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Author Topic: catv hardline matching transformers  (Read 1173 times)
WD8TJM
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« on: January 18, 2006, 02:59:31 AM »

I am looking for 75to50 ohm catv hardline matching transformer's for 6 meter's. I am not having much luck finding these,anyone know who makes these any more.
thanks
tom wd8tjm   wd8@sbcglobal.net
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WD8PTB
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« Reply #1 on: January 18, 2006, 04:04:12 AM »

Kanga USA now makes these. Bill Kelsy N8ET is the man you need to contact. He has adds in world radio and QST. Don WD8PTB
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K5DVW
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« Reply #2 on: January 18, 2006, 05:26:47 AM »

Just a thought, but using a 75 ohm line for a 50 ohm antenna system might not produce enough mismatch to worry about matching transformers. That's only a matched vswr of 1.5:1. With antenna variation, you'll probably be under 3:1 which is on the order of 1 dB mismatch loss. If you're saving more than that by using the 75 ohm line over 50 ohm, then you're ahead!

I'd guess the matching transformers have some loss, probably near 0.5 dB each.

Maybe I'm way off here, but it just seems like it's a wash to me. Especially if you're using a tuner at the rig end.
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KA1MDA
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« Reply #3 on: January 18, 2006, 07:37:10 AM »

You may not need them- way back when I was starting out in Ham Radio, I had an Icom IC-290H multimode 2 meter mobile rig feeding a Mirage B3016 160W amp into a Cushcraft B213 antenna. The amp fed the antenna thru 70 feet of surplus (free) 70 ohm 1/2 inch CATV hardline with home-made PL259 adapters at each end made from plumbing parts (brass compression couplings). I used that set up mostly for SSB and VHF contests, sometimes on FM also. Never had a problem in the 8 years I had it, and SWR was never more than 1.6:1, which the amp didn't seem to mind at all, even after long periods of contesting.

73, de Tom, KA1MDA
www.ka1mda.org
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WA9SVD
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« Reply #4 on: January 18, 2006, 09:05:06 AM »

Provided your antenna is well matched to the transmitter output impedence (i.e.  both are 50 Ohm, or close) the feedline mismatch will probably result in less loss than would be caused by any sort of transformer(s.)  If it really bothers you, you can adjust the length of your 75 Ohm feedline to a multiple of a half wavelength, in which case the hardline would present the same impedence as at the antenna.  At 6 M, even an extra 10 feet (actually less, depending upon the velocity factor of the hardline) of low-loss hardline would not have a significant amount of additional loss.  (But of course, that solution only works on one band, or on 6M, a portion of the band.)
    But the best solution would be to make sure the antenna is properly adjusted for 50 Ohms, and not even worry about a 1.5:1 feedline mismatch.  And then get on the air and enjoy 6M. Good luck and enjoy 6M.
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K9KJM
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« Reply #5 on: January 18, 2006, 10:53:44 PM »

I agree with WA9SVD. Just hook it up and go.
(By the way, A standard CATV hardline "pin" connector can easily be converted to a PL-259 by simply cutting a PL 259 apart. Solder the center pin to the CATV pin, Then cut off the outer threaded part of the PL 259 and screw it right onto the pin connector. Use an SO 258 feed thru and there you are! Simple and cheap.
To match for a purist, Just connect short jumpers of electrical quarter wave length 50 ohm RG 8 type coax at each end.
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W8JI
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« Reply #6 on: January 19, 2006, 04:05:48 AM »

What everyone forgets is the SWR on the 75 ohm line, when feeding a perfect 50 ohm load is 1.5:1 normalized to 75 ohms.

When you mix that 75 ohm line back into a 50 ohm system, the 50 ohm SWR can vary from 1:1 to as high as 2.25:1.

This is generally well beyond what a 50 ohm rig or solid state amplifier would like to see. Since bandwidth between 1:1 and 2.25:1 SWR limtes becomes narrower as the line is increased in electricl length, it may be impossible to cover much of a VHF band by trimming the line.

On the other hand it only takes four pieces of coax a few inches long to match that line to a 1:1 SWR over an entire VHF band.

Something factual to consider when deciding what to do.

73 Tom
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WA9SVD
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« Reply #7 on: January 19, 2006, 08:26:35 AM »

Well, DUH!  Naturally, Tom is correct!  The SWR CAN be as high as 2.25:1.  (A SWR of 1.5:1 at each end of the 75 Ohm coax.)
===================
  W8JI:  "On the other hand it only takes four pieces of coax a few inches long to match that line to a 1:1 SWR over an entire VHF band."
====================

    But I'm drawing a blank before the morning coffee kicks in.  Tom, can you please explain about the four pieces of coax?
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WB6BYU
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Posts: 13353




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« Reply #8 on: January 19, 2006, 02:01:26 PM »

If you go out from the rig at 50 ohms, you can use a
short length of 75 ohm cable followed by another piece
of 50 ohm cable to give 75 ohms.  (There is some magic
length for the pieces, but I don't remember what it is.)
This is called "series matching".

Then you would use the same combination in reverse at the
other end to step the 75 ohm coax down to your 50 ohm
antenna.  Cheap and simple.
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W9GB
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Posts: 2626




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« Reply #9 on: January 21, 2006, 06:34:43 AM »

Tom -

PLEASE bookmark these web pages, Kevin Custer, W3KKC has this inforamtion, as well as others ont eh Internet.

GOOGLE!

http://www.repeater-builder.com/rbtip/matchingstubs.html

http://www.rogerhalstead.com/conn.htm

http://jonz.net/W3DHJ/catvhardline.html

http://www.n1bug.net/tech/catvline.html#conns

http://www.qsl.net/n9zia/wireless/hardline_connectors.html

http://www.qsl.net/k4pwo/making.htm


w9gb
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WA9SVD
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« Reply #10 on: January 29, 2006, 03:52:13 PM »

No offense, but I was hoping for a bit more specific (or at least techincally applicable) response from Tom, W8JI.  "He da man!" as it were!
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