[Lf] Transatlantic Considerations

[Andre' Kesteloot]

WarmSpgs at aol.com wrote:

> 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, tractorb at ihug.co.nz writes:
>
> << Subj:     Re: LF: Transatlantic
>  Date:  00-01-18 21:17:22 EST
>  From:  tractorb at ihug.co.nz (Dave Brown)
>  Sender:    majordom at post.thorcom.com
>  Reply-to:  rsgb_lf_group at blacksheep.org
>  To:    rsgb_lf_group at blacksheep.org
>
>  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
>   >>