LF noise canellation

André Kesteloot akestelo@bellatlantic.net
Sun, 02 Aug 1998 12:26:25 -0400


> Peter,
>  As you already know, Lech, G3KAU and I also suffer serious QRN from the
> pulse spreading of Loran have also been looking at ways to combat it . I
> think that apart from a few kilos of "Plastique in the right place ! ! ,
> there are three posible ways to minimise or eliminate the problem. The
> first is by antenna polar diagram protection,  eg. by the use of a loop
> which you have already been investigating. The second is by synchronous
> pulse blanking [ref. the E-mail today from Paul OH3LWR] and thirdly by the
> use of signal cancelling techniques.
>  I have not been very sucessful in the use of the loop ( 1 metre per side)
> so far since although it will null out the Loran, it does not appear to be
> as sensitive on DX stations as the main Tx antenna with 40ft vertical and a
> total of 400 ft of top loading with 3 parallel wires.
>  Like you, I have also been looking at the effectiveness of the in-built
> noise blankers in my two HF receivers. My old FT902 has a true  "Lamb"
> noise blanker with a parallel wide band IF noise channel and a FET
> attenuator in the main IFahead of the narrow band IF filters to avoid pulse
> stretching. It certainly is very effective when used as a tuneable IF in
> virtually totally eliminating radar pulse interference on 23cm at my QTH
> from the Pease Pottage CAA high power L-Band air traffic  which is almost
> line of sight with my 23cm antenna but it has very little effect on the
> Loran racket. I think that due to the narrow bandwidth of the antenna and
> RX input ccts. the pulse energy from the primary pulse source reaching the
> wide band noise IF is  not sufficient to trigger the threshold circuit in
> the noise blanker. As Paul OH3LWR says, what is needed is a separate
> receiver and antenna input circuit and a wide band detector to trigger a
> noise blanker switch in the main RX channel. I think that there are
> certainly  possibilities in this approach. Some years ago the Crawley Club
> experimented with a similar system designed by G3LNM to suppress severe
> interference on the 70cm band from the French Syledis position fixing
> system at our VHF NFD site at Folkestone on the south coast which looked
> very promising but in the event, it  was never actually used because  the
> offending Syledis station conveniently closed down.
>  I think however that it would be useful to aquire some information on the
> characteristics of the Loran pulse shape etc. to follow this method up as a
> possible solution.
> I have just started to investigate the use of RF signal cancellation
> techniques using a separate antenna in conjunction with David, G0MRF and
> Lech, G3KAU. I have aquired a MFJ 1025 noise canceller unit which works
> quite nicely on the lower HF bands and will null out a unwanted interfering
> signal or noise source from S9 +20dB almost down to the background noise
> level providing the interferer is reasonably stable and not subject to
> rapid phase or amplitude fluctuations due for example from polararisation
> shifts.  However, contrary to what some of the MFJ advertising blurb
> suggests,  the MFJ1025 does not cover down to VLF but has a lower limit of
> 1.5Mhz.  David, G0MRF has designed a twin-channel upconverter from VLF up
> to a 10Mhz IF to feed the MFJ unit and we plan to test it shortly. To this
> end, I have recently put up a G5RV antenna at the home QTH as a second
> antenna up at about 35 ft.running mainly at right-angles to the main VLF
> antenna and which I can load up as top-loaded vertical with feeders
> strapped for VLF or for cross-band working on 80 or 160M. This second
> antenna now gives a similar (or even slightly greater) signal from the
> offending Loran station than the main antenna and thus hopefully will
> enable the two Loran signals to be subtracted and hence cancelled.  Lech
> and I have also today been out in the thunder showers (and got wet !)
> increasing the height of the vertical element of the Crawley club main wire
> antenna to about 60ft., and in addition, installed halliards on other trees
> to enable us in the near future to pull up a second wire antenna  up at
> least 50ft at right angles to the main 290ft. VLF antenna to provide
> simultaneous coverage of 80 or 160m for cross-band operation and also allow
> some experiments on cancellation of Loran QRN using the Club antenna
> system..
>       I will let you know how we get on.       73   de  Derek,   G3GRO