RadioBanter - View Single Post

May 3rd 06, 01:53 PM posted to rec.radio.amateur.dx

I doubt it, but I wonder ..

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