[Linrad] Re: off subject Delta 44

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Jeremy,

> I am curious about using the delta 44 and the Linrad 
> for lowfer listening. What precautions should I follow 
> before connecting antenna to input of the sound card. 
You mean listening in the range 5 to 45 kHz or so?
(Wikipedia says lowfer is at higher frequencies than that...)

> Also I am sure someone is doing this, what kind of 
> success would one have with this setup over say a 
> commercial selective voltmeter type receiver? 
It could make a tremendous difference. It depends on what
you want to hear.

At low frequencies your antennas will be very small compared
to the wavelength so they will always have a dipole pattern.
Electric dipole for E field sensors and magnetic dipole for
H field sensors. The E field and the H field are coupled in
free space, E/H=300 Ohms but in the near field they are not 
coupled (much) and ratio between the E field and the H field
may deviate very much from 300 ohms. A fluorescent light
might give a very strong E field (high voltages) but a low
H field (small RF current in the plasma) while the transformer
in an old TV might leak strong magnetic fields at the line
frequency and its overtones.

Make yourself two antennas. One magnetic. Could be a coil
with a diameter of a couple of meters. It is very important that
it is not capacitively coupled to anything. Connect a low
impedance amplifier through a low capacitance transformer.
High permeability so you can place the windings on opposite 
sides of the core. It will be a magnetic loop and if you place 
it vertically you will have a nice figure 8 radiation pattern.

Also make a magnetic antenna. Maybe a 5 m vertical tube. 
You will need a low capacitance fet amplifier and presumably
a good (high Q) inductor to balance stray capacitances
and make the impedance very high.

Place the two antennas at a good distance away from noise 
sources and connect both of them to Linrad. If the gain is set
to make the signal level similar for stations in the lobe
of the magnetic loop you will linrad splits the received
signal into two groups, backwards and forwards because
the phase relation in the two directions differ by 180 degrees.

All that might be fun, but you could equally well do it
on 80 meters. It is the same as 80m fox-hunters do:-)

Now, what is more intreaguing is that you can adjust
gain and phase to make the pol indicator in Linrad
say the polarization is always left or right circular
for forward or backward. Then you can move the cables
and send the signal from one antenna into I and the other
into Q of Linrads channel 1. Now running in I/Q mode
you will see two spectra. Negative frequencies are 
forwards and positive frequencies backwards or vice 
versa.

Having two more channels you can of course do the 
same thing once more:-) You may place sensors of any kind
in a way that makes them pick up the local qrm that
might enter your antena and use Linrad to phase out the
QRM by setting pol adjustment to manual.

> The dual antenna thing is curious to me at LW and wonder 
> who is doing it.
I have been waiting a long time for HF operators to discover
what they can do. I have even made the RXHFA available with
a hope it would be found to be a useful tool at e.g. 160 m.

I had no feedback or comments on this issue from anyone before 
and yet I honestly believe the potential is much bigger than in
144 MHz EME. Two channels will give +3dB at maximum in a 
white noise background, but when it comes to interference 
suppression the limit is set by the phase stability of
the signals from the interferer. Local signals should
be extremely stable and I would guess one can attenuate
by 40 dB at least. Linrad does not have routines to suppress
a particular signal (yet?) To look at ways of doing it well
I need recordings of typical cases. 



   73

   Leif / SM5BSZ

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