At 08:04 AM 5/11/2005, Tom Rauch wrote:
> > In connection with a post about phasing dissimilar
>antennas in a stack,
> > someone posted a link to a way to determine the phases
>using an oscilloscope
> >
> > http://www.arraysolutions.com/Products/dissimilarants.htm
> >
> > Determining the received phase of signals from stacked
>antennas is something
> > that I've been spending a lot of time on recently at work,
>albeit at 7 GHz,
> > not 14 MHz, but the principals are the same. So, here are
>a few comments:
>
>SNIP>>>
>
>The scope isn't even necessary, but more interesting why
>would I want the source at a mile or more distance?
I agree... the mile away thing makes it tough. I did the calculation as I
did because the original applications note referred to a "another ham's QTH
in town".
>The
>phase measurement would be much more accurate with a source
>several hundred feet away rather than a mile or more. The
>source distance only has to be large compared to height
>differences between antennas.
The distance has to be far enough away that the phase difference is less
than your measurement requirement. In the usual antenna range world, for
instance, we use 2*d/lambda^2, where d is the diameter of the antenna, and
lambda is the wavelength. This gets you to about a few tens of degrees,
which is the diffraction limit for uniformly illuminated circular
apertures, so the gain measurement will be accurate. If you're looking at
sidelobes, though, you need to go farther.
But, taking some "real numbers".. Let's assume 20 meter wavelength, and the
antennas are 1/2 wavelength (30 ft) apart. Further, let's say we want the
wavefront to be apparently planar to within 1 degree (since we're trying to
measure down in the 10 degree range).
1 degree at 20m is 5.6 cm. The deviation is x = sqrt(d^2+s^2)-d, where d
is the distance, and s is the spacing. Solving this is a pain, so I
usually do it iteratively.
Turns out you need to be about 900-1000 meters away, to get the phase
arriving at the two antennas the same, within 1 degree.
One would, of course, also need to worry about whether the wavefront is
distorted by the terrain, surrounding objects, atmospheric refraction,
etc. This precision phase measurement stuff is non trivial.
The effects of the antenna phase patterns would also need to be dealt
with. For the yagi's I was fooling with earlier, the phase near the ground
about 2 or 3 degrees different than the phase at the main lobe.
One might say, why worry about all this one degree stuff?
The problem is that you get a degree here, a couple degrees there, and
pretty soon, the stackup gets to be in the 10-15 degree range.
>What's with this "mile" thing people always use? Is it
>because AM BC station FS is normalized (not measured, but
>normalized) to standard distances like "one km" or "one
>mile"??
>
>Will a scope connected directly to an antenna system with
>horizontal polarization actually display an HF signal
>cleanly from a 100w rig a mile or more away, especially
>enough to measure phase?? I have serious doubts.
I would agree. And, if you take the suggestion in the apnote of the buddy
with a mobile rig, you'll have all the polarization issues to deal with
(hmm, exactly what part of the car is radiating, and what's the phase
pattern of the car).
You CAN make these kinds of measurements, but it's somewhat tedious. I
think that the following would be needed:
1) A tuned receiver for the detector (greatly increases the SNR on your
measurement)
2) A switched multiport combining network, with resistive pads to eliminate
the mismatch effects. The network takes the two antenna inputs and
combines them with several different lengths of transmission lines. (anyone
who's familiar with network analyzer using 6-ports will recognize
this). The various lengths are unimportant, as long as they're known, and
as long as they span a reasonable range of phases from 0-360 degrees.
3) Multiple source position measurements, at various distances (because
then you can figure out the contribution from non-planar wavefronts).
Actually, since what you "really want" is the phase pattern for elevation
angles that are somewhat off the horizon, why not use strong DX signals as
your test source. They need to be strong, because of the pads in your
measurement system. As long as they remain reasonably stable over the
measurement interval (a few seconds or minutes), it should work well.
A calibrated receiver would help, but if you can disable the AGC, you could
probably use a voltmeter on the audio output.
But this all comes back to my original point. Why bother?
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