NVIS techniques, Part 2

Keypounder, a name a few folks may remember, is writing updates to his articles on NVIS that NC Scout published almost 5 years ago at the Brushbeater site.  He has continued his research and study of NVIS, and wants to update  and expand upon his earlier articles on the subject.  This article is the second of what looks to be now 4 articles on NVIS.  The first one is here: https://www.americanpartisan.org/2021/05/nvis-techniques-part-one/

As NC Scout stated 5 years ago-
“…. I will re-iterate that these skills, along with Land Navigation, are among the most perishable and most difficult to learn- under duress, near impossible. So for those of you who feel you’ll do it when ‘the time comes’, you’ll be sadly mistaken.  Please folks, try this at home.”

____________________________________________________________________________________________

Part one of this article on NVIS operation focused primarily on the basics of NVIS; what it is, why it is, how it works, and listed some of the major factors involved in successful NVIS operation, briefly touching on these factors. Now we will start to root around in the weeds a bit, and get more detailed.

Part Two, this article, is geared toward a more detailed explanation of successful NVIS operation, and a basic guide to successful NVIS. We will review operation of a basic NVIS station and will discuss in some detail how to operate it.

So let’s imagine that you have been interested in emergency communications, and have gotten your General class amateur license. One of your ham buddies suggested that you put up an NVIS fan dipole in the scrubby trees behind your house, and came over to help you build and get this antenna up in the air. It was more work than you’d figured, but the two of you got it about 25’ up, just as you read about in that NVIS article online, and hooked it up to your new (or new-to-you!) HF transceiver. Now what?

Well, start operating! BTW, don’t confuse transmitting with operating- If you are listening on your radio, learning and writing down your lessons, you are operating. Any jackass can make noise and occupy bandwidth, but they are not operating. By listening, learning and logging, you are operating even if you don’t make a single transmission, but unless you record your lessons, you’ll lose most of the benefit. You should make records or notes about where, when and what you hear. Over time, you will detect patterns in these data that will be highly enlightening. If it isn’t documented, it didn’t happen; if you are putting in the time to do it, take a moment and write down the results.

It is essential to get into the habit of listening before you transmit. For any radio operator, this is important; if you build good habits today, in today’s permissive environment, then if (when) the stakes get higher good habits will stand you in good stead. Not only can you learn a lot from listening all by itself, you also learn how a good operator can hear things that the untutored ear cannot pick up. I’ve had any number of people new to radio ask me what I was listening to; they COULD NOT hear what I was able to hear. Note: you do not hear with your ears. You hear with your brain, and teaching your brain how to listen takes practice. Take the time to learn how to listen, and also learn from your listening. You can do both at the same time. Unless you are doing an initial fast sweep for a station you know you’ll hear, turn that big knob slowly, and try to pull out scraps of signal. Just as when stalking deer you don’t look for a whole deer, so it is with radio operation.  Anyone can hear a 20 over 9 signal.  The skilled operator can pick out signals that others miss. Listen for a piece of signal, a scrap of sound that isn’t noise.

That is a reason to tune slowly- as you gain experience, you’ll learn that signals usually don’t stay at one steady easy to copy volume. They’ll go up and down, fade and then grow stronger. Sometimes this is a rapid cycle, a few seconds from low to high. Other times it can take minutes. This is one reason to tune slowly; if you happen to tune past a station you are looking for by tuning too quickly, that station’s signal at that time might be down close to or below the noise level, and you’d miss it. If you had tuned more slowly, you might have heard them as their signal came up in strength. There are times when you want to do a quick scan across a portion of the band, but if you have reason to be listening for a specific signal in a particular frequency range for a given time period, then it is usually better to tune slowly.

Here is probably a good point to touch on panadaptors. Panadaptors are a great tool for locating signals across a band. I use them and like them; they are great when running ‘conventional’ NVIS as you see exactly which signals are easy copy, but when trying to pull out a really weak signal, I can hear signals that are almost invisible on the panadaptor if I narrow the passband down and reduce the RF gain a bit. I might also try the receive preamplifiers, but usually those do not help the S/N.  Sometimes, I will get a hint of a signal on a panadaptor, and tune to it and as the signal rises and falls, I’m able to gradually refine my tuning, narrow the filters and improve the S/N ratio. This is an iterative process, and I’ll narrow the filter, tune the signal just a tad, and repeat until either I have decent copy, the signal fades out completely, I run out of filter, or filter ringing ruins my copy. Or, the other station goes QRT. This technique is useful for any weak signal work, not just NVIS, so let’s talk about this for a bit; it’s a digression, but worthwhile.

_________________________________________________________________________________

NVIS operating instructions will tell you to pick a frequency below the MUF, where the signals are loud, but it is well to know that you can make contacts using very weak signals, when the book says that you can’t, as long as the other station has a very good antenna and a skilled operator running the station. Again, this is not NVIS, but in the broader context of operating in nonpermissive environments, an important, even essential, skill.

Some of the modern digital modes, especially the JT modes, can copy signals that are well below the ambient noise level, and the way that they do it is an automated version of the procedure outlined above; the computer runs an extremely tight filter, a few hz wide, and moves the passband electronically to match the change in frequency of the various tones. It is all about Signal to Noise ratio, and what the computer does automatically you can do manually if you have equipment capable of the needed filtration; having listening antennas is another plus for this sort of operation.

The physical reality behind why you can hear these very weak signals is backscatter; remember that the ionosphere is neither flat nor smooth, and it changes as the Earth rotates and different parts of the ionosphere get exposed to solar emissions. Radio signals bounce off the irregularities of the ionosphere and a small fraction of the emitted RF comes back toward the origin of the signal rather than being reflected farther away.  Backscatter varies and there are times when backscatter is stronger than others. In the morning, when the ionosphere is starting to recover from its lack of night-time ionization, and the MUF is still below the frequency you are on, for example on 80 meters, the ionosphere to the East of you has a downward bulge that can reflect a small, relatively weak signal back toward the sending station, or to you. Similarly, in the evening, as the effective height of the F layer rises overhead, and stays low to the West, the ionosphere will backscatter too. During the day, the foF2 is high enough to allow ‘conventional’ NVIS, so any backscatter from incidental ionospheric irregularities is drowned out by the much louder NVIS signal.

A good operator with a good antenna setup and an excellent modern transceiver can usually hear and pull out those faint backscatter signals. Many operators, who may lack the skill, the antennas or the equipment, will say that these signals are “Too weak to copy” or even deny that they exist, but modern transceivers with high end receive filters, mostly using SDR technology, can exchange comms with stations that one would not expect to be able to hear. Sideband, with a passband of 2300 hz or more, and thus 25 to 100 times less punch than digital or CW, may not get through, but if you are running a weak signal digital mode, or CW with a good extremely narrow filter, you can pull backscatter signals out and make contacts even when sideband communication is impossible.  This is a skill set that can be extremely useful in an non-permissive environment.

Not only is backscatter weak, but it is commonly, but not invariably, low angle, and listening antennas more commonly used for working low band DX can be used to help improve the S/N ratio still further. I have pulled in weak sideband ‘NVIS’ signals during my morning nets, using my BOGs when the foF2 was too low for folks with high dipoles to be able to hear these weak signals due to all the noise that high antennas will pull in.  Remember that around dawn, while you are starting to see sunlight to the East, there is still a lot of darkness to the West and all of the thunderstatic from the night side of the Earth is being propagated;  that high dipole antenna pulls in more noise than a proper NVIS antenna.  (We’ll talk more about how to employ listening antennas  and other advanced NVIS antennas in Part four. )

One last point, then we’ll go back to NVIS operating- As Cycle 25 continues to improve, I expect to see higher MUFs from the F2 layer, allowing more frequent operation on 40 meters, notably more attenuation by the D layer of daytime NVIS, and more interference from the E layer, which we have not discussed to date. June QST has an excellent presentation of this in considerable detail, and it is worth the effort to get hold of a copy. In today’s permissive environments, many operators will welcome the return of 40 meter NVIS.

However, in non-permissive environments, wise operators may elect to use 80 or 160 for NVIS comms even though 40 is open, because the increased D layer attenuation and the E layer will limit communication distance and reduce the POI. In such circumstances, reducing your power to the absolute minimum will further reduce your signature, but will make weak signal operating skills essential. That said, let’s get back to NVIS!`

______________________________________________________________________________

As the Earth rotates, and the sun ionizes the F layer, the foF2/MUF will rise to a point that signals rise from just above the noise level, say s3 or S4, to perhaps 10 db over S9. This happens commonly over a very short period of time, perhaps 15 or 20 seconds or faster, almost as fast as someone flipping a switch. If you participate in a regular morning net, as I do, you’ll have plenty of opportunity to experience this. You also can hear this during contests, like your state QSO party, or listening to any of a number of morning ragchew nets on 80 meters. Log the calls you heard, maybe with a audio recorder, and look up where they’re located on QRZ.com. You will find this enlightening.

One of the things I do every time I check in to an NVIS net is to note in my log which antenna I am using and what the ambient noise level is on that antenna, about every 10 or 15 minutes. Higher antennas are almost always noisier, but YMMV. It is interesting to see the noise levels go down after sunrise, and then see them rise in the evening as it gets dark. This is the result of the absorption of RF by the D layer, which soaks up noise, especially low angle noise, as the sun energizes it. If you have directional listening antennas like the K9AY loop or a set of BOGs you can te