MFJ-269 Antenna/SWR/RF Analyzer
 

"John Ferrell" wrote in message
...
After reading the remarks here and researching the past posts to the
newsgroup on the MFJ-269 I feel I have been missing out on a lot. The
best overall description I have found anywhere (including the MFJ web
site) is in the current ad in QST.

I ordered it Saturday morning and am eagerly awaiting its arrival.

John Ferrell, W8CCW

I am happy with mine. Only thing I can add is that it eats batteries, and my
unit does not like to work below 13 Volts. They do sell a tunable bandpass
filter for the lower frequencies. I tried a high pass filter - don't bother,
it messes up the phase.

Tam/WB2TT

MFJ-269 Antenna/SWR/RF Analyzer
 

Tam/WB2TT wrote:
Only thing I can add is that it eats batteries, ...

I run mine off an external rechargable 12v YUASA battery.
--
73, Cecil http://www.qsl.net/w5dxp

MFJ-269 Antenna/SWR/RF Analyzer
 

Tam/WB2TT wrote:

I am happy with mine. Only thing I can add is that it eats batteries, and my
unit does not like to work below 13 Volts. They do sell a tunable bandpass
filter for the lower frequencies. I tried a high pass filter - don't bother,
it messes up the phase.

If you do want to measure antenna impedance with something like the MFJ
and have trouble (as I do) with local broadcasting and other RF, I
recommend using a "half wave" filter. This type of filter has reasonable
out-of-band attenuation characteristics and doesn't disturb the
impedance measurement too much. (Over a limited frequency range it
mimics a half wavelength of transmission line.) The impedance
disturbance will be the least when the filter is designed to
approximately match the measured impedance.

It's simply two cascaded pi sections, with each element having the same
value of X. For example, a lowpass looks like this:

---.---L---.---L---.---
| | |
C 2C C
| | |
----.-------.-------.---

For example, a filter with Z0 = 50 ohms (one which mimics a half
wavelength of 50 ohm line) has XL = XC = 50. One designed for 7 MHz
would have L = 1.14 uH, C = 455 pF. You can wind the coils on small type
2 or 6 powdered iron cores, and use the closest standard value (e.g.,
470 pF) for the capacitors. This filter will cause little impedance
disturbance at 7 MHz and, for load impedances near 50 ohms, at lower
frequencies also. A highpass looks like this:

---.---C---.---C---.---
| | |
L L/2 L
| | |
----.-------.-------.---

Again, XL = XC = the filter Z0. Make filters according to your
particular measurement needs. You can check the amount of disturbance
the filter causes by making a known impedance out of a resistor and
capacitor or inductor which is about equal to the measured impedance,
and measuring it with and without the filter.

If you can read German, there's a marvelous book describing how to make
a wide variety of measurements with an antenna analyzer, with tricks,
tips, and details. It's _HF-Messungen mit einem aktiven
Stehwellen-Meßgerät_ by Gerd Janzen, DF6SJ. It's available directly from
him at Hochvogelstraße 29, D-87435 Kempten, Germany.

Roy Lewallen, W7EL

MFJ-269 Antenna/SWR/RF Analyzer
 

"Cecil Moore" wrote in message
. net...
Tam/WB2TT wrote:
Only thing I can add is that it eats batteries, ...

I run mine off an external rechargable 12v YUASA battery.
--
73, Cecil http://www.qsl.net/w5dxp

Never thought of that. I actually have a 12V, 7 AH Yuasa sitting on the file
cabinet. I would just have to keep it well charged.
Tam

MFJ-269 Antenna/SWR/RF Analyzer
 

"Roy Lewallen" wrote in message
...
Tam/WB2TT wrote:

I am happy with mine. Only thing I can add is that it eats batteries, and
my unit does not like to work below 13 Volts. They do sell a tunable
bandpass filter for the lower frequencies. I tried a high pass filter -
don't bother, it messes up the phase.

If you do want to measure antenna impedance with something like the MFJ
and have trouble (as I do) with local broadcasting and other RF, I
recommend using a "half wave" filter. This type of filter has reasonable
out-of-band attenuation characteristics and doesn't disturb the impedance
measurement too much. (Over a limited frequency range it mimics a half
wavelength of transmission line.) The impedance disturbance will be the
least when the filter is designed to approximately match the measured
impedance.

It's simply two cascaded pi sections, with each element having the same
value of X. For example, a lowpass looks like this:

---.---L---.---L---.---
| | |
C 2C C
| | |
----.-------.-------.---

For example, a filter with Z0 = 50 ohms (one which mimics a half
wavelength of 50 ohm line) has XL = XC = 50. One designed for 7 MHz would
have L = 1.14 uH, C = 455 pF. You can wind the coils on small type 2 or 6
powdered iron cores, and use the closest standard value (e.g., 470 pF) for
the capacitors. This filter will cause little impedance disturbance at 7
MHz and, for load impedances near 50 ohms, at lower frequencies also. A
highpass looks like this:

---.---C---.---C---.---
| | |
L L/2 L
| | |
----.-------.-------.---

Again, XL = XC = the filter Z0. Make filters according to your particular
measurement needs. You can check the amount of disturbance the filter
causes by making a known impedance out of a resistor and capacitor or
inductor which is about equal to the measured impedance, and measuring it
with and without the filter.

If you can read German, there's a marvelous book describing how to make a
wide variety of measurements with an antenna analyzer, with tricks, tips,
and details. It's _HF-Messungen mit einem aktiven Stehwellen-Meßgerät_ by
Gerd Janzen, DF6SJ. It's available directly from him at Hochvogelstraße
29, D-87435 Kempten, Germany.

Roy Lewallen, W7EL

I will have to try your configuration. I did C - L - C/2 - L -C, with
C=1000PF, L=2.2uH. According to SWCad, the gain is flat above 2 MHz, but
there is 120 degrees phase shift at 4 MHz, relative to the phase at 100 MHz.

Tam/WB2TT

MFJ-269 Antenna/SWR/RF Analyzer
 

Tam/WB2TT wrote:

I will have to try your configuration. I did C - L - C/2 - L -C, with
C=1000PF, L=2.2uH. According to SWCad, the gain is flat above 2 MHz, but
there is 120 degrees phase shift at 4 MHz, relative to the phase at 100 MHz.

The center component should be 2C, or 2000 pF for your experiment, not
C/2. The circuit is simply two pi networks in cascade, each having all
component reactances equal to the "transmission line" Z0. Each pi
section mimics a quarter wave transmission line.

Roy Lewallen, W7EL

MFJ-269 Antenna/SWR/RF Analyzer
 

Tam/WB2TT wrote:

"Cecil Moore" wrote:

Tam/WB2TT wrote:
Only thing I can add is that it eats batteries, ...

I run mine off an external rechargable 12v YUASA battery.

Never thought of that. I actually have a 12V, 7 AH Yuasa sitting on the file
cabinet. I would just have to keep it well charged.

I use a 2Ah NP2-12 with a molex connector double-sided taped to
the underside of the MFJ-259 and charge it with an Astron RS-20M.
The battery is smaller than the MFJ and about 3/4" thick.
--
73, Cecil http://www.qsl.net/w5dxp

MFJ-269 Antenna/SWR/RF Analyzer
 

"Roy Lewallen" wrote in message
...
Tam/WB2TT wrote:

I will have to try your configuration. I did C - L - C/2 - L -C, with
C=1000PF, L=2.2uH. According to SWCad, the gain is flat above 2 MHz, but
there is 120 degrees phase shift at 4 MHz, relative to the phase at 100
MHz.

The center component should be 2C, or 2000 pF for your experiment, not
C/2. The circuit is simply two pi networks in cascade, each having all
component reactances equal to the "transmission line" Z0. Each pi section
mimics a quarter wave transmission line.

Roy Lewallen, W7EL

Roy,
I wasn't too clear, but I have 2 T networks back/back. That makes the center
cap C/2. I am going to run SWCad on the Pi configuration later, and see what
that does.

Tam/WB2TT

MFJ-269 Antenna/SWR/RF Analyzer
 

"Tam/WB2TT" wrote in message
...

Meanwhile, I measured the impedance of a Drake 100W dummy load with and
without the HPF. All readings without the HPF are within Drake spec.

FREQ NO HPF With HPF

4 Mhz 47j2 33j13
7MHz 47j2 55j3
14MHz 47j1 50j7
28MHz 48j2 47j3
50MHz 49j2 54j11
144MHz 53j11 74j36

Everything was connected with UHF adapters, and no coax was used. Ignore the
VHF readings,. as the filter was not built that carefully. Capacitors are
mica (actual values 1000, 560, 1000), and inductors appear to be 68-2
(2.2uH).

If I get a chance later today, I will rewire it into the Pi configuration
with the same inductors.

Tam/WB2TT

MFJ-269 Antenna/SWR/RF Analyzer
 

Tam/WB2TT wrote:

Roy,
I wasn't too clear, but I have 2 T networks back/back. That makes the center
cap C/2. I am going to run SWCad on the Pi configuration later, and see what
that does.

If you've cascaded two sections, you have two 1000 pF capacitors in
parallel at the center. That makes a total value of 2000 pF at that point.

Roy Lewallen, W7EL