A few things set NVIS somewhat apart from "conventional" HF ham radio techniques: NVIS can have very short communication distances via ionosphere (less than 150 miles). NVIS generally has no skip zone. NVIS uses frequency agility. Strategies that rely on NVIS often use standardized frequencies so the communications path can be tested rapidly and dependably. Most normal ham radio activity is not really geared for NVIS: Ham nets that are intended to be for a local area, tend to be locked on one frequency. If a station is "too close to be heard", then someone further away in the net will "relay". Reluctance to QSY to another band, for equipment, antenna, hard-headed or lazy reasons. For example, the NoonTime Net, here on the west coast, meets every day at noon on 40 meters. It is a wonderful net, usually with hundreds of check-ins when the NVIS frequency is 7 MHz. But, 40 meters has not been capable of NVIS at noon, most of the time this year, due to changes in the solar cycle. Most of the time, the NVIS frequency hasn't been above 5 or 6 MHz. The "75 meter alternative frequency" for the net has very little activity, even though the local and regional propagation is excellent on it. The net doesn't even have a 60 meter alternative frequency, which often would be the best NVIS frequency, and a lot better than 40 meters for the intended net coverage area. It is noon right now. Here's the ionogram for this area, showing the NVIS frequency is 4.9 MHz. The skip distance on 40 meters is around 400 miles at the moment. "Skip distance" means that you can't copy anyone closer than 400 miles.
I remember one fellow saying: 'NVIS !?!? I talked to a guy 1,000 miles away and if THAT's not NVIS, then I don't know WHAT is!" I assured him they he was correct on his last two points. I wonder if he ever got the joke. 'Near Vertical' - hmmm, I'd just bet that it means something. Go Forth And Radiate- 73 de Ray ..._ ._
Been using NVIS antennas for many years. Most productive and current version is deployed here in Weatherford Texas. My soil conductivity is moderate according to the graphs I found on line. The NVIS antenna now being used is not a dipole. It is a loop configuration. I strung out as much copper clad steel twisted pair as I could manage between four conveniently placed pecan trees on my property. This has been up nearly two years through two ice storm laden Texas winters. Configuration is as follows: 450' circumference 33' above ground 720 ohm, parallel open feeder (home brew) 82' length from antenna feed to shack feed through in shack, wall mounted (by-passable) parallel L network With tuner bypassed and run through a home-brew, also wall mounted two core 4:1 balun the antenna system exhibits an under 2:1 SWR on 40, 30, 20, 15, & 10 meters. Slightly over a 2:1 SWR on 17 and 12 meters. On 160 meters a 1:1 single core, but large balun is used and under a 2:1 SWR is experienced from 1875 to 1930 kC. I use the balanced L network most often just for the sake of any transistorized transmitters longevity. There have been many variations of dipole antennas used on my properties over the years in the NVIS configurations. By far, now that I finally have the room to stretch out some wire - this loop is exceeding ALL expectations in it's intended purpose(s). Not only is it a wonderful NVIS radiator; at it's "low height" it makes a wonderful RX antenna on ALL HF bands when a home-brew antenna interface is used for separate TX and RX antennas. Transmit on a full size 75 meter quarter wave vertical (with coupler designed to load it as a 1/2 wave on 40 meters and a 5/8 wave on 30 meters...) simply amazing ! GREAT STN ratios on 160 to 30 meters (where it really counts). Photos on my QRZ page will fill in the gaps of my (unorthodox) explanation. Thanks for a really good pod cast and article on the NVIS antenna. 73 from Weatherford, Texas de K5NSN Jimmie . .
The setup that 'low band' operators have known for decades. ON4UN details it best in his fabulous how-to book "Low Band DX'ing" https://www.amazon.com/ON4UNs-Band-DXing-Devoldere-ON4UN/dp/087259856X
I agree with you. NVIS was in the General Class question pool, at least on the last go round. Never heard of it until I studied for (and passed) my general.
Well from personal experience.... all my homemade mono band 1/2 wave dipoles (so called NVIS antennas) are no higher then 10-12ft off the ground and I've worked 126+ countries in 24mo. mostly on phone. No bad for NVIS and further distance then expected.
As a ham, I say "Enn Viss". But I'm also in the Army Signal Corps and everyone there says "Nev Isss". It's a universal debate on par with "gif or jif", toilet paper roll direction (over or under), and whether or not pineapple is allowed on pizza. OK, maybe not that big of a debate.... (btw, it's JIF, toilet paper goes UNDER to hide the ugly end, and no pineapple allowed)
The patent does define. It laps over the roll. It's seriously in the patent. Like we needed to know LOL! --Shane KD6VXI
Back in the 80s, I learned about NVIS by accident when I was tuning up a 10 meter yagi on the ground. I was using the reflector on the ground and the antenna pointing up technique at the time and I made a contact while testing. I thought I was working off the side of the antenna, but by rotating the antenna, the signal never nulled as I expected it to. Fortunately, the solar cycle was favorable at the time for 10 meter NVIS, today, I don't think it would happen, but 80 and 40, yes, it is a possibility. What would be interesting to do, would be to add several ground radials under the dipole at 1/4 wavelength height to help as a reflector. Getting a director at 1/4 wavelength above the dipole would be harder, but give more up direction to the antenna system. Too bad the same wasn't effective in the 70s on 15 meters, I had an NVIS 15 meter antenna, but I never made any contacts on it. I figured it was bad propagation at the time, and did not know about needing more height for the band at the time. I also did not know about how the time of day could get you some DX if you listened as the sun was setting or rising.
No. Nowhere in NVIS theory is frequency agility part of the propagation equation. Nor is frequency agility an essential part of NVIS operations. One can remain on one frequency forever. Makes as much sense as saying Line of Sight propagation/operation uses frequency agility.
Choice of frequency is a prime factor when it comes to any type of ionospheric propagation. But, go ahead, stay on one frequency... a broken clock is accurate once or twice a day... and that's about how often a rockbound frequency might work for NVIS (for mid-latitudes at this point in the solar cycle). If you chose 7 MHz, you probably wouldn't achieve any NVIS at all for your location in Maryland this month. The optimum NVIS frequency changes throughout the day/night cycle. At certain times of the day or night, the NVIS frequency may rapidly change. That's the reason why Frequency is the foundation of the NVIS Triangle also known as the NVIS Pyramid:
Frequency agility is no more important in NVIS operations than it is in any other typical HF operations. I'm afraid you are biasing your comments with your interests in ALE.
