>
>measures wrote:
>>
>///snip///
> >>
>> >> ? How do these standing waves remain invisible to the SWR meter at the
>> >> halfwave measuring point?
>> >> >......
>> >
>> >I think only you say they do.
>> >I didn't.
>> >
>> ? If "The Z looking into the cable is 50 ohms", how can there be
>> standing waves at that point?
>>
>> later, Mike
>>
>> - Rich..., 805.386.3734, www.vcnet.com/measures.
>----------------
>Hi Rich,
>I think the main problem here is a lack of understanding as to what a
>standing wave is. It is simply a voltage or current on a line that
>changes amplitude as you move from one end of the line to the other.
>This is due to the Z mismatch at the load/transmission line junction.
>Using your example of a 50 ohm load & 93 ohm line:
>with 100 watts output from the generator the voltage at the generator
>end of the 93 ohm 1/2 wavelength line will be 70.71 volts @ 1.414 amps.
>ExI = 100 E/I = 50 ohms.
>
OK
>At the 1/4 wavelength point on the line the voltage will be 131.52 volts
>@ .77 Amps and 90 dgr phase angle. The Z at this point on the line is
>172.98 ohms... eI=100 e/i= 172.98 ohms and the max E/ min E =
>131.52/70.71 = 1.86 = VSWR
>
? indeed. So when the 50-ohm SWR meter is moved from either end of the
halfwave section to the 1/4 wavelength / midpoint, the SWR increases from
1:1 to 1.86:1. . This is why SWR measurement is an inexact science.
>At the antenna end of the line the voltage is again 70.71 v and the
>current is 1.414 Amps which gives 100 watts and 50 ohms. 93 ohm
>feedline / 50 ohm load = 1.86:1 SWR.
>
? ... as measured with a 93-ohm swr meter. When measured with a 50-ohm
swr meter, the swr is 1:1. // The 93-ohm half wavelength section is
more than a simple transmission line. It is effectively a
reactance-reversing, impedance-repeating, resonant transformer.
>So, The Z looking into the cable is simply the voltage / current at that
>point on the line. If you measure the voltage and current and
>associated phase angles with a scope and the appropriate probes you will
>see a single voltage and a single current at each measurement point.
>
? Agreed, Bob
- Rich..., 805.386.3734, www.vcnet.com/measures.
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|