On Mon, 4 Oct 2004 20:58:07 -0500, "John Smith"
wrote:

|
|"Wes Stewart" wrote in message
.. .
| On Mon, 4 Oct 2004 16:45:31 -0500, "John Smith"
| wrote:
|
|Freq (MHz) Impedance (Ohms)
|
|410 46.4 + 6.0i (50 Ohm terminator)
|
|380 9.7 - 12.5
|390 3.5 - 5.7
|400 5.1 + 1.3i
|410 5.1 + 6.5i
|415 4.0 + 10.0i
|425 2.5 + 15.7i
|
|
| | 3) How well is your signal source terminated, in other words do you
| | know its source match?
| |
| |
| |I only know that the signal source is an HP 3200B. It directly feeds the
| |Narda dual directional coupler through a few feet of RG58.
|
| If I remember that correctly the '3200 is nothing but a p-p oscillator
| and a waveguide-below-cutoff probe. If your VVM reference probe
| readings are changing much between frequencies and/or
| calibration/measurement, try a 6 or 10 dB pad right on the generator
| output and see what happens.
|
| When you're calibrating using a short, the source Z has really got to
| be nailed down.
|
| Wes
|
|
|Okay. I repeated the test using an HP 355C attenuator set for 10 dB and at
|400 MHz got 4 + 3i on the antenna. I also checked my 50 Ohm network
|terminator with this setup and it measured 44 + 4i. The data are different,
|but they're not an order of magnitude different, at least.
|
|So, although my measurements aren't repeatable, they are sloppily
|consistent. That is, although I can't say exactly what the antenna impedance
|is with confidence, I am beginning to believe that it really is very low in
|impedance. Am I drawing an erroneous conclusion too early? I can wait a
|little longer to draw an erroneous conclusion.

First of all, neglecting the feed method, the antenna is a simple
folded dipole. In free space, or an approximation thereof, it should
have a feedpoint Z of about 300 ohm. (See the ARRL Antenna book for a
description of why this is so under "Special Antenna Types", p.2-32 in
the 17th edition) In the presence of other (non-resonant) objects, it
may differ from this but not a whole lot.

In theory, the "natural balun" doesn't change the impedance of the
feedpoint whatsoever. By "feedpoint" I mean the gap between the ends
of the folded element, not the "tee" connection opposite.

At the outside of the tee connection, the voltage is zero thus this
point can be grounded, connected to the boom in a Yagi, etc. without
upsetting anything. Likewise the coax feeder can be introduced here
and run through one side of the element without upsetting anything
either.

But, a nominal 300 ohm load is terminating a 50 ohm line, so the usual
transforming effects are in play. The input Z of an arbitrary length
line is---well, arbitrary. If the line is many wavelengths long, then
when the frequency is changed, the long lines effect kicks in and the
input Z is going to vary rapidly with respect to frequency.

Second. I believe that you need to determine the parameters of your
directional coupler. As Richard pointed out, your B1/A1 numbers are
pretty unstable.

So here's what I recommend. First verify that the "A" and "B" probes
read the same thing when connected to the same source. Then put your
pad right at the input connector of the coupler. Terminate the
reverse port and connect your VVM "A" probe to the ouput connector and
the "B" probe to the forward port.

The ratio reading is the forward coupling factor of the directional
coupler. Vary the frequency and see how this changes and note some
values. Move the "B" probe to the reverse port and terminate the
forward port. Note the readings at the same frequency.

Reverse the input and output ports and repeat the measurements.
Ideally, the data sets will track closely. If they don't then you
have a problem. Serious differences might indicate damage to the
internal terminations. This assumes that this is a true dual coupler
and not single line coupler with the termination applied to the unused
port externally.

If the numbers are consistant, then you can determine the directivity
by computing the ratio between the two readings on a given port when
the coupler is reversed.

I'm going to stop here and assume you understand the consquences of
poor directivity on measurement accuracy. If you don't then I can
expound further later.

Wes