NOTE:
I do not claim to be anything but a student of radio, one with little experience compared to those that really “dig this stuff” and this is just my opinion (likely ill-informed) of how things work, so please correct any errors in logic you see here in the comments below. Like I tell everyone… “I’m just a prepper with a radio.”
It was 1984 and I got volunteered for Forward Observer School while in Infantry Training School at Camp Pendelton, which meant going to a brief radio school to learn how-to call-in indirect support (mortars) and air support.
Surprisingly, I would place second in the class of ~30 other Jarheads, but other than learning the International Phonetic Alphabet I was no more dangerous with a radio than when I started.
The only advantage I got from the school was getting to carry a radio instead of the “Pig” (M60 Machinegun) on Battalion exercises for the Company Commander, which lasted only one exercise while in ITS and then I was shipped out to the Fleet where I never saw a radio again.
The radio carried was the PRC-77 (probably a PRC-25 by then) and was carried with the antenna folded over, which I mistakenly thought years later when I got into HAM Radio was for NVIS use.
Considering that the PRC-77 was a 10M/6M radio, and NVIS is not possible in those frequency bands, meant it was using “Ground Wave” propagation like a CB and not NVIS.
This document (Reconnaissance and Surveillances Leaders Course Communication read ahead) gives insight into what was being done with the manpacks in the day and is THE BEST overview of propagation and wave path I’ve seen, a MUST read IMO.
THE GOAL OF THIS POST
Is to share my experience with NVIS and try to have an understanding of the “theory” behind the results of those experiences. Theory does not matter to me as much as results, but better results can only be had by understanding the theory behind the process.
As an Electrician, “Electrical Theory” is something we all got in the apprenticeship program and something rarely we need to fall back on in our careers as it is usually one of “doing and learning the hard way”, hopefully not too hard!
But having a solid background in theory can prevent many mistakes if you understand the basics (theory) of how electricity works, and I venture that NVIS Theory does the same for setting up systems that will work best.
UNDERSTANDING NVIS
You can Google NVIS and learn all about it, but the video below does a great job of summing it up IMO and the Marine Corps has a pretty good manual called the MCRP 3-40.3C (marines.mil) that I would suggest reading along with the 1996-fiedler-near-vertical-incidence-skywave-communication-book-worldradio-books-2.
I also recommend reading these two articles:
SO WHY USE NVIS?
If you watched the video all the way through, you know that using NVIS makes you hard to “DF” (Direction Finding) when sending traffic (ie. on patrol and sending SALUTE reports back to base) which is good thing.
Since there is no repeater being used in the middle high ground blocking DLS (Direct Line of Sight) transmissions, the Ionosphere is the repeater, there is nothing to be stolen by deer hunters like my X-Band Repeater was last year or have one of your cross-band frequencies made known.
ENTER JS8CALL
Taking things one step farther, combining the power of weak signal digital modes (JS8Call) with low power HF (<5W) transmission you can get past the traditionally required 20W baseline power requirement for reliable voice comms using NVIS.
Voice is considered a valid path when the signal reception is above +10db, I’ve had decodes in JS8Call at -31db and -20db decodes are common.
Lower power equals smaller equipment and less energy consumption, which is critical in the field as recharging via solar panels is not practical, we are patrolling, not camping. The weak link of course is the computer and its power needs.
Armature Radio Montra: Use only enough power to get the message through!
Not only does JS8Call allow for a small signature, you also do not need an operator at the base unit to pass information as its built in “mailbox” feature allows messages to be delivered and picked-up whenever the operators decide to check in on the other stations STATUS and “pull” (request from) INFO to get any messages your base has stored for you.
POORMANS VOICE MAIL
Don’t want to use digital modes? Prefer KISS?
I understand and agree that voice is the way to go, if possible, but only experimentation NOW will let you know what will work when you need it most, spoiler alert, voice rarely works under 20W and that means bigger radios and more power-hungry radios in the field.
One option for capturing SALUTES in an unattended HAM Shack when using voice mode, is to have a VOX digital recorder next to the speaker of the base unit HF rig, which as long as it’s in a quiet room it should work to capture incoming traffic.
An alternative is to have RecordPad running on a nearby computer with VOX mode enabled.
I’m not willing to purchase a dedicated recorder right now as I don’t see voice being reliable enough to be my primary mode so for testing RecordPad (free for private use) will do for testing.
Unless I can have a 20W+ rig in the field with me, which I don’t have at the moment, it’s highly unlikely voice NVIS is going to be reliable.
Someday I plan to get the Xiegu 125 amp/tuner and make the FT-817 my base station and the 20W Xiegu G90 my field unit and will revisit the VOX Recorder option then, but for now, the 5W FT-817 will have to do and will likely only do using JS8Call.
ANTENNA CHOICE
When it comes to keeping things small, simple, efficient and easy to setup…
The “Inverted-V” linked Dipole is the answer IMO. You get the best energy transfer with a tuned dipole, and a 40M/80M linked dipole gives you the ability to transmit NVIS 24/7 with these two bands, in theory, you never know until you try it!
My personal choice was to purchase a quality made 40/80M linked dipole and tune it (cut to length the wires) to be resonant on the freqs I planned on using:
40M (7.078) the default JS8Call freq for digital use.
80M (3.980) the OEN (Oregon Emergency Net) voice net freq.
ARRL has Digital in the 3.5-3.6mHz range, but that’s a suggestion, you can transmit digital in the voice section of the band, especially in an emergency.
They are not difficult to make but for the price I paid for mine, I could not make as nice of one for much less.
NVIS THEORY
My primary source for NVIS information is the Marine Corps MCRP 3-40.3C Manual.
Below are a few highlights I gleaned from it.
How far out your signal travels in NVIS mode is determined greatly by the takeoff angle of the antenna you are using.
My primary concern is to have a takeoff angle that allows for very close in “Tactical” comms between me in the field and my homestead (base station) in the 2-50 mile distance, which means it needs to be steep.
Looking at the chart above, the inverted vee and the inverted L are the best choices to cover those in close ranges, but the inverted vee is the easiest to setup and why I chose it as my primary antenna.
The higher the takeoff angle the better for in close comms, so the next questions is how do we know the takeoff angle of the antenna as we have set it up?
Well, it seems that the inverted vee is about the same as a dipole when it comes to horizontal polarization with very little loss (~ <2db) which is an acceptable trade for ease of setup in the field and why it’s so popular with SOTA.
One thing that take-off charts don’t factor in is the rear reflector effect that lowering it to the ground below the 1/2 wave suggested height does to the signal.
Also, experimentation has shown that the center angle is not so critical and can range from 90-120deg without issue, even down to 60deg with ~ 2db loss.
This is because no antenna is a “laser” and is sending up “lobes” of transmissions that are not just in the take-off angle of design, but throughout the radiation bubble as defined in take-off angle charts as above.
PERFECTION NOT NEEDED
My own setup below has a center angle of 74deg (limiting factor of the 16ft crappie pole) when using the 80M extensions and works just fine. That’s the great thing about HAM Radio, you never know what truly works until you test it.
According to Fig 4-15 above, a TOA of 74deg should be ~.5db less than the 55deg TOA that is shown as the designs “sweet spot” for 3MHz or 80M, but on 9Mhz the angle is much more important (wish they used 7MHz / 40M on the charts, but estimating a line between the two and hoping the profile is the same, a 74deg TOA would be ~2.5db loss compared to the 30deg TOA listed.
HAM Radio has a lot of theory and as an Electrician I can tell you that theory is the foundation, but experience has shown me that electricity does not always behave in a theoretical vacuum, neither does radio wave propagation.
Just using the 40M legs with a lower center height of 9’-0” (collapsing bottom two segments) gives me the same Horizontal polarization of 74degs and a lower profile.
I purposely don’t go below this pole height as it’s been established that 10-15ft is a good / practical height for NVIS.
NVIS can work by laying the wire on the ground, but at what losses? Something I plan to test eventually, but accepted thoughts are that it’s a last resort and inefficient means of signal transfer.
The best NVIS antennas use takeoff angles near 90deg (180deg parallel to ground) with little ground wave emission which is why raising the inverted vee legs off the ground helps, along with not “shorting” the antenna by taking the high voltage ends to earth.
Placing a “radiator wire” under the antenna (along the 114ft portion above) increases horizontal polarization also. Optimally, it should be 5 % longer than the base of the antenna and off the ground by a foot or so. I don’t bother with this as a fast setup is preferred and I don’t want to carry another chunk of wire.
When using a digital mode that can handle the ~2db drop from an inferior NVIS setup lets me go for a quick easy setup without having to worry about a slight loss in efficiency that the mode can handle, verses voice comms where antenna setup is critical with only 5W in the field, and even then, the chances of getting through are still slim.
My target goal for this field setup is to be able to forgo VHF repeaters and still have comms from base out to 10 miles for tactical comms, and across the USA for regional informational gathering.
The testing continues… Get out there and see what it takes!