Is the standard coax for antennas 50 ohm? When to use 75? Does it depend upon equipment? Thanks in advance.
Let's say you have a 2-element quad (driven element and reflector; usual configuration). The driven element's feedpoint impedance will be around 90-100 ohms. If you attach a quarter wavelength (allowing for velocity factor) of 75-ohm coax, with a proper connection to the 50-ohm feedling running the rest of the way to the shack, you'll see the ~2:1 mismatch is now very close to unity.
For higher frequencies and/or wider-spaced quads, you may want to use a 3/4 wavelength transformer rather than a 1/4 wavelength. (DO NOT USE a half-wavelength!!) The 1/4 wavelength transformer might not be long enough for the antenna to turn a full 360 degrees without pulling the feedpoint apart.
Last time I looked any odd multiple of quarter waves will yield the same impedance transformation. Maybe you were thinking of something else.
Last time I looked any odd multiple of quarter waves will yield the same impedance transformation. Maybe you were thinking of something else.
Why not just use one continuous length of 75 ohm coax cut to an odd multiple of 1/4 waves (velocity factor taken into account) all the way to the shack,
saves having any more losses involved in splicing (with 2 pl259-double so239 adapter which incurs losses) a shorter odd 1/4 wave multiple of 75 ohm coax to a 50 ohm cable run to the shack,also eliminates a point where water could ingress into the join.the resultant impedance at the radio will be the same but you'll have a tidier,less lossy and mechanically better cable run.incorporating a 1:1 coax wound rf choke in the single piece of cable at the antenna end would also be wise.