Hi Paul,
The VNA was calibrated at the VNA directional coupler port for 50 ohms
without any cable attached.
All cables used were 50 ohm.
When using the 75 ohm load with no tuner the SWR always read 1.5:1 no matter
which cable was used between the VNA and the 75 ohm load.
With the tuner in line and the 75 ohm load at the output side of the tuner
and the tuner adjusted for a 1:1 SWR at the VNA it made no difference which
length of line was used between the VNA and tuner. The SWR stayed the same.
Even with the tuner "detuned" for 2:1 or 3:1 SWR at the VNA, the SWR stayed
the same when changing cable lengths between the tuner and VNA.
My intent with this setup was to emulate the tuned input of an amp.
However:
Adjusting the tuner for 1:1 with the 75 ohm load directly on the output of
the tuner and keeping the same line after this between the tuner and VNA.
When changing the length of cable between the output side of the tuner and
the 75 ohm load, and leaving the input cable the same, I did see a change in
SWR at the VNA when changing with different cable lengths as I described in
my earlier post.
"with just a 4 foot cable on the output of the tuner between tuner and 75
ohm load the SWR was 1.8:1.
With the 10 foot cable SWR was 1.46:1.
With the 10 foot and 4 foot cable the SWR was 1.4:1"
This makes me wonder if people that are seeing an SWR change with different
lengths of cable between the radio and amp may have the radios tuner
involved?
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Paul
> Christensen
> Sent: Tuesday, January 13, 2015 9:21 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Input SWR revisited
>
> Roger, that's right - I forgot that the VNA was being used and not a
> directional line coupler.
>
> The results are not surprising. However, it would be interesting to
> observe SWR at the VNA with random changes in line length between the
> VNA
> and the tuner when he used: (a) the 4 ft. section between the tuner and
> 75-ohm resistive load; and (b) a 19-inch line section between the VNA
> and
> tuner. If I have his facts straight, that showed an SWR value of
> 1.8:1.
>
> If Gary was trying to use the tuner to emulate an amplifier input
> network,
> then what we want to see is measured SWR sensitivity to change as a
> function
> of line length between the VNA and tuner when a non 1:1 SWR is present
> between the tuner and load.
>
> Paul, W9AC
>
> -----Original Message-----
> From: Roger D Johnson
> Sent: Tuesday, January 13, 2015 9:21 AM
> To: Paul Christensen
> Subject: Re: [Amps] Input SWR revisited
>
> The VNA does not measure the VSWR on the line. It measures VSWR at the
> input to the VNA unless you do a cal at the load end of the line to
> remove
> it's
> effect.
>
> 73, Roger
>
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