A question about OTA antennas

When I was a kid (late 70’s) we had a large antenna installed on our roof, hooked up to our TV. There was a dial control that allowed us to rotate the antenna, which gave us a better signal.
My question… What is it about the shape of my antenna (and every other one I see on roofs, now that I think about it) that makes the direction important to picking up the television signals?

The antenna is designed to be highly directional. It was probably a log-periodic or Yagi-Uda design. If you just look at it, it is obviously assymetric. Either of these designs contain multiple folded dipoles poisitioned along a line. The folded dipoles are directional, but receive (or transmit) over a rather broad angle. When you line them up along a line, the recieved signals add constructively if the spacing is just right for the wavelength of the radio waves. This constructive interference only works over a narrow angle, making the antenna more directional than the individual elements. The advantage is two fold: first, you only pick up the stations in the direction of maximum response, thus eliminating unwanted interference, and second, the received signal is larger because of the constructive interference. The latter is termed “antenna gain”. In a properly designed antenna, gain and directionality go hand in hand.

Geez. That’s right, and complicated (to my simple mind). Simply put, if your OTA antenna is a Yagi (most are), it needs to aim at the transmitting towers of the TV station. Your antenna should pull in the signal you want and null out interfering signals.
TV stations try to make this simple by clustering their transmitting towers together. Not a hard thing to do since they want to be at the highest point they all can get, so there’s the cluster.
This way you can point your stationary tower in one direction and get most of 'em.
Your rotating tower might be considered a luxury.

the gain of a multielement antenna, the types you see on rooftops, increases the signal from a single wire/rod, this allows the reception of a signal from far away. an antenna for use in the same city would have fewer elements (rods) and length compared to a deep fringe antenna for 60 miles away that would have many elements and be 8 feet long.

even for a close station if you might get improvement because reflected signals (from other directions) don’t cause as much interference, with analog tv that would be ghosting and with digital tv might be as if there was no good signal.

depending on the antenna design you might have two active elements in an antenna and a number of passive elements or a string of active elements connected together. some antennas would be VHF or UHF or both combined. now in the USA there are fewer channel 2 through 6 (the real channel broadcast on not the old channel number that still may be the station’s identity), some more but fewer than before on real channels 7 through 13, and lots now on 14 to 51, so old antennas work fine, new antennas might not have to be as big.

in a major metro area that doesn’t have a major mountain nearby the the stations might be in more than one cluster (hills not near air traffic paths). in areas out of the cities (with the tv stations) you might have cities in two or three directions to get signals from.

with analog signals you might get by with one antenna to your weakest stations and get the stronger signals off the side of the antenna; analog signals allowed you to accept a snowy picture and still call it tv. with digital tv a good signal is more important, either it is good enough or no tv; multiple fixed antennas and an antenna switch work very well and is rapid, a rotator/rotor works but is slower.

If you make a non-directional antenna (which you can do), it tends to suck equally in all directions. If you make a directional antenna, it is better than a non-directional antenna in the direction you aim it, and even worse than a non-directional antenna in other directions. A directional antenna therefore allows you to pick up distant signals more clearly, and naturally filters out noise from other signals in other directions. In other words, a directional antenna naturally boosts the signal you want, and naturally rejects signals you don’t want. The only problem is you have to aim it.

Most rooftop antennas are yagi antennas, because they are cheap and easy to construct and they have good directional qualities.

While I have seen a few older Yagi TV antennae , most VHF antennae were Log Periodic Dipole Arrays. (LPDA) Many of these are still in service.

Yagi arrays have a single driven (connected to feedline) element, and multiple passive (No wired connection to feedline) director and usually a single passive reflector element, though corner reflector Yagis are popular in UHF television service.

LPDA antennas have all elements wired to the feedline, in alternating phase. You can see the feeder criss-crossing down the length of the boom.

Yagi antennas offer excellent gain and directivity, but the bandwidth is too limited to cover the entire VHF band. LPDAs offer modest gain and directivity, but can cover l octaves of bandwidth.

The UHF band in narrower when considered geometrically (ratio of bandwidth to absolute frequency) than the VHF band, so Yagis are usable, and with the new Digital service in the US, the VHF band has been reduced/eliminated to where LPDA antennas are no longer the norm. With the new band, planer arrays are feasable.

So what makes them work: The math needed to explain in detail is fairly complex.
Due to the length of the elements, the phase of the RF current flow is altered. Depending on spacing of the elements, the signals from the elements combine either constructively or destructively. This increases the signal in one direction, and partially (fully in some cases) cancels it in others. Passive elements first absorb energy, then re-radiate with shifted phase. Think of an acoustic echo if that helps.

Note: Antennae perform symmetrically when transmitting and receiving. Most antenna literature assumes (as I have done) the transmitting case…thus the element connected to the feedline is called “driven” even though the element is driving the feedline in the case of reception.