Is there a field guide for antenna spotting?April 4, 2012 10:58 AM   Subscribe

What can I identify about antennas based on their shape, location, etc? And is there a layman's explanation for why they are shaped the way they are?

Recently I've been looking at the antennas that adorn the rooftops in my city and am wondering about them.

I would presume that different shapes of antennas are optimized for different frequencies, so I should be able to look and see, "Oh, that one is a vertical folded dipole, so it's best at ___ frequencies which are used for ___ purpose." Where could I learn what I need to know to do this?

I've been skimming through the Practical Antenna Handbook, which is improving my underlying vocabulary, but it doesn't identify many of the types of antennas that I see.

And I've found a couple example shapes named online but it's sporadic.

So, is there anywhere I can find an exhaustive list of antenna shapes and what they are used for, or anything else that could help me research this? What are the most important features to classify an antenna?
posted by RobotHero to Technology (15 answers total) 10 users marked this as a favorite

Wikipedia is full of information about antennas, you just need to dig more.

The main thing to know, though, is that antenna shape has more to do with the pickup or transmission pattern than frequency. Bigger antennas can handle longer frequencies, but fractal designs allow you to handle low frequencies in a compact package.
posted by b1tr0t at 11:11 AM on April 4, 2012

Size, rather than shape, is a better indicator of the frequency range an antenna is designed for. The higher the frequency, the smaller the antenna (relative to others of its same shape/design).

The shape is more about focusing the transmitted (or received) energy in a particular direction. For example, a Yagi antenna (like you linked to) is designed to have high gain in one direction (the direction it's "pointing"), while a dipole will have gain perpendicular to its longitudinal axis. A vertical dipole, therefore, is good for transmitting or receiving in all directions at once.

From your examples, a Yagi antenna could be used for a variety of purposes. If you see a medium-sized one on the roof of a house, it's probably someone's TV antenna. It will generally be pointed in the direction of your city's cluster of broadcast TV towers. If you see a larger one mounted on a tower in someone's backyard, that's likely used for amateur (ham) radio on HF frequencies, which are lower than the VHF & UHF television frequencies (hence the antenna is larger).

Antennas that you see on utility poles or municipal buildings are likely used for public safety communications (police, fire, DPW, etc.). Most such communications these days are UHF, which is line-of-sight (to the horizon) hence the need for elevated central antennas and potentially geographically distributed repeaters.

Are there any specific examples you're wondering about?
posted by Nothlit at 11:13 AM on April 4, 2012

Folded dipoles are the most readily identifiable ones, and from the size you can tell if they're VHF or UHF. There are a lot of log-periodics for television and UHF voice/data links. Tow-truck operator shops seem to have verticals; maybe quarter wave for CB monitoring?

Some yagis can be huge ...
posted by scruss at 11:14 AM on April 4, 2012

Oh, bonus question, why are cell tower antennas shaped like boxes?

And can I assume all box-shaped antennas are for cell-phones?
posted by RobotHero at 11:22 AM on April 4, 2012

Those "box" antennas are sector antennas. They are indeed frequently used for cell-phone base stations,
posted by RichardP at 11:42 AM on April 4, 2012

The box-shaped antennas are sector antennas. They project out in a fan shape, sort of halfway between a omnidirectional dipole and a directional yagi. That way you can have several antennas around a mast (or building, or church steeple, or fake tree...) and have 360 degrees of coverage. The box encloses the actual antenna, which is usually more complex and delicate than simpler antenna types. Here's an example of a DIY sector antenna that someone made for wifi.
posted by zsazsa at 11:49 AM on April 4, 2012 [1 favorite]

Ha! I just noticed that the last 3/4 of my sector antenna comment above got cut off somehow. However, zsazsa's reply is remarkably similar to what is missing from mine.
posted by RichardP at 11:54 AM on April 4, 2012

As an illustration of the "antenna size goes up in accordance with frequency" principle: some VLF antennas are really freaking huge.
posted by Emperor SnooKloze at 1:15 PM on April 4, 2012

Sigh. I meant down. </doingitwrong>
posted by Emperor SnooKloze at 1:16 PM on April 4, 2012

The ARRL Handbook (ARRL == Amateur Radio Relay League) is a good introduction to basic antenna technology, shapes, etc.
posted by introp at 2:12 PM on April 4, 2012

The ARRL Antenna Book is listed as out of stock at their page, but Ham Radio Outlet claims to have copies. I've only ever built one antenna, from a kit & planned one other (a Yagi-Uda for 88.5MHz KQED only like this one) but I've spent hours with this book. There's so much far out stuff people have tried.
posted by morganw at 4:11 PM on April 4, 2012

Seconding the ARRL Handbook. It has everything about antennas, from LF to microwave.
posted by scruss at 5:53 PM on April 4, 2012

Thirding the ARRL Handbook, but in general size is a pretty good indicator. Although fractal designs exist that are (allegedly, anyway) good at lower frequencies than their size would suggest, I have never seen any 'in the wild,' at least for broadcast applications.

In my experience, if you see a giant antenna that is 100' across, it's probably HF; in particular, shortwave broadcast antennas tend to be rather large. N.B. about that one: it is big, so you should think "low frequency." It is low to the ground, so you should think "probably not line-of-sight propagation." It is radially asymmetric, so you should think "probably directional." Also, that antenna is very high power (250,000 W), but that's not necessarily obvious.

Parabolic dishes are sort of an exception. There are some really big dishes used in high-gain applications (e.g. radio astronomy) that operate at higher frequencies than you might naively expect for their size. In general, small dishes will be for the high microwave -- the best example are Ku band "pizzabox" sat TV dishes -- while bigger ones are typically lower-microwave like C band, but I would say the better telltale is the material the dish is made from. A dish made from mesh or perforated metal is limited in its ability to reflect smaller wavelengths, so that suggests a lower frequency than a solid dish (given that solid dishes are expensive and troublesome in terms of wind, snow, water load, etc., so it only makes sense to install one if you need it), but there are always exceptions.
posted by Kadin2048 at 8:37 PM on April 4, 2012

One thing I wonder with things like the ARRL Handbook or The Practical Antenna Handbook is they are focused on amateur radio, while surely most of the antennas I spot are professional, right?
posted by RobotHero at 4:22 PM on April 6, 2012

The difference exists, but hams have bands scattered all over, usually not very far away from the commercial bands in which you might be interested. The difference between a 1900 MHz PCS cellular antenna and an amateur radio 2300 MHz antenna is slight. The trick is that a lot of commercial antennae at UHF and up have covers so you can't see the guts. You have to figure them out by size, shape, mounting, and then work backwards to the specific common antenna design that's hiding underneath all that fiberglass.
posted by introp at 10:04 AM on April 7, 2012