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Why do "vintage" rabbit ears come with 300 ohm twin lead cable?

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For fun, I've been collecting rabbit ears at the thrifts. I find them now and then for 1 to 5 bucks. My collection is now up to a grand total of three. One is a more modern one from RadioShack (Model No. 15-1828) and has a female F-connector on the box to attach standard 75 ohm CATV cables (RG-6, RG-59) with male F-connectors. The other two likely came off an old TV set and have a short length of 300 ohms twin lead cable attached.

Rabbit ears can be formed into half wave dipoles that are resonant with the FM band (and some TV bands). Half wave dipoles have a nominal input impedance of 73 ohms [1] that matches the impedance of 75 ohm CATV cable well, although a balun is required to reduce common mode currents that cause the coax line to radiate in addition to the antenna itself.

On one of rabbit ears, the 300 ohm twin lead is attached directly to the dipole antenna elements- a direct impedance mismatch to a 73 ohm half wave dipole. The other I haven’t taken apart for fear of damage to the case. I wonder if it has a balun/transformer (73 ohm to 300 ohm) inside the tiny box.

So my questions are:

Why do "vintage" rabbit ears come with 300 ohm twin lead cable?

Does the short length of 300 ohms twin lead cable attached form some sort of RF impedance matching circuit? Maybe I’ll have to get out my textbooks to answer this one.

On my FM tuners, do I connect these to the 300 ohm or 75 ohm antenna terminals?

Any comments?

I’m thinking about modifying them to attach a 75 ohm CATV cable with an air-core balun [2].

[1] http://en.wikipedia.org/wiki/Dipole_antenna

[2] http://hamuniverse.com/balun.html

Addendum: Here's a photo of the RadioShack Model No. 15-1828 (top) with CATV cable screwed into the female F-Connector and the Sony AN-18 with 300 ohm twin lead permanently attached (bottom).
antennadipole1200x900.jpg
 
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aabottom said:
On one of rabbit ears, the 300 ohm twin lead is attached directly to the dipole antenna elements- a direct impedance mismatch to a 73 ohm half wave dipole.
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An OPEN half-wave dipole has an impedance of 73 Ohms, a FOLDED dipole (tips joined together to form a closed loop) has an impedance of 300 Ohm.

Yagi-Uda arrays with folded dipoles (the usual type for rooftop mounting because they are strong, and the earthed centrepoint is 'good' for lightning) need a balun at the antenna terminals to be connected to a co-axial cable
 
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steerpike2 said:
An OPEN half-wave dipole has an impedance of 73 Ohms, a FOLDED dipole (tips joined together to form a closed loop) has an impedance of 300 Ohm.

Yagi-Uda arrays with folded dipoles (the usual type for rooftop mounting because they are strong, and the earthed centrepoint is 'good' for lightning) need a balun at the antenna terminals to be connected to a co-axial cable
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Thanks for your response.

Yes, you are correct. The rabbit ears I'm referring to form an OPEN half-wave dipole and came with 300 ohm twin lead cable attached to the elements, thus creating the impedance mismatch.
 
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I'd bet a lot of rabbit ears were made before the days when coax connectors became commonplace on televisions. When I was a kid, cable TV didn't exist, and the only terminals available on the back of the TV were the 300-ohm screw terminals. When cable came along, the 75-300 baluns were still commonly used for those older sets.
 
When I was more into scanners, it was common practice to use RG-6 instead of something like RG-8 because not only was RG-6 cheaper and easier to run, the lower loss apparently more than offset any issue caused by 50 ohm vs 75 ohm mismatch.

Since twin lead is typically lower loss than coax that could have some effect too but probably rather minimal at these lengths.
 
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bmbguy said:
I'd bet a lot of rabbit ears were made before the days when coax connectors became commonplace on televisions. When I was a kid, cable TV didn't exist, and the only terminals available on the back of the TV were the 300-ohm screw terminals. When cable came along, the 75-300 baluns were still commonly used for those older sets.
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Ditto.

If somebody was buying coax at the electronics store they were into CB or Ham radio. TVs only had those two screw terminals.
 
aabottom said:
rabbit ears I'm referring to form an OPEN half-wave dipole and came with 300 ohm twin lead cable attached to the elements, thus creating the impedance mismatch.
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I have both open & closed set-to rabbit ears (bunny ears to us).
One 300-ohm folded is made from a flat ribbon of spring steel, like tape measures are made from and can be rolled/unrolled to tune it - this has a ribbon cable. The open dipole ones are two telescopic rods, with a co-ax cable wired through capacitors for safety.

I don't know the designs of yours, but a balun doesnt have to be a transformer, matching can be done with blind stubs of cable cut to correct lengths.
 
As someone else said, the older tv's didn't have coax connectors. We used the thin 300 ohm twin lead for VHF and a "more insulated" type for UHF to connect to our antennas. Some may have used coax. I'm not sure. I just know that it wasn't used commonly as it is today.
 
yup, old tvs had twin lead inputs. F connectors didn't become commonplace till at least the 1970s, and more likely the 1980s. If you were running bunny ears, you likely had more than enough signal to overcome the mismatch problems anyway so it frankly doesn't matter all that much. For reception outside the city where you needed a good outside antenna, it would have been matched to 300 ohm twin lead to avoid the loss concerns.
 
gadget73 said:
yup, old tvs had twin lead inputs. F connectors didn't become commonplace till at least the 1970s, and more likely the 1980s. If you were running bunny ears, you likely had more than enough signal to overcome the mismatch problems anyway so it frankly doesn't matter all that much. For reception outside the city where you needed a good outside antenna, it would have been matched to 300 ohm twin lead to avoid the loss concerns.
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You are right on the money, and unless the length of the elements was just exactly right (tuned to resonance), the feed point impedance would have been a complex reactance of who knows what value, especially with the random pieces of tin (aluminum) foil adorning the elements and little Joey holding one of the elements just right so dad could watch the Friday night fights..:D:D

Not much point in using a matching transformer in that event.:scratch2:
 
steerpike2 said:
I have both open & closed set-to rabbit ears (bunny ears to us).
One 300-ohm folded is made from a flat ribbon of spring steel, like tape measures are made from and can be rolled/unrolled to tune it - this has a ribbon cable. The open dipole ones are two telescopic rods, with a co-ax cable wired through capacitors for safety.

I don't know the designs of yours, but a balun doesnt have to be a transformer, matching can be done with blind stubs of cable cut to correct lengths.
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Interesting ... I've never seen "folded rabbit ears". My rabbit ears are open dipole with the two telescopic rods, but the have 300 ohm twin lead cable attached directly to the two telescopic rods. I'll post a picture of mine soon.

I'd like to see a picture of your 300-ohm folded made from a flat ribbon of spring steel, like tape measures are made from and can be rolled/unrolled to tune it - this has a ribbon cable.
 
Thanks all, for your replies.

So it looks like the answer to
Why do "vintage" rabbit ears come with 300 ohm twin lead cable?
is simple economics. 300 ohm twin lead was a cheap way to hook up rabbit ears to a TV, dispite the impedance mismatch.

Ok now this question is intriguing me.
Does the short length of 300 ohms twin lead cable attached form some sort of RF impedance matching circuit?
Now where's my copy of Schaum's Outline on Transmission lines. I hope I don't have to dig into my graduate texts on E&M (electricity and magnetism).

No one commented on this questions.
On my FM tuners, do I connect these to the 300 ohm or 75 ohm antenna terminals?
Any suggestions?
 
bmbguy said:
And we had those cool little 'clothespin' lookin' things to clip the antenna leads onto the terminals...
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Geeze, I haven't seen any of those since the early 70s, I bet. And they weren't all that common then. I'd like to find at least a couple, just because they looked so cool, AND worked very well. :thmbsp: Seems to me they were in sets of 2, one yellow & one blue. :scratch2:
 
To the OP, I added a photo of the RadioShack Model No. 15-1828 (top) with CATV cable screwed into the female F-Connector and the Sony AN-18 with 300 ohm twin lead permanently attached (bottom).

Enjoy!
 
On the why, could be that it is a simple physical connection to each dipole, and twin lead has less loss per foot for it's bulk-weight-flexibility than average coax.

Most tuners have a balun to match the balanced 300 ohm lead to the unbalanced 75 ohm resistance that the front end of the tuner is designed for.
 
aabottom said:
No one commented on this questions.
On my FM tuners, do I connect these to the 300 ohm or 75 ohm antenna terminals?
Any suggestions?
Click to expand...

I'm not usually one to get too hung up on the details. So if it were me, I'd just try them both and see which one worked better for the stations I wanted to listen to.
 
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ericson38 said:
On the why, could be that it is a simple physical connection to each dipole, and twin lead has less loss per foot for it's bulk-weight-flexibility than average coax.

Most tuners have a balun to match the balanced 300 ohm lead to the unbalanced 75 ohm resistance that the front end of the tuner is designed for.
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Yes, at least the twin lead is balanced on the way to the tuner.

And yes, most of my tuners have 300 ohm and 75 ohm antenna inputs.

bmbguy said:
I'm not usually one to get to hung up on the details. So if it were me, I'd just try them both and see which one worked better for the stations I wanted to listen to.
Click to expand...
That sounds like a good plan.
 
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