A NUTTER COAX QUESTION?

[yama junk owna]

I have heard some guys talking about running long runs of cable say up the side of a mountain, using 75 ohm coax for the longest part of the run, with a specific length of 50 ohm at each end. Not sure on lengths but the thinking is that 75 ohm is cheaper and the runs at each end with 50 is supposed to match it back to 50 ohm all the way? What do some of you think? :idea:

 

[Captain Kilowatt]

Most guys that do that are using 75 ohm cable TV trunk line,the large low loss stuff that they get for next to nothing from the cable company as reel ends.Any impedance cable can be used if it is the proper length.In order to present the true antenna impedance at the TX end the cable must be in multiples of 1/2 waves long.You don't really need the 50 ohm cable on the ends,however if using the CATV trunk cable which is really hard to bend,it would be much easier.

 

[W5LZ]

owna',
Well, if you know the length of the 75 ohm stuff, and adjust it accordingly, you wouldn't need the 50 ohm stuff at the beginning or the end of the run. Just adding 50 ohm cable to the emds of a long run of some other impedance cable isn't going to change the input or output impedance to 50 ohms (unless the 'other' stuff happen to be exactly the right length). It just doesn't work that way. Sorry 'bout that...
- 'Doc

 

[Jay in the Mojave]

Hello Yama Junk Owna:

You got some real good advice here from QRN and Doc.

You didn't tell us what type of antenna you wanted to use or the length of coax you want to use.

Its possible that some antennas can be tuned to work with the 75 Ohm Coax.

No there is no magic length of coax that will easly transform it back to 50 Ohms. But there where articles in the ARRL Handbook showing how to make 50 to 75 Ohm Toroid Transformers, for just the use you want. Using the 75 Ohm coax.

Jay in the Mojave

I have heard some guys talking about running long runs of cable say up the side of a mountain, using 75 ohm coax for the longest part of the run, with a specific length of 50 ohm at each end. Not sure on lengths but the thinking is that 75 ohm is cheaper and the runs at each end with 50 is supposed to match it back to 50 ohm all the way? What do some of you think? :idea:

 

[nomadradio]

This trick gets used in the eastern part of our fair commonwealth. Biggest headache they seem to have is with the rotator-control cable. Was never meant to be used with a wire that long. And 500 feet of 8)FAT conductors makes the coax look cheap. Two of the wires used in the old American-made CDE 8-wire rotators hook to the 'run' capacitor in the control box. One slick fella figured out how to move that capacitor to the motor end of the cable, and saved himself the need for two of the 8 wires. Gotta figure six conductors is cheaper than eight.

I never had any trouble just using the 75-ohm coax with 50-ohm toys at each end. Biggest reason I quit using it is that the stuff is brittle. Bend it a LITTLE too much and the aluminum shield crimps and cracks. Unless you support the stuff REALLY well, it's much more fragile to handle than flexible-shield coax.

Jay, somewhere around here is an ancient wattmeter that was selectable for 75 or for 50 ohm use. Seems to me this was a popular feature 40 years ago. If you're just gonna convert the antenna to present a 75-ohm load, you'll need a 75-ohm meter to tell, right?

73

 

[yama junk owna]

Thanks, I personally don't have any need of this setup I just heard some folks talking about it a while back, I just didn't see how a length of coax was going to change impedance, but I thought I might be over looking something, I know back in the 70's a lot of guys would use the old flat ladder tv wire for mountain lines up to their antennas and I have seen all kinds of different types of matching coils made for it don't really know if any of them really worked or not! Thanks for the replys !!