[NLRS] Tropo Maps

[Dr.Gerald Johnson]

I don't want to be taken as totally negative, just that I'm sure there are more 
weather phenomena showing at VHF than at MF to affect propagation. First 
off the scale of a localized event needs to be a wavelength or so to have 
minimal effect and many wavelengths of thickness and breadth are needed 
for a significant signal effect. Many are refraction rather than reflection so 
the thickness has to be significant in wavelengths. 2m vs 160 m meters makes 
me think events that affect 2m are 80 times more likely than at 160m, but 
then 2m Sporadic E events are far less common than 6m and 10m E events. 
Ionization intensity which isn't a NWS measurement is important. Sometimes it 
can be inferred from wind shears, but with the relative sparsity of the 
sounding network vertical events like sprites won't have been detected.

I wish that for severe weather forecasting and for understanding of odd 
none precip radar echoes (like the blobs of summer evenings) that there 
were thousands more soundings and that they were every few minutes. There 
are some UHF vertical sounders that return data continuously, but I've 
neglected to remember all their operational characteristics.

As we've just been discussing on the microwave list, it may well be important 
to look at IR satellite images for clouds of cold crystals (though there's no 
elevation information other than temperature and the satellite temperature 
includes surface through thin clouds or clear skies) and to look at storm tops 
(as K0SM is doing for predicting rain and snow scatter opportunities at 10 
GHz) from radar data.

It appears to me that most of the accepted phenomena at MF come from 
solar (e.g. UV) ionization of layers too high above the ground for balloon 
observations. I can't discount there being pressure variations on those 
heights and ionization intensities. Most often VHF and up requires a great 
deal greater intensity of ionization that is very uncommon so other factors are 
predominant.

If we did know all there was to know about propagation and the effects of 
solar output and atmospheric weather, we could always know whether it 
was possible or impossible to work a particular path on a given frequency. 
The fascination of amateur radio is that the forecasts may show RELIABLE 
paths, but we only need sporadic paths to make a contact that may never 
be repeated.

At the most recent Central States VHF Conference in Cedar Rapids, a Collins 
Radio Systems designer (not a ham) pontificated that going for 0.3 dB noise 
figures at VHF was a waste of effort. I wanted to respond that as hams we 
weren't going for the 99.99999% reliable path, but we were interested in the 
.001% reliable path that could only occur when noise and propagation both 
having the characteristics of (time variable) random noise worked together 
to provide a short term path and we sure don't want receiver sensitivity to 
be the limit on those occasions where the atmospheric noise drops way 
below the average noise level in our narrow band receivers. Where he 
wanted a signal at the receiver antenna 10 dB above the peak noise, we 
can be elated with a signal 20 dB below the average noise level.

To me, it sounds like path skewing, spotlighting, path nulls, terminator effects, 
and reverberations are characteristics of ducts created by solar ionization or 
varying air densities or humidities. Long ago, Nicoli Tesla propose that there 
was a resonant frequency of the atmosphere that he wanted to excite. He 
also wanted expected the atmosphere at a fairly low altitude to be far more 
easily ionized by high voltage and so could be used as the conductor to 
supply electricity to all users who could fly a kite. Power companies weren't 
exactly ecstatic about the prospect of customers getting power without a 
meter to help pay for its generation.

Don't discount the possibilities that a duct might be from ionization some 
places, from temperature gradients in other places and from humidity 
gradients in other places and a random collection of all three (and probably 
a couple I've not mentioned) are required to make a particular path.

I've not studied it but it seems like the same ionized particles that create 
aurora can accumulate in the atmosphere to make charged and refractive 
layers, most often thin for VHF propagation, but one can't rule out MF 
propagation either. A lot depends on the ionization intensity and weather 
balloon soundings won't show that and its unlikely the ionization will cause 
any weather shift, but its worth looking.

Then for unexpected directions at MF, what about polarization shifts in 
propagation? Many long wire antennas have significant cross and angle 
polarized lobes well off the main axis from the preferred signal polarization 
that can confuse the observer of the apparent incoming direction.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.