ENVIS

Frequencies for NVIS

Selecting optimum frequency for NVIS operation depends upon many variables e.g. time of day, time of year, sunspot activity, atmospheric noise, and atmospheric absorption.
In Australia we would expect to work the frequencies allocated for emergency services [WICEN].
Propagation prediction software is also available on the web, giving the current critical frequencies.
It's important to prepare a backup plan between operators with alternate frequencies, or agreement to follow a pre-arranged pattern to "find each other" i.e. begin at frequency, then steps of +/- 5kHz.
Whatever works for your emergency comms group!

Practical considerations

1. Remember your amateur radio training and basic antenna propagation theory? An antenna sends out a ground-wave which may travel up to 300km. The antenna is also radiating a sky wave that is refracted by the ionosphere and returns to earth (some greater distance away).
There is a “dead zone” of "no signal" between where the ground wave ends, and where the sky wave comes back to earth. The dead zone is called the Skip Zone.
The NVIS antenna is for "short-distance" HF communication into the Skip Zone. You're attempting to communicate within your region/state/country.
2. Research... the frequencies and performance varies with a number of factors.
Location: NVIS Propagation maps indicate that 40M is not generally available above 30 Degrees North or South of the equator... (Qld = good for me, maybe not as successful in the southern states of Australia).
Height above ground: Technical specs say anywhere from 1.5metres to 5metres, but there are logistical differences in raising an antenna to those greater heights. My final antenna design is kept to a low height, 1.8metres (6foot) above the ground and this is easy for a solo operator to erect and manipulate.
Practical tests & modeling [available on the web] show that better coverage is obtained if the antenna is mounted at about 1/20th wavelength above ground and at this height the background noise level is lower.
This feels counter-intuitive as we're always told... height, height, height... but an NIVS antenna will be less efficient if the height is increased.
Ground: Look at the type of the soil below the antenna? i.e. what's the conductivity of the earth/ground you are operating above?
Poor conductivity will attenuate up to 3db more of your signal than high conductivity soil. If the ground is wet after rain the signals are likely to be greatly increased due to better conductivity.
So better soil conductivity = better performance so soil may be the single most important parameter affecting propagation.
Reflector [counterpoise] Another approach is to lay a wire on the ground under the NVIS antenna wire, to act as a reflector. Simply a wire laying on the ground...the full length of the dipole. It can be connected to a ground rod at one end, or not.
Also consider “watering” your earth stake for a noticeable improvement in both receive and transmit signals.
About Frequencies: Convention says you should use 40M during the day and 80M at night. This is not necessarily correct, especially during sunspot minimums.
Check the propagation maps and notice some areas are actually open for 80M but not for 40M in the morning, so don't discount 3.5Mhz up to about noon local time, and we work 7Mhz into the evening and reach required distances.

That's if folks, as far as I'm interested in taking the theory.
Research and modelling is available on the web will prove that an NVIS Dipole at less than 3 metres is pretty impressive with constant omnidirectional coverage.
As I stated in the introduction, emergency communications was my goal; a solo operator using a 'quick and easy' to erect antenna.
This one keeps the antenna height manageable and utilises the NVIS science to put out effective signals.
It ticked all the boxes for me.