On Jan 7, 2012 8:13 AM, "James Moritz" <[email protected]> wrote:

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