Re: 40M band closed ?

I doubt it, but I wonder ..

shan_rish@xxxxxxxxx wrote:
As for Doug's question, i am able to receive the broadcast stations
very strongly, but all through the band there is QRN which sounds likes
continuous shot noise. If i disconnect my antenna, then there is
absolutely no noise except the slight hiss from the preamp. I am not
able to come to any conclusion about what is the reason for the noise
which is very high, even broadcast stations are difficult to receive
sometimes.

Regards
Shan

There is a technique in the radio world called Over The Horizon radar or, for short, OTH. It is a pulse type radar operation which is used in the high frequency spectrum, often in the 40 meter area of the spectrum. The transmitter pulses are fired off at a repetition rate which, when heard by a 'normal' receiver, like shots being fired over and over. In common use of this system, these normally very high power pulses, at least in the uses in older times like this, like a woodpecker bird battering at a tree. Hence the old name "Woodpecker" for it. A number of countries have used the technique which fires these high power bursts of radio energy over the horizon and then listens for the return echo from far beyond the transmitter/receiver site(s).


The intent has been to track ships at sea, other moving objects, sea wave and other weather issues .. from far beyond what normal direct line-of-site radar service would be able to deliver from beyond what the curvature of the earth would allow, irrespective of the path length of the 'normal' radar beam.


Amateur radio operators and SWL's have long had very unpleasant feelings about this use of radio spectrum. If you think about it, since this service depends on the radio pulses being able to use 'skip' techniques, and being reflected off the ionosphere to reach and return to the agency which uses this, the pulse-type transmissions are 'competitive' for whatever frequency is in use. For their purposes, these OTH services are not going to get back much energy from the 'echo' from hundreds or thousands of miles away. Surely far less signal than you get from commercial broadcast SW transmitter stations and for SURE less from distant ham radio stations. As well, since the return signal is so naturally weak for this OTH operation, the use of OTH is usually focused on using WHATEVER frequency area which is. at any given time, the optimum for propogation over the desired path to and from the target of interest to the OTH operations. And high power, yes VERY high power transmissions, are used for this plus HUGH antenna arrays as felt are needed by those who use OTH.


As well, since there is no control over virtually any other radio service in use on any frequency in the general range for the optimum needed for these terribly weak echoes produced from the normally huge transmitter pulses, the operator of the OTH operation has, at least in the past, used multiple frequencies or 'nodes' in any given frequency range. That to optimize the possibility of getting back an echo. And as well, the technique has used a sliding transmission frequency technique which continuously wiggles the channel all over the optimum path channel frequency area.


Much to the disgust of any 'normal' user of the band in question, such as the ham operators, SWL's and so forth. Since you are Internet oriented enough to post here for help or education, you ought to be able to use a search engine such as Google or whatever to collect more information on OTH. If you have an operating system and client software for audio, I think you can even find a few audio samples of the OTH operations from at least a few of the very disgusted hams who have been plagued with this.


It used to be a MAJOR problem for the 40 and 20 meter ham crew back in the 70's or so. And lately, it has again apparently become a tool of choice for gathering whatever military or weather use is thought 'proper' by whomever has started using this radio technique again. I've read that the transmitter site or sites is or are now focused in the Indian and Pacific ocean area, for those which are creating major interference to hams and SWL's wanting to use 40, 30 and 20 meter services.


If this isn't a 24 hour a day issue with you, look all this up. If it is an all day long every day issue, then if you really want to track it down, you'll need to learn about directional receiving antennas such as loops. You'll need to be able to tool up to take you receiver and the directional receiving antenna out in the area around you. You'll need to be able to track the direction of the interference down to a general area. Then there are other tools which can be used to narrow the search for the interference to the specific cause, depending on what is causing it.


W5WQN

--


--> Sleep well; OS2's still awake! ;)

Mike Luther
.

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