Exploring the Really Low Bands – Robert AK3Q

I find myself becoming more and more interested in chasing some of the experimental stations and beacons which reside down in the 200-500kHz portion of the band, as well as signals in the 136kHz band. This might
sound odd for someone who has a terribly small lot and has yet to find a way to get a decent signal out on 160m. But, then again, receiving is not transmitting, and there are a number of ways to get big waves to fit on small antennas.

In receive mode we are not worried quite as much about resonance (although it is still
important for optimal reception), nor are we forced to be concerned about transmitter inefficiency or arcing over coils in our tuners. We are just listening. In fact, many people around the world are limited to antenna lengths which are mere fractions of a wavelength and still do quite nicely. Additionally, folks who listen only like myself can contribute to DX spotting, testing, and propagation information gathering.
With that in mind, I have begun exploring LF antenna designs which hold promise for achieving my goals, and I think they might be worthy of examination by anyone interested in catching some of these “monster waves”!

A Few Caveats

Several issues face the intrepid LF band explorer unless one happens to be in a very quiet region of the world. The first is, not surprisingly, man-made electrical interference from almost every type of electrical device imaginable. I live in an area like this and so it might seem any hope of hearing beacons or experimental stations on these bands is a waste of time. While there are more challenges to this type of situation, rest assured signals can get through.

The second major hurdle is the sensitivity of our receivers at low frequencies. I have no doubt for those who get serious about chasing LF (and VLF) signals they will eventually build or buy receivers designed specifically for LF work. In the meantime, just be aware that many receivers are a bit deaf in this range, particularly if they automatically insert an attenuator in below 500 kHz.

The most obvious solution would be to add a preamplifier to boost signal strength to compensate for the lower sensitivity of many receivers, but this can also boost the unwanted noise. A solution around this is to use preamps with tight bandpass filtering to block signals out of the desired bandwidths, thus reducing the noise and overload. SDR receivers can be a good alternative, particularly if they have filtering capability down in the lowest regions of the bands. A preselector circuit may also be used to filter out some of the stronger AM signals. A significant advantage of an SDR receiver or a standard radio with a true panadapter / waterfall display is the ability to see the code pattern (or digital mode pattern) when it might be hard to distinguish aurally.

Regardless of whether analog or digital radios are used, filtering is key. CW filters are perfect for LF reception as they are narrow-banded by design. Many radios now come with CW filters built in, and of course many more allow for adding optional filters. Since most signals in the 136 kHz through 500 kHz are CW signals, this will work quite well. If AGC can be turned off in the rig this is a bonus, but if not, some selective use of the RF gain will help.

There are numerous homebrew projects online for building simple preamplifiers for LF, as well as preselectors and LF-to-HF converters for radios which either do not tune down to 136 or which have poor sensitivity.

Use What You Have

My first piece of advice would be to use what you have already, both in terms of antennas and radios. I have used a basic multiband vertical barely 5 feet off the ground and have been able to receive beacons and an experimental amateur station out of Connecticut outputting just a couple of watts. Local noise levels will always be a factor, but what I have found is that even noise levels are unpredictable. There might be a string
of 5 days in a row where the noise makes it hard to hear any but the strongest signals, and then a day pops up where there will be stations which have never come in before. On days where the AM broadcast band is coming in well is also a likely time for LF reception, as many of the stations we are trying to hear are just below the AM band.

Keep in mind there are many different ways to improve reception. One of the easiest ways to begin exploring the lower bands is by listening for non-directional beacons.  These beacons usually only put out a few watts at most, but they have surprising coverage. I use them to test propagation as well as local conditions.

As typical with most antennas, but especially so with LF antennas, move them as far away from local interference sources as possible. These interference sources include electrical wiring and metal surfaces, but they also include environmental sources as well. The lower the frequency the more likely there will be some interference from tall trees, buildings, etc. This is less of a problem on receive than on transmitting in that these sources can block a lot of radiated power, but they are also a concern for keeping signals from getting to the receiver.

Grounding is also important for lowering noise and increasing signal strength. For vertical antennas radials are essential, but sometimes there are creative ways to increase grounding capacity such as using a chain link fence for the ground portion of the signal. A ground rod will work better than nothing, but the larger the grounding area the better.

While it might seem that larger antennas and bigger radial systems would improve signal reception, the fact is, most folks cannot put up an antenna which represents more than 1% or 2% of a wavelength for 136 kHz, and no more than 4% to 8% of an antenna for 500 kHz. The ideal antenna for 136 kHz would a vertical 1800 feet high (and do not forget the ground radials!!).
At 500 kHz we would still need something on the order of 500 feet up in the air and an equivalent ground system. Let’s see: 10% of 1800’ is 180’, and 10% of that (or 1% of 1800) is 18’; so a 1% length antenna is roughly a 1/256 wavelength. You get the idea. If we are looking to transmit with these antennas, we are looking at fractions of a percentage of efficiency for 136 kHz, and maybe if conditions are good for our 500 kHz antenna, 1% or 2% efficiency.

Next time around I will look at some different antenna types for these low bands. Until then, give these low bands a try – you just might be surprised at what you hear!

73, Robert AK3Q 

Originally published in the March 2017 issue of the Q-Fiver.

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