On 10/30/20 10:17 PM, David Gilbert wrote:
I would think that testing antennas today should be easier than it was
when the K7LXC/N0AX report was done, and I don't see why an antenna
range is even needed anymore. A wideband noise source mounted on a
drone with an appropriately polarized short antenna could fly around the
antenna while a receiver on the ground connected to the antenna took
measurements. Drones are very stable these days even in stiff winds,
and while I don't think GPS positioning would necessarily be accurate
enough to define near field location, various ways of doing so from the
ground (optical triangulation, tethers, etc) should be sufficient. Far
field measurements out several wavelengths could most likely just rely
on the GPS coordinates.
It doesn't need to be a noise source - a CW tone works just fine.
or you can fly the receiver and radiate from the antenna.
This is a pretty standard technique these days - They use it for
calibrating radio telescope arrays, for instance.
One would probably want to calibrate the probe's antennas. Polarization
purity is going to be a real challenge. What you probably want to do is
actually do three orthogonal "short dipoles" (or loops). Or a "tripole"
(three monopoles, with a somewhat more complex feed system), and then
use an electronic switch (i.e. diodes) to switch among the three.
Then calibrate against something like a dipole and monopole, so you can
get the antenna pattern of the "probe"
All of that assumes reciprocity, of course, but even if that was in
question I'd bet that it would be relatively simple and inexpensive to
design/build a small receiver for the drone that forwarded signal
strength data to the ground via an RF or optical link.
An RTL-SDR would do, or, if you want to fly the Tx, one of those SiLabs
sources. It's a lot easier to fly the Tx, and just have it step through
your test frequencies and antennas in a "recognizable" pattern. For
instance, do 1 second no antenna, 1 second axis A, 1.5 second axis B, 2
seconds axis C, step to next frequency.
You'll need telemetry from the UAV that gives X,Y,Z position and
attitude - but that usually comes down over the control link. Whether
it's easily retrievable is another thing.
Let's talk required accuracies
GPS easily gives position within 10 meters in all three dimensions, and
at a distance of 100 meters, that's about 6 degrees. If you move out to
1km, you're in "sub degree" territory.
Power measurement accuracy (assuming we're interested in relative
measurements, not absolute) is determined by:
1) How accurately do you know the distance (inverse square)
2) How stable is the Tx
3) How stable is the Rx
4) What's the SNR.
Let's assume that the SNR isn't limiting - it's easy to get 50 dB SNR -
Path loss for 1km for 10 MHz between isotropes is 52 dB. You're making a
narrow band measurement (100 Hz?), and even a crummy receiver with a 34
dB NF will have -120 dBm noise floor. So radiating 1 mW (0dBM) will
give you a 60-70 dB SNR, and 40-50 dB in the -20 dB nulls. Yes, you're
radiating from a "short dipole" and there's mismatch and all that, but
if you had to radiate 10mW or 100mW that's not a deal breaker.
How stable is the Tx - this mostly a matter of power supply voltage and
"mismatch" stability as the antenna moves. Let's say the power supply
voltage is stable to 1% (easy with a 3 terminal regulator) - that's
about 0.08 dB
How stable is the Rx - probably similar to the Tx, 1% voltage gain
stability is plausible.
Distance uncertainty.. 10meters at 1km is 1%, so another 0.08 dB.
These are all random, so RSSing is suitable - and that comes up as 1.7%
or around 0.2dB. for relative pattern measurements.
If you want absolute measurements, you'd need to either have a standard
gain antenna (hard at HF) to compare against, or calibrate your Tx and
Rx, which is a lot tougher. Rx can be calibrated with a signal
generator, but you have a mismatch issue (Rx input Z is typically only
"nominally 50 ohms" - you'd need to measure that somehow )
For Tx, getting an accurate "radiated power" is tough.
I've not looked at it, but you might be able to do something like they
do for MIL-STD-461 Measurements - a "voltage probe" that is a 1 meter
monopole or a bicone of similar dimensions. You should be able to
calibrate this to E-field levels. Then measure your UAV transmitter
against that.
The SNR won't be as good, but you can make longer measurements to drive
the noise floor down. A test antenna into a high Z receiver with 1
meter effective height would be down 20 dB from a full size dipole for
40m (heff=~10m), but if you can make 1 Hz bandwidth measurements, and
maybe at 100 meter distance you can pick up 40 dB of SNR.
One big advantage of using the drone, of course, would be that the
measurements could also be taken at the actual operating site, thereby
including the effects of nearby structures.
I haven't seen any reports of anyone doing this yet, but I assume that
people already have
Pretty standard in the commercial and research worlds. A big 18 meter
deployable boom for Europa Clipper is being tested this way, for instance.
"Calibration of the SKA-low antenna array using drones"
https://dial.uclouvain.be/memoire/ucl/fr/object/thesis%3A14813/datastream/PDF_01/view
"A new approach for in-situ antenna characterization, radome inspection
and radar calibration, using an Unmanned Aircraft System (UAS)"
https://ieeexplore.ieee.org/document/7944287
UAV-Based Calibration for Polarimetric Phased Array Radar
http://www.caps.ou.edu/reu/reu16/finalpapers/Boyer-Paper.pdf
(see "Twitching Eye of Horus")
https://www.haystack.mit.edu/astronomy/astronomy-projects/edges-experiment-to-detect-the-global-eor-signature/
is using UAVs - there's probably something there in the hundreds of
memos - look at the memo list link in the above.
Actually there's a fair amount of UAV stuff at Murchison Observatory in
Australia.
Low frequency radio astronomers obsess about calibrating all the details
out because the things they are looking for are deviations from the
expected power spectral density. So small variations in the
"chromaticity" are important - Small, here, is parts in 1E4 or 1E6 - so
things like slight mismatch causing ripple in the cable losses over
frequency are important.
And then, those that are looking for "objects" are looking for faint
sources near strong sources, and understanding the phase and amplitude
of each antenna in the array, in every direction, is important, so you
can do the "beam forming" implied in calculating the images.
For myself, I'm looking into calibration of large arrays of small
satellites in space that cover 0.1 to 30 MHz (because that's blocked by
the ionosphere, and can't be observed from Earth). My first array in
space is looking at the Sun with 6 crossed dipoles, which is by far the
brightest source in the sky, so my calibration doesn't have to be that good.
However, if you're looking for the remnants of the big bang, highly
red-shifted down to HF and VHF from the original 1420 MHz emission
frequency, you need to start worrying about measuring to 1 ppm kinds of
uncertainties.
73,
Dave AB7E
On 10/30/2020 8:35 PM, chetmoore@cox.net wrote:
Not a FLAME. But You would likely benefit from reading the k7lxc N0ax
tribander report. After raeading it I ordered the C31XR. Force 12
is no more but some of their antennas are mechanically improved and
sold by JK and there are A lot newer tribanders I would like to see
them test on their antenna range. Hint, tests of some of the mosely
and hy gain antennas did not fare all that well. I had a TH6, a TH3
, TH7 and a classic ta33 all of which worked
Pretty well..........in their day. My th-3 is still a good FIELD
DAY antenna and great as a mult antenna to grab south American
mults so I don’t have to rotate the c31.
N4fx
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