kb8tq@n1k.org said:

What does "precise" mean in that context?

I'm not an antenna-nut.  Can an antenna miss-match change anything other than
the amplitude?

How do you automatically tune something like that?  The manual way would to
twist the knob while watching a meter.  If the meter goes down, you are going
the wrong way.  If it goes up, keep going until it starts going down, then
back up to the peak you just passed.

How do you even know that it needs tuning?  Can you measure something
accurately enough?  If so, what?

--
These are my opinions.  I hate spam.

Yes, the weather can change the phase of the transmitted antenna signal unless corrections are performed. From the WWVB (60 kHz VLF time/frequency station at NIST in Fort Collins, CO) website at https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwvb (see second paragraph about automatic antenna tuning):

Changes to the antenna and tuning network affect the transmitted phase, and of course phase changes over time affect the fractional frequency error. At WWVB they control the actual transmitted phase so that it doesn't effect the received frequency accuracy.

--
Bill Byrom N5BB

On Sat, Aug 8, 2020, at 3:15 PM, Hal Murray wrote:

Hi

Sure, it also changes phase ….

Measure it and feed the result into a servo ….

S11 back into the antenna if it’s an RX antenna. If it’s a transmit antenna SWR is the
normal thing to monitor ….

Bob

On 8/8/20 1:15 PM, Hal Murray wrote:

You bet it can change - a lot. The reactive component of the antenna
impedance will change the phase of the signal.  In fact, that's the
dominant source of phase change of a resonant antenna with temperature.
The length changes, the R part stays pretty constant, but the X changes
quickly.

The farther away from resonance, the worse it is.

Granted, we're talking tiny phase changes for big temperature
changes.  If the CTE is 20 ppm/C, then a 50 degree swing is 0.1% in
length (or, equivalently, 0.1% in frequency).

I've been doing some analysis of a drooped fan dipole for the OVRO-LWA -
it's about 3m total length, and resonant around 50 MHz, but it's used
from 30-80 MHz. I've attached 3 plots of the (modeled) phase difference
into the LWA's 100 ohm input impedance receiver.

That 0.1% change in length is about 0.2 degrees phase at 30 MHz (18
picoseconds?), about the same phase difference at 70MHz (9 picoseconds).

That's for tiny changes.

For a lot of low frequency receive antennas - the antenna is wildly off
resonance, but it's feeding a high Z input, so the phase of the voltage
at the input (= Zload/(Zant + Zload)) is somewhat reduced by circuit (if
Zload >> Zant).  But a tuned loopstick - that's another story - now
you've got a highly reactive antenna, where the reactance is tuned out
with a highly reactive component (a capacitor) - both of which might
have significant temperature coefficients, and you're back in the "small
change in a curve which has a steep slope" territory.

But you can also get bigger reactive changes - a bird sitting on the
antenna can change the reactance a lot.  The moisture content of the
soil under the antenna will also change it (I'm still running the
analysis for that one for OVRO-LWA).

Recently at work, I've been doing lots of analyses for a variety of low
frequency interferometers in space for radio astronomy - There's
interesting phenomena between say, 100kHz and 50 MHz, and those
frequencies are not observable on the surface of Earth because of the
ionosphere.

In general, we're looking at lots of "electrically short dipoles" spread
over 10s of km, either on the surface or in space. Since it's
interferometry, the phase is important - the phase uncertainty is what
sets the fundamental resolution of the instrument. And, of course, you
need to know what the baseline is (where the two end points are)
geometrically.

I've been looking a LOT at what the phase response of a dipole is, vs
temp, orientation, bending, etc.  For one thing, we can use the dipoles
to measure the position and orientation of the receiver, i.e. use
interferometry with a source of known location, and back out the other
terms, then we can measure the locations of the unknown sources.

You can measure both voltage and current very accurately on the antenna.
The radiated field is related to the current, so that's usually your
starting point for adjusting. You could set up a control loop to make
sure that the feedpoint current phase always matches a reference phase.

BTW, the way most of the ham autotuners work is effectively looking at
the phase of the antenna current vs the phase of the Tx voltage - rather
than comparing Fwd and Ref power.  The phase difference tells you "which
way to turn the knob" for one thing.

Fwd and Ref power is historically useful - the Bird wattmeter or a
simple bridge will give you powers (or voltages) the magnitude of which
is an easy RF measurement to make with a thermal sensor that is
independent of frequency, phase, modulation, etc.

Making good RF phase measurements over a wide frequency range is
something that's been "easy" only for the last few decades. And doing it
cheaply really required wideband monolithic ICs.  Those Analog Devices
LogAmp on a chip (like the AD8302) really changed a lot of things. A $20
chip instead of a $10,000 hybrid module makes a difference.

Hi

Yes … but ….

There’s two antennas. They each have their own matching setup. Each are fed by this or that
transmitter (= there are backups …).

As long as everything is stable it all works fine. Wind blows and the antenna(s) move. Matching
network ( one or the other ) responds. Since the other antenna is tightly coupled, the other matching
network responds to the first one’s adjustments. They fiddle back and forth …

While this is all going on, who knows what the phase is in this or that direction. Once it all balances
out, you can do things to servo correct.

Fun !!!

Bob

Hi

The main point is: If you are looking at a VLF system, phase matters a lot. If your
objective is a 100 ns @ 1 second sort of accuracy, you need a very stable phase.
At 100 KHz, you are looking at 3.6 degrees of phase shift. Go down to 60 KHz and
you are right at 2 degrees. Head to Omega sort of frequencies and it just gets worse.

If you are trying to be “as good as” GPS / GNSS, this is still an order of magnitude (or more)
away from the goal ….

Since fairly normal propagation effects can get you into  “which cycle am I on?”, antenna
effects are not the dominant issue. Toss in things like skywave and …. yikes ….

Bob