I'm completely out of 50ohm coax at the moment besides short bits to use for interconnects. Was considering getting creative and trying some random twin-lead or even 75ohm CATV cable. Now I know CATV can work in a pinch with slightly higher SWR, but even that can be adjusted by tweaking the final transistors output tank circuit. My question is would connecting twin-lead directly work?
I have a roll of old zip line twin-lead used originally for TELCO lines. I think the stuff is 150 or 200 ohms. It's a belden type. When my shed door is unfroze I will crawl in there and see what number it is exactly. I have around 250 feet of this stuff!
Cutting into it is another story. Last time I tried I kept snapping the internal copper. They must have a special tool for this stuff they use at the telephone company.
My experience is that most coax or twin-lead can be made to work, sometimes suffering from SWR mismatch or feed line loss, but this being twin-lead I can't imagine it would be too bad even with a balun?
I did some searching on the interwebs to see if people have used twin-lead before on modern CB radios and found very little information.
Any ideas or experience?
I'm not worried about ruining the finals in this radio. The thing is built like a tank on the inside.
BTW before someone mentions it, I do realize the antenna has to match the feed line or suffer loss and higher SWR. If using an antenna with a matching stub like a j-pole or zepp that problem is already solved by finding the hot spot for impedance matching so that wouldn't be an issue. My concern is with matching the twin-lead at the radios side. I'm assuming a balun would be required since CBs expect an unbalanced load correct? Twin-lead being a balanced feed line and all.
Classic case of going from unbalanced coax line to balanced twin lead feed line.
You need way to convert from the unbalanced 239 or N to balanced. AKA balun. ARRL handbook would come in handy in this case. Look up info for the 10m band and that will get you in the ball park.
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Post by cptzulu158 on Mar 17, 2013 11:12:35 GMT -6
Yep you can do it with a balun. remeber the power loss though. dont know much about the feedline youre talking about but the 75 ohm tv stuff is usally very poorly shielded. if you can see through the shield its not worth crap.do what you want but I wouldnt screwdriver the radio just to change the feed point impedeance. The balun will work we use them alot in amateur radio. however you could direct feed the 75 ohm stuff and prob only suffer about a 2:1 swr. About the arrl book, these are great and should have plans to build a simple match. The 75 to 50 ohm match can be made by using two shot peices of 50 ohm coax soldier together and you can find plans on internet. if you have some decent tv coax this will have the least amount of loss and will be a closer match.
"As people become more intelligent they care less for preaches and more for teachers."
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Well I finally dug out that twin lead wire I got but can't find any information on it. Maybe someone here may have a guess or actual specifications for it. Only thing the cable says is..
"Superior Cable 1993" and "E-39169"
It looks like this stuff may actually be 75ohm twin lead that I have!? If that is the case then it would match a standard dipole flawlessly I gather.
Well spent some of the night in the shack and ended up trying the RG6 CATV coax out of curiosity and to my amazement it's extremely low loss. With 45 feet of it I notice almost no power drop at 27MHz and no readable SWR above 1.0:1 into a 50ohm dummy load which seems amazing to me since 50ohms is not what that coax should want to see as it's 75ohm cable.
Of course when I connected it to a dipole that's when things got weird. First trying with a 1:1 balun at the dipole into the coax I could not get a low SWR. Then I realized that it's possible the balun is somehow not doing its job properly because of the impedance mismatch so I just flat out removed it and connected the 75ohm coax directly to the dipole. This went against everything I have been taught but for some odd reason it worked this way! I am unable to get an SWR lower than 1.4:1 but that could have something to do with the height of the dipole above ground since I know height does change the impedance of a dipole slightly. Even at that it's still a very tolerable reading!
The loss of this RG6 is still far less than RG58. It's on par with RG8X if not better from what I can tell. Only issue so far is not being able to get the SWR lower but that may be the price I have to pay for using coax never intended for this purpose.
I did play with the twin lead wire but couldn't get it to play nice so I gave up on that for now. It's far easier just to use the RG6 coaxial cable if I have to, and besides the stuff can usually be had for free if one knows where to look.
Ah silly me, I forgot that SWR meters are built normally to read 50ohm lines and will show 1.5:1 using 75ohm coax even though it's actually 1.0:1 unless the SWR meter has a button for 75ohm use. Just read this on another site...
If a 50 ohm load is fed with 1/2WL of 75 ohm coax, a 50 ohm SWR meter will read 1:1 even though the actual SWR is 1.5:1. (My old Heathkit SWR meter could be calibrated for either 50 ohms or 75 ohms.)
Something very odd is happening with this setup. When I use RG58 on the dipole with or without a balun I notice my DTV TV reception gets crapped out. The strange thing is that the SWR meter says all is good and my field strength meter confirms it. Checking for harmonics the suspected ones are in the dirt.
When using the RG6 75ohm coax instead with or without a balun the TVI problems go away, yet I still get the same field strenth and almost no harmonics.
This has me completely baffled. I tried rearranging the coax figuring that maybe it was the placement difference between the 50 ohm RG58 and the 75 ohm RG6 but nothing even budged a meter.
How on earth is it possible for impedance of a coax to affect TV reception when the CB SWR meter reads clean on both lines, field strenth is the same, and harmonic issues are equal?
This freaks me out even more thinking about how the 75 ohm coax is actually causing less TVI than the 50 ohm cable designed for ham/cb use.
I know all cables aren't created equal but this seems completely backwards.
Something very odd is happening with this setup. When I use RG58 on the dipole with or without a balun I notice my DTV TV reception gets crapped out. The strange thing is that the SWR meter says all is good and my field strength meter confirms it. Checking for harmonics the suspected ones are in the dirt.
When using the RG6 75ohm coax instead with or without a balun the TVI problems go away, yet I still get the same field strenth and almost no harmonics.
This has me completely baffled. I tried rearranging the coax figuring that maybe it was the placement difference between the 50 ohm RG58 and the 75 ohm RG6 but nothing even budged a meter.
How on earth is it possible for impedance of a coax to affect TV reception when the CB SWR meter reads clean on both lines, field strenth is the same, and harmonic issues are equal?
This freaks me out even more thinking about how the 75 ohm coax is actually causing less TVI than the 50 ohm cable designed for ham/cb use.
I know all cables aren't created equal but this seems completely backwards.
I think what you'll find is the RC network in the radio is the culprit. This is VERY common in cb and 10 meter units that use mosfet final and driver sections. These units see anywhere between 65-70 ohms at the so-239 connector. It's also common in am only radios that use the 2sc2078 final. Does the TVI 'change pattern' on TX if you ground the chassis of the radio?
Well a bit has changed since then. I still haven't setup an outdoor antenna yet but will real soon. The whole issue as far as I could tell was the mismatch from the CB 50 ohm output which was matched and tuned to a dummy at the factory and the 75 ohm coax. Luckily it was a simple tuneup, or rather a tune-out job to match it to the cable and now it feeds the 75 ohm cable flawlessly as if it was built for it. To make matters more interesting the impedance of the cable matches a dipole perfectly, way less SWR because of the natural match being that the dipole runs around 60-70 ohms depending on height. So all I needed as a simple 6 turn current balun or .. air wound choke at the antennas input and it tunes up perfectly and gives me an SWR that is lower than the folks online who swore I would never get under 1.5. I am easily getting 1.2 across the 11 meter band, and even 10 and 12 meters. This all being RG6 CATV coax used for satellite installs. Best of all is the low power loss! People think I am insane using 75 ohm anything, but the reality is.. and the meter doesn't lie.. there is about the same loss as RG8/u at 27 megs. In other words you put the 4 watts in and 100 feet later you damn near get that same 4 watts out! But hey man I am nuts for using 75 ohm cable haha! It can work, just takes a bit of dinking around, and when you get it working... it works great!
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