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Author Topic: 75 ohm coax as a feedline  (Read 7788 times)
N2TMC
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Posts: 15




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« on: February 23, 2004, 06:56:17 PM »

Help...I am puting up my first wire dipole. I have read in the ARRL Operating Manual to feed with 72 ohm coax. I asked a few guys locally and they say if you want to use 72 to 75 ohm coat you need to have 2 runs tied together at braid and use the centers to my tuner as a balanced line.  My brother-in-law has a satellite installation company and I can get lots of RG-6/SAT cable and if I can just run one coax back to the tuner I would be very happy.

Thanks so much
George
N2TMC
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KD5MJJ
Member

Posts: 29




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« Reply #1 on: February 23, 2004, 07:12:44 PM »

 
go ahead and use the coax normally.

 hook one end of the coax  to a pl259 connector for your rig  and the other end of the coax to the dipole . at the antenna, solder the center conductor  to one side of the dipole and the braid to the other side of the dipole.

If you want a more refined attachment at the antenna use a commercially available center insulator  which also accepts a Pl259 plug.

no need for the elaborate splice job. some one is pulling your leg.

in theory the dipole has an impedance of 75 ohms but this can vary a little due to height above ground and influence of nearby objects.

73 hank
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WB2WIK
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Posts: 21818




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« Reply #2 on: February 23, 2004, 07:14:42 PM »

Either the local guys who gave you the advice about using two runs of 75 Ohm coax didn't understand the question, or they're all wet.

A half-wave dipole can be efficiently fed with 75 Ohm coax (or 50 Ohm coax, it doesn't matter) without the need for any sort of balancing or tuner if the dipole is used on its fundamental resonant frequency (and often on any odd harmonic of that frequency, as well).

No need to use two lines, or a balanced tuner, at all.

RG6/U quad-shielded cable TV line is quite good, and very well shielded -- just be careful about it flapping around in the wind.  RG6 cable TV coax always has a solid, copperweld (copperclad steel) center conductor which cannot take repeated flexing -- it will break.  Inside the coax, it's somewhat protected from sharp bends because the coax itself is fairly stiff and won't make sharp bends.  But once you "strip" it to expose the center conductor and make a connection to a dipole wire, most of that strength is lost.

This is about the only reason *not* to use this kind of coax to feed a dipole.  Its electrical characteristics are just fine; its mechanical characteristics might be undesirable for the application.

WB2WIK/6
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AA4PB
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Posts: 14754




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« Reply #3 on: February 23, 2004, 08:55:05 PM »

If you will be using your dipole only on the band which it is cut to 1/2-wavelength on or odd multiples (like a 40M dipole on 15M) then you can use a coax feed with an unbalanced tuner. A dipole in free space is around 73 ohms. If the dipole is 1/2 wavelength or more high then the actual impedance will be close to 73 ohms. If the dipole is closer than that to ground or sourrounding objects then the impedance will be lower, often closer to 50 ohms. In practice it doesn't much matter whether you use 50 ohm coax or 75 ohm coax.

If you want to use that dipole on all bands then you must use a very low loss feedline. It is common to make the dipole at least 1/2-wavelength on the lowest frequency that you wish to operate and feed it with balanced, open wire line and a balanced tuner. Some people resort to using two runs of coax cable in lieu of the open-wire feed line. The loss in the coax is still quite high on some frequencies however and makes for a very inefficient antenna system on those frequencies. You really should use open-wire feeders in this application.
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Bob  AA4PB
Garrisonville, VA
KB0GU
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Posts: 130




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« Reply #4 on: February 23, 2004, 09:29:27 PM »

Actually for a balanced antenna, dipole or loop, what your friends recommend will work and no balun will be needed at the feed point to the antenna.  But, this approach is not required because you are using 75 ohm coax.  Rather it will permit multiband antenna operation with your tuner and a shielded balanced feeder.  I did this with a large vertical loop and it worked quite well.  At the antenna, tie the two shields of the coaxes together but leave them floating and unconnected to the antenna.  In the shack, tie the shields together and attach to the chassis of the tuner and put each of the center wires to one side of the balanced output of the tuner.  I used small nylon zips every 6 inches or so to hold the twin coax feeder together out to the antenna.
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K0ZN
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Posts: 1826




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« Reply #5 on: February 23, 2004, 10:44:47 PM »


I second the foregoing comments. 75 ohm is FINE. You are lucky to have a source of this good stuff! What works quite well in the real world and what meets perfection in theory are not necessairly the same. For most hams the feed point impedance is neither 50 or 75 ohms!!...it is something between due to height above ground.  Don't sweat it if you find your SWR is not "one to one" and is something like 1.5 to 1 at resonance; your antenna will radiate just fine. If you "just can't stand it" because it is not 1:1, get a decent UNbalanced tuner and you can tweak it to let your rig see 1:1. I would strongly suggest you pick of a copy of the ARRL antenna book and look over the sections on basic antenna theory. It is NOT that complicated and will help you get the most out of your station. Your antenna is literally your connection with the world...and a little time spend understanding "how and why" will be a big help and likely also help your signal too! This will be time WELL spent.

73,  K0ZN
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