K1TTT Technical Reference

Questions about my use of 50 vs 75 ohm cables:



Robert Shohet wrote:


> Ok, now I am really curious....

> > >

> > > Are you saying that you run 75 ohm coax from all the antennas to the

> antenna

> > > switches and then from the antenna switches, run 50 ohm to the

> wattmeter?

> >

> > between the switches on the towers and the outside wall of the shack all

> the

> > lines are 75 ohm.  from the switches to the antennas is 50 ohm, and from

> just

> > outside the shack wall all the way to the radio is 50 ohm.

> >


> Let me ask a naive question.... Let's say that the antenna really is at 50

> ohms with 1:1 swr.  The you add the 75 ohm piece to bring it inside the

> shack.  SWR is now (at least) 1.5:1 not including the effects of any

> reactance since the 75 ohm piece is not a multiple of a 1/2 wavelength.  Now

> you go back to 50 ohm.  Doesn't that multiply the mismatch by at least

> another 1.5:1?  Or does that somehow reduce the mismatch by 1.5 :1?   But

> then since the piece inside the shack is not a multiple of 1.5:1 and

> reactance is a factor of some unknown quantity, doesn't this create a total

> "witches brew" of the swr and impedance at the radio and amp?  Please help

> me understand this.


 _  50 ohm             75 ohm                      50 ohm    |||

|_|------------================================--------------||| 50 ohm antenna




ok, from right to left...

on the 50 ohm line attached to the antenna the swr is 1:1

on the 75 ohm line the swr is 1.5:1


at the left end of the 75 ohm line the impedance can be anywhere from 50 to

112.5 ohms due to the impedance transformation along the line.  every 1/2 wave

the 50 ohms will repeat, and half way in between at the odd multiples of 1/4

wave you get 112.5 ohms. 


so on the 50 ohm line to the transmitter you can have anything from 1:1 to

2:1(approx) swr.


on a monoband system there are 2 easy fixes if you get an impedance you don't

like... i always leave a bit extra hardline anyway so that can be trimmed to

bring it closer to the 1/2 wave multiple.  or in one case (160m where trimming

1/4 wave wasn't very practical) i made the line from the hardline to the tx out

of 75 ohm rg11 which made the impedance much better.


all my systems are made like this and the only ones i have had to play with were

160m and 15m a bit, i guess i was just lucky that the others were close enough

to 1/2 wave not to bother with.  you could of course measure out the hardline in

advance with an analyzer.  there are also other methods like one that puts

coaxial transformers at each end, or the 50-75 ohm un-un... for hf though both

of these seem like overkill and add complexity with extra connectors and other

things to break or cause more loss than the mismatch does.


btw, on mine i always stop the hardline just outside the shack and bring rg8/213

into the shack.  its just too hard to bend that catv stuff around without

kinking it.



> > >

> > i never measure swr on the 75 ohm cables, only one the 50 ohm ones inside

> the

> > shack, or on the 50 ohm pieces going to the antenna when i am tuning them

> on the

> > ground.  i have never had a problem with this.

> >


> Now does this also mean that with my hardine stubs (75 ohm fused disc CATV),

> when I am making 1/4 sections of coax (to connect the stubs to each other)

> that I should have used  75 ohm coax (RG11 say) instead of 50 ohm coax

> (RG213)?


no, because the 1/4 wave stubs are meant to look like an open or short at the

end where they connect to the 50 ohm feedline the actual impedance of the stub

cable doesn't matter.  so you want to keep the section of coax between them the

same as the rest of the cable that the goes to the transmitter.