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LF: Re: 137 Revival

To: <rsgb_lf_group@blacksheep.org>
Subject: LF: Re: 137 Revival
From: "James Moritz" <james.moritz@btopenworld.com>
Date: Tue, 22 Jun 2010 20:04:29 +0100
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Reply-to: rsgb_lf_group@blacksheep.org
Sender: owner-rsgb_lf_group@blacksheep.org
Dear Tony, LF Group,

What you need for a 136kHz transmitter depends a lot on the antenna it is to be used with, and what you want to do with it.
Antenna efficiency is usually low on 136k - a small fraction of a percent is 
the norm - and very variable. If you have access to 100 foot high antenna 
masts, then you will be able to achieve the 1W ERP limit with probably less 
than 100W of TX power. But if you are restricted to "back garden" sized 
antennas, you will probably need a kilowatt or more to reach this radiated 
power level. If you are restricted to lower powers and small antennas, your 
ERP will be well below 1W, although contacts over quite decent distances can 
be had with tens of milliwatts ERP. Much lower powers are viable at 
relatively short ranges within ground-wave propagation distances. But due to 
the low numbers of active stations on the band, this severely limits the 
number of contacts that will be possible, so 136k is not really a band for 
QRP transmitters, unless you have a huge antenna available.
For CW operation, transmitter design can be pretty simple - a reasonably 
stable frequency source, a class D PA stage, and some way of keying it on 
and off. For extreme-narrow-band QRSS use, improved frequency stability is 
needed. Usually this means some sort of synthesiser, although simpler means, 
such as ceramic resonator based VFOs, are quite viable unless extreme 
stability is required. If you are interested in data or other "visual" 
modes, it gets a bit more complex, requiring either a home brew modulator 
operating at 136k, or a transverter to shift the frequency of an existing 
rig. Also, amplitude modulation of the signal is needed for some modes, 
which requires either a linear PA, or techniques such as "EER".
Go to G3YXM's pages at http://www.wireless.org.uk/, and look at the 
"Circuits", "Features", and "Build a TX" pages for some ideas. The RSGB 
books "LF today", and recent editions of the "Radio Communication Handbook" 
contain several transmitter designs. I have written up a 200W multi-mode 
transverter design using the EER technique at 
http://uk500khz.googlegroups.com/web/EER_Transverter_v3.pdf . There are some 
designs for 500kHz transmitters also at 
http://groups.google.com/group/uk500khz on the "files" page, which could be 
easily adapted to 136kHz with appropriate component changes. Google-ing for 
"136kHz transmitter" will get you quite a few results.
Shoving a lot of power into an antenna that is really too small for the job 
is never going to be very reliable or foolproof ... but it is sometimes 
quite exciting ;-). The class D designs have become popular because they use 
simple circuits and cheap, readily available, switching MOSFETs to achieve a 
large output power, but one should be aware that most class D circuits will 
try to deliver very high output currents into low impedance loads, which can 
lead to damage when the antenna is mis-tuned, as it inevitably sometimes is. 
With electrically small elements and large loading coils, antennas are 
unavoidably high Q with critical tuning which is easily disturbed. Current 
limiting of the DC supply or RF output is very desireable. Class D designs 
are very efficient and cool-running during normal operation, but can 
dissipate a lot of power when badly mis-matched, so conservative design is 
needed for reliability.
Good luck on 136kHz, hope to see you on the band this winter!

Cheers, Jim Moritz
73 de M0BMU



From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Tony Baldwin
   Sent: 22 June 2010 11:49
   To: rsgb_lf_group@blacksheep.org
   Subject: LF: Re: 137 Revival



   I fully intend to be QRV on 137 by the winter.

On that note, can anyone recommend a reliable foolproof transmitter that uses easily available components ?


   Tony, EI8JK.





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