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Author Topic: MIXING RG8 AND RG58 COAX  (Read 6210 times)
N8CHR
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Posts: 34




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« on: April 27, 2010, 11:31:57 AM »

I have a vertical antenna. The bottom connection is with the center conductor attached with a screw as well as with the ground end.  Can I use RG58 here and go to my lightning arrestor with a PL 259 and come off the other end of the arrestor with RG8 to the transmitter? I know the velocity factor is different between the two. Will mixing the two have any great effect on the power to the antenna? I only use 100 watts.
Thanks TOM N8CHR
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N3OX
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« Reply #1 on: April 27, 2010, 11:37:57 AM »

I know the velocity factor is different between the two. Will mixing the two have any great effect on the power to the antenna?

Velocity factor doesn't change how much power makes it through the coax and it doesn't cause any mismatch between the two lines.  The impedance is the same, and that's what matters.  You can go ahead and mix them.

The only difference in power is if you replace a big fraction of the total run with RG-58 because it has higher loss :-)  You won't have any SWR problems or anything.  I do exactly what I think you're doing all the time: using a short piece of smaller coax to make a more flexible connection and then transitioning to bigger coax for most of the run. 

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

Monkey/silicon cyborg, beeping at rocks since 1995.
K5DVW
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Posts: 2193




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« Reply #2 on: April 27, 2010, 11:50:05 AM »

I've seen a lot of people get bent out of shape because of velocity factor and I dont know why. All that means is the speed of the electromagnetic wave inside the coax due to the dielectric material. It's important if you care about how many wavelengths are contained inside the coax (for matching, phasing, etc), but for practical use just feeding a matched antenna, it's not that important.

Connecting one 50 ohm cable to another wont cause any impedance bumps. The loss between them might be different... generally smaller cable is more lossy.
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WB2WIK
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« Reply #3 on: April 27, 2010, 11:50:30 AM »

N3OX is correct, it makes no difference.

Even if the velocity factors were different (and they're actually not, RG58/U and RG8/U are both 0.66 Vf; if one of those cables is "foam" dielectric, then it's probably closer to 0.8 for that one), that wouldn't matter, either.

The velocity factor inside your transmitter, in the cable, and once the signal launches from the antenna are all "different."
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WA3SKN
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« Reply #4 on: April 27, 2010, 11:56:37 AM »

This is not an issue.  Long runs of RG58 might be, but this will be OK.
73s.

-Mike.
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N8CHR
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Posts: 34




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« Reply #5 on: April 27, 2010, 12:06:01 PM »

Thanks for all your help. Think I was more conserned with impedence match. Your right they are both the same. RG58 run will be 2 feet at most. Reason I want to mix is I have 100 foot of RG8 with factory ends and I REALLY couldn't bring myself to cut them off.
Thanks again Tom N8CHR
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AD6KA
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Posts: 2233




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« Reply #6 on: April 27, 2010, 02:26:35 PM »

The only exception I can think of would
be is if the vertical is a Butternut HF6V, which requires a
75 ohm RG6 stub to load on 20m.

(Hey, you said you had a vertical. Maybe somebody
gave or sold you an HF6V w/o the stub....it could happen)  Smiley
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N7ZM
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Posts: 131




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« Reply #7 on: April 27, 2010, 03:32:32 PM »

Go ahead and use them both as they are both 50 ohm coax. But if you ever get an amp and go over that 100 watts, change the RG-58 to RG-8 or RG-213 that can handle the power.
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VK1OD
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« Reply #8 on: April 27, 2010, 04:12:45 PM »

Go ahead and use them both as they are both 50 ohm coax. But if you ever get an amp and go over that 100 watts, change the RG-58 to RG-8 or RG-213 that can handle the power.

The implication here is that RG58 cannot handle more than 100W.

In fact, on the HF bands at low VSWR, RG58 fly leads can handle 1500W PEP on SSB easily and Morse code (based on 50% transmit cycles). Some modes (eg continuous RTTY) would be limited to lower power.


Owen
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WB2WIK
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« Reply #9 on: April 27, 2010, 06:07:00 PM »

Go ahead and use them both as they are both 50 ohm coax. But if you ever get an amp and go over that 100 watts, change the RG-58 to RG-8 or RG-213 that can handle the power.

The implication here is that RG58 cannot handle more than 100W.

In fact, on the HF bands at low VSWR, RG58 fly leads can handle 1500W PEP on SSB easily and Morse code (based on 50% transmit cycles). Some modes (eg continuous RTTY) would be limited to lower power.


Owen

I'd say that's true for "real" (solid PE dielectric) RG58A/U or C/U.  But in "ham circles," there's a lot of "imitation" RG58 cables about, including many that have cellular PE "foam" dielectric.  I've had those break down and flash over at much less than 1 kW, into a perfect 50 Ohm load, especially if it has been through a "bend" in the cable.
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VK1OD
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Posts: 1697




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« Reply #10 on: April 27, 2010, 06:18:47 PM »

...
I'd say that's true for "real" (solid PE dielectric) RG58A/U or C/U.  But in "ham circles," there's a lot of "imitation" RG58 cables about, including many that have cellular PE "foam" dielectric.  I've had those break down and flash over at much less than 1 kW, into a perfect 50 Ohm load, especially if it has been through a "bend" in the cable.

1kW in 50 ohms is 223VRMS, 316V pk. I am very surprised that you have had a quality cable of that type flashover at 320V or less (much less than 1kW).

We cannot comment on sub-standard cable other than to note that you cannot comment on sub-standard cable! (prophetic!)

On the heating front, the lower loss foam coaxes have slightly less heating, but slightly less maximum operating temperature, the net effect is little difference where outer diameters are the same. Fig 2 at http://www.vk1od.net/transmissionline/RG6/index.htm shows the difference between RG58 and RG8X (which is a larger foam coax), not much, perhaps 10% at 30MHz.

Nevertheless, I do not favour foam coax for fly leads, some radios run with a case temperature up to 60°C which is right up there at the limit of many foam coaxes. For that reason, I would choose RG58 (which implies solid PE dielectric) over a foam version of the same, it has higher operating temperature and better tolerance of tight and permanent bends.

Owen
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