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Re: LF: Re: LF Propagation thoughts (and antenna, too)

To: [email protected]
Subject: Re: LF: Re: LF Propagation thoughts (and antenna, too)
From: [email protected]
Date: Mon, 7 Feb 2000 13:15:42 EST
Reply-to: [email protected]
Sender: <[email protected]>
Rik writes:
Regarding the groundresistance the rules was : the lesser the better. No
only for the sake of antenna effeciency but also to avoid radiation at
higher angles. This 'high angle radiation' was for the commercials, who
were only interested in surface wave propagation (stable 24h/day, 365
days/year) not only pure waste but also a source of possible interference
(via ionospheric wave).

But if we want to make 'DX-QSOs' with 1 Watt ERP we defenitely need
ionospheric wave propagation, so producing some higher angle radiation
might be usefull.
Yes and no. Commercial and military LF stations do not depend on surface
wave alone, especially not at transatlantic distances. One cannot achieve
globe-spanning distances with surface waves above roughly 100 kHz; and even
some ways down below 50 or 60 kHz, conventional ionospheric refraction still
plays a significant role. It is not until one gets down to VLF that the
propagation model changes dramatically.
The commercial guys do suppress high angle radiation to whatever extent
possible, but that's not because they are only interested in groundwave.
High angle radiation causes problems with the more desirable low angle
skywave at intermediate distances, and wastes power that would be more
beneficial for long-distance coverage if it could be radiated at the lower
angles.
If one were able to radiate most of one's power at, say, less than 5 degrees
above the horizon, the signal would stand a far better chance of making it
across the ocean than if the bulk of the power were 20 or 30 degrees above.
That's where the region of the ionosphere is that we want to excite. (I keep
saying "if," though, because in practice, any short vertical already has
plenty of high angle radiation well above 30 degrees, whether we want it or
not.)
Further increasing high angle radiation with horizontal runs of wire may have
interesting propagation effects, as described in another post, but this is
over medium distances. We must take into account how much "D" we are really
talking about when we speak of transatlantic DX.
I'm not speaking of this from a theoretical standpoint so far as amateur
efforts are concerned, either. Nor do I mean, in what follows, to detract
*in any way* from the excellent work being done by the AMRAD group. They
have opportunities to experiment with power levels and antenna lengths that
we 1 watt / 15 m antenna LowFERs can only dream of for now, and I have no
doubt that much valuable information will result.
I merely point out that the recent successful copy of the AMRAD beacon in
Ontario, despite the long horizontal antenna and the power levels involved,
does not quite yet match some of the DX achieved by stations working within
the 1 w / 15 m limits.
Utilizing much patience and only moderately slow digital techniques, one-way
copy has been achieved from California to Minnesota, Georgia to Minnesota,
Texas to Quebec, and more. (I nearly hesitate to mention the old CW LowFER
record from California to Hawaii, as it required a once-in-a-lifetime
combination of good propagation, ideally situated stations, and remarkably
low noise levels. Yet, there it was.) Apart from the latter example, the
stations were sending Coherent CW or BPSK. All, without exception, employed
vertical antennas, top loaded to the maximum extent allow by regulations,
working over the best ground systems that their operators were able to
achieve!
At no time would I suggest that anyone is wasting their time pursuing new
ideas. However, I don't think it would be productive to arbitrarily set
aside the commercial operators' examples as being irrelevant, either.
Especially, in the face of such strong experimental evidence that what's
sauce for the commercial goose may also be sauce for the amateur gander.
73,
John  KD4IDY


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