Dan,

I repeated your test with the 50 ohm load at 10 mHz and got R 006 S
127 Z 126 which is pretty close.

With a 75 ohm termination reading is R50 S102 Z152
With a 200 ohm termination reading is R147 S50 Z204

With a 100 pf cap at 10 mHz I read R = 0 X=145 109 pf and R245
S083 Z242

With a 100 pf cap at 3 mHz I read R = 0 X=554 95 pf and R246
S023 Z255

Here are some things to check:

1. Try 75 and 200 resistors and see if you get readings close to mine.
If you do your diodes are probably OK but you need an alignment.

2. Use the 100 pf cap and starting from 3 Mhz start increasing the
frequency. Does it read R=0 some of the time - most of the time?? Try
another type/size of cap.

3. Attach about 20 feet of coax to the unit and put a 75 ohm resistor
at the other end. Your SWR should be about 1.4 to 1.5 depending on
coax type/length. Start at 2 mHz. and start increasing the frequency
to 30 mHz. The SWR should gradually get lower as you increase
frequency due to cable loss. However it will go up and down a bit (+/-
..1) at certain frequencies due to alignment and instrument error. R &
X should change in a smooth fashion as you change frequency (the cable
is an impedance transformer)


If you attempt alignment here are some tips.

1. Follow the procedure at W8JIs site - not MFJ. Tom was the designer
and his site has the latest info.

2. Use precision resistors placed deep into the connectors and short
leads for minimum inductance. I used metal film 1% and picked the best
ones with an accurate ohm meter.

3. Use a stable power supply to power the unit. If you try to use
batteries alone you will find the voltage dropping with time and you
will not get consistent results.

4. Be prepared to repeat the process several times in order to get the
best results.

5. It will never be perfect. The 259-b is an 8 bit implementation,
the bridge has inherent inbalance due to resistor/diode characteristics
and the log buffer does not match the diode characteristics perfectly.
This isn't a VNA but it is a useful low-cost instrument.

Hope you keep us updated. I am interested in your results.
Roger




wrote:
At 10MHz, I get the following:

Open: R 255 S 003 Z 248

Shorted: R 232 S 255 Z 001

Decent 50 ohm termination: R 001 S 127/128 Z 121

At 3 MHz my 100pF +/- 10% capacitor reads164-j530 ohms on the Impedance
display ... it seems to get X right... Furthermore, setting the meter
to capacitance makes it read about 108pF... but there's this big
resistive component.

In the test menu, this cap reads R 255 S 023 Z 247 at 3 MHz

So maybe I just need an alignment... I suppose it wouldn't hurt to try
it if I can scare up some precision terminations.

I wouldn't consider these numbers "way off" ... this reading of
resistance at low frequencies had got me worried that something was way
wrong, but will an alignment fix this? I guess I can check through
the W8JI alignment procedure with the proper loads before I start
twiddling things.

Dan

200 (197 on my DVM) ohm termination:

R 155 S 051 Z 197

75 (74.5 on my DVM) ohm termination

R 047 S102 Z 146

With the cap sweep, with two different caps (a 100pF doorknob and a
30pF air variable trimmer), R is almost never zero until I get up into
the 60MHz+ range and even then, rarely.

With the coax check, the SWR oscillation amplitude is about 0.2 or so;
1.1-1.3. R and X vary smoothly and more or less cyclically.. I guess
the oscillatory SWR variation with frequency is incorrect... I do get
more than you mentioned and more than one might expect... i've got
about 40 feet of RG-55U terminated in the 74.5 ohm termination that I
used above.

- - - - - -

From what you've said, it seems like alignment time. I guess the large
resistive component on capacitors at low frequency is a numerical
artifact of a slight misalignment... because the R S Z menu values are
really fairly close to yours. I'll build or find some precise SMT
terminations and go at it. Thanks much for your help and I'll post
after I do the alignment.

73,
Dan