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Author Topic: RG-6 75Ohm coax cable; what can it be used for  (Read 7049 times)
BPONG
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« on: November 06, 2009, 09:13:22 AM »

i have access to alot of this cable, but most antenna articles i have read call for RG58 50Ohm coax cable.
has anyone used RG6 for antennas and such ?
bp
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N3OX
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« Reply #1 on: November 06, 2009, 11:15:10 AM »

I use a fair amount of it.  I'm using it to feed three of my four HF/MF antennas, one receive-only and two transmitting antennas.  

The impedance mismatch you get when you mix in 75 ohm cable into a "50 ohm" system may or may not matter to your situation in a way that depends on a whole lot of details.  The exact impedance of the antenna can pose problems.  If you feed a dipole high above the ground you'll probably have it "just work."  A high dipole is close to 70 ohms impedance, so a very good match to the 75 ohm cable.  The worst case SWR referred to 50 ohms at the radio end of any length of 75 ohm cable feeding a 70 ohm antenna would be about 1.6:1.

But I had a big problem with a 40m ground plane antenna that was ~30 ohms.  30 ohms by itself is a 1.67:1 SWR and acceptable if you use 50 ohm cable.

But the *transformation * caused by 75 ohm cable could make the SWR at the radio end of the cable as high as 3.64:1.

I encountered this when I tried to feed my 40m ground plane with some coax I found, that turned out to be RG-11.  I tried to tweak and tune out at the antenna and could never get it below 3:1 SWR.  

 The mismatches are never going to be severe enough to cause noticeable extra losses, but they can cause some  issues with SWR...   you can probably usually sort that out on a single band by changing the length of the 75 ohm line but sometimes that becomes impractical.  Adding an extra quarter wave of coax to a 160m feedline is not the best solution, for example :-).  The use of transformers can be helpful to transition in and out of 50 ohms in some cases.

There's a very simple transformer called a "twelfth wave transformer" that is just alternating sections of 75 ohm and 50 ohm coax:

http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm

I've built some of those using RG-6 and RG-58 or RG-8/X and they work well enough across a few ham bands.

You can also build ferrite broadband UNUN transformers. You can buy them too but that almost certainly defeats the cost savings of using the cheaper coax...

Here's a good resource:

http://vk1od.net/transmissionline/RG6/index.htm

Anyway, that's a long story.  The short one is yes, I use it, yes I use it on transmitting antennas, and yes it's worth a shot.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N5LRZ
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« Reply #2 on: November 06, 2009, 01:08:23 PM »

The main problem with using such coax, 70 ohm, is that it is constructed with non copper materials which while working great with crimp on TV connectors, is a bitch to solder.  Aluminum does not solder very well using conventional methods.  There are certain soldering paste compounds that enable one to solder to aluminum.  Do a google on Soldering Paste and you will find several retailers.

NOW they do make a coax that is 70 ohm coax that has a more normal copper shielding that solders just like the normal 50 0hm coax more commonly used.

Can you use it, yes you can.

BUT you will probably have to forget about any normal SWR meter giving you any kind of accurate readings--normal meters are designed for 50 ohm coax.

The SWR on any given center fed half wave conventional/proper length dipole will be 70 ohms or so average and would match the coax BUT most radio specs state 50 ohm coax so the SWR will still be 1.5 to 1 due to the mismatch where the coax is connected to the radio--see ARRL magazine, the doctor is in, about 6 months ago.

Basic bottom line, forget the 70 ohm coax unless it is the one with the copper shelding and you are going to make a phase line for an antenna array.

Amateur Radio is a very expensive hobby if you want to do it right and proper, so just as soon break down and break out the ol credit card (you are going to be using it a hell of a lot before you are finished building your station) and get the best quality, lowest loss coax you can possibly afford--suggested you get and use absolutely nothing lower than the RG8X mini coax for HF.  For VHF use only the lowest loss coax possible and dont worry about the price (its all damn expensive).
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K9KJM
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« Reply #3 on: November 06, 2009, 11:49:58 PM »

This question comes up frequently.

The simple answer is, Yes, 75 ohm coax like RG 6 (Or RG 11, etc)Can be used for ham radio, Usually with great results. (Many HF antennas are closer to 75 ohms than 50 anyhow)

The big advantage of using it is the super low cost, Usually available from your local cable TV installer for a bag of donuts, Just ask for the "spool ends" that they normally toss in the dumpster.

One of the downsides is to adapt the coax to standard PL 259 type connectors. The simple, Easy way is to just get some adaptors from cable "F" type direct to PL 259, And use the type "F" on the coax, Crimp style.
(With any reasonable VSWR, Type F connectors will handle full legal limit power.

As pointed out, Another way is to buy a short section of copper braid shield RG 6 to solder a PL 259 directly on, Then just use that short jumper with a type F connector on the other end as the adaptor. Either way works fine.

If you install the PL 259 directly on the aluminum shield RG 6 or RG 11, Sooner or later there are usually oxidation problems, And it is pretty hard to solder to aluminum, Any decent solder will melt the foam dielectric before the solder flows.
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