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Author Topic: DFing HF SSB  (Read 6391 times)

SWL377

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DFing HF SSB
« on: April 16, 2008, 04:57:11 PM »

I am a brand new ham AF6IM, licensed last Monday. I love DFs and have used them a lot in commercial fishing both for navigation (back in the day when every lighthouse had an LF beacon) and for locating other boats and net/longline buoys that are beacon equipped.

None of the DF/ADF gear I used worked very well on HF SSB as the effective rotational speed the goniometer was too slow compared to the syllabic speech rate. You couldnt tell a null from a speech pause. If there was a loud constant background noise, as from a genset on constant RPM engine, then you could see a consistent pattern on the CRT and get a bearing on an HF SSB signal, especially when the guy transmitting paused in his speech and you only had the constant noise modulating the xmtr. The ADF works great on AM and slow CW/MCW.

The ADFs were typically Taiyo Gonio types with a crossed loop antenna. Any suggestions for improving HF SSB DFing?
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WB6BYU

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DFing HF SSB
« Reply #1 on: April 19, 2008, 09:49:00 AM »



Most HF DF equipment can be used on SSB, at least in manual mode.
You can'd DF the station when the operator isn't talking, of course,
because there isn't any power output (unless the signal is strong enough
that you can hear the suppressed carrier or some background noise.)
But that doesn't mean you can't still take a bearing.

I got started in DF many years ago hunting SSB on 75m with a loop.
You kept turning the loop back and forth to hear the null while the
operator was talking and waited during pauses.  Now I use a hand-held
DF receiver on keyed CW and the situation is similar:  I can only take
a bearing when the transmitter is on, but with mental averaging I can
find that point fairly easily.  It does, however, require a slower sweep
than can be used on a continuous carrier.

(Or a faster sweep if that means I can cover the whole range of interest
in the time of one DAH on CW.  Most of the time I know the rough
direction and am simply refining the bearing.)

Note that all of this is done by watching the S meter or (preferably)
listening to signal strength by ear, so I'm using my built-in signal
processor rather than an external electronic one.  Though with an ADF
system it should rotate while the operator is talking and stop during
pauses (depending on the time constants) and it may still be able to
give a good bearing if you are willing to wait a bit.

If you want a more elaborate system, the Watson-Watt method will give
an instantaneous readout, even on a varying signal.  In theory it requires
two receivers (and a third for sense) that are exactly matched in phase
and amplitude response.  Modern implementations use a single receiver
and modulate the antenna inputs so the resulting signals can be sorted
out later.  The readout is a line on an oscilloscope face that is calibrated
in degrees around the perimeter.  I wonder if that is the type you were
using when you talked about watching the CRT?

Systems that switch between two antennas will have problems unless
the switching speed is very high.  Methods that compare the phase
between two antennas in real-time should still work on an SSB signal,
though the time constants may require adjustment.  Rotating a simple
loop for a null is probably hard to automate, since it is difficult for the
hardware to tell the difference between a null and a pause in speech
(unless there is a reference non-directional antenna as well) but can
work just fine using the processor between your ears.

SWL377

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DFing HF SSB
« Reply #2 on: April 22, 2008, 02:36:44 PM »

Thanks for the helpful info. The CRT I was describing is on the Taiyo LF MF HF marine ADF. It gives a polar plot of signal strength. In the ADF mode you see a propellor or figure 8 pattern on the CRT. You can remove the 180 degree ambiguity by switching in the sense antenna.
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WB6BYU

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DFing HF SSB
« Reply #3 on: April 23, 2008, 09:49:21 PM »

I'm not familiar with those commercial units - presumably the antenna
is motorized and the plot is created by rotating the antenna?  I understand
the principles but don't know which manufacturers' model numbers
use which method.

I did come up with a phase detector receiver for 80m that indicates
whether to turn the antenna right or left.  Of course the indication goes
away when the operator stops talking, but eventually it should swing
the antenna around to point in the right direction.

SWL377

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DFing HF SSB
« Reply #4 on: May 02, 2008, 10:32:32 AM »

The Taiyo HF ADF antenna (two stationary loops crossed at 90 degrees) has no motor. The manual (really poorly tranlated from Japanese) appears to say that the loops are "electronically rotated using Goniometer techniques." The CRT prsents a polar plot with direction vs amplitude. When you look at a CW groundwave sig you see a propellor pattern on the screen. Pushing the sense button resolves the 180 degree DF ambiguity by switching in a sense antenna (short whip). Freq range (from recollection) is 200 Khz to 16 Mhz. Some later models went up to 30 Mhz but required a smaller loop for freqs in the upper range.
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