On Mon, 10 Oct 2005 08:04:47 -0400, bjs555 <aaa@bbb.com> wrote:

>Antennas are new to me so the questions I ask may not be entirely
>sensible. But here goes anyway:
>
>My understanding is that a dipole antenna is in the shape of a T where
>the length of each horizontal branch is equal to a quarter wavelength.

Correct. Actually, it's more like 0.95 * 1/2 wavelength because of
"end dispersion" effects.

>So, for 2.4 GHz, each horizontal branch would be about 1.2 inches
>long.

Watch your accuracy. At 2400Mhz a wavelength is about 125mm. However,
each MHZ is equal to:
125mm / 2400 = 0.052 mm/MHz
The band is 83.5 MHz wide, so your overall tolerance on cutting the
elements is:
83.5 * 0.052mm/MHz = 4.35 mm.
it doesn't take much cutting error to end up with a non-functional
antenna.

>But I've seen articles and spec sheets that call the stock
>"rubber duck" antennas that come with many routers and client radios
>dipoles. They don't seem to be in the shape of a T at all. Rather they
>just seem to be a length of wire. What gives?

Well, there's many ways to make a dipole. In the case of the rubber
ducky, it's called a coaxial antenna or vertical colinear. The
antenna consists of a 1/4 wave driven element and a 1/4 wave sleeve
fitted over the coax cable. If you built your dipole out of tubing
instead of wire, and shove the coax cable feed down one of the tubes,
connected at the center as usual, you would have a coaxial antenna.
It's very cheap and easy to make out of just coax cable. It yields
about 2.1dBi of gain.

However, if you disembowl some of the rubber ducky antennas, you'll
find several different types and additions. The simple dipole is
found in the antennas that are about 100mm long. The 200mm long
antennas have an extra sleeve soldered to the coax cable braid about
1/4 wavelength below the feed point. This is intended to reduce VSWR
and radiation from the coax cable feed. It also improves the gain
slightly.

>Would it be possible to build my own T-shaped dipole by soldering a
>couple of 1.2 inch pieces of wire on to the end of a coax cable?

Yes. It's done all the time in feeds for dish antennas which are
often simple dipoles. The antenna will be 75 ohms instead of 50 ohms
but the mismatch loss in nominal. A balun (balance to unbalanced)
transformer might be a good improvement.
http://www.poynting.co.za/tech_training/tut_balun.shtml

>Well,
>it would be possible, but would it work ok and why or why not? :)

It would work no better than the rubber ducky antennas. however, it
could be used to position the antenna in a better location. Try to
use fairly low loss coax and not junk.

>Would I get more signal if I made the dipole a full wavelength long or
>longer?

No. Without phasing the additional 1/4 wave sections, you would end
up with a pattern that vaguely resembles a cloverleaf at 1 wavelength
per element.

What you should try is building a vertical colinear as in:

a b b b b a
===== =========== ========< >======== =========== =====
| | | | feed | | | |
a | | a a | | a a | | a a | | a
| | | | | | | |
=== === === ===

The long pieces (b) are 1/2 wavelength long. The stubs (a) are 1/4
wavelength long. The short cross pieces at the 1/4 wavelength stubs
(a) are as short as possible. The end pieces are 1/4 wavelength long.
The antenna can best be made by bending copper wire or brass rod. A
sleeve balun might be used at the coax feed point if you want to
squeeze every bit of gain out of the antenna.

It's not a perfect or great antenna but is very easy to build. You
can expand it forever, but there's a catch. Doubling the size of the
antenna only yields 3dB of gain. Most of the radiation comes from the
two sections near the feed point.

>How about many wavelengths like a couple of wires running all
>the way across a room?

A "long wire" antenna has problems with matching to 50 ohms. It
starts to look more like a big inductor than a proper antenna. Getting
RF to the end points of the antenna is difficult. Don't bother.
Bigger antennas don't necessarily imply better antennas.

>I know this borders on "turn your electrical wiring into a giant
>antenna"

That's how I got my start in electronics in the 1950's. There was
this crook in New York that was selling "Turn your House Wiring Into a
Giant TV Antenna" kits. It had a "capacitator" inside and was rather
dangerous with the AC/DC TV's of the era.

>but, as a novice, I can't help thinking that I can "grab" a
>larger signal by putting up a bigger antenna.

Nope. Think phase cancellation. If all the parts of your bigger
antenna received the signal at exactly the same time, and exactly in
phase, then you would have some gain. (Double the antenna gives 3dB
gain). However, just a random wire antenna doesn't do that. Different
parts get the signal at different times. That causes them to randomly
cancel as well as add. I can model anything reasonable with an
antenna modelling program (4NEC2). Methinks you'll find that long
antennas for 2.4GHz don't work at all and are actually worse than a
1/2 wave dipole or rubber ducky. Think phasing.

If you want to build something, I suggest a Biquad antenna. This is
basically two full wave loop antenna in parallel with a reflector.
http://martybugs.net/wireless/biquad/
http://www.weijand.nl/wifi/
http://homepages.ihug.co.nz/~redwood4/


--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558