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Author Topic: SWR varys with power  (Read 1016 times)
VE4BLB
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Posts: 31




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« on: October 04, 2012, 01:07:39 PM »

I have a vertical (R8) and a 3 element SteppIR. They are both fed through RG213 from the desk to outside. On the desk I have used RG8X to connect the antenna switch, linear, and the external SWR/wattmeter (LDG TW1) I also use the meter on the K3.
At ~90W the SWR is higher than at 25W. The amount of difference varies with the band but always is higher on either meter.
Why?
I suspect I inadvertently put 500W through the RG8X while on 6 meters. This may have upset its characteristics a bit.
Shal I install RG213 all the way?
73 de me BLB
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73 de me BLB
W4OP
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Posts: 388


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« Reply #1 on: October 04, 2012, 03:39:00 PM »

How much higher? VSWR is a function only of the mismatch between the transmission line and the antenna. Power is not in the equation. almost always  when I see higher VSWR at higher power it is because of poor design the instrument used to measure VSWR.
Dale W4OP
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KD0REQ
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Posts: 836




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« Reply #2 on: October 04, 2012, 04:57:24 PM »

unless you are using a double-needle SWR meter that autocorrects by using a "crossed over scale Z means SWR of Z" scaling, both the forward and reverse power will increase as you crank up the skywarmer.  that is why there is a "calibrate" knob on the single-meter jobs with a FWD/REV switch, like the old Heathkit and other vintage reflectometer designs.  you have to adjust for full scale in FWD power to get an accurate REV, or SWR scale, reading.
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