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

heh

 

I believe that this report was among the first more systematic efforts made here about finding out the properties of different mobile HF antennas.
Previous work was somewhat uncoordinated and consisted more of trial runs where communications reliabilities were subjectively compared.

My own views of this come out of a discussion at a Nordic HF conference in the 90s where the topic of bending whips over vehicles was handled.
It was a consensus among the users that bending did not improve NVIS communications as expected.

I have a former colleague (ex-captain in the Armoured Corps) that witnessed such an mishap. It resulted in soiled underwear for the APV crew, a lot of damaged radios and onboard electrics.

73/
Karl-Arne
SM0AOM

 

Soiled..underwear?

Are we talking 'relaxed muscles' by electroshock here?

 

Depends on your expectations. An arced whip on a vehicle is not a great antenna at HF. That would have been my expectation.

Qoes it fill in a deep null at zenith?

A: Some.

I am convinced that the main reason some folks arc whips is because it looks cool.

 

I have had great luck with mobile NVIS using an angled antenna element, compare to vertical. However, the element was hanging off of the side of the vehicle, over the ground. Not over the vehicle itself.

I did a short, not technical write-up (not very technical myself)... I am about to run out for a few hours, but if the original thread owner doesn't mind and if anyone is interested, when I get home I can share my results.

 

Nothing like several million dollars of radio equipment in military equipment that no one knows how to use it what it's supposed to be used for! Betcha the Chinese know how to operate and listen! de k5oke. When I was in Navy on AGB-1, we threw perfectly good over the side in preparation for Captain's inspection. (Ya can't go broke spending somebody's else's money)!

 

Okay, sorry for the double post, but this is the summary from my testing. Again, I am new to HF (about 14 months) and still learning, but this is what I observed. No scientific proof by any means, but the anecdotal experience seems to support the appearance of proper NVIS performance. Feedback is welcome.



With EmComm being one of my main interests with Amateur Radio, I found that I had a need to fill a gap in my mobile setup. While at home, I can talk to just about anyone almost anywhere in the world. When mobile, I had a gap from about 20 miles away to about 350 miles out.

With my portable antenna systems, I cover this gap with HF and an inverted V, using NVIS propagation. The problem there is, setting up the inverted V while doing portable operations is very time consuming. Especially if there is wind and it needs to be guyed. Plus, who want to spend an hour doing a setup and tear-down if you only need to operate for a few minutes to either have a voice call or send a quick email/text message? I know I don’t. So, I got a little creative.

The short of it, I used a hamstick to make a single element NVIS mobile mounted antenna. Digital testing was conducted with 2 Winlink Peer to Peer messaging sessions and a connection to an RMS. I also conducted some voice tests.

The Winlink testing was mobile to base at distances of 145 miles, 96 miles and 87 miles. All well outside of LOS propagation and too close for traditional reflection and refraction of low-angle skywave propagation. NVIS propagation was also confirmed by placing the antenna fully vertical and there was zero target station signal reception on either end.

Additionally, voice testing was conducted with multiple stations at 20 miles, 35 miles and 190 miles. The plus side is, there was/is a little more to it than NVIS, as I was also able to speak to people well outside the NVIS coverage area, as far as 746 miles. This is a good indication that there is still significant low-angle skywave propagation occurring at the same time NVIS propagation is occurring. Providing a bit of flexibility in communications.

Another advantage of the design is, though you cannot drive while the antenna is fully deployed, you can swing the antenna over the cab and drive. When you get to where you need to operate, you can simply swing the antenna out, lock the jam nut and start transmitting.














If you would like to replicate what I have, it’s fairly inexpensive. The total cost was about $70 and an hour of work. The parts are…

A UHF mount to the bed of the truck (for flexibility with other antenna options)
MFJ PL259 to 3/8x24 adapter
Wilson 10" Replacement CB Antenna Shaft (actually 12 inches long)
Two jam nuts
One 3/8x24 barrel connector
A 75m hamstick with a 3/8x24 detachable upper whip
One 102 inch steel whip 3/8x24 antenna

To get the angle I needed for the antenna, I copied my successful portable inverted V element angles, using a press to bend the antenna shaft to about a 40 degree angle. Once the antenna is assembled, the weight of the antenna leads to sagging to the correct position/angle. I mounted the MFJ adapter to the antenna connector, put jam nuts on each end of the antenna shaft and spun the bent antenna shaft onto the adapter. I put the barrel connector on the top of the antenna shaft and locked it in place with the top jam nut.

Once the foundation was in place, I screwed the loading coil from the 75m hamstick on the end of the extension. From there, I used the steel CB whip as the main element, since the 75m whip is too short for the bottom end of 80m. I cut the whip to about 2 feet longer than the whip that came with the hamstick, and then used my antenna analyzer to cut the whip to the proper length to be resonate on 3.58 MHz with the best possible SWR.

So, I have been asked by several people already, why not just use an 80m hamstick? Good question… and the answer is simple. Using a 75m coil and a CB antenna style 3/8x24 whip gives you less coil and more straight radiating element, being a slightly more efficient antenna. Also, the whips that come with 75m and 80m hamsticks are flimsy and sag over too far when the coil is at the proper angle. This causes an arch, not a straight element. The steel whip stays straight for a better performing antenna that provides a more consistent/stable SWR, due to moving less in subtle winds.

I found that the closer the whip is to the vehicle, the worse the performance was and it was harder to get a good SWR. The best performance and SWR is when the antenna is perpendicular to the bed of the truck. You see in the images that the antenna comfortably hangs off the side of the vehicle.

 

Our military has purpose-designed NVIS antennas that are more varied and sophisticated than the ones shown or described so far. For example, I'm aware of a contract awarded in the late 80's for what was essentially a wire that was thrown out on the ground--specifically sand (think Desert Storm)--that was capable of being run over repeatedly by tanks. A poor ground below the antenna was essential. Lots of interesting devices and corresponding research out there, but not necessarily in the public domain. The guys with the red stars on their helmets also had/have similar stuff. There will always be a need for idiot-proof, quickly-deployed and effective antennas and related technology.

 

Nice job!

The 'antenna' here has two components: 1) the slanted whip; 2) the vehicle. The vehicle radiates substantially and is not a ground plane at HF. The vehicle--as a counterpoise--is a horizontal radiator above poor to fair ground. That's what is giving most of the contribution at the high (approaching zenith) angles.

 

A drill is all you need. Airplane microphones for decades have had a hole in the back side, the engine noise hits the microphone element from both sides and your voice hits it from one side. Another way to do it was a thing roadies came up with for rock concerts with speakers behind the band instead of next to it - you use two microphones on the stand wired out of phase and sing into just one of them

 

The AN/MRC-138 is an HF radio so, yeah, the antenna in a tied-down position will radiate in an NVIS pattern. However, most mobile military comms are on VHF frequencies, which don't radiate in NVIS patterns. Most of the time the antennas are tied down for safety.

 

That's good information to know. I appreciate the feedback!

 

why do I need a password to read this document?

 

What??? What password? Where?

 

"To appreciate this we show, in Fig. 2, both predicted and measured results at 2.8MHz using a 4.8m whip antenna positioned at the rear of a Land Rover@. It is evident that the performance is degraded in all directions, not just toward the zenith, when the antenna was tilted 60" forward (as is practicable) across the vehicle. Not only does the zenithal null remain but the radiated field strength is also decreased significantly by tilting the antenna. Rearward tilting is preferable since the substantial out-of-phase current induced in the vehicle bodywork is reduced. However, the practical disadvantages of a rearward tilted antenna are clearly formidable" [Emphasis mine]

The "practical disadvantage" primarily being that the antenna usually can't be rearward tilted while the vehicle is in motion.

From: B. A. Austin and W. C. Liu, "Assessment of vehicle-mounted antennas for NVIS applications," in IEEE Proceedings - Microwaves, Antennas and Propagation, vol. 149, no. 3, pp. 147-152, June 2002.