SOME THOUGHTS ABOUT NVIS
The pronunciation of the NVIS initialism as “Enn-Viss” is probably the most common, or else spelled out: “Enn-Vee-Aye-Ess” (N.V.I..S)
I’ve also heard ops say: “Knee -Vuss” and “Nev-Iss”.
Give it some time, someone will call it Knee Vice 🙂
NVIS IS ABOUT FREQUENCY CHOICE
NVIS: Frequency choice is way way more important than antenna height.
Often, just moving 250 kHz can make a huge difference in NVIS signal.
Commercial and Governmental stations that intentionally utilize NVIS may have a variety of channels to choose from throughout the 2 MHz~13 MHz range.
For hams to take advantage of the NVIS at this point in the solar cycle, operators need frequency agile capability mainly on the following bands:
160 metres 1.8~2 MHz (NVIS Top Band late night when 80m doesn’t work)
80 metres 3.5~3.8 MHz (NVIS evening and some daytime)
75 metres 3.8~4.0 MHz (NVIS evening and daytime)
60 metres 5.3 MHz (Primary NVIS mid-latitude daytime on the Rock Band)
40 metres 7.0~7.3 Mhz (NVIS mid-latitude daytime)
30 metres 10.1 MHz. (NVIS daytime mainly in equatorial latitudes)
EVOLVING NVIS FREQUENCIES AND THE SOLAR CYCLE
Recently, the only daytime NVIS ham bands here in California have been 60 metres and 75 metres… either one or the other… rarely both at the same time.
IONOSONDES AND IONOGRAMS
Ionosondes capture the critical frequency of the F2 layer of the ionosphere (foF2).
For all practical purposes, we consider the foF2 as “the maximum possible NVIS frequency”.
The foF2 may be different at another location. Look for the foF2 is the ionosonde that is near your QTH.
A dipole at 1/10 wavelength high is a popular misconception and mis-reading of NVIS recommendations for optimum antenna height.
For RECEIVE NVIS, 1/10 wavelength (0.1 wavelength) is the height for optimum SNR (Signal-to-Noise-Ratio) , in a perfect ground site.
For TRANSMIT NVIS, 0.2 wavelength is approximately the optimum height.
There is only a 1dB NVIS advantage at 1/10 wavelength for Receive (compared to 0.2 wavelength).
But, there is a 3dB NVIS advantage for 0.2 wavelength for Transmit.
What this all boils down to, is that 0.2 wavelength is probably the best overall height for an NVIS dipole, above typical (farmland) damp soil. Over other types of ground, it varies quite a lot.
Generally, the lower the dipole, the more of your transmit power (gain) gets absorbed by the ground soil.
But, with HF receive, the SNR is often more important than gain or loss.
Then, there’s the modern reality of SNR… the RFI received due to nearby RF noise sources.
Whatever height for the receive antenna which achieves the lowest receive RFI, is probably now the best NVIS receive height 🙂
Unless your QTH is totally noise-free, you may need to throw out the NVIS myths about low dipoles being better.
Also, a little-known fact is that a transmitted signal from a dipole, when it goes thru an NVIS path, gets converted to approximately circular polarization by the ionosphere 🙂
See antenna height NVIS chart attached. source: B. Witvliet, 2015
In the chart, “hRX” is a good Receive height, and “hTX” is a good Transmit height.
NVIS TECHNIQUES VS HAM TECHNIQUES
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.
LOCAL HF HAM NETS AND NVIS
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. Looking at the ionogram for this area (attached), it is showing the foF2 optimum NVIS frequency is 4.9 MHz.
There’s no NVIS on 40 meters now, regardless of what type of antenna you have.
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.
NVIS isn’t an antenna.
73s , Bonnie Crystal KQ6XA