[Elecraft] P3TXMON VSWR Measurement Accuracy

[Robert Nobis]

Don,

Thank you.  I tested separately. When using the P3TXMON, the calibrated tester was not in the transmission path.  I actually did what you suggested in your second paragraph.

As another test, I borrowed another ham’s Elecraft W2 SWR meter.  I replaced the PSTXMON with the W2.  The readings using the W2 software utility were close to the readings from my tester, about 1.08:1. Significantly, different than the reading form the P3TXMON of 2.0:1.

73,

Bob


Bob Nobis - N7RJN
n7rjn at nobis.net


> On Sep 8, 2015, at 15:23, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> 
> Bob,
> 
> How are you testing?
> If you are testing with the P3TXMON and your 'calibrated VSWR tester' in series with your feedline, then those numbers may be incorrect.
> The 'calibrated VSWR tester' is reading the SWR seen looking into your antenna, but the P3TXMON is looking at the SWR looking into the input of your VSWR tester - and that may be two different things.
> 
> If you want to do a valid test, obtain the SWR reading on your VSWR tester, and then take the feedline off that tester and connect it to the P3TXMON directional coupler and compare the SWR reading obtained from that combination.
> 
> In other words, while your VSWR tester may be calibrated at its output, there is no guarantee that its coupler does not introduce an SWR at its input.  Switching the same coax from the output of your tester to the output of the P3TXMON is the only valid test.
> 
> 73,
> Don W3FPR
> 
> On 9/8/2015 1:49 PM, Robert Nobis wrote:
>> I am having a VSWR measurement accuracy issue with my P3TXMON.  I have a calibrated VSWR tester that indicates my antenna and feedline VSWR is 1.05:1. When I connect the P3TXMON and transmit, the PS display shows my VSWR is 2.0:1 into my feedline and antenna.
>> 
>> I also tried testing into a dummy load. The VSWR tester shows a VSWR into the dummy load of 1.01:1, whereas the P3TXMON shows a VSWR of 1.29:1.
>> 
>> Power level readings on the P3TXMON are within 5% of measured power levels using a power meter, so the P3TXMON is reasonably accurate measuring the transmitted power.
>> 
>> In reviewing the P3 manual, I do not see any methodology for calibrating the VSWR measurement accuracy for the P3TXMON.
>> 
>> Any suggestions on how to calibrate the P3TXMON for better measurement accuracy?
>> 
>> 
> 
>