-ear grin, I was hearing and working DX over the noise of my QTH that I did not even know was there. Although I learned to flip the switch carefully (and transmit on the correct antenna!), I was no longer, “that guy,” and could work most of what I heard.


Watch and Learn

Unfortunately, that ham, Seymour (W6CCP), is no longer with us, but I will always remember how he helped me overcome a difficult problem. I have since graduated to yagi building, and am studying about listening antennas. If you aren't there yet, and live at a noisy QTH, I hope you watch these two video examples, and try listening on an antenna not meant for the band you are working.


Using a 20m antenna to listen, while transmitting on a 17m vertical, after a switch:



Using a 20m yagi to listen on 40m, and pull signals out of the noise:

 

Excellent tip.

What the OP describes is a nice feature of non-resonant (and non-resonated) antennas used for receive. Beverages and receiving loops all exploit some facet of this feature, although usually at longer wavelengths. Even the short verticals used in a receive-only four-square system can exhibit similar S/N improvements if they are resistively loaded and lossy. W8JI discusses this on his website [w8ji.com]. For HF and MF receive antennas, a very lossy antenna actually means better copy. Using an off-band vertical as a receive antenna achieves resistive loading through the many reflections up and down the feedline before the signal reaches the receiver. Don't use a desktop tuner with a receive antenna, because resonating the antenna circuit allows it to raise the on-frequency noise, as well, because the tuner raises the overall Q of the antenna circuit.

In addition to the popular receive antenna types, like beverages, there are some antenna designs that can achieve similar S/N improvements and even similar directionality improvements in a much smaller footprint. See here, here, here, and here [kk5jy.net]. The loop-on-ground is the antenna I use the most for low-band receiving. I wouldn't even attempt to work 160m or even 80m these days without it. I don't have space for a proper beverage antenna, but I get a very similar improvement of S/N with the on-ground loop antenna. The single small untuned loop (this isn't a magloop/STL, it's just an electrically small piece of wire, properly choked at the feedline) did a fantastic job on 40m and 80m cutting through the neighborhood QRM. It's not magic, or a cure-all, but it does seem to optimize the S/N ratio in a way that resonant antennas can't, even when they are properly attenuated on receive. The nonresonant receiving element is just a good tool for skywave reception.

Even if you live on a small lot, and even if you live in a noisy neighborhood, there are plenty of options to better receiving on HF/MF.

 

You've got some great points.

With the Elecraft, in the video, I was using a simple A/B switch. The 7300 was using the balanced line switch, as shown. You are certainly right, in that the tuner has been known to add a bit of noise, through coax. Faint, but noticeable, at times. It did sound cleaner through the basic switch, in at least one known circumstance.

Also, I've been toying with the idea of using a small loop for receive-only, to cut through the QRM. NH7RO keeps preaching the virtues of the loop, and he'll probably have me converted, when I get off my duff and try one.

Lastly, I appreciate the mods approving the post, but am kind of sad that they used a poor freeze of my Elecraft, instead of the provided picture of the boy plugging his ears.

 

The photo of the boy is there. You may need to clear your browser cache or something....

 

Derek,
Thanks for your informative articles. They are refreshing and are providing classic information to a new generation of operators. I appreciate your crisp delivery and the videos don't involve any slight of hand.
73 Sid K3SX

 

Oh, I realize that I posted the opposite link names above the video titles. Should have checked that more closely! The 40m version with switch is the top video. The 17m antenna with 20m listening antenna, is the Icom on the bottom.

 

You still did a fine job. You did a much better job documenting your thoughts than most folks. I see all kinds of "news" posts on QRZ that don't even have a single link to the article they talk about. Your post is well above average.

As the sun continues to dip into the low-band years, people are going to need more ideas like the one you suggested here.

 

I wonder if some of the difference might be comparing a vertical antenna to a horizontal antenna.
Another thing I noticed is that he had the preamp on while he was reducing the rf gain.
I would turn off the preamp before reducing the rf gain.

But in general, I usually reduce my rf gain to where the it cuts off the noise.

73,
Larry KB0R

 

Yes, that was a point I made in one of the videos, if memory serves (I think the Icom one, unless I made it in the initial recording, and accidentally edited it out).
I use a vertical for transmit, and horizontal on receive. The vertical is a better transmitting antenna, but noisy on receive. I can assure you, however, that I also have a horizontal up (down, at the moment) for the same band as the transmit antenna, and it is also noisier than the off-tuned horizontal. This is not a new thing....hams have noticed this for years.

 

B

I don't get how a non-resonant or otherwise lossy antenna would enhance signal-to-noise ratio,
the desired signal and noise being close in frequency. I suspect the dimished noise is all about
location, loction, location. A Beverage, being close to ground, is equally lossy for all signals (and
noise) on a given frequency. The advantage being its directivity, toward the desired signal, and
away from the QRM, QRN, and local noise.

 

Then maybe you should try it, and see.

 

I only disagree with the reasoning. How dose a non-resonant antenna differentiate between the desired signal,
and noise that's on the same frequency? It is location and directivity, not resonance that makes the difference.

 

Location and directivity are definitely important, as is proper selection of receiver antenna gain (up to and including using attenuation to lower the noise floor level). I'm not a practicing physicist, so I can't derive a proof for you that there is more to it than that. That said, I am convinced there is more to it than that. My experience is that there is a difference between an antenna that is resonant, and an antenna that is not, when trying to receive skywaves. I started noticing it while tinkering with magloop antennas that were slightly off-frequency, and the effect appears to translate to other antenna types, as well.

Anecdotal? Sure. Effective? Yes.

 

My HF dipoles are fed with open line, and with a matching netork, can be resonated on any frequency.
If you come up with a working model of a receiving antenna which can increase s/n ratio by simply
detuning it, you could be a rich man.