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home-made twin-lead

dewdude

dewdude

High on resolution.
Hey guys,

It's raining here, so I'm sitting around looking for projects I can do inside the house.

Recently, with the acquisition and repair of this Kenwood shelf system, I now have some form of tuner. There's two problems....a) I don't have an FM antenna. b) i don't have any twin-lead.

I know I've heard about hams making their own transmission feed-line; and that the impedence depends on the distance. With that in mind, what's my idea about trying to make some 300 ohm twin lead?

I know it'll probably take forever to make sure the spacing of the wire is even; but i've got time.

The tuner's input accepts both 300 ohm and 75ohm. Oddly enough, they both use the same kind of connector they used to put on stereos, flip tab down, insert wire, flip tab up.

I got a ton of speaker wire and tape. I'm thinking I might go ahead and give it a try. But before I get involved in that, thought I'd ask suggestions/advice.
 
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I have made miles of open wire feeder or ladder line as it is called by amateur radio operators.

The spacing between the wires will depend on the gauge of wire that you use.

Zo = 276 log (D/r)

Where Zo = characteristic impedance, D = distance between conductors (center to center) r = radius of one conductor.

So 18 gauge wire with a spacing of about a quarter of an inch will get you in the ball park, so to speak.

D/r=12.5

log 12.5=1.096

276*1.096=302.5 Ohms.

It is 300 Ohm open wire feed line.

For spreaders, I have used pieces of plastic coat hanger. It is easy to cut and easy to drill the holes that the wire is fed through.

If strength is not an issue, you may use thin strips of plastic. I have used the clear blister pack plastic from items that I buy at the story.

The wire is taped to the plastic strips at the desired wire to wire (center to center) spacing.

You may look up the physical characteristics of wire gauges on the internet.

Now that you have your 300 Ohm feed line, you can make your antenna and connect to your tuner.

It is nice to see someone with an interest in more than just plug and play.

Good luck with your project.

Let us know how it turns out.

RF is fun...
 
Last edited:
JBL GUY said:
I have made miles of open wire feeder or ladder line as it is called by amateur radio operators.

The spacing between the wires will depend on the gauge of wire that you use.

Zo = 276 log (D/r)

Where Zo = characteristic impedance, D = distance between conductors (center to center) r = radius of one conductor.

So 18 gauge wire with a spacing of about a quarter of an inch will get you in the ball park, so to speak.

D/r=12.5

log 12.5=1.096

276*1.096=302.5 Ohms.

It is 300 Ohm open wire feed line.

For spreaders, I have used pieces of plastic coat hanger. It is easy to cut and easy to drill the holes that the wire is fed through.

If strength is not an issue, you may use thin strips of plastic. I have used the clear blister pack plastic from items that I buy at the story.

The wire is taped to the plastic strips at the desired wire to wire (center to center) spacing.

You may look up the physical characteristics of wire gauges on the internet.

Now that you have your 300 Ohm feed line, you can make your antenna and connect to your tuner.

It is nice to see someone with an interest in more than just plug and play.

Good luck with your project.

Let us know how it turns out.

RF is fun...
Click to expand...

My interest isn't more than plug and play....I just can't see spending $20 for a 100-ft roll of 300 ohm twin lead when really...all I need is about 15 ft.

I've seen the half wavelength dipole a LOT, it's the standard FM antenna. However, in an antenna book from 1941, they refer to a 1/6 wavelength "T" antenna as having exceptional broadband properties, and I've found these work quite well.

I got distracted with other things...so I haven't even started trying to construct my twin-lead...chances are I'll dig in the junk pile at work tomorrow and find some.
 
Sorry that you got distracted.

Even if you do not want or use the information that I took time to post, maybe others will find it interesting and helpful.
 
18AWG w/ 1/4" spacing is darn close to lamp cord. I've read some antenna plans that say lamp cord is OK. For a receiver project I have to agree and say go for experiment phase one with lamp cord. If that works but someone wants better then wait to scroe some actual 300 twin lead sections.

Note that I would never, ever, recommend transmitting though lamp cord. That could break stuff. But receiving is fine.
 
No, I actually do appreciate the information. I was going off something that quoted a specific measurement and didn't take in to account the guage of the wire. I also do in fact want to make some, just to add to that whole DIY/geek decor I've got going on in that room; part of me was looking for something i could maybe throw together in about 10 or 20 minutes...my idea was securing some seperated speaker wire to two stationary objects at the right spacing, and sandwitching them in duct-tape. I probably will try that, and your information will come in handy.

I've also seen lamp cord mentioned quite a bit. I've got some zip-line from an old extension cord that I was using as speaker wire. The conductors are maybe 2 or 3mm apart. But, I already did something similar to that. The FM reception here is, poor, so I was just trying to maximize my chance of a fun tedious hack having the highest chance of good success. That is the great thing about receiving; a major impedance mismatch just won't work well, not burn up transmitters.
 
The 18 gauge lamp cord that I have checked turns out to have an impedance of about 220 Ohms.

Of course this will vary depending on the thickness of the insulation (conductor spacing).

For those that are interested that works out to VSWR of about 1.36 to 1.

Or a return loss of about 16.33 dB.

Or to put it another way, the loss in signal would be a little over 2% (due to the mismatch, not taking into consideration any dialectic loss due to the insulation). Not all that bad.

If one builds the antenna as a folded dipole using 18 gauge lamp cord, it may work OK.

There is one unknown. The velocity of propagation of the lamp cord.

I may be around 0.6c. to 0.7c

The velocity of propagation must be taken into account if one wants to cut the antenna to the correct length.

BTW, another factor. The dialectic properties of the insulation. May not be a large concern if the feed line is not all that long (in terms of signal loss.)

If my RTDR and network analyzer were not in storage I would check some lamp cord just for fun.

Anyway fun stuff and good luck with your antenna project.
 
JBL GUY said:
Sorry that you got distracted.

Even if you do not want or use the information that I took time to post, maybe others will find it interesting and helpful.
Click to expand...

I for one found it both interesting and useful! :thmbsp: A long time ago I was in a similar situation as the OP. Couldn't locate 300 ohm twin lead. I had made a folded dipole using whatever gauge I had handy and using some pine trim stock as a spacer guessing it would be about right. Your #3 post above shows me why it probably wasn't as effective as it could have been.

by the way, I snap up twin lead when ever it shows up in usable lengths at my local habit for humanity re-store, and occasionally at garage sales. I'm set for a while :)
 
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