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75 ohms to 50 ohms

M

michaelt501

Member
Nov 8, 2010
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I have 400 feet of 75 ohm coax.Looks like rg8.Has solid copper center wire,foam insulation,alumn.foil sheild and braided alumn.sheild.The company that made the coax is Comscope.That's all that is written on the wire.No model or part number.It has some kind of sticky sealer stuff running through it.
I have a 5/8 wave alumn.antenna (HALCOM 27).What I want to do is make a 150' to 175' run.I want to run it into my MFJ tuner (mfj 949c) and then into my Kenwood ts 140s.
What I want to know is.Can I streighten out the match through the tuner?Or use a balun or WHAT?
 

75 ohm coax is ideal for feeding a half wave dipole but not a 5/8 wave cb antenna or other antenna with an input impedence of 50 ohms.. I assume that's what your talking about. Never heard of a Halcom 27.

Even with a tuner on a coax run that long you would probably see a lot of your power wasted in feedline radiation and in the tuner.

Another problem I ran into using 75 ohm coax, even for dipoles, is that the aluminum shield is impossible to solder and you'll probably have difficulty finding a connector for it inless it's the same size as RG-8.
 
I've used 75 ohm coax (RG-6, RG-11 and RG-59) for a long time, with various antennas including dipoles, and haven't had any problems at all. The impedance "mismatch", numerically, is 75/50, or 1.5:1. That shouldn't be a show stopper for any decently-built and decently-operated equipment.

If you have a matching network (what some would call a "tuner"), you can transform whatever mismatch you have into something your transmitter will be perfectly happy with. 100 watts, not a problem. I've run 1200+ to a dipole with straight RG-11 and NO "tuner". And I lived to tell about it! And my equipment didn't complain at all.
 
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Beetle has it right. The loss due to a mismatch of 1.5:1 is very small and will not be noticed in most applications. The loss in that long run shouldn't be too bad at 27 MHz especially since it looks like you have RG-11 cable. While you can't solder a PL-259 to the braid of most brands, you can cheat a little and get a normal connector on this wire with no problem. Just use a file or knife to dull the threads inside the connector. Fold the braid over the black jacket and screw the body of the connector right over the braid. Keep in mind this is not the right way to install a connector but it works with this cable. If you have it at your disposal, don't be afraid to use it.
 
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I agree, no reason at all not to use the 75 ohm cable. In fact depending exactly what the impedance of the antenna is you may not even need the tuner to effect a good match. I have used long runs of 75 ohm cable TV trunk line,one inch diameter aluminum hardline, to feed various antennas and always have had good results. One of the greatest low band DX'ers is Jack Leahy, VE1ZZ, and that is pretty much all he uses is cable TV trunk line. His splices are simply soldered together and a two liter pop bottle inverted over the connection. He is a word class low band operator and the 75 ohm cable works great for him.
 
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Shockwave said:
Beetle has it right. The loss due to a mismatch of 1.5:1 is very small and will not be noticed in most applications. The loss in that long run shouldn't be too bad at 27 MHz especially since it looks like you have RG-11 cable. While you can't solder a PL-259 to the braid of most brands, you can cheat a little and get a normal connector on this wire with no problem. Just use a file or knife to dull the threads inside the connector. Fold the braid over the black jacket and screw the body of the connector right over the braid. Keep in mind this is not the right way to install a connector but it works with this cable. If you have it at your disposal, don't be afraid to use it.
Click to expand...

He's not going to use it wih a dipole. Read his post.
 
If this feedline is only going to be used on one band just cut it for a number of electrical half wavelengths. It will present the same impedance to the rig as the antenna itself.

1/2 2/2 3/2 etc. electrical lenghts long will do.

All transmission lines will do this.

You can use any impedance cable. It will present the load impedance at the source end if it's multiples of an electrical half wave long.

I had a 124' dipole up. Fed it with 110' of open wire line. Down in the shack a 1:1 balun (made with a coil of coax) presented a 1:1 SWR on 75 meters.
 
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RatsoW8 said:
He's not going to use it wih a dipole. Read his post.
Click to expand...

Type of antenna doesn't really matter. His "tuner" will transform the complex impedance and provide the conjugate for the transmitter to allow it to transfer the maximum power into the feedline. It doesn't "tune" the antenna if it's way down in the shack, but it will allow the full transmitter power to get into the feedline, whether the feedline is 50 ohm, 75 ohm or any other value or combination of values.
 
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Kamikaze said:
If this feedline is only going to be used on one band just cut it for a number of electrical half wavelengths. It will present the same impedance to the rig as the antenna itself.

1/2 2/2 3/2 etc. electrical lenghts long will do.

All transmission lines will do this.

You can use any impedance cable. It will present the load impedance at the source end if it's multiples of an electrical half wave long.

I had a 124' dipole up. Fed it with 110' of open wire line. Down in the shack a 1:1 balun (made with a coil of coax) presented a 1:1 SWR on 75 meters.
Click to expand...


The 1/2 wavelength of coax idea does work but it gets hard to simply measure and cut when you are dealing with several wavelengths. In the case of 200 feet we are talking almost 15 electrical halfwaves. I use electrical halfwaves to tune my antennas when I install them to ensure the impedance I am seeing is the actual antenna impedance. Using several halfwaves in series affects the overall length as well as the Q of the line. The fewer used the better.
 
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Kamikaze,
Fortunately, a correctly used tuner will not have more loss than the 'worst case' SWR, and that's why they are so handy. That's like saying that the impedance matching device used with most beams would have more loss than the directly fed beam would have without that matching device. Just not so, sorry.
All this assumes that the tuner is 'large' enough to handle whatever that SWR is, and the other things associated with that situation.
- 'Doc
 
W5LZ said:
Kamikaze,
Fortunately, a correctly used tuner will not have more loss than the 'worst case' SWR, and that's why they are so handy. That's like saying that the impedance matching device used with most beams would have more loss than the directly fed beam would have without that matching device. Just not so, sorry.
All this assumes that the tuner is 'large' enough to handle whatever that SWR is, and the other things associated with that situation.
- 'Doc
Click to expand...

Most tuners are lossy at lower impedances due to wimpy inductors and switches. A typical built-in riceburner tuner can easily lose 20% of transmitter output power on 10 meters.

Same tuners do well with meduim impedance loads.

Matching devices for beam antennas usually use components properly sized for low loss.

I've owned many a tuner due to estate sales and that's the way it is.
 

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