>
>On Monday, October 04, 1999 2:56 PM, Jon Ogden [SMTP:jono@enteract.com]
>wrote:
>>
>> Vic Rosenthal wrote:
>>
>> >>
>> >> >What is the SWR as measured at the generator with a 50-ohm
>> >> >characteristic-Z SWR meter?
>> >>
>> >> ? At the end of an any characteristic.-Z halfwave transmission
>line, the
>> >> termination Z repeats itself -- with a reactance reversal. .
>Since
>> >> there is no reactance in this termination, the Z at the end of the
>93-ohm
>> >> halfwave line is 50 +/- j0 ohms, the SWR is 1:1.
>> >
>> >No. It is correct that the impedance seen by the generator is 50
>ohms but
>> >the
>> >SWR is unchanged, regardless of the line length. The SWR is
>entirely
>> >determined by the impedances of the line and the load.
>>
>> This isn't correct, Vic.
>>
>> In a 93 Ohm system, the SWR is unchanged. However, we are attempting
>to measure a 50 Ohm load in a system where 50 Ohms is our base (the SWR
>meter has been specified as being 50 Ohms and the exciter is also
>assumed to be at 50 Ohms).
>>
>> At a half wavelength of coax, you rotate completely around the VSWR
>circle and thus see the impedance of the load at the input of the coax.
>In this case, the load is 50 Ohms. At the half wavelength of coax, the
>load is 50 Ohms as well. The SWR in a 50 Ohm system then is 1:1. In a
>93 Ohm system you would have the SWR mismatch of 1.86:1, but in a 50 Ohm
>system, our VSWR is 1:1.
>>
>
>Jon - Your statements seem to contradict the most important concept I've
>learned in the last few days.
>
> The SWR in the cable is set by the load/cable mismatch.
>PERIOD.
>
>If this isn't true, then I haven't learned a thing :(
>
>If it is, I'll continue:
>
>Remove the SWR meter and the generator. Cable and load only. What is
>this black box?
>It's 1.86:1 VSWR regardless of line length. The cable could be 2 meters
>or two miles (yes, ignoring attenuation).
>Why would putting a SWR meter at one end change this?
>
>In general you normalize to the cable, in this case 93 ohms.
>
>
>> So the bottom line is that if you have a 50 Ohm load and a 1/2
>wavelength of coax that it matters not what the impedance of that coax
>is! It's inherently narrow band, but that's the case.
>>
>
>Yes, to transform the impedance back to "itself" it matters not what the
>cable Z is.
>The SWR is a different matter.
>
? So even though the Z at the halfwave point is 50 +/- j0 ohms and we
are using a 50-ohm meter, there are in fact, standing waves?
- Rich..., 805.386.3734, www.vcnet.com/measures.
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