Omnidirectional Antenna

What is the most appropriate length of an omnidirectional antenna of a radio system that operates at 900 MHz (c = 300e6 m / s)?

Please include references of books or web sites

TELECOMUNICATIONS
I have five options:

a) 0.33 cm
b) 3.3 cm
c) 33cm
d) 3.3m
e) 33 m

The question says that I have to find the approximate answer.
I thought there was a formula, so am I wrong?

Almost any conductor will radiate electromagnetic energy. In general, we like to use resonant antennae for our transmissions but a non-resonant antenna will work just as well. A non-resonant antenna could be any length. The problem with non-resonant antennae is in feeding them.

An omni-directional antenna will almost certainly be a vertical (any horizontal antenna does not radiate off the ends so is not omni-directional). Omni-directional in this sense means it will radiate in any horizontal direction (it still will not radiate of the ends but we ignore that case since that direction is straight up).

The shortest length that will give us a reasonable impedance to allow the transmission line to couple to the antenna is usually accepted as being one quarter wavelength or 8.328cm. In practice, slightly shorter than this would be used. You will also need a ground plane. This antenna would be vertically polarized.

Another length that would be suitable would be a dipole which would be one half wavelength or 16.65cm. No ground plane would be required and the transmission line could feed the center of the dipole (still vertical).

Many other lengths and feed systems are possible.

2009-05-06 19:34:55 by bita_cookie

In this episode of Wi-Fi Busters, we bust the myth that omni-directional antennas are as effective as directional antennas. Starring Violet Smith ...

trying to find something i can mount to a pole???

Hi Rudy,

I don't have a particular name for you... It is my opinion that an antenna is an antenna and you buy what you need, as opposed from a particular company. There is not a lot of technology involved. Also, keep in mind an amplified antenna setup may be what you need. A final thing to keep in mind is that if you have an antenna now, it should also work for high def... (especially if you hang an amplifier on it!)

I used to live up the street from you in Visalia.

http://www.antennaweb.org/aw/welcome.asp x

2008-09-17 20:06:12 by rudy c

Price: $36.99

Warranty - 1 year
Polarization: Linear, Vertical
Product Type - Antenna

Should I get better equipment or an antenna. I currently have the Hawking 54g wireless router and an Airlink101 card. The router cost $30, and the card $10, maybe I made a mistake and should have spent at least $100-200 on equipment, I dont know better but I figured it would work.

The problem is the signal is sporadic, it comes and goes as it pleases.

I've not heard of Hawking before. I'd do some Google searches on reviews of that model to see what other have to say about it; however, as you have alluded to -- you get what you pay for.

You also may have interference problems from cordless phones that use 2.4GHz range or neighbors on your same WiFi channel.

Annorax64

2007-03-18 11:53:56 by cjc

Price: $753.99

Works up to 70 miles or more from transmitters
Impedance of 300/75 ohm
Weatherproof construction
Range of up to 70 miles or more
Very flexible aiming characteristics

Assuming clear line of sight, and the AP/router is using a 3dbi omnidirectional antenna.

You ought to be able to get a couple of miles out of it if the yagi is aimed carefully.

With carefully aimed parabolics and wave guides instead of coax at both ends, I've managed 22 miles. One link has been running for over 4 years now.

2009-03-19 10:19:18 by luckyman

A yagi antenna is directional. It has non energized reflectors. Look at the pictures, directional.

2006-12-01 23:51:50 by halocon