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On 22 June 2010 20:04, James Moritz <[email protected]> wrote:
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: [email protected] [mailto:[email protected]] On Behalf Of Tony Baldwin
Sent: 22 June 2010 11:49
To: [email protected]
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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