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

[steerpike2]

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.

Not my photo, but these are they:

There were also ones sold here that had a closed loop made of something similar to coat-hanger wire. Totally ridgid and un-tunable. But that was in a time when we had only three broadcast channels on the VHF band 3, and the channel spacing was close enough.
These types has co-ax cables fitted - but I don't know if there was a balun in the antenna base. South African TV sets were only ever sold with co-ax RF connectors, right from inception.

 

[aabottom]

Not my photo, but these are they:

There were also ones sold here that had a closed loop made of something similar to coat-hanger wire. Totally ridgid and un-tunable. But that was in a time when we had only three broadcast channels on the VHF band 3, and the channel spacing was close enough.

Cool, thanks for the picture. They do look much more like "bunny ears". I can imagine Bugs Bunny falling in love with them.

These types has co-ax cables fitted - but I don't know if there was a balun in the antenna base. South African TV sets were only ever sold with co-ax RF connectors, right from inception.

That's funny. Exactly opposite of what's needed. As you say, these are folded dipoles and would work fine with 300 ohm twin lead, cheaper and better matched than coaxial cable. :sigh:

 

[aabottom]

OK, I've been reviewing my transmission line theory and, as I suspected, there's good news.

If the length of the 300 ohm feedine is a multiple of a half wavelength (inside the transmission line) , then the impedance measured at the end of the feedline (at the receiver) is equal to the impedance of the antenna (the load), that is 73 ohms for an open half wave dipole. In that case it would be best to connect to the 75 ohm input of the receiver.

Using 0.82 as the velocity factor of 300 ohm twin lead and frequency 98 MHz (the center of the FM band), one half wavelength inside the transmission line is 48.4 inches. Thus for 98 MHz, a feedline length of 48.4 would be best, but any multiple of that would work too.

Alas, the feedline length on my Sony AN-18 rabbit ears is 19.065 inches, corresponding to a frequency of 253.8 MHz, way beyond the FM and TV bands. :sigh:

PS: Yes, I'm a geek.

 

[bmbguy]

So, the obvious conclusion is that Sony really messed things up trying to save on their feedline costs! Now we geeks know the real truth...

 

[aabottom]

Well, I measured the 300 ohm feedline length of my other rabbit ears, it is 33.625 inches, corresponding to a frequency of 143.9 MHz. At least that one is in between the VHF TV bands [1]. At least that make sense.

[1] http://en.wikipedia.org/wiki/North_American_broadcast_television_frequencies
VHF low-band (band I, channels 2 to 6): 54-88 MHz
VHF high-band (band III, channels 7 to 13): 174-216 MHz

 

[KentTeffeteller]

In the early days of television and FM reception, standard antennas used 300 ohm twin lead. Coaxial cable existed but cost vastly more for years. And didn't catch on until sometime in the 1970's.

 

[Stereo_type]

Originally Posted by bmbguy
And we had those cool little 'clothespin' lookin' things to clip the antenna leads onto the terminals...

Spcmn:

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. Seems to me they were in sets of 2, one yellow & one blue.

Ha! That brings back memories from waaay back! I did some TV repair in my early days, learning the basics on the job and I, along with all the other TV techs loved the clothespin clips. For some reason, I seem to remember light colored red ones, along with maybe light blue. Instead of fooling with the tedious twinlead and screws every time, we could clip an antenna connection on the back of a TV in a snap. I wouldn't call those clips very 'good' for a permanent conn. tho, as they were kind of flaky, connection-wise...just sayin'...