Elecraft

SWR Readings:Differences

Classic

List

Threaded

22 messages Options

Wes (N7WS)

Re: SWR Readings:Differences

I'm talking about what goes on in the transmission line.

You're bringing up a change in the "antenna". That's another subject :-)

Wes N7WS

On 9/3/2014 2:41 PM, Alan wrote:

>
> On 09/03/2014 02:12 PM, Wes (N7WS) wrote:
>> Oh dear me!
>>
>> If I take a lossless 50-ohm line and terminate it in 100 ohm and measure the
>> VSWR using an ideal bridge/coupler/VNA/etc that is calibrated for a 50-ohm
>> system, I will measure 2:1 SWR no matter how long the line is, from zero to
>> infinity. The transformed Z will change with length, but the SWR will not.
>> That's why one can draw a circle of constant SWR on a Smith Chart. Any point
>> on the circle will have a different Z from another, but they all have the
>> same SWR.
>>
>> If you change line length and the SWR reading changes, then: 1) the line has
>> loss, 2) the line Z and the SWR meter Z are different, 3) the source match is
>> poor, 4) the bridge/coupler directivity is poor, or 5) all of the foregoing.
>> With most ham stuff, it's 5.
>
> Or 6) there is common-mode current on the feedline. In other words, the
> feedline is part of the antenna so when you change its length you change the SWR.
>
> Alan N1AL
>
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

Wes (N7WS)

Re: SWR Readings:Differences

In reply to this post by Stephen G4SJP

Stephen,

Thanks for your note.

If hams would think of mismatch in terms of return loss and coupler directivity
then they might be less surprised at the variability of SWR measurements between
different 'instruments', such as the built-in SWR measurement circuits within
our transmitters/amplifiers.

This is an old discussion. See:
http://marc.info/?l=elecraft&m=125734496911585&w=2 and:
https://www.mail-archive.com/elecraft@.../msg76426.html

Wes N7WS

On 9/3/2014 2:35 PM, Stephen Prior wrote:

> Wes,
>
> As someone involved in the design and manufacture of couplers for VSWR
> measurement for the aviation industry (admittedly some 30 odd years ago!),
> I would say that you are 100% correct, although I suspect that in amateur
> gear number 4 in your list is probably the biggest culprit of all. We had
> people on the production tweaking bridges to maximise directivity and it
> was a job that required some skill, e.g. bending leads of matched zero bias
> Schottky diodes until the spec was achieved - admittedly this was in the
> days before the large scale adoption of lead-less components, which must
> have made things a bit easier.
>
> 73 Stephen G4SJP
>
>