LF: Re: LF Propagation

["James Moritz" <james.moritz@btopenworld.com>]

Dear Doug, LF Group,

> I
> was under the (admittedly simplistic) understanding that, the lower one
> goes in frequency, the more the signal propagates via groundwave.
In amateur experience, communications via ground wave up to perhaps 2000km 
is feasible, so a much more significant propagation mode than at HF. The 
ground wave signal strength is strongly dependent on ground conductivity, so 
paths including a lot of sea water are particularly favoured.
> I do know
> that almost all military installations transmitting in the VLF/LF bands 
> use
> vertical antennas (very, very large ones with huge capacity hats).
Probably most amateurs are using top-loaded verticals of one sort or another 
for transmit. In some circumstances, large vertical loops may be a better 
option - especially in locations with many trees, where the losses in 
verticals are likely to be higher. Several W & VE stations have successfully 
used loops supported by trees.
> Do
> signals at 136 kHz experience "skip" from ionispheric reflection, similar
> to HF signals?
Reflection takes place at much lower heights in the ionosphere at LF 
frequencies, so more involving D and E layers rather than the F layers 
important at HF. This results in shorter hops for sky-wave, and overlap with 
ground wave coverage. So interaction between multiple modes of propagation, 
with resulting QSB, can be present at any distance greater than a few 100 
kms, with some sort of propagation existing, especially after dark, at 
almost any distance.

Alan, G3NYK has studied LF propagation in depth - see his web pages, 
including at http://www.alan.melia.btinternet.co.uk/hcf2001.htm
Cheers, Jim Moritz
73 de M0BMU