antenna-theory.com

[rtso]

Maybe you've seen this advertised - an indoor, "paper thin" flat antenna for HDTV. It is only 9" x 11.5" and it is very flat. It got very good reviews on Amazon. Looks to be passive.

I though it would be fun to hear antenna designer's guesses as to what they think is inside.

My #1 guess - a bowtie where the elements and transmission lines are made of metal foil, like copper tape. It's advertised as being omni-directional, but I find that is hard to believe. But it's also hard to believe a simple bow-tie without a reflector would get such good ratings.

What do you think?

[bigSteve]

The antenna is very likely omnidirectional. "Omnidirectional" in the antenna sense means it has a donut shaped radiation pattern, so that at least the radiated (or received) power doesn't vary in the azimuth direction (circularly around the antenna). An example omnidirectional pattern is shown Figure 4 on this page:
http://www.antenna-theory.com/antennas/dipole.php

For antennas, it is easier to make an antenna omnidirectional the thinner or more "wire-like" the antenna is. If the antenna takes a 3D shape (like a horn or satellite dish) it becomes more directional (less omnidirectional).

In terms of what is inside, it is probably a bow-tie type antenna made of some copper or aluminum foil. Expanding the dimensions in two-dimensions (relative to a simple wire antenna) greatly expands the bandwidth. To get more bandwidth, they could have gone into the 3rd dimension, but it isn't necessary - the paper thin version will be sufficient.

One other possibility: they could be using the chord itself to increase the effective length of the antenna, to enable better reception at lower frequencies. I can't tell without handling one though.

In regards to a reflector: a reflector only serves to increase the directivity of the antenna. That is, it doesn't make the antenna more efficient; it simply makes the antenna's radiation pattern more directional, or sensitive in certain directions and not others. In addition, for a reflector to be effective at HDTV frequencies, it would have to be pretty big, probably larger than a DirecTV dish, which is too big to put in your house .

[rtso]

thanks BigSteve! Great answer!

You're right! I was thinking omni-directional would be in the useful sense, but in this case it's a technicality that lends itself to more marketing hype than anything else, since hdtv stations are not going to be coming from the earth or straight up from the sky!

But this leads me to my next question. Why are TV station signals horizontally polarized (TV antenna dipoles are horizontal) whereas I see many small non-TV application Yagi's deployed where the elements are vertical?