NVIS Techniques, Part 4
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 third of what looks to be now at least 4 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 will review some basic NVIS antenna characteristics in detail, and discuss different types of operation and touch on the implications of these differences on antenna selection. This will be followed by Part 5, which will discuss more advanced NVIS antennas and their uses in non-permissive environments.
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Antennas. The single most important feature of any station; apart from the skill of the operator, the antenna largely governs the effectiveness of the station. Your station may have a great brain (rig and computer) but when it comes to putting the RF where it is wanted, antenna systems do the work. To continue the (imperfect) biological analogy, they are the circulatory, musculature and skeletal systems of your station, controlling the strength and direction of the RF your station sends and receives.
When it comes to NVIS, a great many antennas may be made to serve, but some are better suited than others. Before we get into NVIS antenna selection, let’s review some of the basics of NVIS:
- Intentionally low horizontal antennas to maximize upwards gain.
- No requirement for high power.
- No satellites or repeaters required.
- Use of skywave propagation on lower HF to provide terrain-independent continuous radio coverage out to perhaps 300 miles away, inside the traditional upper HF skip zone and outside LOS.
The key to effective NVIS operation is Signal to Noise ratio; while different modes have different characteristics the key to maximizing the signal and minimizing the noise for any given mode is proper selection and use of the NVIS antenna. We’ve touched on basic NVIS antennas in Part 1, but let’s get a bit deeper into this topic, probably the most important factor in effective, efficient NVIS operation, especially when operating in non-permissive environments. We’re going to look at some basic antennas, then examine some more advanced NVIS antenna options.
Before we do, consider your expected or intended operational environment. As a tool for emergency communication in a more or less permissive environment, NVIS can be used to support communication from either fixed locations or temporary locations. We talked in Part 3, rig selection, about HF rigs suited for NVIS operation- both fixed, and temporary or portable operation. When we talk about antennas for NVIS, we are going to distinguish between antennas for operating from temporary locations, and antennas for operating from semi-permanent or fixed locations. The operational imperatives are different, and drive different antenna selections.
A temporary operating position must be portable; all of the equipment must be quickly and easily set up and quickly taken down and carefully stowed for the next use by people who must be assumed to be tired, stressed and in suboptimal circumstance and condition. Speed and ease of setup and TAKEDOWN are requirements, when considering antenna selection for portable NVIS operating. Given the size of an NVIS antenna, this is a non-trivial concern. My goal when setting up portable NVIS wire antennas in trees is to get each hoisting point for that antenna in the tree in 15 minutes from a standing start.
This means, string or cord through a good crotch or over a reasonably stout branch, hoisting rope looped into the tree, antenna laid out and connected to the hoisting rope, transmission line connected to the antenna feedpoint and laid out ready to run to the radio. If there are two hoisting points, this takes 30 minutes; three points, 45 minutes, and for 4 hoisting points I allow an hour, minimum. For NVIS, high lifting points are neither required nor desired, but it all takes time and the more hoisting points required the more time required. If you can transport surplus military masts or an extendable fiberglass mast, you can speed this up. Having a trained helper speeds things up too, and if you have an open area in which to put your antenna, you don’t have to worry about tree branches being where your antenna wants to be. Threading a 160/80/40 fan dipole through 250’ of scrub forest is work; when considering an operating point it is often better to select a spot inside mature forest, or in between trees across a meadow since there will be less brush and undergrowth to compete with.
On the other hand, at a fixed location, a semi-permanent or permanent station, speed of setup is not the primary concern; antennas which have more sharply defined vertical gain patterns, especially those which minimize vertically polarized RF are more of a priority. We will get a lot further into this shortly.
You will need to balance the speed of deployment with reduction in Probability Of Interception and especially DF. If you are part of a small patrol expecting to use NVIS for reporting in from an extended patrol in a non-permissive environment, the less time used and the lower the profile of your radio communication efforts, the better. On the other hand, if you are erecting an NVIS antenna array for a fixed location, time may not be a primary concern. Maximizing your vertical gain, reducing your transmit power, and reducing your vertically polarized RF to the greatest extent possible may be more important. We are going to take a more in-depth look at the basic NVIS antennas, starting with the easiest to erect and moving to the more complex and slower types.
To that point, the easiest antenna to set up and recover is the inverted vee:
- Single hoisting point;
- 2 low height and low stress attachments, one at each end.
You can use a variety of dipoles as inverted vees. The Cross dipole, with one central hoisting point and multiple dipoles cut for different bands at different azimuth angles reduces the interaction between different dipoles, improving bandwidth somewhat. You also can deploy a Fan dipole (multiple dipoles more or less along the same axis) as an inverted Vee, which has two low height attachments. If your operation requires instantaneous availability of transmission on multiple NVIS bands, then either of these are good basic choices. You can receive 80 on a 40 meter antenna, and vice versa, but if your SOI requires rapid shifts in band for transmissions, then the multiband resonant antenna is the preferred option.
Both the cross and the fan can be something of a challenge for man-portable operation and take more time to deploy, You will have to balance the added time to deploy and recover the antenna against the operational requirement for instantaneous multiband use. For portable HF operation, especially man-portable operation, including but not limited to NVIS, the linked dipole is a good choice if you are only going to operate on one band per session or if you can take a few minutes to switch bands between contacts. With a linked dipole, bandswitching requires that you lower the antenna and disconnect the parts of the antenna you are not going to use, but it is a single wire and weighs less than a fan dipole. Mine is made from 18 gage polystealth, and I cut it to allow operation on 80, 40, 30, 20, and 17 meters. I can add extensions to allow 160 operation, as well.
Let’s take a moment and go back to the height of the F2 layer. Remember this graph?
This is a printout of the height of the F2 layer as measured at Wallops Island on May 9th 2021.
The average height of the F layer during daylight drops from over 300 km down to between 200 and 250 km, then rises again as evening comes on and the illumination from the Sun decreases. What this graph does not show is the attenuation from the D layer, which rises from ~0 at night to a max at local noon.
Keep the above graph in mind while we look at a printout of what the broadside radiation pattern of an 80 meter inverted vee with the feedpoint at 25’ AGL over average ground looks like:80m_invVee_25fthfp_broadside
Note:
- This pattern is broadside to the axis of the antenna;
- The gain is about 2.5 dBi;
- The 3 db points are ~50 degrees down from the peak or 40 degrees up from level.
- The -10dB point on this graph is more than 70 degrees down from the peak!
This is a pretty broad pattern so your daytime NVIS signal will not be limited by your antenna pattern. What will limit your normal daytime communication is the attenuation of your signal by the D layer. This is important, bearing both on both your ability to communicate and your vulnerability to DF, so let’s touch on this a bit more.
Normally, the D layer is about 40 km thick, but that varies depending on the sunspot cycle. The density of the D layer also varies depending on the intensity of solar radiation, both visible and ionizing (soft and hard X-rays, among other things.) Normally, D layer attenuation varies more or less predictably, with attenuation low at night, when only cosmic rays