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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