Yes, I agree that the computation needs to be done in the frequency
domain with an FFT. I have no clue whether or not the K3 and K3s have
enough computational power to do so, but Lyle (Elecraft's DSP guru)
briefly discussed this idea with me when I suggested it several years
ago and he seemed to think it might be feasible. At the time the K3
synths wouldn't preserve lock when you changed frequency so it wouldn't
have been very practical back then, but the new synths are fed by the
same oscillator now.
Regarding the direction finding part of my proposal, I once fed a 40m
broadcast band carrier into both receivers of my K3 in diversity mode.
I used my 2 element 40m yagi for one receiver, and the tribander below
it for the second receiver. I then fed the line output of the K3 into
the sound card of my computer running an dual trace oscilloscope
application. I didn't know the actual delay of either feedline so I
just set the app to trigger on one of the signals and then observed how
my trace delay there was on the other signal. I knew the audio
frequency of the signals, the frequency of the RF, and the distance
between the two antennas. While I wasn't able to calculate the actual
arrival angle without knowing the actual phase delay of each feedline, I
was able to calculate/observe roughly how much it changed over time. I
had to wait for pauses between the voice modulation of the AM signal, of
course, but after watching for several minutes I could see that the
short term jitter was roughly 5 to 10 degrees and the longer term
changes maybe about 15 to 20 degrees. The results were enough to
convince me that a better setup could give interesting information, and
I suspect that the azimuth angle would be more stable than the arrival
angle.
73,
Dave AB7E
On 1/13/2019 8:45 AM, David Woolley wrote:
Assuming that there is enough processing power available, and the
architecture physically allows the mixing, manually steering either
the peak or the null should be achievable. There is a slight subtlety
in that one is trying to achieve a constant time delay at RF, not a
constant phase change, but, if RF is sufficiently higher than the the
baseband, it will be a good approximation, and the hardware devices
being quoted probably have the same defect.
Automatically getting a direction is only accurately achievable if the
interfering noise is isotropic, at least in the plane containing the
line between the antennas. As such, it will not be able to
automatically get a direction for an interfering signal, unless it can
separate that signal from everything else, and if it can do that,
there may be more direct ways of removing the signal.
The only way I can see of performing the computation is in the
frequency domain. Although everything starts as just a time delay,
the down conversion preserves phase not time, so I think one needs to
do a Fourier transform, rotate each value by a fixed amount (or one
accounting for the offset from the carrier not being negligible) and
then invert the transform. (There may be better algorithms.)
Because of the discrete nature of FFTs, you would probably want to run
several overlapping transforms and average their results.
I suspect the current DSP tries to avoid doing a full Fourier
transform, and even panoramic adapter FFTs don't need to run
overlapping transforms.
I think the big question is does the machine have enough processing
power to do this.
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