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 being posted as the fifth of what looks to be now at least 6 articles on NVIS.
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 series 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. Link here: https://www.americanpartisan.org/2021/05/nvis-techniques-part-one
Part Two of this series on NVIS operation looked at HF listening and transmitting techniques, some specific to NVIS. Link here: https://www.americanpartisan.org/2021/05/nvis-techniques-part-2
Part Three discussed how to decide which HF radio to purchase. Several common civilian amateur radios will be reviewed in some detail, and general characteristics desirable in an NVIS station specifically was discussed. Link here:https://www.americanpartisan.org/2021/05/nvis-techniques-part-3/
Part Four reviewed NVIS antenna characteristics in detail, and discussed different types of operation and a brief discussion of the implications of these differences on antenna selection. Link here: https://www.americanpartisan.org/2021/06/nvis-techniques-part-4/
Part 4 1/2 is some further discussion about simple NVIS dipoles, antenna height and related topics folks have asked questions about regarding simple easy to put up NVIS antennas. We’ll get to more advanced NVIS antennas shortly.
More on basic NVIS Antennas
We’ve covered the simple and easy to erect NVIS antennas, all of which are dipole variants, in Part One and in more depth in Part Four. There have been some questions about how high is high enough, and how low is too low. We’ll get into the answers to these questions before we delve further into more advanced NVIS antennas, as it is important to understand the fundamentals if you want to master the subject.
In particular, Part 4 provoked some questions about how antenna height affects propagation, how I got the numbers I was tossing around, and whether the data presented are reproducible elsewhere. Taking the last couple first, I use an antenna modeling program, EZNEC, which is a computer model that abstracts reality and produces an idealized antenna pattern.
It is probably worth repeating that this is a model, and that this model IS NOT a completely accurate representation of reality. Among other departures from reality, the ground is assumed to be homogenous, flat, and of infinite extent. None of these things are true for the vast majority of cases in the real world, operation from the middle of the surface of the Great Salt Lake in Utah on a calm day possibly excepted. The model also neglects the effects of trees and topography, two other factors that are hard to quantify but which nonetheless have significant effects on RF propagation, as we’ve touched on before.
In Part Four we’ve already seen that the height of the antenna, and of the tips of the antenna, has a significant effect on the radiation pattern, especially the low angle pattern of the antenna, and on the overhead gain of the NVIS antenna. In Part Four we also discussed in some detail the reason that low angle vertically polarized RF at lower frequencies can be a threat in a non-permissive environment. With these reminders firmly in mind, let’s take a closer look at the effect that height has on antenna pattern:80m_FT_combined_BS_3ft_to_70fth_broadside
The above is a graph showing the broadside pattern of an 80 meter flat-top dipole at heights ranging from 3 feet off the ground, a height mentioned in the questions on Part 4, to 70 feet high. This covers the range from well below 1/10 wavelength (~25’ high) to about ¼ wavelength (~70 feet high). I have also included azimuth traces of both the 70 foot high and the 3 foot high antenna at 6 degrees elevation. Remember that the height above ground level in feet will be different in wavelengths when considering different bands.
The pattern shown by a 50′ high antenna on 80 meters would be about the same as the pattern for a 25′ high antenna on 40 meters, and a 100′ high antenna on 160 meters. In other words, if you have a 160/80/40 fan dipole at 25 feet, 40 meters will have more overhead gain, more like the 50′ high 80 meter pattern, the 80 meter dipole will have the same gain as shown on the 25′ high 80 meter graph, and 160 will have a tad more gain than that shown on the 10′ high graph.
There are a number of things to note in the graph above:
- Over average ground, the pattern of the 80 meter 70 foot high flat top has just started to flatten out- the maximum gain for this height, indicated by the straight lines at about 60 degrees above the horizon, is NOT directly overhead, and the gain pattern (the black line) is the broadest of any of the patterns shown. Remember, this pattern is about 1/4 wavelength at 80 meters.
- The -3db elevation for the 70’ high antenna is about 25 degrees above the horizon. This means that the half power beamwidth is 130 degrees wide! This pattern is effective for NVIS and for low angle skywave DF, and is useable for long haul communication on 80 meters at night.
- The 50’ high pattern, the dark blue line, actually has slightly more overhead gain than the 70′ high antenna and significantly less gain at lower angles.
- The 25’ high antenna provides within 2 dB of the gain of BOTH of the higher antennas. This is significant and one reason I was modeling 25’ high antennas for 80 meters; it will be hard to distinguish the performance of a 25’ high antenna and a 70’ high antenna for NVIS as a practical matter, but the 25′ high antenna is a lot easier to put up, and in the real world, that matters a lot.
- Antenna patterns and overhead gain vary somewhat depending on the ground type; poor ground gives more loss as you approach the surface, and you may want to set your NVIS dipole a few feet higher in that circumstance. You can also use reflector wires, too, or chicken wire to improve the ground. Higher antennas do have some advantages, too, and we’ll talk about those in a bit.
- The 3′ high antenna has lost about 12 dB or about 2 S units of gain relative to the 25’ high antenna referenced in parts 1-4. This means that if your signal was an S9 at 25’ it would be about S7 at 3’, still easily usable for working stations within 300 miles. Stations farther away may see more attenuation, depending on your terrain.
- There are two bilaterally symmetrical traces; these are the 6 degree elevation patterns, showing what the antenna is radiating at a 6 degree elevation. Note that the vertical axis on this graph is the direction that the antenna itself is oriented; the RF trace along the vertical axis for both of these 6 degree elevation graphs show vertically polarized RF radiating from the antenna at 6 degrees elevation.
- Examining these two bilateral patterns. the higher antenna has a much bigger broadside signal at low angles than the low antenna; this is mostly horizontally polarized RF.
- Both the 3’ high and the 70’ high antenna have the about the same absolute magnitude signal off the ends of the dipole. This is vertically polarized RF, and this is likewise very important, as the low antenna is about 13 dB down in overhead gain from the 70 foot high antenna. We’ll come back to this shortly.
- The broadside patterns are similar in shape except for the 70 foot high dipole which starts to flatten out.
Here is an overlay of the 6 degree azimuth patterns for 3′ high to 70′ high antennas:80m_FT_combined_6degEL_3ft_to_70fth
As mentioned above, the absolute gain off the ends (up and down in the graph above) at low angles is about the same, within a couple dB give or take,but when compared to the overhead gain, which I showed in the graph above, the higher antenna has a much higher ratio of NVIS useful overhead gain to dangerous low angle vertically polarized RF.
if you were to cut your power to the 70’ high antenna by a factor of 20 (from 100 watts to 5 watts) you would be reducing the overhead gain by 13 dB, to the same as the 3’ high antenna with 100 watts, reducing the vertically polarized RF radiating off the ends of the antenna by 13 dB also, reducing it from a peak of -17 dB to -30 dB, or over 2 S units. The 25’ high antenna would do almost as well, giving up only 3 or 4 db to the 70 foot high antenna, (from ~-15 dB to -26) but being MUCH easier to set up!
In a permissive environment, this won’t matter much. But in a non-permissive environment, on the battlefield or after TSHHTF, antenna height and configuration can make a significant difference in your electronic signature and can be the difference between sending the response to a critical message without being located, or receiving a drone missile or a barrage of 105mm rounds in the middle of passing traffic. This is also why CW or especially digital communication, which can be near 100% reliable at much lower power with faster transmit times than voice comms, is being emphasized by savvy military radio operators. It is entirely feasible to transmit CW or digital text comms with power under 1 watt, especially on 80 and 40 meters. Not only does this reduce your POI and location, but it saves power and makes batteries last longer, another big plus.
- Low height antennas do work and they are fast to put up. The trade-off is increased POI and probability of DF.
- Antennas about 1/10 wavelength high work better and are a very good compromise height for NVIS antennas, offering good overhead gain and allowing the operator to use less power for the same signal strength.
The classic advice for the height of an NVIS antenna is to put it between 1/10 and ¼ wavelength high, but as you can see from the examples above you can, and may decide to, go lower; let’s look a bit more at that. For digital, which you should be planning on using, on 80 meters your antenna should be cut for 3.55 mHz; this will cover from 3.50 to 3.60 mHz at less than 1.5:1 or the entire CW/digital part of 80 meters. A wavelength at that frequency is about 277 feet, so 1/10 of that is about 28’, and ¼ wave is about 69 feet. For 40 meters, at around 7.075 mHz, that’s about 14 to 35 feet high; for the middle of 160, 1.9 mHz, that’s 52 to 130 feet high.
These three bands are the common NVIS bands, those where the ionospheric conditions will permit NVIS operation during some portion of the day during most of the sunspot cycle, and many folks, me included, have a three band fan dipole that allows operation on any of these three bands without any need for band switching or adjustment. Having this combination allows instantaneous band changes and saves on feedline, too. This is a very common NVIS configuration, and a great place to start.
You will note, however, that a fan dipole that includes these three bands cannot be operated within the classic recommended height ranges for NVIS on all three bands for several reasons, including these:
1) the recommended heights do not overlap; the maximum height on 40 meters is less than the suggested minimum on 160;
2) the minimum height range for 160 is high up in the air, and issues related to the length of the 160 antenna are compounded in many locations by the lack of high enough hoisting points. 50 feet can be impossible for many operators and is difficult for most. Getting a line into a tree crotch 80 feet up is a significant challenge even with experience.
So, what to do? If an antenna gets above ¼ wave, then it becomes less effective for NVIS as the overhead gain drops off, as the low angle gain increases and the noise level rises and as the antenna gets more directional. The graphs above begin to show this happening. You can run separate antennas, and at a permanent fixed site this may be practical if you have the space, but many do not. Unless you have access to a very tall structure, like a tall building or tower, getting antennas to this height is difficult on 80 let alone 160, but having your antenna too high can be an issue on 40, and towers are not low profile; getting access to the tops of tall buildings is not generally a low profile activity either.
As noted, another factor is noise, both manmade and natural. In general, the lower the frequency, the more noise there is, especially when it is dark and the D layer is essentially gone. If we put up a 3 band NVIS fan dipole antenna as a Vee with the feedpoint at 25′ high and the tips of the antenna farther up in the air, then on 160 (1/20 wavelength high) you will have a better Signal to Noise ratio than if you had set this up as an inverted vee. On 80 meters (1/10 wavelength high) , the 80 meter dipole (half the length of the 160 section) will be more or less flat at 25’, maximizing overhead gain on 80 meters, and again helping S/N. On 40 meters (about 1/5 wavelength high) this antenna will function as a decent mid-height NVIS antenna, being also more or less flat.
It is better to run low on 160 than high on 40 meters, as more people operate on 40 and 80 than on 160. The increase in POI on 160 means nothing to most amateurs, while the ability to run NVIS on 40 when the foF2 will support NVIS is a big advantage. This is why my NVIS 160/80/40 fan dipole has the feedpoint at about 25 feet; that works well for 40 and 80, and is useable on 160 NVIS. My local topography and antenna orientation reduce the threat from DF significantly. Longer term, I am planning on erecting a higher performance antenna farm specifically for NVIS, and we’ll see what those antennas look like, and provide several workable options for higher performance antennas in Part 5.
I hope that this relatively brief followup to Part Four is of use.
Hope I will hear you on the air,