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W5LZ · #11 10-20-2009, 06:10 PM

That will certainly work! There are 'catches' to it though. Is that 75 ohm coax as cheap as the 50 ohm stuff? How about losses compared to the 50 ohms stuff? Those connectors do mean some slight losses, but they are NOT as much as they are made out to be most times. One connector is certainly better than 20 or 30 of them though!

- 'Doc

Beetle · #12 10-20-2009, 07:34 PM

Quote:

Originally Posted by jazzsinger

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.

That might work well for a single band quad, but I'm talking about a 5-band quad. As QRN described, I run a single RG-213 to a remote coax switch near the top of the tower, and I have three 75-ohm 1/4-wave lines and two 3/4 wave lines from the switch to the feedpoint of each driven element. Pretty standard fare, and it works perfectly.

Captain Kilowatt · #13 10-20-2009, 10:37 PM

What beetle said. When running multiband antennas you cannot run a single length of tuned feedline to act as an impedance transformer. 1/4 wave on 20m is 1/2 wave on 10m etc.

wambulance · #14 10-23-2009, 01:55 PM

Cool. Sticking with 50ohm strictly myself.

Question; ok to use RG-58/U 50ohm in place of other 50ohm coax for radio?

Beetle · #15 10-23-2009, 05:45 PM

Just keep in mind that RG-58 will not handle high power levels, and that it has higher losses than, say, RG-213.

wambulance · #16 10-24-2009, 12:33 PM

Ouch! RG-213 is expensive hehe. I'll stick with the RG-58/U for now unless I need to run a longer cable or put more than 15 watts into it. Thanks for letting me know about that though, as I had no knowledge of RG-213 before you said something about it.

W5LZ · #17 10-24-2009, 04:45 PM

Nothing wrong with RG-8 either.

Beetle · #18 10-24-2009, 09:43 PM

I haven't seen any genuine RG-8 coax for decades. It's no longer a Military Specification cable, so ANY bozo can throw together something and call it "RG-8", "RG-8/U" or whatever he wants to call it. REAL RG-8 hasn't been made in about 30 years, and any Cold War/Viet Nam era surplus is way past the pull date.

W5LZ · #19 10-24-2009, 10:45 PM

'Real' or not, most every manufacturer of coax still makes some kind of version of it. Typically has the same characteristics as that of 30 years ago. What's the difference?
- 'Doc

Beetle · #20 10-25-2009, 10:34 AM

Radio Shack "RG-8" has probably 70% shield coverage, IF THAT. I've seen "RG8 TYPE" coax with a center conductor about 2/3 the correct wire size (and very skimpy shielding).

Dielectric layers broken or just not there, like "foamed in place" construction that skipped a few inches.

Because it's no longer MILSPEC cable, the manufacturers don't have to comply with ANY of the old RG8 criteria. And far too many of these fine manufacturers seem to agree with that statement.

"Genuine BELDON [sic] RG8 TYPE" coax.

No thanks. I'll stick with name brands that are at least spelled correctly!

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When to use 50ohm, or 75?