Jim Lux 2023/06/15 12:17
On 6/15/23 8:55 AM, N2MS wrote:
> The characteristics of coax vary with the lot, temperature, installation
> and aging. I know that hams use coax for phased arrays and I have seen
> it used in CB and Marine arrays. Are there any commercial applications
> that use coax for matching or phasing these days? I would assume
> commercial installations would want something adjustable and rated over
> tempetature.
Pretty much every broadcast station uses a phased array of radiators,
whether it's a stack of dipoles or bowties for VHF, FM, etc.; or a
directional array for AM.
But aside from that (where the phasing tolerances can be pretty loose -
your Tx pattern doesn't change much from small phasing errors):
Phase matched assemblies are quite common in direction finding or
multichannel systems. There's the notorious bump for PTFE/Teflon
dielectrics around 25C, so if you want good phase matching, you need
good temperature control (or matching) at least.
I built an experimental system at work which measured the received phase
from 4 X-band (7.15 GHz) antennas several meters apart to control the
transmitted phase from 4 Ka-band (32 GHz) antennas. The idea was to be
able to compensate for pointing errors and do limited "retrodirective"
beam steering. When you have a the equivalent of a 6 meter diameter
antenna (0.1 degree beamwidth), you need to be able to point back to
earth very accurately. We were looking at tenths of a degree
variations. Just measuring and calibrating the system was non-trivial.
Multichannel test systems (both VNAs and things to test multi port
devices) use phase matched (and stable, repeatable) coax assemblies.
There's a reason those test port cables for a good VNA cost $5000 each.
So anything where you are measuring the phase between two signals -
monopulse feeds for radar, for instance - you want good phase behavior.
Near field scanners for antenna ranges rely on the precise measurement
of phase as received/transmitted at the probe, because the phase and
amplitude is transformed into the far field radiation pattern.
Any precision measurement is probably going to care about the cable -
probably more about stability in general, and repeatable behavior over
temperature. You can calibrate out the phase shift vs frequency, but if
it changes, then your cal is all fouled up