Dear Peter, LF Group,
In response to your questions...
What is the longest sec/dot size that is used on amateur LF ?
I noted that Rik's QRS program offers up to 60 sec per dot.
Are there programs with much longer sec/dot times out there ?
The longest dot lengths regularly used are 120s - much longer than that,
and DX propagation "lifts" do not last long enough to transmit a useful
number of characters - also the symbols get broken up by QSB. The
software could utilise much longer dot periods, but it does not seem
practically useful to do so.
Do the various cheap car audio amps ( OP up to 600 W rms ) do a
reasonable
job on 137 Khz ( after possibly removing audio filters etc. ) or is
the
frequency roll-off disqualifying ?
Audio PA modules can be made to work on 136k, but generally need
modification to achieve anywhere near full output power. The heat sinks
are usually a bit too small for continuous key-down output. By the time
you have devised and tested suitable mods, keying circuits, PSUs,
matching transformers, filters etc, the amplifier itself is only a small
part of the construction - so it is probably just as easy to build from
scratch.
If disqualifying, does someone have a suggestion c.q. reference for me
to
build a Tx ?
In the junkbox I have lots of Fairchild Power MOSFETS 75345P 55V 75 A (
ex
various 24>220 V AC ships power sinus converters ) mounted on heavy alu
cooling blocks.
So, if I have to build the Tx PA myself, I prefer to use these FETS.
Has someone a suggestion for a possibly suitable circuit diagram ?
For a start, see:
http://www.wireless.org.uk/136rig.htm
http://www.g0mrf.freeserve.co.uk/300w.htm
With an aerial effenciency that low, I guess the heat dissipation in
the >PA
OP stage will be tremendous.
Where is most of the heat dissipated ?
Though I don't know as yet what outputstages are used on LF, I guess
this
will be in the tank/pi coil (?) and therefore I would welcome
references to
information as to minimizing dissipation loss in coils ( or wherever
else
the dissipation losses appear ).
If you use one of the class D or other switching-mode TX designs,
efficiency is often better than 90%, so the TX does not need to get very
hot. All the RF power goes out of the TX output socket, provided the
antenna is properly matched. A significant amount is dissipated in the
matching network - see Rik's remarks - so a big loading coil with high Q
(or at least big enough to dissipate the power without melting!) is
needed. Most of the power actually reaching the antenna is probably
dissipated in the ground (or in the sea), so we don't have to worry
about it too much!
If advantageously, onboard I could build a very big multiturn loop
aerial.
Does big multiturn LF loop aerials have an advantage over straight wire
aerials ?
Loops have their adherents, but I expect the advantage for a ship
antenna would be with top-loaded verticals. For a lot of antenna info,
see Rik's pages at:
http://www.qsl.net/on7yd/136ant.htm
Is the maximum E(I?)RP still restricted to 1 W ?
Is this restriction applicable to /MM offshore as well ( say midway on
the
North Sea between G and PA )?
ERP (in the UK it is ERP with respect to a dipole) is still 1W max - I
suspect it would still be the same for /MM. But usually it is quite
difficult to get even 1W ERP. With 1.2kW and 40m long, 10m high inv L, I
get around 0.4W ERP - to get 1W, I have to increase the height of the
antenna with a temporary extra mast. But things might be easier on the
high seas...
Does seagain ( if any at all ? ) and etc... add substantially to LF
performance ?
I think the concept of "sea gain" at VHF really only applies when the
separation between the antenna and the sea is large compared to a
wavelength. At LF, your ship would need a very big mast for this to be
true! But having sea water for an antenna ground plane should
substantially reduce antenna losses, and also propagation losses, so in
that sense, there could be a large "sea gain" at LF compared to the same
antenna used on land.
Cheers, Jim Moritz
73 de M0BMU
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