[Lowfer] LF Daytime Propagation (was Morning Lowfers)

[John Andrews]

Sorry that I couldn't jump into this earlier. And I apologize for the 
length of this post...

First off, I can second most of what Bob has said about AM broadcast 
band FS measurements. Done my share of tromping through people's back 
yards ("It was a field, two years ago..."), avoiding dogs, wire fences, 
overhead lines, etc.

At these frequencies, winter temperatures definitely produce higher 
readings. That may or may not be a good thing, depending on what you are 
trying to prove. I have never seen a book or paper that properly 
describes the effect. It seems to be a combination of things that all 
work in the same direction. Frozen ground helps, as Bob describes. But 
air temperature is definitely part of it. About 40 years ago, I took 
daily readings at mid-day in the winter on a 1000 kHz station about 25 
miles away. It was amazing to see the lower levels on warm days, when 
there hadn't been time for a change in snow cover or ground frost. 
Perhaps the dielectric properties of the air make a difference at these 
frequencies, even though they are less of a factor in the ground than at HF.

Consulting engineers and now, the FCC, have become less reliant on AM 
broadcast field strength measurement due to all of the variables. 
Re-radiation from nearby wiring, piping and fencing complicate things 
even worse than the temperature. But they are still a good way to prove 
a point, particularly in difficult real-world situations. Enough of the 
broadcast stuff...

Surface wave (= "ground wave") propagation over a perfectly conducting 
flat surface has the E or H field dropping linearly with distance. In 
dB, that's -20 dB for each 10X increase in distance. If you start at 10 
miles from the antenna, take a reading, and go out to 100 miles, the 
reading will be 0.1 of the original (-20 dB). At 1000 miles, it will be 
0.01 (-40 dB), and so on. Can't do any better than that, regardless of 
frequency. On your nice copper flat world, if there are any noise 
sources, they will probably clobber your signal before the noise in your 
receiving setup becomes the principal issue. But either way, there will 
be a practical limit.

All of the "real world" factors such as a spherical world, finite and 
sometimes miserable ground conductivity, etc., all increase that 
attenuation. The ground conductivity issue is frequency-sensitive. 
Hence, as you already know, it takes more power at the top end of the AM 
broadcast band to produce a ground wave signal equal to one at the 
bottom end of the band. Over lousy ground, it takes about 10X more power 
over a bottom-end signal than one in the middle of the band. 
Broadcasters tend to notice the hardware difference between 5 and 50 kW!

BUT, as you go lower in frequency, the differences are less. Consider 
these examples from Laporte, describing the attenuation at 1000 miles vs 
10 miles over a smooth, spherical earth.

Over sea water: -42dB at 50kHz, -46dB at 100kHz, -66dB at 200kHz, -78dB 
at 400 kHz.

Over good soil: -43dB at 50kHz, -48dB at 100kHz, -69dB at 200kHz, -90dB 
at 400kHz.

Over poor soil: -49dB at 50 kHz, -64dB at 100kHz, -113dB at 200kHz, 
-134dB at 400 kHz.

Note that at 50 kHz, we only stray 9dB over lousy ground compared to a 
perfect -40 dB over copper. That's a huge difference from 400 kHz, but 
not such a great difference compared to 100 kHz. So, the best you can 
hope for going down in frequency is to **approach** the magical 
1/distance line.

Of course, by going down to 73 kHz from some higher frequency, you pay 
some penalties in smaller ERP from the same antenna, and generally 
higher noise levels at the receiving end. There's no free lunch, and I 
see no reason to look for anything really dramatic.

So, Jay's observations of a lack of summer/winter variation on WWVB 
appear to be valid.

John, W1TAG