[Fwd: Receiving on 136 KHz]

André Kesteloot akestelo@bellatlantic.net
Mon, 27 Apr 1998 09:54:35 -0400

Peter Dodd wrote:

> Hello Andre
> > I am sending a quick reply to your note with copies to
> > - Dave Borden K8MMO
> > - Bob Bruhns WA3WDR
> > - Elton Sanders WB5MMB
> > - Glenn KA0ESA and Terry WB4JFI also own DSP boards, but their business schedules
> > are rather hectic for the moment.
> > These are five members of the AMRAD core group here in the Northern Virginia
> > region. All these good people are equipped with suitable receivers and DSP boards.
> Thanks for the QSP. Could you also QSP this e-mail?
> > Yes we are definitely interested in monitoring your transmissions and eventually
> > trying for a transatlantic QSO.
> > Do not forget that we do not, as yet, have permission to  _transmit_  on 136.  We
> > can of course receive anything we want.
> Yes, this is understood. I have had several cross-band QSO with
> European stations who have not yet got 136kHz authorization.
> > Paul Rinaldo W4RI is busy applying for a special authorization for us to be
> > allowed to transmit on 136 KHZ.  That is still possibly a couple of months away.
> > We will try for 1 watt ERP, like Europe.
> > Question: is the BBC 198 KHz Tx on the air 24 hours a day?
> yes
> > Is this a 500 KWatt tx?
> yes
> I understand that the BBC can often be received on your side of the
> Atlantic even with a broadcast receiver. The signal level from this
> station needs to be monitored for the best propagation times.
> There is also a powerful LORAN C station in northern France on 100kHz
> (whose spuri cause me lots of receive problems on 136kHz). I
> understand that the USA does not use this system any more.
> As regards LF receiving setups the best advice comes from "Optimizing
> receive systems for
> 73kHz and 136kHz" by John Moore G4GVC, in Chapter 3 (LOW FREQUENCY
> RECEIVERS) of the LF Experimenters Source Book. Here is an excerpt
> The following notes are a summary of my personal, empirical
> observations over several years experimenting with receiving systems
> at LF. They are not intended to discourage those who want to
> experiment with simple receive systems, which can produce perfectly
> acceptable results, but are intended to encourage those interested in
> fully exploring these frequencies to develop more effective receive set-ups.
> "Optimum sensitivity
> The complete receive system, i.e. the receiver and antenna, must
> provide enough sensitivity to receive the very weak amateur signals
> on these bands. Unless the station is reasonably local, say less than
> 50km away, the signals will be much weaker than the other signals
> audible on the band such as navigation beacons, data transmissions
> and time signals. Many of these are very strong indeed, so the
> ability to receive non-amateur signals well does NOT in itself
> indicate adequate sensitivity. Even during the daytime, on clear
> frequencies loud static crashes should be audible which affect the
> receiver AGC and frequently cause the S-meter to 'kick' - similar to
> listening to 160m on a humid summer evening. If these crashes are not
> plainly audible, the receive system is not sensitive enough and steps
> should be taken to improve the antenna and/or overall receiver gain.
> An optimum sensitivity must to be achieved however, as too much
> front-end gain can be disastrous, causing very strong non-amateur
> signals to overload the receiver with resultant crossmod/intermod
> problems, as described below.
> "Maximum signal to noise ratio
> At these frequencies, electrical noise sources, both in the shack and
> close to the antenna, can totally swamp a wanted signal. Typical
> noise sources are computers, televisions, switching PSUs, fluorescent
> and 'long-life' lighting, dimmer switches, multiplexed displays (e.g.
> the one in the rig!!), alarms etc. - all the things you are likely to
> have around the home. The obvious precautions to be taken here are
> minimizing the number of possible interference sources left running
> whilst listening, siting the antenna as far away from noise sources
> as possible, making careful use of a directional antenna such as a
> loop to null out noise and interference, and using screened or
> balanced circuitry where appropriate.
> "Too high receiver sensitivity, poor receiver gain distribution
> and/or receiver non-linearity can have disastrous results at these
> frequencies, where very strong adjacent signals and nearby broadcast
> stations are almost certain to cause overload, cross-modulation and
> intermodulation products in a poorly designed receiver. (Remember,
> Long Wave starts less than 15kHz above the 136kHz band!) These
> problems cause effects such as high general background noise,
> broadcast modulation audible on other signals, 'phantom' data signals
> and signals 'keyed' by time pulses - all very common problems in poor
> LF receivers.
> "Except when using a very narrow-band, low-gain loop antenna, it is
> best to avoid front-end preamps. Any noise generated by the preamp
> itself will be amplified by the rest of the receive system, but far
> more important, the preamp or following stages will almost certainly
> be overloaded by strong unwanted signals received on a good external
> antenna, even when filtering is used. The best strategy, whether
> using a converter or general coverage receiver, is to reduce or even
> omit any gain until after the first mixer and concentrate on good
> hi-Q tuned circuits directly following the antenna to give maximum
> attenuation of out-of-band signals. Any lack in overall gain can then
> be made up in later stages where these problems are less troublesome.
> "Choice of the most suitable IF and AF bandwidths is very important.
> Most amateur signals on these bands are either CW or narrow-band
> data, so the use of good CW filters is more or less mandatory (a
> 'standard-width' SSB filter is wider than the whole 136kHz band and
> almost as wide as the 73kHz band!). The IF filter bandwidth normally
> needs to be 500Hz or less, and something around 250Hz is preferable,
> provided it does not ring. Ringing is particularly troublesome on
> these frequencies due to the high impulse static noise, and causes a
> further increase in noise and a reduction in signal readability.
> Receivers with the ability to cascade filters at different IFs and a
> 'slope-tune' control are particularly useful, as this allows the
> overall IF bandwidth to be reduced still further with a minimum of
> ringing. Another handy feature is to be able to swap carrier
> insertion oscillator to the other side of the filter (often called
> "CW reverse" mode); if a signal is suffering adjacent interference,
> swapping to "reverse" mode often shifts the interference outside the
> passband, or at least changes its pitch to something less intrusive.
> Analogue audio filtering may also be used with care (but NEVER
> instead of adequate IF filtering), although this often has a tendency
> to worsen readability when used excessively. DSP audio filters can be
> effective on some signals, but my own limited experience of early
> units was that they didn't always perform well in the presence of
> very impulsive LF noise, which may not conform to the filter's
> noise-elimination algorithm and may also upset the filter's operating
> threshold. No doubt later units are more effective in this respect.
> "High receiver local oscillator noise can cause problems,
> particularly on 73kHz where there are often very strong in-band data
> signals present. The most obvious result of this is the inability to
> hear weak signals close to these strong signals, due to oscillator
> noise sidebands. The use of well-designed crystal oscillators and DDS
> techniques are effective ways to minimize oscillator noise.
> "Antennas
> My main weak-signal receiving antenna for both 73kHz and 136kHz is a
> long end-fed wire. There is a consensus of opinion that large outdoor
> receive antennas on LF are unnecessary as reception is ultimately
> limited by band noise, not signal strength. My own observations
> suggest that whilst this may be the case for reasonably strong
> non-amateur signals, some amateur signals are so weak that a good
> antenna is required to receive them at all, particularly on 73kHz.
> There are certainly a number of amateur signals I can copy on the
> wire but not on the frame-loop, so I feel that a large, quiet,
> outdoor antenna is required when looking for the weakest signals. My
> wire is approx. 60 metres long and 8 metres high, although at these
> frequencies, such dimensions are still tiny and by no means critical.
> The aim should be to get as much wire in the air, in the clear and as
> far away from noise sources as possible. Obviously this will be
> limited by what space is available at each individual location.
> Another theory suggests that vertical antennas work best for the
> surface waves used at LF, but in practice I have found this mostly
> horizontal antenna works superbly for receiving weak amateur signals
> on these bands at ranges varying from tens of kilometres to 1760km,
> whether transmitted from verticals, loops or even ground electrodes!
> I haven't found it necessary to match the antenna to the receiver
> input, but what is very important is to resonate it with a high-Q
> series-tuned circuit. This not only provides a very large increase in
> wanted-signal strength but also strongly attenuates the out of band
> signals which are likely to cause receiver overload problems with a
> good outdoor antenna (see previous section):-
> "I use a different inductor (L) for each band; the one for 73kHz is
> 30mH and the one for 136kHz is 7mH. These values should provide a
> starting point for similar antennas. It is well worth experimenting
> with different types of inductor if you have a junk-box, as I have
> found large variations in received signal strength when using
> different inductors in this circuit. Also, I can't emphasize enough
> the importance of a good ground system when using an antenna of this
> type on these frequencies. Your ability to receive weak signals will
> be limited by the effectiveness of your ground, so put plenty of
> effort into improving that, especially if you live in an area of poor
> ground conductivity.
> "Receivers
> Having tried a number of different receivers over the years, I have
> now settled on the Kenwood TS-850S as my main LF rig. Despite the
> receiver being specified only down to 100kHz, mine works to well
> below 70kHz with excellent performance, and is certainly the best
> off-the shelf receiver I have used on LF. There may be others which
> perform as well or better, but this particular receiver has
> practically all the desirable features mentioned earlier, with
> excellent sensitivity, strong-signal handling and selectivity when
> fitted with a full complement of crystal filters, plus the
> significant advantage of not requiring an external LF converter.
> I have also tried the Yaesu FRG7700 general-coverage receiver on
> these bands, but the sensitivity and selectivity are nowhere near up
> to the required standard, so I have never heard any amateur signal on
> 73kHz and only G3LDO on 136kHz, at very poor strength, when using
> this receiver.
> "Very slow CW
> If the transmitted Morse is slowed down considerably so that each dot
> takes for example 20 seconds or more, the bandwidth of the
> transmitted signal becomes (ideally) very small. Using computer DSP
> techniques it is possible to analyse signals across part of the
> receiver passband at the sub-Hertz level. Noise is (usually) random
> and cancels over time, but a coherent signal will slowly build up a
> trace on the screen , even when no signal can be heard in the
> loudspeaker. Using these "Waterfall" or "Spectrogram" techniques the
> Morse signal is displayed as the sequence of dots and dashes down or
> along the computer screen.
> "A number of systems and/or software are available for very slow CW
> reception. I use the relatively simple and inexpensive design by Andy
> G4JNT, which gives very good results when used with care. The radio
> interface is based on a PIC microcontroller and I use a 486 PC for
> the main computer. Using Andy's WFALL16 program I have obtained
> several excellent traces of the DA0LF signal from Germany on
> 137.100kHz. These systems can also be used to detect and analyse any
> very weak signals buried in the noise, so are ideal for finding new
> stations just setting up on the bands."
> I trust that this info will be of some help to you. I have found the
> Kenwood TS-850 to be the 'hardest' amateur radio receiver I have ever
> used, even outperforming the old Drake R4C with all Sherwood filters
> in looking for weak signals in the presence of strong signals.
> --
> Regards, Peter, G3LDO
> <g3ldo@zetnet.co.uk>