[time-nuts] eLORAN in the Antipodes ?

[jimlux]

On 8/8/20 1:15 PM, Hal Murray wrote:
> 
> kb8tq at n1k.org said:
>> Same basic issue, lots of weird interactions and a need to keep the signal
>> very precise. Not as easy as it might seem.
> 
> What does "precise" mean in that context?
> 
> I'm not an antenna-nut.  Can an antenna miss-match change anything other than
> the amplitude?


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.





> 
> 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?

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.


> 
>    
> 

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