Several real world comments on NVIS antennas from Kurt KD7JYK
Kurt popped up out of his lurkers hole to make the following comments that I totally agree upon with my comments:
- “NVIS is not an antenna, a frequency or a radio”
There is no special NVIS antenna as its just a dipole
NVIS usually happens below 8 Mhz when the FoF2 layer is in play as it is dependent on atmospheric conditions to work
Any HF radio can be used
2. “RF going thataway, in this case pretty much up”
If the RF signal is radiated at between 70 and 90 degrees there is a good chance it goes up and is reflected back down by the F2 layer within the regional range the resulting signal pattern effects. Kurt explains the RF going up in a narrow beam of the bandwidth between 10 and 2 on a clock face.
3. “The more signal going up the better it becomes”
In this case we need a yagi or a beam but that itsnt practical on 80m is it? Kurt suggests a Un-beam, a partial beam or a 1/3 beam but it can simply be a wire underneath the dipole acting as a reflector.
4. Reflector Design
The reflector is 105% longer than the driven element and is optimally spaced at 1/10 of the wavelength behind the director.
For 80m its 8m high, 6m for 60m and guess what its 4m at 40m. Ok so with two poles 25 feet in length will cover 80m and shorter 16 ft Painter poles will be awesome on 60m. I suggest the reflector be about 1-2 ft off the ground to avoid ground losses.
That is totally doable for a permanent or portable NVIS set up.
5. A two band antenna
I agree with Kurt that one must have separate dipoles for each band. If a third band is needed then 3 dipoles is warranted.
6. “HMMVV Antenna Placement”
Kurt has convinced me that his theory is correct. These Humvee’s with the antenna pulled towards the front or towards the back have NOTHING TO DO with NVIS at all but are so they don’t snag power lines and electrocute the passengers and driver.
It is represented that this antenna set up is to increase low angle of radiation from a vertical to be up in the 80 degree angle to shoot up and shoot back down.
My adhoc skunkworks testing has proven that the mobile antenna actually works better as its horizontal than in this pulled design thus confirming that the FM 24-18 Antenna Field manual has been wrong from day one. An EZNEC study shows the vertical gain decreased from 2-15db and power down 3db at 30 degrees proving this is a poor choice of an NVIS antenna but is better than nothing I suppose.
A Vietnam RTO explained to me that in the LRRP missions NVIS was critical to get a signal above the canopy and reach the FOB and horizontal antennas on HF (PRC-74s) worked best when the PRC-25s low 1 watt and vertical antenna didnt work properly.
The PRC -74 had a 10 ft whip for short range comms and a adjustable dipole in the kit bag (doublet configuration). I have the doublet antenna and as a 44 ft version it works great on the HF bands with a tuner of course