ANTENNAS HF and NVIS Communications .... only for the Military?

ANTENNAS HF and NVIS Communications .... only for the Military?


Here too nothing new, indeed, and maybe you have already talked about it....

An in-depth and fully annotated discussion of Near Vertical Skywave Incidence (NVIS) is available in the research paper, “Radio Communication via Near Vertical Skywave Incidence Propagation: An Overview,” by Ben A. Witvliet, PE5B / 5R8DS, and Rosa Ma Alsina- Pagès.


First investigated in 1920, NVIS propagation was rediscovered during World War II as "an essential tool for establishing communications in large war zones, such as D-Day in Normandy," the paper notes, adding that the Army of the States United later sponsored a lot of NVIS field research, particularly between 1966 and 1973.


More recently, NVIS has become a popular means of enabling first in communication on the Amateur Radio HF bands between 3 and 10 MHZ.

NVIS can be used for radio communication in a large area (range of 200 km) without any intermediate artificial infrastructure, and has been found to be particularly suitable for distress communication, among other applications, according to the paper.

"A comprehensive overview of NVIS research is given, covering propagation, antennas, diversity, modulation and coding," explains the abstract. "Both the big picture and important details are shown, as well as the relationship between them." As the paper describes, in NVIS propagation of electromagnetic waves are sent almost vertically towards the ionosphere, and, with the appropriate frequency selection, these waves are reflected back to Earth.

"The large reflection height of 80 to 350 km results in a large footprint and homogeneous field strength across that footprint," says the newspaper. "Due to the steep radiation angles, large objects such as mountain slopes or tall buildings cannot block the radio path."

As with NVIS antennas, the paper predicts that the important parameters are antenna diagram, polarization, and bandwidth. "Since only high elevation angles contribute to NVIS propagation, optimizing the antenna pattern for these elevation angles will increase the actual transmitted power and improve the signal-to-interference ratio at reception."

The article states "More recently, NVIS has become a popular means of enabling first in communication on Amateur Radio HF bands between 3 and 10 MHz." Hams used NVIS before the acronym was coined, primarily because not many stations have the means to get a high enough HF dipole.

Yes, this is what is said in the text, in fact it is said that it is even before that (Second World War):

"First investigated in 1920, NVIS propagation was rediscovered during World War II as an essential tool for establishing communications in large war zones, such as D-Day in Normandy,"

PS The military use HF antennons bent in an arc, mounted on wagons or mobile vehicles. Either very low dipoles, or verticals bent downwards, the reflection towards nearby areas is exploited. Excellent on 80 and 40 meters, forget about using it on 6 meters! ...

in fact it goes great on 40 and 80 meters, already at higher frequencies it no longer works.
The military must necessarily arch the styli in the armored vehicles, they certainly cannot move with 6 and + meter wipers vertically apart from that after a few kilometers they would no longer communicate without the NVIS effect.


To those who may be interested I talked about it on NVIS also in a MY article available on my website ... here, good reading!

https://rodolfo-parisio.jimdofree.com/nuovi-articoli-2016-1/


73 de IW2BSF - Rudy

 

and .........

 

Several of us have been doing this on 40 meters in the day time from our mobiles using a Scorpion set at 45 degrees, it also allows you to run a longer whip.

 

NVIS antenna on military jeep ....

 

This is an interesting topic that has been discussed here before but I think it’s great when it comes up because propagation is such a fascinating aspect of this hobby. This is great for new hams and those that haven’t experimented with this before. Besides my portable outings testing antenna heights, radiation angles etc., I had an accidental test of NVIS when my inverted V center support broke and I was too busy to re-string it. For several weeks I was operating with my antenna about 5’ off the ground. It was interesting that I was able to work closer stations on 40 and 80m. Fun topic! What a great hobby this is!

 

Military antennas are put into that position simply to keep the antenna from hitting low obstructions like tree branches. Not for NVIS operations. Most, not all, but most military vehicle radios operate in the low VHF between 30-88 MHz. NVIS is non-existent at those frequencies.

Now, for the antenna on the white Toyata: that is an HF antenna, but it is most likely in that tied-down position for the same reason that the military vehicle antenna is: low obstructions that would damage the antenna.

 

The state of the art is much advanced beyond Ben's paper, which has been available for some time. But your point is well-taken: NVIS is not dead and is an important approach that benefits with better understanding of wideband antennas, and beam shaping. That is not found in any antenna handbook.

Not all NVIS antennas are 'built on an arc'.

Practical NVIS frequencies are from about 1 MHz to about 15 MHz. Lower is better.

73
Chip W1YW

 

And, when the military trucks are in the field, you’ll see the antennas in a full vertical position.

 

On 80 meters those whips are just about point radiators, and their configuration will have little if any effect on their radiation pattern. Bending them down over the body of the vehicle is likely to reduce ERP because of cancellation, thus reducing whatever NVIS radiation there is. It does keep them from hitting tree limbs!

 

@AJ4WC & @WA8FOZ

Correct on both points.

Anecdotal evidence follows.

After being active Army for nine years, I was recruited out into the defense contractor arena as a Field Engineer working on US Army Artillery command and control systems. A lot of radios of various types involved in that. Got a lot of experience with the field deployed radios. During my stint at Ft. Lewis, WA. I was asked by the fellows to show them how to operate their new HF radios. They knew by then that I was a ham and these new radios had been installed in the HUMVEE's assigned to them and they had no clue on how to operate them.

I had no clue either, but what the heck, right? No manuals either and I don't remember the nomenclature either... But I remember that they were made by Harris and contained a small HF radio connected to a big amp and tuner combo in the back of the truck. There was a big helical wound fiberglass type antenna on the side of the truck. It looked a lot like that one on the front of the white Toyota in post #1. Probably operated in a similar manner but with an antenna tuner built into the system.

So... We jumped into the back of the truck and turned this monster on! It came on and it was noisy as heck, with big fans blowing and all kinds of commotion like that. The front of the little HF radio had a series of push buttons to set an operating frequency. I recognized that at least. Ran a quick visual on the other equipment and it was all Greek to me, but again, I figured what the heck nothing ventured, nothing gained. At any rate, not seeing any transmit / receive switching I just keyed the supplied microphone, which was one of those telephone style things we were all familiar with. The radio beeped at me once on PTT and beeped again as I listened on the microphone / hand set.

I had selected 28.400 as a starting point for testing just because I like ten meters. So I figured what the heck: call CQ and immediately had a east coast station reply to my CQ. I did the usual and asked the other ham if he could hear any of the (loud) fan noise coming from this rig. He said no, no background noise which surprised the heck out of me because it was very loud in the back of the truck. Must have been some very sophisticated noise reduction stuff going on inside the "exciter".

Then, I cleared off with that fellow and tried fifteen meters. This time, we hooked up with a JA station! By this time, the GI's eyes were bugging out and the questions began...

Can we do this too? No, not without a license. I was careful to explain to them what Amateur radio was an why I could use those frequencies while they couldn't. At least not without the permission of the unit commanders and all that stuff... Military has their own set of rules.Turns out that someone had an idea to use HF radio for just what the OP said, local communications out to about 500 - 800 miles. Brigade level communications.

Problem with that idea is that it was implemented and sent to the field with absolutely no training on the equipment or how it was supposed to be used... They did ask me to do some training with them, but it was not a thing I could train them on without stepping on a lot of toes. It was left up to their command and as far as I know, the radios were never used. Dunno what happened to the radios either. A total waste of taxpayer dollars, so what else is new?