75 ohm coax on UHF - VHFUHF Forum at eHam.net

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eHam.net Forum : VHFUHF : 75 ohm coax on UHF

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75 ohm coax on UHF	Reply
by K9GRN on May 6, 2008	Mail this to a friend!
What will happen if I feed 25 watts into a six-element UHF yagi, using approximately 50 feet of 75 ohm coax as the feed line? I've searched the forums and see several related discussions; most saying 75 ohms is OK for HF and up to VHF, but I can't find a specific answer to UHF. What I've learned so far is that the mis-match will reduce radiated power and increase reflected power and the latter generates heat. Is the reduction in power enough to render the antenna useless or just less than optimal? Way less or a little less? Most important, will the heat be enough to damage the radio? Damage the coax? Or start a fire? The coax goes through several inaccessible spots on its route from the basement to the attic and would be very difficult to replace with 50 ohm, so I'm hoping I can use it. It's CommScope coax, but shows no model number. The center conducter is not copper. I have adapters to take it from the PL-239 to the cable TV fitting and back again. Thanks in advance for any advice.

RE: 75 ohm coax on UHF	Reply
by AA4PB on May 6, 2008	Mail this to a friend!
Frequency = 440MHz Coax = RG59 Length = 50-feet Transmitter output = 25W SWR on feedline = 1.5:1 Total feedline loss = 3.574dB Power at antenna = 10.97W Impedance at transmitter = 83.66 Ohms (82.81+j11.89)

RE: 75 ohm coax on UHF	Reply
by N3OX on May 6, 2008	Mail this to a friend!
If it's RG-6 type coax (bigger than RG-59) it's got less loss, 2.3dB at 440MHz in 50 feet. That's about 11W lost (which certainly isn't enough to start fires!!!! In fact, all 25W lost along a length of coax wouldn't cause appreciable heating, really) But to put that in perspective, RG-213 coax at that length has 2.4dB matched loss at 440MHz. So if you do have a decent grade of CommScope coax, it might not be too bad... you might have to go to LMR-400 to see improvement! So it's actually, in some sense, not bad coax for casual UHF use if it's the lower loss "RG-6 type" stuff . Are you sure it doesn't have a model number? It won't be "RG-59" or "RG-6" it'll be, I think, a C##### type thing. In the absences

RE: 75 ohm coax on UHF	Reply
by N3OX on May 6, 2008	Mail this to a friend!
Sorry, posting problem. Picking up where I left off: In the absence of a model number, I'd try to measure the loss. If it seems acceptable, run with it. If not, the old coax might be a good PULLER for the new one ;-) 73, Dan

RE: 75 ohm coax on UHF	Reply
by K9GRN on May 8, 2008	Mail this to a friend!
The cable has no number on it. Only CommScope and a symbol of a telephone handset. But I found the spool that the cable came on. It says CommScope 1 gHz performance and has the numbers 4517603 and F660BEF-PR1691. I searched the CommScope website and neither number corresponds to anything they make currently. The cable is at least 11 years old. Now I'm not sure what I've got. I'll just try it. Thanks to both of you for the reassurance that I won't fry anything.

RE: 75 ohm coax on UHF	Reply
by N3OX on May 8, 2008	Mail this to a friend!
I found a datasheet for CommScope F660BEF ... probably the rest of the numbers aren't so important. For that, the specified loss is 4.4dB/100ft @ 450MHz. That jives with other RG-6 type cables. You'd have to go to something kind of fancy to do much better: 9913 is 2.8dB/100ft, LMR400 is 2.6dB/100ft Of course, age might have taken its toll... One other issue, if you get bad SWR, try changing the line length five and a half inches or so. An odd multiple of a quarter wavelength line length will transform a 50 ohm antenna impedance up as high as a bit over 100 ohms... but if you add another quarter wavelength on to that, it'll come back down to 50 ohms again. 73 Dan

RE: 75 ohm coax on UHF	Reply
by N8EKT on June 7, 2008	Mail this to a friend!
Most modern RG6 75 ohm feedlines use aluminum foil shields and sparse aluminim braid that will not solder. This makes connectors an issue. 25 watts is not an issue and neither is the impedance difference. If it DOES use copper braid shield, it is likely RG59 and would not be a good choice.

RE: 75 ohm coax on UHF	Reply
by N8EKT on June 7, 2008	Mail this to a friend!
If F660BEF is what it is, forget it! As I said, it has a aluminim shield. It also uses a copper plated STEEL center conductor which would make this cable very stiff and the center conductor my rust off if exposed to moisture. It also is meant for direct burial. These are usually full of a gel that believe me you DO NOT want to get on YOU or your clothes!

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