What happened to 72/75 ohm TRANSMITTING twin lead?

[k4kyv]

I recall some years ago seeing transmitting type twin lead for sale pretty much everywhere as a common item.  It looked similar to heavy duty zip cord but the insulation was the same stuff used with 300 ohm TV ribbon.  The conductors were stranded, about #12, but not spaced as closely as the tiny TV receiving stuff.

I searched the web but couldn't find any mention of it anywhere.  Anyone know of a source?

Looks like there should be a major demand for it for single-band resonant dipoles.  No balun needed and the stuff is lighter in weight than RG-8 or RG-11.

[WA5VGO]

I used this stuff extensively back in the 60's and 70's. I don't think it's made anymore. A couple of years ago there was a guy (in Canada, I think) that was selling spools of it. That's the last of it I've seen.

Darrell, WA5VGO

[Bill, KD0HG]

Oh, yeah, that was Belden 8210 72 ohm stuff. Pair of close-spaced #13 wires. Capable of mucho power when matched, but easily fries out QRO with a bad mismatch. The conductors are less than a pencil width apart.

They quit making it maybe 25 years ago.

Better off using a high-Z balanced line.

[Sam KS2AM]

I recall some years ago seeing transmitting type twin lead for sale pretty much everywhere as a common item.  It looked similar to heavy duty zip cord but the insulation was the same stuff used with 300 ohm TV ribbon.  The conductors were stranded, about #12, but not spaced as closely as the tiny TV receiving stuff.

I searched the web but couldn't find any mention of it anywhere.  Anyone know of a source?

Looks like there should be a major demand for it for single-band resonant dipoles.  No balun needed and the stuff is lighter in weight than RG-8 or RG-11.

If its Belden 8210, there was an earlier thread on the subject.  http://amfone.net/Amforum/index.php?topic=21126.0

[k4kyv]

I would only use a dozen or so feet of it, to run from link coupled transmitter to link coupled tuner.  I have used zip cord with the conductors pulled apart, or twisted pair made out of a couple of pieces of hookup wire, but figured that stuff would have lower loss. I suppose a short run of coax would work just as well.  No need to ground the shield.

[Bill, KD0HG]

Don, you could use a parallel run of 50 ohm coax. Connect the shields together at both ends. Makes for a shielded, balanced line of 100 ohms. RG-213 is good for at least 10 KV.

[k4kyv]

For a very short run, a few inches or a few feet (depending on frequency/wavelength) connecting two links together, I don't think the impedance of the line makes much if any difference, so the extra piece of coax to make the double-barrel stuff wouldn't serve any useful purpose.  I was just thinking that plain old rubber or plastic insulation might be more lossy than the  stuff they use on the low-Z twin lead. Two links tied together with a short line merely simulates physical proximity between the coils, and allows mutual coupling when they are located some distance apart. Once the length of the line exceeds somewhere between 1/20 and 1/10 of a wavelength, then it begins to act more like a transmission line with line length and standing waves beginning to have a noticeable effect on the coupling.

I have several rolls of #12 and #10 insulated stranded wire.  I might try making up a length of twisted pair out of several samples and comparing the loss on 40m and maybe on 10m with a piece of RG-213.  IIRC, transmitting grade 72Ω twin lead was physically stiff, about as stiff as coax.

I once had a roll of two-conductor coax, but don't know what ever happened to it.  I wish I had a few hundred feet of it to use as the transmission line for the beverage. It should be more resistant to common mode coupling than is regular coax. I suppose two conductor coax is about as unobtanium as 72 ohm twin lead.

[K5WLF]

Don,

Belden 89555 and 9555 are both 75 ohm dual coax. There are several others in the Belden catalog that are also 75 ohm, but I didn't see any 50 ohm dual coax as I scrolled through the online catalog.

ldb
K5WLF

[KA2DZT]

Did anyone ever hear of using twin conductor telephone drop wire.  The stuff that is used from the pole to the house.  I think that stuff is close to 70 ohms.  I think I remember hearing or reading about it years ago.

Fred

[KM1H]

Im running 2 x #18 Telco wire as a 1000' 2 wire Beverage. That computes at 220 Ohms rounded off.

Here is a good program for that
http://www.csgnetwork.com/antennaerpcalc.html

Carl

[K5UJ]

If you only need a short run, maybe you can find the formula for computing spacing and if you know the diameter of the wire and desired Z you can compute the spacing with an air dielectric and make up a 12 foot run of 72 ohm blanced feed on your own.  it might only need to be separated by an inch.   Get yourself no. 10 stranded THHN.  If the run is straight through the air you could mount two pairs of ceramic post insulators with the necessary space between them and string up the wire.

[KM1H]

#10 at .25" spacing is 190 Ohms and 108 Ohms at .125"

With #12 its 218 and 135 Ohms respectively.

I believe Ive read that the old TX 72 Ohm twinlead was rather optimistic.

Carl