Worldwide DX Amateur Radio Forums - Ham - CB - HF - VHF - UHF

Is the standard coax for antennas 50 ohm? When to use 75? Does it depend upon equipment? Thanks in advance.

You can use 75ohm coax to make phasing lines for antennas, or balun to "fit" antenna impedance to transmitter impedance.

Usually if transmitter impedance is 50ohm antenna impedance is 50ohm than feeder should be 50ohm as well.

You can use 75ohm coax, but you need to use specific length to transform antenna impedance to transmitter impedace. In this case you can use 75ohm feedr.
Mike

__________________

'sp5it' pretty well says it. The only time 75 ohm coax is really all that desirable is with receiving systems (TV/cable) and with phasing or impedance changing transformers (what those 75 ohm sections in a feed line do). That particular value, 75 ohms, os a sort of compromise dealing with typical impedances with radios, sort of. Wish it were 100 ohms, but oh well...
- 'Doc

then again, it just depends on the antenna and where you wanna deal with that impedance transformation. A flat-top dipole's input impedance is very close to 72 ohms, give or take a few. That would mean that the SWR at the feed point using 75 ohm coax would be very nice. Of coourse, on the other end you'd have to deal with changing that 75 ohms to 50 ohms for the transmitter. And then again, having a tuner at the transmitter end would make that a snap, now wouldn't it? If there ain't but one way to do something you probably just aren't trying hard enough... sort of.

When to use 75 ohm cable?

When a friend calls and says they widened the highway and ripped down 6000 feet of 1" Foamflex. It's laying in the scrap pile. How much do I want?

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.

__________________
If you're like everybody else,
you're about average.
73 de K7KBN

Last time I looked any odd multiple of quarter waves will yield the same impedance transformation. Maybe you were thinking of something else.

Beetle is talking about the fact that a single 1/4 electrical wavelength of cable on 10 or 12m is not long enough to go from the driver element to below the rotator on a tower. Most guys that run multiband quads use a remote antenna switch mounted at the top of the tower and run matching lines out from there. You could use 1/4 wave lengths but then you would need to add in another short piece of 50 ohm cable to finish the run to the switch. Why bother with another two cable connectors, a barrel connector, and a short run of cable,all potential problem areas, when you may as well simply run a single piece 3/4 wave long.Additional losses are nonexistant.That's my plan for when I build my two element (maybe three) five band quad next year. Hopefully.

__________________
Front/back ratio comes from the antenna. Forward gain comes from the wall socket.

That's exactly what I said, I think. THREE-quarters wavelength IS an odd multiple of one quarter wavelength. Does the exact same transformation, but the 3/4 wave is long enough to allow the antenna to rotate a full 360 degrees with no binding.

I pointed out that using a half wavelength transformer (2/4 = EVEN multiple) would be a BAD idea.

__________________
If you're like everybody else,
you're about average.
73 de K7KBN

Makes sense if you are using hardline and want to save a jumper.

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.