[RFI] What antennas are good for HF DFing?

[Roger Parsons]

You can use either a loop or a ferrite rod antenna.
The performance will be quite similar. The sensitivity
will be greatly improved if you use a tuned loop, and
that may or may not matter in your particular
situation. It probably will matter, as you listen for
a NULL rather than maximum - the null will be a few
degrees wide and the maximum perhaps 90 degrees. It is
important that the antenna be constructed in as
symmetrical way as possible, and kept a reasonable
distance (a few cms) away from any metallic objects,
such as the receiver. If this is not done, the null
positions will probably not be 180 degrees apart, and
shielded loops are somewhat better in this respect
(although probably not worth the extra effort in
construction). (There must be a break in the shield,
for a loop directly opposite the feed point, and for a
ferrite rod use a slotted tube.) 

Ferrite rod antennas are quite tricky to get working
properly - you need the highest permeability rod
possible, _but_ the loss must be low at your
operational frequency. Most medium wave rods will work
OK at 2MHz, but probably not at 3 or 4. To increase
sensitivity you can use several rods, _but_ just
strapping them in parallel or lengthwise will only
produce a miniscule improvement. You can get a 3dB
improvement for 2 rods by spacing them parallel about
5cm and splitting the tuned and coupling winding
between the two rods with a single tuning capacitor.
Probably not worth the effort, as a 30cm square loop
will be just as good and is a lot easier to make.

Your bearings will have a 180 degree ambiguity, which
makes life somewhat tricky. This can be resolved by
adding a sense antenna. This is an omnidirectional
antenna (such as a whip), the output of which is
combined with the loop. If the outputs from each
antenna are the same, you will then get a broad
maximum in one direction and a broad minimum in the
other - broadside to the rod, or in line with the
loop. 

The technique is to obtain a bearing first, without
the sense antenna in use - this should be within a few
degrees - then turn the antenna at 90/270 degrees and
see from which direction the signal is arriving.
Obviously you need to practice first on known signals!

All of the above works wonderfully if 
(1) The signal being DFed is arriving solely by ground
wave. This will probably be true, but is a good reason
to only do your tests during the day when sky wave
propagation on mf is minimal. 
(2) There are no extraneous conductors, such as power
lines around. The latter is most unlikely to be the
case if you are trying to track down noise sources.
Power lines can carry mf signals for tens of
kilometres with low loss (BPL...), and because there
are standing waves present, and because the poles
often have a nice grounded vertical (the ground
connection), there will probably be multiple maxima
and minima. The solution is to get well away from the
power line to take your initial bearings - several
wavelengths. This can be very difficult of course if
the signal can only be heard close to the power line.

The professionals all advise to use the highest
possible frequency at which the signal can be heard -
there is most unlikely to be ambiguity DFing on
150MHz. mf DF can be done, but it is very, very easy
to literally go down the wrong garden path!

GL and 73
Roger VE3ZI





	
	
		
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