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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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4th Dec 2021, 6:21 pm | #1 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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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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4th Dec 2021, 6:27 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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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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4th Dec 2021, 6:28 pm | #3 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
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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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4th Dec 2021, 7:30 pm | #4 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Re: Joining two feeders of different velocity factor
Thank you.
Steve
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11th Dec 2021, 4:24 pm | #5 |
Hexode
Join Date: Dec 2018
Location: Lugo, Spain
Posts: 477
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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 |
12th Dec 2021, 2:08 pm | #6 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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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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12th Dec 2021, 5:43 pm | #7 |
Hexode
Join Date: Dec 2018
Location: Lugo, Spain
Posts: 477
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Re: Joining two feeders of different velocity factor
Ok
Should read AT and T . |
13th Dec 2021, 10:54 am | #8 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,420
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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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13th Dec 2021, 6:36 pm | #9 |
Pentode
Join Date: Nov 2021
Location: Laurencekirk, Kincardineshire, Scotland, UK.
Posts: 182
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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. Chris R |
14th Jan 2022, 8:06 pm | #10 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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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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14th Jan 2022, 10:42 pm | #11 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
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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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14th Jan 2022, 11:12 pm | #12 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
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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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14th Jan 2022, 11:43 pm | #13 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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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.
Steve
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