On Wed, Nov 1, 2023 at 10:48 PM, mfle2001 wrote:
Thats interesting Bill, I guess VSWR read by an SWR meter varies along the length of coax because of the changing impedance / voltage ratio, but the actual SWR does not. I am currently building a GG amp and wondering about having a VSWR meter coupler directly at the input.
Mike
VK2AMF
I don't buy that for one second.? ?SWR won't change at all with different line lengths.? Only the R / X? will change, but swr remains the same.? ?Having said that, with real long lines to ants, like say 200-300'? on upper HF,? swr at the shack end will decrease, simply cuz of line loss.? IE: say 3 db of line loss.? Reflected power (after line loss to the ant,? ?gets attenuated by the same 3 db).? IE: joe ham puts put 200 watts frwd power.... but only 100 w gets to the ant.? With a 3:1 swr ( measured at the ant), he has 25% reflected power...( 25 watts reflected).? ?That same 25 watts gets attenuated by 3db, and only 12.5 watts is shown as reflected power at shack end.?
?
200 frwd and 12.5 watts reflected is the same as? 100w frwd and 6.25 watts? reflected = 1.4:1 swr.? ?Meanwhile his real swr is 3:1? at the ant, and he only has? 75 watts radiated.?
?
In the case of coax to the amp, we are only talking abt real short lengths, like 3' to 10'...and coax loss is dick.? ? The input Z of the amp didn't change... and the stray C, between cathode? and chassis? (via the grnded grid)? didn't change, it's constant....and it's also directly in parallel with the C2 cap of the tuned input for each band.? On the L4B, a single .01uf @ 2 kv disc coupling cap,? directly between the tuned input...and the cathode.? There is NO coax? between the output of the tuned inputs...and cathode of tube....just the single coupling cap.? ?
