[TowerTalk] Antenna Analyzers

[Doug Ronald]

Okay, as long as we're talking VNAs, I have a question:

I am making an automatic antenna tuner, the sensor of which is a pair of
AD8302s, and a pair of AD8362s. I made a 36 dB Tandem Coupler which has a
forward and reverse RF output. The two 8362s input the Fwd, and Rev ports,
and output a DC potential proportional to the power received from each port.
>From those I calculate forward power, reverse power, and VSWR which all
works according to plan.

Onto the AD8302s. Both have one input from the Rev port of the coupler, and
the other AD8302 input is a voltage sample of the transmitter input port,
one at no phase shift, and the other AD8302 gets a 90 degree phase shifted
transmitter input port to resolve the ambiguity inherent in the AD8302
design.

So, at the microprocessor end of the RF head, from the AD8302s I have a DC
voltage proportional to the phase relative to the incident transmitter's
phase, and from the AD8362s, I have a DC voltage proportional to the power
received from the forward and reverse ports of the coupler. My A/D converter
gets these voltages into the microprocessor.

My end aim at this point is to read the complex impedance of the antenna,
but what I'm looking for is the exact math's to get me from the above
defined DC voltages to a (x, jy) number. 

Can anyone help with this, or refer me to an online reference? Needless to
say, I've exhausted my search capabilities, and have only found the source
code for a TAPR VNA which I could plow through to get the answer, but that
would take a lot of time...

Thanks,
-Doug, W6DSR 

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Paul
Christensen
Sent: Wednesday, December 5, 2018 15:47
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Antenna Analyzers

>"The VNWA is a nice unit but it's WAY WAY WAY to delicate for serious field
work.  The newest variation with the micro USB is a step back because that
connector is far more fragile than the old hunk-square type A USB connector.
Guys who use them "all the time" in the field are apparently much more
disciplined than I am.  The other issue is that it requires a computer to
work.  So it's workable on the ground, but if you have to go up the tower,
then it's a problem.  And the software is a long way from intuitive - but it
does work fine once you figure out how to use it.  The myVNA software used
with the N2PK requires virtually zero learning curve.  I love the GUI and
the N2PK but this is even more non-portable in that my N2PK requires a
parallel port to drive it as well as the computer.  So it's attracted to the
bench and never goes for a field trip."

My $0.02.  I own the USB version of the N2PK VNA and as Jeff points out, the
myVNA GUI from G8KBB is very intuitive.  I have tried the VNWA GUI with the
N2PK but found that setup is more complicated and graphics quality isn't
nearly as good as myVNA.  

I used to lug around a tablet PC and gel-cell when making field measurements
with my N2PK VNA, but now use RigExpert 230 and 55 Zoom models in the field.
They've worked very well in high AM BC fields, display graphics is good, and
the batteries seem to last forever.  Probably 95% of what I need in the
field can be accomplished with either RigExpert single-port CIA rather than
2-port VNA measurements.  Most of my S21 measurements are done on the
workbench.

Another excellent VNA is the Array Solutions VNA2180.  For detailed S11 and
S21, I like it as much as the N2PK and graphics quality is excellent with
W5BIG's software.  All the analyzers mentioned have a HF or VHF frequency
limit but I rarely need measurements beyond VHF.   

Paul, W9AC 


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