Your probably better off with vertical antennas at both ends on a 30mi HF path which will work ground wave 24/7 where NVIS relies on propagation that will change from day to night.
With that said, most articles would be incorrect recommending no higher than 3 to 7ft for a horizontal NVIS antenna. The max efficiency will be close to 1/4 wavelength off the ground or about 33ft for 40m and up to about 62ft for 80m. For multiband NVIS, use about 1/4 wavelength at the highest operating frequency and since the highest amateur band where NVIS would normally work is 40m, 33ft would be the highest you would ever need.
Going lower is ok if you have plenty of signal at the other end and your receive noise will go down when the antenna is lowered, partially from the efficiency being reduced and some from rejecting signals and noise arriving at a low angle from afar.
Going from 30ft down to 10ft can easily loose 6 to 10dB of efficiency on 80m but if propagation is good then it usually works fine. When the bands are hopping I can usually toss 30 to 50ft of wire on the ground and make contacts out to a few hundred miles with my 25w manpack radios but when the bands are lousy that's very difficult without much higher power.
There is quite a bit of controversy about placing a reflector wire under your NVIS antenna. Sometimes it helps and sometimes it can make things worse so I don't bother. My main HF base antenna is a G5RV horizontal at about 30ft which falls in the NVIS category, although it also works fine for DX. I use several portable NVIS antennas that are a 40 and 80m dipole at right angles to each other that wind up on a 33ft portable mast and the dipole elements become the guy wires for the antenna. I also use a military AS-2259 modified with resonant wires for 40 and 80m with extra mast sections to raise the feedpoint from 15ft to 30ft.
prcguy
I have been reading numerous articles on NVIS antennas from dipoles, long wire to a t antenna.
Some point out a reflector below the antenna....I see they show a wire below the antenna , a variant of 3 reflectors below a dipole.
I am wondering if just a steel pipe under the longwire will suffice.
Height wise all or most articles recommend from 3 feet off the ground to the max of 7 ft for a dipole.
Anyone have experience in this type of antenna.
My goal is to communicate a short range of 30 miles or less on 40 meters digital to a friends qth.
I know one can use VHF/UHF repeaters etc., but want to do it on 40...
DOCTOR/795
