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Old 4th Dec 2021, 6:21 pm   #1
Panrock
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Default Joining two feeders of different velocity factor

A quickie question, if I may.

Say you are feeding an antenna, and for various reasons you want to use a different type of feeder (in series) for the final run up the mast. The two types of feeder involved are both of 50-ohms characteristic impedance but they have different velocity factors.

What would be the effect, if any?

Thank you.

Steve
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Old 4th Dec 2021, 6:27 pm   #2
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Default Re: Joining two feeders of different velocity factor

None - unless, that is, there's some specific wavelength-dependent issue [such as the feeders being used as a power-divider or resonant transformer/matching section].
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Old 4th Dec 2021, 6:28 pm   #3
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Default Re: Joining two feeders of different velocity factor

None unless you are tuning the feeder for a particular length such as a multiple of half waves. Then your calculations shift a bit.
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Old 4th Dec 2021, 7:30 pm   #4
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Default Re: Joining two feeders of different velocity factor

Thank you.

Steve
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Old 11th Dec 2021, 4:24 pm   #5
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Default Re: Joining two feeders of different velocity factor

Panrock , grab a coffee and watch this old Bell Labs film, i hope it makes things clearer and add,s to your understanding . i enjoyed it .

https://www.youtube.com/watch?v=DovunOxlY1k
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Old 12th Dec 2021, 2:08 pm   #6
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Default Re: Joining two feeders of different velocity factor

Thanks. A clear explanation of how all types of wave are related. Nice demonstration of SWR!

Steve
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Old 12th Dec 2021, 5:43 pm   #7
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Default Re: Joining two feeders of different velocity factor

Ok
Should read AT and T .
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Old 13th Dec 2021, 10:54 am   #8
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Default Re: Joining two feeders of different velocity factor

Don't forget every join will add in a small loss which is inevitable.
I've seen amateurs with many joins, antenna switching boxes, power meter, filters and VSWR bridge between transceiver and antenna with a more than a slight loss overall.
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Old 13th Dec 2021, 6:36 pm   #9
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Default Re: Joining two feeders of different velocity factor

On installations with a long run, when feeder losses become important, it is quite normal to have a "tail" of something like UR 67, followed by long run of low loss LDF 450 / 550 or similar, then a further tail to the aerial. This is normal practice on radio towers.
As murphy310 notes, there will be a small loss at each join. The joins will show on a TDR but, if everything is done correctly, these are unimportant.

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Old 14th Jan 2022, 8:06 pm   #10
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Default Re: Joining two feeders of different velocity factor

Hi again. Here is a view of the cross sections of the two cables. The overall diameters are both correct. But the relative sizes of the inner core and screen on the more slender RG213/U looks like it might be a 75-ohm cable, rather than 50-ohm, to me. Am I worrying needlessly?

Thanks. Steve
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Old 14th Jan 2022, 10:42 pm   #11
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Default Re: Joining two feeders of different velocity factor

If it's really RG213 then it should be 50 ohm. It looks plausibly 50 ohm from the cross-section - I would expect a larger ratio of core to screen diameter for 75 ohms. The other cable looks like it has a different dielectric, hence the somewhat lower diameter ratio.

Chris
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Old 14th Jan 2022, 11:12 pm   #12
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Default Re: Joining two feeders of different velocity factor

These appear different because they have different dielectric One is foam PTFE the other Polyethylene. If both are 50 Ohm impedance, and I suspect they are, a decent joint is all that is needed.
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Old 14th Jan 2022, 11:43 pm   #13
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Default Re: Joining two feeders of different velocity factor

Yes, I had hoped this was down to the different dielectrics rather than being 'pirate' cable passed off as something else. Thanks for the reassurance.

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