[TowerTalk] Probably dumb question

[Jim Rhodes]

Notice the huge difference between the two sets of calculations. They are
"ratios" as long as all the measurements are in the same units, measured on
the same system you are good to go.


On Tue, Aug 14, 2018 at 1:08 PM jimlux <jimlux at earthlink.net> wrote:

> On 8/14/18 9:19 AM, k7lxc--- via TowerTalk wrote:
> >      If there are 100 watts out and 10 watts reflected, what's the swr?
> >
> >      If there are 200 watts out and 10 watts reflected, what's the swr?
> >
> >      Tnx.
> >
>
> Since SWR is the ratio between high and lowest  "voltage", you need to
> calculate that.
>
> For a 50 ohm line, 100W is 70.7 V, the 10W reflected is 22.4V - so max
> voltage is 70.7+22.4 =93.1 and min voltage is 70.7-22.4 = 48.3 so VSWR =
> 93.1/48.3 or 1.93
>
> (the answer is impedance independent.. I just used 50 for an example)
>
> This is handy to remember a 2:1 is about 10dB return loss  (actually
> it's 9.5 or something...)  1.5:1 is about 14 dB
>
>
>
>
> For the 200W and 10W case the forward voltage is 100V, the reverse is
> 22.4 so it's 122.4/77.6  = 1.58
>
> (this is 13 dB return loss, so it's a bit worse than 1.5:1, which would
> be 14 dB)
>
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-- 
Jim K0XU
jim at rhodesend.net