Cable TV HardLine
Amateur Radio Application(s)
| Cable TV
notes by Jonesy W3DHJ
First posted to usenet (rec.radio.amateur.antenna) circa Jan-02.
I recommend that you check with the field technicians at your local cable company. Find out what time they 'show up' in the morning prior to taking off for the first job assignment. Introduce yourself and explain your (modest) needs. You may even discover a ham in the bunch!
What you want to ask for is the 'scrap ends' from the reels they have used. The 100-to-150 foot left-over ends are usually of no use to them. They either stuff it in a dumpster (a heart-rending thought - HI! HI!), or sell it to a scrap dealer (which also brings tears to the eyes of this Ol' Scrounger!).
In this manner, I got the 1/2" hardline that feeds my 6 meter beam and the 3/4" hardline(2) that feeds my 2 meter horizontal and vertical beams.
Connectors -- dig out an older ARRL "Hints And Kinks" (the one I have is in the H&K 12th edition -- pages 5-18,5-19) (or research back QST issues) and find the article that describes using brass compression fittings to make the connectors. Use it as a guide - each assembly may be (will be) different. I fabricated N connectors for my use (to get the "weatherproofed-ness"). In a nutshell: you turn down the outer threads on a 1/2" brass coupler to fit inside the coax connector, and/or bore out the inside of a 3/4" brass coupler to allow the coax connector to slip into it. Sweat solder the coax connector and brass coupler together (which might screw up the plastic center pin collar in some coax connectors).
Then do trial assemblies to determine the exposed foam length, and center conductor length -- before you solder the tip on and lock it all down with the compression ring.
The center conductor of the 3/4" hardline is too big to fit the center pins of PL-259/SO-239 and Type N connectors. But, you do not want to shave or grind it down to size -- since you'll lose the copper cladding you need to get a good solder connection. What you do is cut the center conductor a 'wee bit' shorter than is needed. Then, using duckbill pliers (or, the smooth jaw section of some large needle nose pliers (or, the 'armpit' of a pair of Lineman's Pliers)), you work your way around the center conductor -- firmly squeezing it into a smaller, and smaller diameter. Since the center conductor is made of soft aluminum, you'll end up with a nice fit for the connector pin and it will still have plenty of copper cladding to solder to.
I 'cheat' by using a grid dip meter to cut the final length to a multiple 1/2 wavelength at the frequency of interest. This means (for me) assembling the outdoors end first; running it up the tower with a shorted connector ( as simple as a piece of wire held in place with a rubber band ); fish the other, 'extra long' end through the wall/window/whatever; and, using a small (VERY small at VHF! Just a "hump" at 2M!) single turn loop on the end of the coax, dip and cut the coax to length before doing the indoors connector. (Hint: I find it somewhat easier to dip it for a sub-multiple frequency. For instance, I figured out I would be cutting the 6M hardline to 6 half wavelengths. I was able to dip and cut more accurately at 25.1 mcs [ 3 half wavelengths ] to get my desired 50.2 mcs length.) Move up in frequency and cut in increments: Each inch ( or centimeter ) you whack off has a greater effect as you go up in frequency!
How do I rotate a beam with hardline? Again, I use a n-multiple half-wave length of RG-8 coax from the beam, down to the hardline attached to the tower. For me it was 3/2 wavelength on 2 meters, and 2/2 wavelengths on 6 meters. Alternately, you could design your whole system -- from the back of the rig, up through the gamma match, for 72 ohms.
I am impressed at how 'flexible' the 1/2" hardline is -- not that it is floppy -- but it will bend to a very nice radius and can probably be arranged to come directly to the back of your rig. (However, I am using a 1/2 wavelength RG-8 pigtail for both 6M and 2M.) Do not bend it too tightly, as the center conductor may/will tend to migrate through the dielectric insulation to the inside wall of the bent hardline. You should use a pipe bender for any really tight bends to prevent it from kinking.
It is worth considering the 1/2" stuff for all your hf applications, as well. Seeing as how cheaply (free!?) it can be acquired -- and how much better it will hold up to outside exposure than RG-8 -- it can make sense for use as feedlines for hf wire and beam antennas.
I have used the 1/2" hardline as a gamma match for a 5-element 6M beam that the local club uses on Field Day. Just start with a piece that you know in your heart-of-hearts will be too long. Then, while alternately whacking small amounts off the end -- and re-adjusting the shorting strap -- zero in on your frequency of interest. If you over-shoot -- well, no big deal. You did get 95' of it for free, didn't you?
The higher Q of the 3/4" stuff (versus RG-8 and the like) makes its use in quarter-wave suck-out traps a practicality. I have built suck-out traps for 6M and 2M and attached them to my TV antenna feedline right at the point where it enters the TV equipment. It works reasonably well. I have suggested its use to ham friends who have solved bad rf overload problems from nearby TV and FM broadcast stations by attaching the trap right at the antenna terminals. Be aware of a trap's ability to suck-out at odd multiples of its designed wavelength.
Having said all that, Your Mileage May Vary!!
(Corrections, additional tips, comments appreciated.)
Regards, es 73! Jonesy
Last updated: 18-Jun-08