dansawyeror wrote:
Now that the setup is reading consistently I will 'test' a loaded 2m
monopole over a 1 m**2 ground plane.

I suggest that you start with an unloaded monopole or some very simple
antenna with a well known impedance. (You will of course have to know
and allow for the effect of the finite ground plane.)

You also need to take measures to prevent coupling between the antenna
and the outside of the feedline. The ground plane you mention will help,
but there can still be substantial coupling. Some high impedance ferrite
beads at the feedpoint and another set about a quarter wavelength down
should reduce the coupling to a small value.

Roy Lewallen, W7EL

I proceeded before reading this note. The procedure was to zero the phase meter
on an short and then to test the loaded 2m vertical. The result was +10 dbm
forward (before the 20 dbm pad) and -50 dbm reflected. The coupler measures
about -14 dbm. The total was about -60 dbm, with 34 db of that due to the pad
and coupler. The net is -26 db forward - reflected.

(The phase angle and reflected ware very touchy. It was almost impossible to
adjust by changing frequency. It was easier to 'adjust' it by sitting very still
and moving my arm.)

The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch
long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2
foot square aluminum plate. This antenna should have an input impedance less
then 20 Ohms.

How can it measure very close to 50 Ohms? Is there something wrong with this
analysis?

Thanks - Dan

Roy Lewallen wrote:
dansawyeror wrote:

Now that the setup is reading consistently I will 'test' a loaded 2m
monopole over a 1 m**2 ground plane.


I suggest that you start with an unloaded monopole or some very simple
antenna with a well known impedance. (You will of course have to know
and allow for the effect of the finite ground plane.)

You also need to take measures to prevent coupling between the antenna
and the outside of the feedline. The ground plane you mention will help,
but there can still be substantial coupling. Some high impedance ferrite
beads at the feedpoint and another set about a quarter wavelength down
should reduce the coupling to a small value.

Roy Lewallen, W7EL

dansawyeror wrote:
I proceeded before reading this note. The procedure was to zero the
phase meter on an short and then to test the loaded 2m vertical. The
result was +10 dbm forward (before the 20 dbm pad) and -50 dbm
reflected. The coupler measures about -14 dbm. The total was about -60
dbm, with 34 db of that due to the pad and coupler. The net is -26 db
forward - reflected.

(The phase angle and reflected ware very touchy. It was almost
impossible to adjust by changing frequency. It was easier to 'adjust' it
by sitting very still and moving my arm.)

The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a
1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is
mounted over a 2 foot square aluminum plate. This antenna should have an
input impedance less then 20 Ohms.

How did you arrive at this figure? I wouldn't hazard a guess without
modeling it.

How can it measure very close to 50 Ohms?

1. Inductor loss.
2. Effect of finite size ground plane.
3. Coupling to feedline.
4. Measurement error.

Is there something wrong with
this analysis?

I don't know. What should the impedance really be?

Roy Lewallen, W7EL

On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror
wrote:

The coupler measures
about -14 dbm.

What does this mean? Are you trying to tell us that the power on the
coupler port is 14dB less than the through power? What has dBm got to
do with it?

You didn't report the power in the coupler port with a s/c and / or
o/c at the measurement plane. Did you perform this cal?

(The phase angle and reflected ware very touchy. It was almost impossible to
adjust by changing frequency. It was easier to 'adjust' it by sitting very still
and moving my arm.)

That is understandable. How much coax between the A probe and the
load, and the B probe and the load... how many degress does this total
electrical length change for a 1% change in frequency? Does that
explain some of the phase sensitivity?


The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch
long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2
foot square aluminum plate. This antenna should have an input impedance less
then 20 Ohms.

Don't you need to measure some "known" loads. Why not try a 50 ohm
load tee'd to a s/c stub (quarter wave at a known frequency) and see
if you get the predictable results at different frequencies around
resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
half that frequency it should be 0.85+j50, etc...


How can it measure very close to 50 Ohms? Is there something wrong with this
analysis?

See if you can trust your measurements on known loads before wondering
why the unknown load isn't what you expect when using unknown
measurement technology... too many unknowns.

Owen
--

On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy wrote:

(The phase angle and reflected ware very touchy. It was almost impossible to
adjust by changing frequency. It was easier to 'adjust' it by sitting very still
and moving my arm.)

...Does that
explain some of the phase sensitivity?

Hi Owen,

Being very touchy, especially to the specifics of sitting very still,
sounds like classic common mode problems.

73's
Richard Clark, KB7QHC

On Mon, 02 Jan 2006 23:06:43 -0800, Richard Clark
wrote:

On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy wrote:

(The phase angle and reflected ware very touchy. It was almost impossible to
adjust by changing frequency. It was easier to 'adjust' it by sitting very still
and moving my arm.)

...Does that
explain some of the phase sensitivity?

Hi Owen,

Being very touchy, especially to the specifics of sitting very still,
sounds like classic common mode problems.

Yes it does Richard. I saw Roy's response regarding isolation of the
feedline, and it is a valid comment. My comment was towards the
reported frequency sensitivity... until the effect of the propagation
delay is removed from the results, the underlying impedance is
obsured.

Owen
--

Thanks,

I went back through the calibration procedure.

1. The meter appears to be performing as it is supposed to. The 'static readings
work repeatably and predictably. With a balanced configuration shorts and
opens read correctly.

2. The pad I was using appears to have a frequency shift. With the 'legs
balanced for length' the measurement would change with frequency. This really
confused the measurement process. I put it aside.

3. I have some 23 Ohm couplers. These of course confuse readings when used as
normal couplers. However if they are used in a truly balanced configuration -
that is the legs are configured as close to identical as possible then they
appear to work.

I am going to shelve this until I find or make a 50 Ohm bi-directional coupler.

Thanks again - Dan


Owen Duffy wrote:
On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror
wrote:


The coupler measures
about -14 dbm.


What does this mean? Are you trying to tell us that the power on the
coupler port is 14dB less than the through power? What has dBm got to
do with it?

You didn't report the power in the coupler port with a s/c and / or
o/c at the measurement plane. Did you perform this cal?


(The phase angle and reflected ware very touchy. It was almost impossible to
adjust by changing frequency. It was easier to 'adjust' it by sitting very still
and moving my arm.)


That is understandable. How much coax between the A probe and the
load, and the B probe and the load... how many degress does this total
electrical length change for a 1% change in frequency? Does that
explain some of the phase sensitivity?


The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch
long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2
foot square aluminum plate. This antenna should have an input impedance less
then 20 Ohms.


Don't you need to measure some "known" loads. Why not try a 50 ohm
load tee'd to a s/c stub (quarter wave at a known frequency) and see
if you get the predictable results at different frequencies around
resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
half that frequency it should be 0.85+j50, etc...


How can it measure very close to 50 Ohms? Is there something wrong with this
analysis?


See if you can trust your measurements on known loads before wondering
why the unknown load isn't what you expect when using unknown
measurement technology... too many unknowns.

Owen
--

I now have use of a b-directional coupler for HF and would like to perform the
following 'tests' however I do not understand them. Can you explain the last a
different way. I don't understand "two 50 Ohm loads in parallel with the stub".

Tests of stubs are now clearly reading correct for various lengths across
frequencies. That is once the 1/4 wave is determined the next odd and even
is very predicable.

Thanks,
Dan


Don't you need to measure some "known" loads. Why not try a 50 ohm
load tee'd to a s/c stub (quarter wave at a known frequency) and see
if you get the predictable results at different frequencies around
resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
half that frequency it should be 0.85+j50, etc...

On Thu, 05 Jan 2006 22:24:54 -0800, dansawyeror
wrote:

I now have use of a b-directional coupler for HF and would like to perform the
following 'tests' however I do not understand them. Can you explain the last a
different way. I don't understand "two 50 Ohm loads in parallel with the stub".

Parallel two 50 ohm loads with a tee piece, then use another tee to
put the stub in parallel. The tees will be imperfect, but at HF, the
impact should not be major.


Tests of stubs are now clearly reading correct for various lengths across
frequencies. That is once the 1/4 wave is determined the next odd and even
is very predicable.

Do you get the correct answers for 1/8, 3/8 wave stubs in parallel
with a 50 ohm load? (1/8 wave s/c stub has an impedance close to
0+jRo, and you need to put it in parallel with the dummy load, so
series equivalent is 25+j25. 25 ohms in parallel with 1.8 wave s/c
stub should be around 20+j10. Put a 25 ohm load at the end of a metre
of coax and check its transformation at different frequencies
(equivalent to quarter wave, eight wave etc) against your Smith Chart
prog.

They are some examples, work some out for what you have at hand.

Prove that your measurement system works on predictable loads.

Owen


Thanks,
Dan


Don't you need to measure some "known" loads. Why not try a 50 ohm
load tee'd to a s/c stub (quarter wave at a known frequency) and see
if you get the predictable results at different frequencies around
resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
half that frequency it should be 0.85+j50, etc...


--