Receiver Antenna Selection

[Pheezy]

Hello there,

I am presently trying to select what antenna to purchase for this project I am working on. I am trying to receive 2.4 GHz signals from an access point in my university. Its a Cisco 3600 Series AP operating at dual frequencies (2.4 GHz and 5 GHz) (I am only interested in the 2.4 GHz). Its max transmit power is 23 dBm and the integrated 2.4 GHz antenna has 2 dBi gain.
Also, it is omni directional.

Link:
http://www.cisco.com/c/en/us/produc...ironet-3600-series/data_sheet_c78-686782.html

Based on this information, I am trying to decide what receiver antenna to purchase for my circuit. I know the following:
1) I need an omni-directional antenna
2) length of receiver antenna has to be 1/2 of the wavelength of RF signals I am receiving, so for a 2.4 GHz signal, length of my antenna should be 6.25 cm.

I need assistance in deciding the following:
1) the gain of the antenna
2) the type of antenna (yagi, whip, dipole, etc)

Your suggestions and why?

Regards

Pheezy

 

[(*steve*)]

why do you want an omnidirectional antenna?

Is this homework?

 

[Pheezy]

Hi steve,

This is part of a research project I am doing relevant to my studies. I am building a circuit which essentially captures wifi signals from the APs at my university. So since there is no particular direction, I was thinking of buying an omni-directional antenna.

Regards

 

[davenn]

well you need a dipole or a whip then. Both will have low gain between 1 and 2.4 dBi

A Yagi is not omni-directional

Dave

 

[(*steve*)]

Any gain at all makes the antenna directional. dBi is a measure of gain over an antenna that is isotropic -- that is, is uniform in all directions.

To put it simply, if an antenna is twice as sensitive in some direction then it mist be less sensitive in another.

What you want to do is to maximise sensitivity in the direction you want and reduce it in others.

The type of antenna Dave mentions has even sensitivity all around the antenna, but far less in the direction along the axis of the antenna. You can imagine the peak sensitivety like a wedge. In this case it's a short fat wedge. You can be anywhere around the antenna, and also substantially above or below the plain of the antenna.

You can increase the gain (consider colinear and waveguide antennas) and that wedge gets thinner and linger. Now you have more sensitivity in the plane normal to the antenna, but sensitivity falls off far more quickly above and below that.

You might not consider that and find that a high gain antenna gives you better gain to far off AP's, but much lower gain to nearby ones. As an example, I have a waveguide antenna on my roof that allows me to use my wifi for a significant distance outside my house. But I get no signal inside the house or in tha park next door.

You also need to consider polarization. My waveguide produces a horizontally polarised signal, and a vertically polarised antenna will have a hard time picking up a signal. In your case the signals are likely to be vertically polarised, so a colinear (or a whip) would be the sensible option.

 

[flippineck]

I've had the best experience with collinears. I've tried the type that involves 3/4 wave vertical sections joined together by circular 1/4 wave - long loops in the horizontal plane, with the highest vertical section being 10% less than 3/4 wave and the lowest vertical section being a half wavelength long. Continuing Steve's analogy, the more 3/4 wave vertical sections, the thinner the wedge & the greater the gain. Take the feed straight off the bottom of the half-wave section, and attach the coax shield to 4 equidistantly spaced horizontal ground plane wires joined together at & by the shield. ISTR I made my ground planes a full wavelength long, the antenna seems to work ok, but I guessed that part. Someone might know the formula for the proper length of the ground planes.

Martybugs collinear: http://martybugs.net/wireless/collinear.cgi

My antenna works at 2100MHz not too far from your frequency. Your dimensions would calculate out a little shorter than mine for 2.4Ghz but not much (actually the ant linked above IS for 2.4GHz, so you should be able to use the dimensions direct from there). Also don't fail to see http://cheatthezone.tripod.com/wireless/colinear.html

http://www.wavelengthcalculator.com/

There's another kind of collinear which involves joining a bunch of short lengths of coax together so that the inner & outer conductors cross over at each join. Google 'coax collinear'. Looks like harder work, never tried it, maybe this type works better?

Steve, any more details about your waveguide ant? I was wondering if something like that might help me bring 2.1GHz mobile signal into the house. I'm going off topic.