Today in Northern Virginia I saw a tower that was maybe four stories tall, atop which sat an antenna. It was one distinguishing feature of an otherwise nondescript building, and in NoVa, that usually means cool stuff is going on inside. So, I was hoping the antenna wonks in here could clue a hapless MechE in.
The antenna was shaped like a glockenspiel, or perhaps a triangular fishbone, and it was laid on the tower’s spire so that all of its elements were horizontal. More precisely, the antenna was made of a metal central “spine” that looked to be about twenty or thirty feet long, with pairs of thinner metal whiskers set along its length at regular intervals. The shortest whiskers stuck out to either side of the spine by about four feet. The longest whiskers were so long they drooped significantly! However, I’d estimate that they were ten feet long. So that’s thirty feet in length, perhaps twenty feet wide at the base, and maybe eight feet at the other end.
I’m a mechanical engineer and have very little E&M theory in my background. What wavelengths was this antenna designed to transmit or receive? What sort of information is transmitted on those wavelengths?
Yagi is possible, but I’m guessing a log periodic dipole array. (LPDA) They provide fair directionality over an extreamly wide bandwidth. (often more than an octave). The FAA uses BIG LPDAs to communicate with transocianic aircraft via HF (AKA shortwave).
I used to own a Yagi antenna and travelled it in my Steadicam sled case. It looked exactly like this.
It was used to pick up RF video transmissions from my onboard transmitter. The delicate Yagi antenna pattern was embedded in a resin substrate. Worked great.
Bolding mine. Not necessarily. Buildings rent space for antenna arrays on a regular basis. My fire company does that and earns a nice piece of change, but what we are doing inside is no cooler than it was before the antenna.
Definitely a log periodic dipole antenna, and similar in size to the one in the overhead photo, I think. I’ll try to find a nearby object for size comparison next time I’m in the neighborhood. As for cool stuff going on inside, well, that depends on what kind of antenna it is, I guess.
(Edit: Anyone who measures “fun” or “cool” based on anything you can do with antennas goes down in my book as a geek - including me.)
