I left (grades weren’t good enough) the EE program at UT for the CS department before I could take E-Mag and so I never found out why TV aerials are shaped the way they are. I know this is a complicated question, but can someone give me a high level overview?
Thanks,
Rob
The very short answer is because VHF TV channel is a different frequency, and each one of the “bays” on the antenna was optimized to pick up a different range of frequencies.
Don’t ask me why UHF antennas were a small loop or bow-tie, though.
I know about the different-sized antenna elements being optimized for different frequencies, but I had a TV with a simple rabbit-ears antenna (built in, I know the OP means the big ones you’d see on roofs) and it worked fine without adjustments for most of the channels it could receive; thus such an elaborate antenna doesn’t seem very necessary (see the UHF antenna example you gave).
Rabbit ears work fine if you have a strong signal, such as if you are living within the optimal range of the transmitting tower.
However, if you are farther away, or if the signal is blocked by tall buildings, mountains, etc. you will need the bigger antenna to pick up the weaker signal.
When you say “TV aerials,” do you mean this?
If so, that is a Yagi antenna, and it is a high-gain directional antenna. It consists of “directors” and a reflector., which are tuned to a fairly narrow band.
The wiki page on the Yagi-Uda Antenna has a detailed explanation (assuming that’s the type of antenna you are asking about). Very basically, one of the cross-bars is the actual active antenna; by itself it’s a simple dipole antenna. The other bars make it directional; the bars behind the active element act as reflectors, and the ones in front act as directors.
VHF used Yagi design. Very directional. UHF was a loop, and picked up on the magnetic component of the EM wave if memory serves me correctly.
the Yagi antenna was most common early. another type that was used a lot more in later years was the log periodic. from the ground they look very similar. the bandwidth of tv signals is large and the log periodic made a better antenna for that purpose.
rooftop UHF antennas might also have a corner reflector with a single antenna. UHF not propagating as well might have multiple antenna vertically ganged together (might look like part of a bed spring vertically).
though having an antenna the right length for your desired signal, other lengths will get a strong signal. so rabbit ears left non adjusted, or a UHF loop or bow tie will get many channels.
