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.
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
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.
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.
I hope this helps.
73, Bob, W5AH
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