Keypounder continues his updates on NVIS, a subject he first wrote about 5 years ago, which NC Scout published at the Brushbeater site. This article is being posted as the 6th of what looks to be now at least 7, more likely 8 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, was an amplification and further discussion of simple NVIS antennas in more detail, including the effects of height on antenna performance in response to questions. Link here: https://www.americanpartisan.org/2021/07/nvis-techniques-part-4-1-2/
Part 5 is a discussion of advanced NVIS antennas primarily for fixed locations and beginning to touch on NVIS operation in non-permissive environments.
Advanced NVIS Antennas
So far, we’ve covered the simple and easy to erect NVIS antennas, all dipole variants, suitable for man-portable operation. Now we are going to examine some more advanced antennas that offer advantages for fixed permanent or semi-permanent locations.
First up are the family of double dipoles. These can be deployed either in parallel, spaced about half to 5/8 wavelengths apart, or in line. As one might expect from the examples seen to date, the flat-top versions maximize gain and minimize low-angle ground wave off the ends of the antenna, at the price of more effort to erect. The Inverted vee types take less time to erect and take down, but have significantly more low angle radiation. The Vee antennas are lower noise, with less end-fire vertical RF than the Inverted Vee, but more than the Flat-top.
However, if you space two dipoles, both fed in phase, a bit over ½ wavelength apart from feedpoint to feedpoint and in line, regardless of the type (Inverted Vee, Flat-Top or Vee) the vertical radiation off the ends is significantly attenuated, while providing broad coverage off the broadside of the antenna. If you have a preferred direction for comms, or if you have a particular direction that concerns you with regard to being DF-ed, then this antenna offers some advantages if you have the space for it.
Here is the broadside pattern:80M_2el_inline_InvV_broadside
Here is the elevation plot off the ends of the dipole:80M_2el_inline_InvV_offends
Here is the azimuth pattern at 6 degrees of elevation off the horizon:80M_2el_inline_InvV_AZ_6degEL
Note the significant reduction in vertically polarized RF off the ends of the antenna; there is an extremely deep notch off the ends of this in-line array, a significant plus for local noise reduction. Even where the vertically polarized RF is at a maximum (the 4-lobed red line pattern) the vertically polarized RF is down over 20, approaching 23 db of attenuation. The average is much less; 30 db reduction of vertically polarized RF reduces the ERP by a factor of a thousand, reducing the received signal by over 5 S units from the peak overhead gain.
Advantages of the dual in-line dipole are:
- Low amount of low-angle vertically polarized radiation- much less ground wave than other antennas;
- Deep notch off the ends allows nulling of a single local noise source, improving S/N ratio.
- A broad pattern of broadside horizontally polarized high angle RF, covering a wide area while reducing POI and improving S/N ratio over the basic NVIS antennas;
- About 5dBi of gain, a bit more than 2.5 dB better than a single inverted Vee.
- Easier to deploy in mature forest than a cross dipole or a loop
- Coax feeds are lower profile than other antenna types that use window or parallel feeders.
- Requires more feedline than a single antenna or the side by side double dipole below, adding cost and weight;
- Requires an impedance transformer for best performance;
- Requires an area more or less in a straight line, with trees or other supports properly spaced, for a full wavelength on the band of interest;
- Not suitable for multiband use.
This antenna is a good choice for a semi permanent or permanent installation, especially if a flat-top configuration, which gives another dB or two, is employed. This requires only 5 lifting points at most if you elect to set up a flat-top array, as the center hoisting point can be shared.
Next is one of the classic NVIS antennas, often cited in the literature on the subject, the Shirley. This was one of the first advanced antennas I modeled because it was a classic, and I was surprised at what I found. Here is the pattern of the Inverted Vee version of the classic Shirley antenna- This pattern is off the ends of two side by side Inverted Vee antennas:80M_Shirley_InvV_offends
Note that this setup reduces the low angle vertically polarized RF off the ends only about 15 dB at low angles. The horizontal Rf off the sides looks more like the pattern off the ends of the double in-line above, see the pattern here:80M_Shirley_InvV_Broadside
So, the Shirley limits the range of the horizontally polarized RF, but the vertical RF off the ends looks like the pattern from the single Inverted Vee, down only about 15 dB from the peak, about the same reduction from the peak overhead gain as the inverted Vee. You can improve this by going to a flat-top Shirley, but not by much. This antenna radiates distinctly more vertically polarized RF than the dual inline. While the overhead gain of both the double inverted Vee antenna arrays is about the same, a tad over 5 dBi, the reduction in vertically polarized groundwave RF of the double in-line antenna array is much higher than the classic side by side array. The Shirley poses a significant risk of being DFed by ground wave. So why is this antenna still published in books on NVIS?
When the Shirley was first developed and used, there was little concern about the possibility of DF being employed against the users; the Shirley was developed by Major Shirley, a British Army officer during the Malayan crisis around 1950. The Malayan guerillas did not commonly use HF DF and did not have airstrike or artillery capability, so the operational advantages of the Shirley far outweighed the negatives. In the 21st century, those assumptions are no longer prudent for those operating in non-permissive environments. We’ll get a lot further into this when we review operating in non-permissive environments in one of the upcoming posts, but there is a lesson here. Not all classic solutions are still applicable, and you should not make the same mistake I did at first by ignoring the implications of modern technology, even when selecting something as simple as your antenna.
It is possible to set up a resonant dual-band broadside NVIS array, a multiband Shirley which does not require bandswitching and which can be used for cross-band simultaneous operation. I did a fair amount of modeling of this option including the feedline arrangements required. The radiation pattern for this is the same as for a single band, so there is no particular advantage to setting one up if you do not contemplate multi-band ops. It requires multiple supports (10 for an 80-40 setup; 12+ for a 160-80 setup) and is not a low profile installation if erected in an open area. If erected in a wooded area it can be difficult to get the elements properly arranged with proper spacing needed for best efficiency. With all that said, if you have a fixed location where the topography limits the potential for ground wave DF, AND you need the operational flexibility for instantaneous multi-band operation, then the Shirley might be worth a look, but I would make sure that I checked to see that I was not propagating ground wave if I did elect this option. Caveat Operator.
Now we move to a variant of the Shirley, the Jamaica, an adaptation of the Shirley, essentially a classic “Lazy H” with doublet elements a full wavelength long spaced a half wave or a bit more apart. The Jamaica significantly reduces low-angle vertically polarized RF off the ends. It does require a balanced tuner, however, and ladder or window feedline. Here is the broadside pattern:80M_Jamaica_FT_Broadside
Here is the pattern off the ends:80M_Jamaica_FT_offends
Here is a horizontal slice taken at 6 degrees elevation- note that the pattern is more complex than with a shorter antenna:80M_Jamaica_FT_AZ_6degEL
advantages of the Jamaica:
- Significant reduction in low angle vertically polarized RF, around 33 to over 40 db down from the peak gain, both reducing the POI and improving the S/N ration;
- Significant overhead gain, over 9dBi;
- Relatively narrow gain pattern- the -3dB boundary ranges from 42 to 53 degrees, and this drops off very quickly as the elevation angle decreases.
- Can be used on 160 when cut for 80 meters, or for 80 when cut for 40 meters, although with less gain and a less defined pattern.
- As it is a tuned antenna, it can be used ‘out of band’ with reasonable efficiency +/- at least half a mHz; MARS operators, take note.
- The Jamaica is a long antenna; each half will take 3 hoisting points for a total of 6 minimum, possibly 7.
- Efficiency requires open wire or window line feeders, which can snag on vegetation, and which MUST be connected so that the two doublets are in phase.
- This antenna requires a balanced tuner capable of handling open wire or window line; if you plan to use it on 160, that rules out a number of tuners.
In summary, the Jamaica while it is a large array offers significant advantages for NVIS operation from a fixed or semi-fixed location; its reduction in low angle vertically polarized RF is significant, approaching and in some azimuths exceeding 40 db. When combined with the high overhead gain, it is possible for users of this antenna to reduce their power by 75%, taking another 6 dB off the ground wave signal, while still providing a strong overhead NVIS signal. It can be used on 160 but the reduction of vertical RF is not nearly as pronounced. For a station transmitting in a non-permissive environment, the Jamaica is a very good choice as long as the stations to be worked are within about 250 miles; past that the signal will drop off quickly due to the narrow overhead pattern at all azimuths.
Next up we get to loops. The fullwave loop is another one of the classic NVIS antennas, requiring 4 lifting points and a wavelength of wire. It offers the same gain as the double inverted Vee antenna, with only a single feedline required, where any of the double dipoles take more feedline, and ideally need an impedance transformer for the best match. The single loop can be matched with a ¼ wave stub of 75 ohm coax. Here are the patterns for the loop- first the ‘broadside’- the pattern 90 degrees to the leg being fed:80M_Iel_loop_FT_Broadside
Now the ‘endfire’ pattern:80M_Iel_loop_FT_offends
and now we look at the low angle RF pattern:80M_1el-loop_FT_AZ_6degEL
You will note that the pattern is similar to the Shirley, although the loop does attenuate low angle vertically polarized RF a bit better than the Shirley does, about 19 db down from the peak.
Advantages of the loop:
- Gives about 3 db more gain than an inverted vee, or about the same as a pair of inverted vees;
- Requires only one feedline and no impedance transformer;
- Takes less space than any of the double dipole antennas, only ~1/3 by 1/3 wavelength- you can easily erect an 80 meter loop in a 75’ square space;
- You can nest loops for different bands inside one another. I have not found it optimal to feed them simultaneously, however, so they would need to be switched.
- A 160 or 80 meter loop can be fed with open wire line and used across the HF spectrum for long haul HF communication, reducing the number of antennas required to cover both NVIS and longer distance communications;
- As a closed loop antenna, can be kept ice-free by running DC through the loop, a unique plus in cold climates.
Disadvantages of the loop:
- Requires twice as many supports as the double Inverted vee, so it is slow to put up;
- Difficult to erect in a wooded area, as the loop wire must be threaded around the loop if there are trees inside the loop.
- does not provide as much attenuation of vertically polarized RF as the in-line or Jamaica; at high angles the pattern is more or less omnidirectional, and fairly broad, but at low angles it does emit ground wave RF.
Summary- The loop is a good option for a relatively compact NVIS antenna with reasonably good characteristics. Unlike others, it tends to be less directional, possessing more uniform radiation patterns in all azimuths.
After looking at the patterns for the single loop, and looking at the relatively high vertically polarized RF emitted, it occurred to me to model what happens when you run an array of loops. I looked at a large number of loop variants, changing feedpoint location, number of loops and spacing of multiple loops, but I think the best option to improve the pattern of the loop with the least effort is the double loop broadside array, essentially placing 2 loops in line, with the feedpoints spaced about half a wavelength apart, similar to the dual in-line dipole. This option, which takes two wavelengths of wire, as the Jamaica does, but does NOT require a tuner or a transformer, provides better gain and significantly reduced vertical radiation compared to the single loop. It shows narrow overhead beamwidth, similar to the Jamaica, is more compact, and can be fed with coax, with less visual signature, another plus.
Here is what a double loop array broadside pattern looks like:80M_2el_loop_FT_Broadside
Notice the overhead gain, which is close to that of the Jamaica. Now look at the amount of vertically polarized RF off the ends of the double loop:80M_2el_loop_FT_offends
lastly, see how much less ground wave would be induced:80M_2el-loop_FT_AZ_6degEL
Advantages of the double loop array:
- Gives about 3 db more gain than the single loop, approaching the gain of the Jamaica while using less area;
- As a closed loop antenna, can be kept ice-free by running DC through the loop, a plus in cold climates;
- Significantly less low angle VPRF
Disadvantages of the loop:
- Requires many supports; setting up a loop array is a multi-day project, and if you use towers or masts to support it, definitely not a low profile system;
- Even more difficult to erect in a wooded area than a single loop, as the loop wires must be threaded around the loop support locations.
Summary- The double loop array is a good option for a relatively compact NVIS antenna with reasonably good characteristics.
Let’s take a break from looking at antenna patterns and talk about listening antennas for NVIS. I’ve touched on this before in this series; the first radio article I wrote for publication was about about listening antennas. The idea behind listening antennas is that the operational requirements for a transmit antenna are often different from the requirements for a receive antenna. While the physical principles that make antennas work on transmit or receive are the same, the desired characteristics are different. Transmit antennas do not need to be concerned with noise, manmade or natural, but are all about maximizing signal output, especially in a desired direction. Transmit antennas are usually designed to maximize gain. Receive antennas, however, maximize signal to noise ratio. For a receive antenna the absolute amplitude of the signal is not nearly as important as the ratio between received signal and noise. This is why low band DX chasers often employ listening antennas with low or negative gain, such as Beverage antennas, K9AY loops, or other flag or pennant type directional listening antennas. Such antennas maximize that Signal to Noise ratio on receive, and the station operator will use a low angle transmit antenna to maximize the RF radiated in the preferred angle (and sometimes, direction).
With regard to NVIS operation, while the particulars are different, the principle remains the same. A low height horizontal dipole, like a Vee dipole, does a fine job of reducing noise pickup and making it easier to hear weaker signals, but in a situation in which transmit power is constrained, during an emergency, down-grid event, or while operating in a non-permissive environment, a savvy NVIS operator at a fixed location may elect to raise his transmit antenna and improve its gain and set up a low listening antenna for improved S/N on NVIS. There are two main types of NVIS listening antennas I’d consider for sure, and a third that might be useful if you have the skills and interest:
- A loop on the ground- About a 80 or 90 foot loop a couple feet off the ground, 20 feet or so on a side, on some electric fence stakes, will do a fine job of pulling in signals and keeping incoming noise to a minimum. Wire for this sort of antenna does not have to be large or sturdy; 17 gage aluminum electric fence wire works fine; you can also use 22 gage wire stripped out of Cat 5 or Cat 6 data cable.
- The low height Vee dipole. I would keep the feedpoint low, perhaps three feet above ground, and have the ends perhaps 30’ up. An 80 meter version works well on 160, 80 and 40 meter signals; it does an even better job when you use a tuner to peak it on each band.
- If you are a serious DIY type, you may want to consider a low height circularly polarized receive antenna, again in a Vee configuration. I don’t have the space to get into those in this article. Those interested should look at the the design found in the ARRL book “Receiving Antennas.”
Listening antennas are probably not suitable for short term stations; a patrol on foot or even a vehicle borne patrol is not likely to want to spend the added time to set up even a LOG. For a fixed station, however, having the operational flexibility to listen on a quieter low antenna, and to maximize your gain on transmit by using a higher flat-top or Vee antenna is worthwhile. This way you can get both higher transmit gain AND high S/N ratio. Keep in mind that a 3 dB antenna gain is the same as doubling your transmit power, while reducing noise pickup and improving your S/N ratio with a listening antenna means you will be able to pull in signals that others cannot hear. In a non-permissive environment, being able to operate effectively without being located is key. Having separate listening and transmit antennas is highly desireable.
Let’s start to wrap things up by looking at some combination graphs.
Here is a combination of the broadside pattern of all of the antennas we have reviewed, listening antennas excepted:80M_combined_advanced_broadside
Here is the combined endfire pattern:80M_combined_advanced_offends
Keeping these combination graphs in mind, for a low profile high performance NVIS antenna array, I would recommend considering the following for permanent or semi-permanent fixed locations:
- For lowest POI/DF on a single band, depending on the space available, I would choose the double loop, the dual in-line, or the Jamaica. The Jamaica requires parallel feeders, which are more visible from the air, while the dual in-line uses coax and can be very low profile if set up in woodlands with a Vee configuration. Both of these can be deployed in Inverted Vee, Vee or Flat-top configurations.
- The double loop takes two wavelengths worth of wire and requires a wavelength of coax to connect the two loops and match impedance, but still does not require an impedance transformer. Relatively compact, it takes a space 3/4 wave by 1/4 wave or about 200′ by 75′, more width than the in-line but about half that of the Jamaica, while still approaching the performance of the Jamaica. As it is coax fed, it is lower profile than the Jamaica, especially if the effort is made to erect it in mature woods. It does, however, take at least 8 hoisting points, and routing two loops through mature trees requires some effort and will expose the antenna to the hazards of falling limbs.
- dual inline takes only a wavelength’s worth of wire, and a wavelength of coax to connect the two elements, plus a 2.25 to 1 impedance transformer at the feedpoint. Placement of the antenna at any distance from the radio results in feedline losses in the coax that can be significant and should be considered. It is possible to homebrew impedance transformers to allow the use of 450 ohm window line for coax fed antennas at a remote location.
- The Jamaica takes 2 wavelengths of wire, and a half-wave of open wire feeder or window line. Like the dual in-line this can also be set up in a Vee, with the connecting window line at a low height; if you are going to move the feedpoint up to 25’ or so, IMO you may as well spend the extra effort to deploy this as a flattop and get that additional gain. You will also need open wire line to get from the array feedpoint to a balanced tuner, but with the low loss of open wire line, especially at low HF frequencies, you can easily set up your antenna as much as 1000 feet from your radio with no practical effect. The Jamaica is particularly worth considering if you have a farm or ranch with a woodlot some distance from the operating position. Both the dual in-line and the Jamaica require a wavelength of space longitudinally, while the Jamaica also requires at least half a wavelength of width. For 80 meters, this means a 270 foot long array.
- For all around use in a relatively compact location, with the potential for multiband operation, the loop, or nested loop array. Loops can be set up at the edges of a clearing with relative ease and since they need only be 25’ high on 80 meters to be very effective, as long as the trees around the clearing are significantly higher it becomes very hard to identify a loop made with brown wire from any altitude. If you add higher frequency loops inside the largest one, then you would be well advised to deploy this antenna in the woods and accept the increased difficulty of deployment. A loop array supported by masts in the open is not a low profile antenna. (The Luftwaffe paid especial attention to British radio and radar stations early in the Battle of Britain. Today’s technology has advanced far beyond the Stuka…)
- For locations where the potential for ground wave propagation is limited (in deep valleys, or near cliffs or other topographic discontinuities, or in rain forest) then you may want to consider the Shirley in a flat-top configuration. Just as with the loop, it is possible to nest shorter wavelength dipoles for instantaneous multiband operation, and just as with the loop, if you want discretion, woods are your friend. If you do elect to proceed with a Shirley in a fixed location, I would strongly recommend that you test the ground wave propagation away from that antenna particularly. It is good practice always to verify model predictions, but especially so with the Shirley to ensure that the expected effects of terrain are in fact realized. Note that in areas with seasonal vegetation that ground wave attenuation can vary signficantly throughout the year.
- Especially for fixed locations, it is also worth considering a low height NVIS listening antenna; a low dipole can be set up on electric fence stakes, with the balun marked to look like a fence charger; same with a listening loop.
In closing, I would be remiss if I did not remind everyone reading this that the gain numbers and the suggested elevations presented here come from a computer model. Models are an abstraction of reality. Reality is more complex than any model, and real world results WILL vary from those I’m showing here. That is why it is so important to get out in the real world, build and deploy your antennas, and SEE HOW THEY WORK. If the results are not to your satisfaction, make changes!
Right now, mistakes and miss-steps may cost time and effort, and maybe some money. But the time may arrive when mistakes in setup or operation of your station carry a much higher price. Make your mistakes while they are cheap, and do your learning while you have the ability to easily reach out and get expert assistance. On that note, take advantage of the resources in experience and knowledge available to you. Get training, get a mentor, and get operating! If you have questions, post them here- if I have failed to explain something properly, then you are certainly not the only person wondering.
Time is not on your side- Do not waste it.