MFJ-921 tuner question
 

With a [DC-500MHz] dummy load on my new MFJ-921 2M tuner, both
adjustable capacitors have to be set at around 3 to get SWR=1.0, with
the minimum being quite sharp.

I took off the cover and verified that cap settings of zero are indeed
minimum capacitance.

See: http://www.hy-gain.com/man/mfjpdf/MFJ-921.pdf

Shouldn't both caps be at the minimum setting for SWR=1.0 with a dummy
load connected? What does it mean if I need to add capacitance at
both points to tune a dummy load?

Should I reset the knobs so that they show zero when a dummy load is
connected?

Ken KC2JDY
Ken
(to reply via email
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ith a [DC-500MHz] dummy load on my new MFJ-921 2M tuner, both
adjustable capacitors have to be set at around 3 to get SWR=1.0
Shouldn't both caps be at the minimum setting for SWR=1.0 with a dummy
load connected?

The best way to get a 1:1 SWR with a tuner connected to a dummy load would be
to put the tuner in bypass (if it has one). Adding a tuner between your dummy
load and swr bridge will make the bridge see something besides 50+-j0 ohms even
with the caps set at minimum.
What you are having to do is adjust the tuner until the bridge once again sees
50+-j0 ohms. Apparently with the 921 this is with the caps around 3.
Should I reset the knobs so that they show zero when a dummy load is
connected?

Not unless you just want to. The numbers are for reference so you can quickly
reset the tuner for a given antenna or impedance.
73 Gary N4AST

Dee D. Flint wrote:

"Ken" wrote in message
...

With a [DC-500MHz] dummy load on my new MFJ-921 2M tuner, both
adjustable capacitors have to be set at around 3 to get SWR=1.0, with
the minimum being quite sharp.

I took off the cover and verified that cap settings of zero are indeed
minimum capacitance.

See: http://www.hy-gain.com/man/mfjpdf/MFJ-921.pdf

Shouldn't both caps be at the minimum setting for SWR=1.0 with a dummy
load connected? What does it mean if I need to add capacitance at
both points to tune a dummy load?

Should I reset the knobs so that they show zero when a dummy load is
connected?

Ken KC2JDY
Ken
(to reply via email
remove "zz" from address)


Unless you have a tube type radio where the radio itself may need tuned to a
50 ohm output, the dummy load should be connected to a bypass port on your
tuner. Then the settings of the capacitors is unimportant. You then use
the dummy load to make adjustments to your ALC level, mic gain, processor
gain, etc without transmitting over the air.

Dee D. Flint, N8UZE


This tuner dosen't have a position to bypass the unit. I would guess the
reason for having to add some capacitance is because there are three
fixed inductors in series and you have to tune out their reactance to
get 50 ohms.

"Ken" wrote in message
...
With a [DC-500MHz] dummy load on my new MFJ-921 2M tuner, both
adjustable capacitors have to be set at around 3 to get SWR=1.0, with
the minimum being quite sharp.

I took off the cover and verified that cap settings of zero are indeed
minimum capacitance.

See: http://www.hy-gain.com/man/mfjpdf/MFJ-921.pdf

Shouldn't both caps be at the minimum setting for SWR=1.0 with a dummy
load connected? What does it mean if I need to add capacitance at
both points to tune a dummy load?

Should I reset the knobs so that they show zero when a dummy load is
connected?

Ken KC2JDY
Ken
(to reply via email
remove "zz" from address)

Unless you have a tube type radio where the radio itself may need tuned to a
50 ohm output, the dummy load should be connected to a bypass port on your
tuner. Then the settings of the capacitors is unimportant. You then use
the dummy load to make adjustments to your ALC level, mic gain, processor
gain, etc without transmitting over the air.

Dee D. Flint, N8UZE

Ken,
What you are seeing is fairly 'normal', nothing to be
worried about. I also wouldn't bother resetting the
controls on the tuner, just be aware that '0' doesn't
always represent '0', the numbers are just for reference.
'Doc