This is of course
pure
nonsense, since these audio feedback loops are simply caused by keyed
up HF-transmitters picking up audio from separate HF-receivers
or receiver control
units that are located in the same radio room and tuned to the frequency
of the radio transmitter. Keep in mind that this effect will only appear
on circuits that
have separate TX and RX units.
The following screenshot will illustrate how this effect is caused.
In this example, a remote station sends
continuous feedback loops on 6940 kHz USB. This is caused by the fact that
the transmitter is tuned to 6940 kHz USB
and a separate receiver, colocated
with this transmitter, is tuned to 6940 kHz USB as well, causing the transmitter
to pick up audio from the receiver, thus creating
feedback loops and simultaneously retransmitting
incoming audio from the RX frequency.
So how do we know the station is retransmitting audio?
If you look at the below screenshot,
you will see an ALE signal, originally being transmitted on 6944 kHz (ISB
= USB + LSB) and at the same time being retransmitted
by the feedback source on 6940 kHz
USB. We know that the lower part of the ALE signal is a retransmission,
because of the difference in signal strength between the
original USB signal and that of the
retransmitted LSB-component of the ALE signal
The ALE station in the screenshot by
the way, is a Saudi MIL circuit (callsign "JCI") that transmits on
6944 in ISB mode. In the below screenshot, the LSB component
of the original ALE signal is however
covered by the much stronger retransmitted LSB component, coming in via
the remote radio receiver tuned to 6940 kHz USB.
Another good example illustrating that
the transmitter on 6940 kHz USB simply retransmits audio from a separate
receiver, is the following
screenshot:
You can see that static crashes, hardly
present at my location, are being amplified and retransmitted by the station
on 6940 USB!
Click here
to actually listen to the section of the audio file shown in the screenshot
below
But how do we know that the remote receiver causing the feedback loop is tuned to 6940 kHz USB and not to 6944 kHz USB?
If you look at the following screenshot,
you will see a STANAG 4285 signal being transmitted on 6943 kHz USB.
If the receiver was tuned to 6944 kHz
USB, it would retransmit parts of the STANAG 4285 signal on 6940 kHz USB
- but
as you can see below, this is not the
case!
If you want to learn more about feedback loop effects, simply look up "Larsen effect" in your favourite search engine!
The Larsen effect can also be easily
simulated at home - all you need is a simple two way radio and an additional
receiver tuned to the
TX frequency of the radio. As soon
as you press the PTT, you will get similar feedback loops (provided
the receiver audio is set
correctly and the receiver is close
enough to the microphone of the transmitter).
And last but not least - some additional
feedback related audio samples can be found under the following link:
http://www.hfunderground.com/wiki/XM
Happy monitoring!
//Leif