[TowerTalk] RG-149: 50 ohm/70 ohm - what does it matter?

[James Wolf]

>Suppose his "50 ohm" antenna has a 1.5:1 SWR and its impedance is actually
33.3 ohms.  Suppose he operates on a frequency where his
>75 ohm feedline is an odd multiple of a quarter wavelength.  Then the
impedance his transmitter will have to drive will be 169 ohms, an SWR of
3.4:1.  This will require a transmatch.

>It really does matter whether the line is 50 or 75 ohms.

>Rick N6RK
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Exactly Rick,

If 75 ohm coax is cut to 1/4 wave multiples on the ham bands, the bandwidth
of the coax/antenna "system" will be narrower than with 50 ohms on the ham
band edges because of the coax mismatch additive component.
And to compound that, on a mismatched coax cut for 1/4 wave, the longer the
coax (or the more multiple wavelengths) involved, the possibility of getting
a good VSWR eventually diminishes. I doubt this will be seen on HF, but
rather on VHF and UHF.

Jim, KR9U