[Lowfer] LF Daytime Propagation (was Morning Lowfers)

[jrusgrove at comcast.net]

John

Thanks for the additional information and data from Laporte ... it certainly supports the WWVB 
signal level plots throughout the year. The WWVB SNR plots clearly show an advantage in winter as 
expected, and somewhat surprisingly a peak in groundwave SNR from Boulder to the west coast (both 
San Diego and Seattle) at around 1800Z to the tune of about +5 dB vs. a few hours earlier or later.

Jay


----- Original Message ----- 
From: "John Andrews" <w1tag at charter.net>
To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands" <lowfer at mailman.qth.net>
Sent: Saturday, January 04, 2014 4:48 PM
Subject: [Lowfer] LF Daytime Propagation (was Morning Lowfers)


> 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
>
>
>
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