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Author Topic: CATV hardline question  (Read 1483 times)
N4UE
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Posts: 291




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« on: June 11, 2008, 01:59:02 PM »

Hi. I am the fortunate owner of many THOUSANDS of feet of CATV hardline, (yes, obtained leagally, thank you), that is brand new. It is Timewave and still coiled up. This 75 ohm hardline is the rigid foam type that will not allow water entry.
OK, I have used it in the past at several locations and it's great. However, I have always had this nagging question in my mind....could I make it better?


Realize I have a machine shop and can fabricate very nice connectors/adaptors for the 5/8, 7/8. and 1 1/4 inch cable I have. It has a PVC jacket....which helps with noise on the tower.

My question is this.... If you were putting it up, how would you go from the hardline (just below the rotor), to the antenna...? Yes, I have all the literature on making 'matching transformers' from sections of different impedance line. BUT, there is some loss associated with every connector, etc.

Here's an example..... One station (back in Ky), had 7/8" CATV hardline running to the base of the rotor. From there, I went with one of my connectors to 9913 to the antenna. In this case it was an M2 6M7JHV. It worked great. The SWR was < 2:1 and the 756PRO's tuner handled it with NO problem. I also have a legal limit amp for 50 mHz and since it has a pi-network output, it will easily load up the system.

I am in the process of putting up several towers. Since 6M is my favorite band, I will be sending the 7/8" CATV line up the tower. I really need to buy some NEW coax. My 9913 (non flex), has not been outside except for maybe 5 years... but is 20 years old! LMR400 looks good, in spite of the 50 ohm imp. Should I buy some new 75 ohm cable to go from the CATV hardline to the antenna?

Get it? 50 ohm radio, 75 ohm CATV hardline, 50 ohm flexable, to 50 ohm antenna....or
50 ohm radio, to 75 ohm CATV hardline, to 75 ohm coax to 50 ohm antenna....Huh

Believe it or not, the 5/8" CATV stuff is so light, I thought about running it down the boom of the JHV, but then there comes the 'flexable' portion around the rotor.

Yes, I KNOW I'm splitting hairs here, and I should probably just put up "what I have", but I thought I'd opt for other opinions.

PS, YES, I know that the sight SWR and resulting mismatch really doesn't amount to anything, but I've just retired and I just want to do it and get it right (and forget it!!) I just wanted some differing viewpoints, other than "just put up the stuff and forget it". That's what happens when you have a Type A personality. I want EVERTHING to be PERFECT....

ha ha

thanks,

ron

N4UE
EM80pa
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K9KJM
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Posts: 2416




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« Reply #1 on: June 11, 2008, 09:16:56 PM »

Congratulations on a good stock of low loss feedline!

The easy way to make connectors is to just get a bunch of common "Pin" cable TV connectors. These are the connectors that go from your hardline to amplifiers, etc.  The cable crews toss LOTS of them into the dumpster every day. A few boxes of donuts etc should get you lots of them.
If you do not have access to the correct core tool for the hardline, Just take a pliers and pry off the extra sleeve on the connectors. You really dont need it anyhow.

Then get some PL-259 connectors, Cut the outer sleeve in half and note that it screws RIGHT ONTO THE cable TV pin connector!!!!     Pry out the center pin from the PL 259 connector and solder it to the CATV pin connector. Cut off the excess pin length. You now have a PL 259 connector.   Get some "Feed through" connectors and you can connect your flexible LMR-400 for VHF/UHF or Belden RG-213 for HF right to the hardline!

By cutting a multiple of a half wave length of your jumpers, you will end up near 50 ohms at your radio.

This stuff works VERY VERY well as low loss feedline!

But don't expect "perfect"........   It is still 75 ohm line......

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WA7NCL
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Posts: 625




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« Reply #2 on: June 12, 2008, 08:36:08 AM »

SWR between 50 and 75 ohms is only about 1.5.  Why the big deal.

Secondly, you could tweek the antenna for 70 ohms on 6m maybe?  If is has a gamma match or some sorta adjustable match, maybe there is enough range to get to 70 ohms instead of 50?

Make the fexible parts of 75 ohm coax and then everthing will be 75 ohm except your TX.  The TX probably wont really care about the 1.5 SWR.

Remember its all about loss and coupling power, not SWR.
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WB2WIK
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Posts: 20547




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« Reply #3 on: June 12, 2008, 09:53:21 AM »

RE: CATV hardline question       Reply
by WA7NCL on June 12, 2008    Mail this to a friend!
SWR between 50 and 75 ohms is only about 1.5. Why the big deal.<

::Actually, SWR would be 1.5:1 worst case only if the antenna were 75 Ohms and you were measuring at the transmitter end of the coax.  If it's a 75 Ohm line and a 50 Ohm antenna, SWR at the transmitter end of the line can be as high as 2.25:1, if the line were lossless (and this would improve with cable loss).

Still not a calamity.

This is the dilemma: Make band-specific matching devices to improve this and then accept whatever losses they introduce, or just ignore it and use the cable.

Since I've used tube-type amplifiers on 6m for many years, and they can transfer full power into a 2:1 mismatched load just fine, I never worry about this stuff and just use the 75 Ohm cable.

WB2WIK/6



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N8EKT
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Posts: 371




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« Reply #4 on: June 14, 2008, 03:21:32 PM »

I fear attempts at baluns will produce more loss than the slight mismatch you will encounter.
However, be sure and leave room for expansion and contraction when installing the hardline.
Hardline is notrious for developing breaks if not allowed to expand and contract freely.
If all else fails you could start your own cable company.
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N4OGW
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Posts: 289




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« Reply #5 on: June 16, 2008, 12:47:20 PM »

I also use CATV hardline at my station (all <30 MHz). Mostly the 1/2 inch stuff.

I either do one of two things for matching:

1) don't worry about it
2) make the 75 ohm cable a multiple of a half wavelength long.

For connectors I generally use UHF type soldered into plumbing compression fittings.

Tor
N4OGW
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