Thanks, Dave. I think we agree on all but a few points, which may have to do
with perspective. After commenting on that, I have an announcement that may
hearten those preparing to undertake this bold venture. But first--
You make a very important point about the ground system. And, it is
certainly true that an emphasis on antenna height will not be the conclusive
factor in transoceanic success. But wherever a vertical antenna is employed
at the transmit end, height will surely help!
As I mentioned in my earlier post, for military and commercial antennas at MF
and LF, we attempt to concentrate as much energy as possible in the first few
degrees above ground. Higher angle radiation contributes to skip zones at
moderate ranges, and is largely wasted for long-distance skywave because it
suffers so much attenuation with each hop.
Perhaps I overemphasized takeoff angle, because as we both agree, amateurs
won't have opportunities to utilize vertical masts of a sufficient fraction
of a wavelength to suppress high-angle radiation. The point I was trying to
make is that, while we may not be able to suppress high angle radiation to
the extent commercial users do, we still need to maximize radiation at lower
angles.
What's good for the groundwave is (generally) good for the low-angle skywave.
And, within our limitations, anything one can do to increase both the real
and effective height of a vertical antenna will help those signals.
You ask, "Increasing the height will reduce the takeoff angle as you
described, but is
this necessarily a good thing for amateur LF DX work?" Yes, although we'll
never be able to do it well enough to affect the outcome based on that factor
alone.
The Galveston NDB is a good example. I haven't seen the facility myself, but
I've talked with people who have seen it. If I remember correctly, it is a
bit taller than average, but is very heavily top loaded as you suggest, and
with a serious ground system in highly conductive soil. I'm sure it does
have quite a bit of high-angle radiation, but the key to its success is the
powerful groundwave, and the skywave component immediately above ground.
At the frequency of GLS, the groundwave itself is effectively useless by
150-200 miles, and contributes nothing further to DX. Skywaves 20 to 30
degrees above the horizon do, in fact, interfere with the groundwave at that
distance, but provide nighttime coverage subject to considerable QSB between
that distance and about 1000 miles. I live within that zone, and as a
result, I'd have more trouble copying GLS consistently than you might have.
:-)
At even 20 degrees departure, it could take 60 to 100 hops to reach ZL; yet a
dozen hops is generally enough to attenuate their signal too much to be
useful. For anything remotely approaching regular reception, the hops cannot
be less than about 1000 miles each. This indicates the part of their signal
being received in ZL is launched at angles of less than 5 degrees.
For ham purposes, we can follow the Galveston model and not worry excessively
about suppressing high-angle radiation, but still must concentrate on
maximizing groundwave and low-angle skywave. To do this with a vertical,
both height and top loading (to improve current distribution and, hence,
effective height) are essential tools. A taller vertical has higher
radiation resistance than a shorter one, reducing the amount of power lost in
any real-world ground system.
Both real and effective height also reduce the losses in surrounding
vegetation and other objects that seem to plague many amateurs on LF.
As with all engineering problems, practical compromises have to be made. It
won't be possible for everyone making the attempt to have the same antenna
efficiency. If some have to obtain 1 watt out by pumping kilowatts into a CB
whip, well, so be it. <humorous exagerration> But wherever one is using a
vertical and it IS feasible to have a bit more height, then the extra effort
could pay off.
Note also that I am not advocating verticals for transoceanic reception.
Wave antennas or loops are clearly better suited. The great Atlantic
longwave stations of the past used verticals of one type or another for
transmission (the chief differences being in how they were top-loaded and
tuned) but employed Beverage antennas at remotely located sites for reception.
As amateurs, we may not be able to appreciably separate our transmit and
receive antennas, but it will behoove us not to fit a square peg into a round
hole. Given the daunting demands of this particular effort, we can and
should use the most appropriate antenna we can install for each task.
Now, the encouraging note I mentioned. Perhaps I missed it during the
holidays, but I don't recall seeing mention on this reflector of an item that
made big news in BPSK circles here: On December 27th, VE2IQ in Quebec
received LowFER beacon TEXAS, just northeast of Dallas, at a distance of 2438
Km, or 1515 statute miles. Now, granted that the message at the time simply
consisted of the characters TX followed by a space, and also that there were
relatively few hits out of ten hours of recording, it's still pretty
remarkable.
WD5CVG has spent quite a bit of time optimizing the TEXAS antenna, ground
system, and tuning network. Even so, with the U.S. limits of a 15 meter
antenna and one watt d.c. input on 1750m, this only amounts to 5 milliwatts
EIRP.
Half the width of the Atlantic with 5 mw--almost makes a whole watt seem
extravagant, doesn't it? :-)
73,
John
In a message dated 00-01-18 21:17:22 EST, [email protected] writes:
<< Subj: Re: LF: Transatlantic
Date: 00-01-18 21:17:22 EST
From: [email protected] (Dave Brown)
Sender: [email protected]
Reply-to: [email protected]
To: [email protected]
Tnx to John (KD4IDY) for the useful comment.
My point was that given the normal constraints on an amateur antenna LF
setup, bigger ain't necessarily better for verticals, when gunning for real
DX (2000 km plus) Even a 'large' amateur antenna will still be a small
fraction of a wavelength at LF so will have significant high angle takeoff
as well as it's more immediately obvious (and useful) ground wave component.
Increasing the height will reduce the takeoff angle as you described, but is
this necessarily a good thing for amateur LF DX work?
Crossing the pond with amateur antennas will require some thinking outside
the square and it may be that trying to minimise the number of hops is not
the best way to go? Sounds stupid? Yes !!
But consider this.. what sort of NDB antenna is in use at Galveston, Texas?
The GLS signal is heard quite regularly down here in ZL. OK, they run a bit
more transmitter power (2 kW? I think) than the usual NDB but not
significantly more than some LF amateur stations and I doubt the tx antenna
configuration is much different to the usual top loaded vertical NDB setup.
It will have significant high angle radiation which would normally be
consided undesirable but I suspect is the main contributor to the signal we
hear down here.
A typical larger amateur vertical antenna will have to have more than just
size to compete when it come to real LF DX, (2000 km plus). High efficiency
is paramount and we all know the biggest loss factor by far in any vertical
antenna system used for LF is the ground loss. Big towers with poor or even
mediocre grounds will fail miserably. I can speak with experience on this
having used ex BC band tx masts in both situations for LF experimental
transmissions. The one with the far better ground system gave an impressive
improvment (2-3 'S' points at 2000 km) over that with the poor ground. Got
similar reports from locals( 300km) as well.
Perhaps the most interesting thing to consider is the fact that for the last
8 or 9 years, the biggest LF signal out of ZL has consistently been that of
Bruce, ZL1WB. His antenna is quite extensive, but it is NOT vertical! He
has 3500 feet of wire strung across a gully in a roughly north-south
direction and with a 40 watt transmitter gets excellent night-time reception
reports in Eastern VK as well as all over ZL. He almost certainly will
have a good signal in many areas of the Pacific but there are no listeners
there to confirm.(Yet! Maybe we need a few Dxpeditions- any volunteers for a
Pacific Island LF listening 'holiday'!!)
Another aspect that has been noted already is the problem of LF reception
with large antennas. Reception requires a useable sig/noise ratio, and large
antennas usually don't perform that well at LF in this regard because of all
the QRM and QRN that they tend to pick up. Working VK a year or two ago we
had to forget all about reception on the big vertical. We could hear the
signals but QRN/QRM made it virtually impossible to copy. Static crashes
and electric fence interference were literally pegging the 'S' meter.
Switching to a smaller 'random wire' antenna gave R5 sigs with what appeared
on an aural basis to be perhaps only an 'S' point drop in signal level but
very little QRN. End result- probably a 30 dB improvment in sig/noise. You
can probably only begin to appreciate what this means when you have actually
tried using a really big antenna for LF reception.
So while 'bigger is better' may be the catchcry for some I'll be surprised
if they are the ones who actually make it across the pond first. My money
will be on those who have efficient (not necessarily big) antennas, located
close to or on, the respective coasts, that can as KD4IDY sez, really run
the full gallon on transmit, are good operators, and above all, are
prepared to keep at it!
73
Dave
ZL3FJ
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