Understanding Amateur Radio NVIS Antennas and Propagation
Copyright © 2002-2011 by Harold Melton, KV5R. All Rights Reserved. Feel Free to Link to This Page.
NVIS Antennas: Page 1
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The material in this article is derived from The ARRL Antenna Book, 1957, 1974, and 1999 editions; Army Field Manual 24-18 and others, several QST articles, and various web articles written by other NVIS experimenters and HF antenna and propagation experts such as W4RNL. Various specifications given as examples are rough estimates, some with wide variability. They do not represent the absolute limits of performance, but practical applications where a high percentage of messages may be passed accurately, under average conditions.
This article is not intended to be a complete primer on HF radio propagation or emergency communications. Users needing background in HF radio propagation should see several of the excellent links at www.eham.net/DX/propagation, the many links at the bottom of this article, and the ARRL Antenna Book. The author assumes that the audience for this article has a general understanding of HF antennas and ionospheric propagation. Users needing background in emergency communications should see the various web sites of ARES and RACES groups, as well as FEMA and MARS sites.
The author wishes to thank all that have contributed useful information on this subject, and encourages comments, in writing, that may be used to improve this article. To quote a great writer from the 1830s, “Before you can convince me of error, you must first convince me you understand what I say.” Please read carefully.
I have a 40 meter home double bazooka for a year now and it”s working pretty good. It is 6 feet above the ground with a reflector underneath the antenna.I was able to contact Europe on 100 watts ZS6CCY he gave me 57 and the 40 meter double bazooka is 20 feet at the apex from the ground putting 300 watts 5×9, these
are NVIS home brew antennas. This K7OHM viva Las Vegas & 73 all.
By the way, I video record it.
I’m pretty sure some of my contacts are NVIS but maybe not. I’m using an end fed half wave dipole from “My Antennas” their EFHW-8010. I’ve talked to people 200 miles away behind 2 sets of mountains, and also a couple of thousand miles away in Northern BC and also Illinois from Kingman AZ. My feedpoint is at the peak of my roof, about 35 feet up and it slants up to a pole about 150 feet away that is 10 feet above ground level at that end. The pole is mounted on top of a knoll that is about even with the top of my house. At the mid point it’s probably 40 feet in the air.
Does that make sense as an inadvertent NVIS antenna set up?
Chris (AKA The Kingman LOP)
Yes, any horizontal antenna at that height will be NVIS on the lower bands.
I have a similar situation. I’m surrounded by hills on three sides so I don’t have a good ground wave signal for local comms. I have a homebrew EFHW about 132′ long in roughly an inverted L. The verticle is about 36’up out of a Spiderbeam pole and terminates at about 20′ up. I’m able to participate in a statewide net, all points being under 250 miles. But I’ve also worked stations in Washington and Hawaii from Ohio. I know the EFHW isn’t the most efficient design, but I’m still getting 59 reports. I’d like to get the antenna a little higher, but I’m afraid I might degrade the NVIS performance.
The high angle lobe doesn’t start to collapse until you get close to half-wave high; 130′ on 80, 67′ on 40. The optimum height for nvis, considering ground losses versus elevation pattern, is about 40-55 feet. Lower is more ground loss, higher (60+) and you lose high-angle power on 40 meters. Very low, like 10-20 feet, is more ground loss, and a smaller average range. The change is small though, so even going from 20′ to 50, you’ll only see the difference on weak signals at max range.
I know this is going to sound like a dumb question: since you guys on here are talking about NVIS would anyone know if NVIS can be used with TV signal? I’m wondering if anyone though of making an antenna for TV since I know people who live in a valley, and if it works for radio…why wouldn’t it work for UHF/VHF TV…I want to set up NVIS for my Yaesu FT450D – it would rock being able to get out in to NY/NJ states from Pa (since I’m on the back side of a mountain).
The reason that NVIS will not work for TV is that the frequencies used by TV are too high to be reflected from the ionosphere
I have been using a 1/2 wave 75 meter Inv- Vee up abt 17 feet directly above a couple mile long 4-wire barbed wire fence. Operate every Sunday morning 7Am ’til about 9AM. This is a casual net every week. Props range 250 to the south to 400 to the north!!!!! East and west also!!!! 50 ft RG-58, So-239 to the antenna. Using speaker wire used for the Elements!!! Total cost minus the coax abt $8.00!!!! Can’t bet this antenna!!!! LEN WA6IQO, Bishop,Ca
Darn! Right when I’m getting ready and attending Church. Any other times?
Chris in Kingman, AZ
Great article and educational comments. Has anyone developed a 3ft diameter nvis dish? For mounting on the roof of a truck, or camper? Or in the attic.
No, a 3ft dish is like for 12 GHz (microwave) and NVIS is like 1.8 to 7 MHz (shortwave).
Hello everyone !
Have you had the opportunity to test this NVIS ???
Thank you in advance for your feedback,
I take this opportunity to ask you, as part of a construction where I can find this resistance of 50 ohms, because this antenna is given up to 1 KW!
In region 5 of Army MARS we regularly use NVIS. It does help to have frequencies available in the 2,3,4,5,6,7 Meg range under the current solar conditions.
I have been using an 8o meter NVIS horizontal full wave loop for several years now and it is a very good low noise antenna on receive and a mediocker transmit antenna. It has a low signal-to-noise ratio ! I mostly trasmit on a dipole array and receive on the loop. Some bands I use only the loop for transmit/ receive. I feed the loop with a 1 to 4 balun on 80 meters and in series with the balun, an electrical 1/4 wave of 75 ohm RG-6 coax to match it on 40 meters. Check the impedence of your loop to determine how to feed it. Willy K3VW
I’ve also worked stations on 30m during daylight hours (Noon +/- 2 hours) from 20 to a few hundred miles via NVIS propagation. Although 30m NVIS mode is less available than 40m or 80m, when it’s open signals are quite strong with very little QSB.
I ran into NVIS antenna operation by accident. I use a 90 ft. clothes line , about 12 ft. off the ground. I couple that to a 35 ft. long single #12 copper wire I found. So now its a low slung dipole. . I use a YAESU FT.-817 radio with a Z-817 tuner. . .My average distance is more that 300 KM and its not unusual to reach nearly 1000 km. Last week, My QTH was Mid Vancouver island BC, and I had a good QSO with a party in Cocolalla Idaho!!
I like NVIS for short haul stuff on 40 and 80m. I have had good experience with a horizontal loop about 30 ft above ground. I fed it with 450 ohm ladder line and an SGC230 tuner. Very reliable for emergency communications type work. I would have put the loop lower but stuff was in the way. You just do what you can. Not sure what the guy was harranging about who said they just don’t work. Maybe he was attempting DX. In NVIS 40 and 80 seem to be the only two bands worth working. No shallow angle transmission for long distance. The signal goes up and reflects back down like an umbrella. If the ionosphere does not cooperate then you are out of luck.
I should mention I’m in the process of assembling another full wave 80m loop antenna. The first one got damaged in a horrible storm. I’ve tried other antennas for NVIS and nothing seems to work as well.
One of the best primers on NVIS I’ve encountered. Thanks for sharing all the efforts.
I have read several articles on NVIS and spent time in the Military who also use NVIS for Tactical command and control situations… Believe me, if it did NOT work as it is touted to, the Military would NOT waste their time with it… That is the reason most of the Military Vehicles you see have their mobile antennas bent over and tied down… To take advantage of the NVIS method of sending signals… On 40 meters I have no desire to send signals to Europe, Mexico, South America or any other DX so an NVIS would suit my purpose perfectly… I’ll know how it works within the next couple weeks…. Thanks… WV8RS…
WHAT TYPE OF REFLECTOR ARE YOU USING?
Unfortunately he didn’t say… But generally speaking, a reflector is a linear conductor that is ~5% longer than your half-wave dipole, spaced ~.2-.25 wave behind. It also needs to be >8 feet high to (a) decouple it from earth and (b) keep it out hair/mower/tractor/wife/dog, etc, thus, if your dipole is <~60 feet high don't mess with a reflector under it.
What are you using as the reflector?
well been using a 10ft high 40mtr NVIS for abt 3 mo. I have 10ft wide ‘reflector’ under ant. BOY! it makes me look like am running a kw or more(rig is kenw 520, 100 wts). I get called a lier by some of those old timers when I tell them abt the ant. my 100wts is as strong to them as I see them. hi hi. it’s the ONLY 40 mtr ant I have………….WB5HDS
I would also like to know what you’re using as a reflector??? A piece of chain-link fencing?????
Galvanized electric fence wire, right on the ground, 5 strands 70′ long. I live in the Lincoln National Forest of Southern NM at 8200msl. 70 ft pine trees all around me.