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The bunny-ears antenna is similar to a dipole, which has an impedance of ~75 ohm.

Why are the elements of such antennas connected directly to 300 ohm ladder line? Wouldn't that cause mismatch losses (even with the other end going into a balun)?

Mark
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    You might want to include a picture of such an antenna just to avoid any confusion. Why would a dipole be 75 ohms ? I know 50 ohm dipoles exist. No there is no mismatch as this is a **balanced** antenna of 300 ohms balanced. If your TV needs 75 ohm unbalaced in (COAX) then a **balun** is needed to convert 300 ohm balanced to 75 ohm single-ended. The balun **prevents** mismatch losses. – Bimpelrekkie Aug 21 '17 at 14:37
  • The feedline to the ~75-ohm dipole is 300-ohm (and connects to the **dipole** without a balun) – Mark Aug 21 '17 at 15:55
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    Where on earth do people still have 300Ohm antenna connections for their TVs? – PlasmaHH Aug 21 '17 at 21:26
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    This *really* needs a picture. – pjc50 Sep 22 '17 at 10:20

2 Answers2

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bunny-ear, rabbit-ear antenna"Bunny-ears" antenna is a non-technical term for a dipole antenna. At its resonant frequency, a dipole has about 70 ohms impedance at its feed-point. To avoid signal loss and reflections, a matching transformer is needed. This simple bunny-ear dipole should include a 300-ohm-to-75-ohm balun inside, at its feedpoint, if its feedline output is 300-ohm balanced transmission line. However, any modern TV has 75-ohm input, usually identifiable by its "F"connector:F-connector
Some very old TV's have antenna impedance of 300 ohms impedance: these do not have "F" connector, but two screw terminals instead. These TVs are meant to accept balanced 300-ohm twinlead directly. A balun transformer matches 75-ohm to 300-ohm. Balanced 300 ohm feedline was used at VHF frequencies because it has low signal loss, when its path from antenna to TV is chosen with care. Today, 75-ohm coaxial cable is often used instead for two reasons: The path to TV is not as critical, and antennas often have preamps, making cable loss less important.
75-ohm to 300 ohm balun

A simple dipole antenna with 300-ohm feedline should include a balun at the feed-point. However, be aware that TV channels are spread over a wide frequency range, so that fixed-length dipoles used for TV reception on many channels are often not resonant.
A folded-dipole antenna is often made with 300-ohm feedline, and has resonant impedance near 300 ohms, and can feed a 300-ohm transmission line directly, with no balun. Where the 300-ohm transmission line connects to a modern TV having an F-connector, a 300-ohm to 75 ohm balun would be required.
folded dipole, 300 ohm

edit: At resonance impedance matching should help deliver maximum signal power. But nobody ever adjusted element lengths to resonance for the TV channel they were watching. Dipole impedance would be unknown for different TV channels...very likely much higher than 70 ohms.
Given this situation, standing waves along the transmission line that connects antenna to TV will cause signal loss. To make the best of a poor situation, the lowest-loss transmission line is used. 300-ohm twin lead can be very low loss, better than coax. And its higher impedance has a better chance of randomly coming close to the ideal 1:1 standing-wave (SWR).

glen_geek
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  • The problem is that the feedline doesn't have a balun. – Mark Aug 21 '17 at 15:40
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    @Mark ,Electronic-geeks, who would ensure resonant dipoles, a balun would be satisfying. To a regular TV-watcher, an OK, noiseless TV image is satisfying - the mis-matched impedance is a moot point, so the "rabbit-ear" maker can skimp. – glen_geek Aug 21 '17 at 16:22
  • So they've saving cost on one balun of the two normally required? – Mark Aug 21 '17 at 16:23
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    Nope, only **one** balun is needed, one like this: https://2.bp.blogspot.com/-hUsD3IBQi0g/VLwb-e_RMpI/AAAAAAAABi0/SslTHi60SxU/s1600/balun2.png – Bimpelrekkie Aug 21 '17 at 19:01
  • But the other end of the 300-ohm line is connected to the dipole (which should be about 75 ohm). Isn't that a mismatch? – Mark Aug 22 '17 at 12:17
  • Where is the answer to the question? – RoyC Sep 22 '17 at 07:44
  • @RoyC Fair comment. Have edited the answer, addressing the issue of mis-match. – glen_geek Sep 22 '17 at 12:48
  • There are many cheap dipole folded antennas available ( https://www.aliexpress.com/item/FM-Dipole-Indoor-Antenna-Aerial-for-HD-Radio-Bang-Olufsen-others-75-Ohm/32616706477.html) Sellers claim to be 75Ohms, but it is unlikely that there is balun at the end. So are these products fake? – Pygmalion Jan 31 '18 at 08:58
  • @Pygmalion Your aliexpress example *could have* a balun transformer hidden inside the connector...it can be made quite small. If not, it has a mis-match loss, but will still provide a signal. Do you still call that fake? – glen_geek Jan 31 '18 at 14:14
  • @glen_geek Thanks. Well, if it is 300 Ohm and specification says 75 Ohm, then it is definitely misleading. – Pygmalion Jan 31 '18 at 14:20
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Yes, there are indeed mismatch losses because the feed line is 300Ω and the antenna impedance is in the range of 50-73Ω. The mismatch losses are around three decibels, or half of the input power.

The reason for the simple direct connection is because it works - in most cases the losses are acceptable because the maximum range of the TV signal is only reduced by ~30%.

neonzeon
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  • Why would anybody deliberately introduce 3 db of attenuation into a system at a point where it is critical to avoid this? You will always have mismatch losses as you can only get a perfect match at one frequency with a fixed antenna but not this much. – RoyC Sep 22 '17 at 07:42
  • Because, as pointed out, in most cases it does not matter. When it does, better matching and higher gain antennas are used, at higher cost and less convenience. – neonzeon Sep 23 '17 at 18:51