Question about antenna types and connection strategy with access point.

[netcraftjason]

Hi all,

I have two question that can the 2 RP-TNC connect 2 different gain or kind of antenna on Aironet AP?

And can the 2 antennas, which both connecting to one AP , locate at two areas which are distence between two points that are far away from each other (e.g. 50 meter with low loss cable)?

Thanks!

Jason

[Rob Huffman]

Hi Jason,

The purpose of the dual RP-TNC connectors on the AP is not to cover 2 separate areas, but rather to give better coverage in a specific type of area by configuring the antennas for Diversity.Have a look;

In order to increase coverage, conduct a site survey to determine the RF coverage of the antennas. Place access points in the appropriate areas of the installation site. The purpose of diversity is to overcome multipath reflections. Diversity antennas that share the same physical housing are placed at an optimum distance apart. The maker of the particular antenna determines that distance based on the characteristics of the antenna. When you use a pair of antennas with matching characteristics to provide diversity for cell coverage in your facility, the guideline is to put those matched antennas at a distance apart from each other that is equal to a multiple of the wavelength of the frequency that is being transmitted. The 2.4 GHz wavelength is approximately 4.92 inches. Therefore, to support diversity on a 2.4 GHz radio with two separate antennas, the antennas should be spaced approximately 5 inches apart. The antenna pair could also be spaced at multiples of 5 inches, but the distance between should not exceed 4 multiples: reflected waves farther apart than that are likely to be so distorted and different in delay spread that the radio could not work with them.

From this doc;

Multipath and Diversity

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml

The farther the antennas are separated, the more the radio coverage cell for each antenna is different. If the coverage cells become too different, the client or end node can experience signal loss and poor performance. An example of different coverage cells would be a directional antenna on one antenna port with an omnidirectional or higher-gain antenna on the other port.

The purpose of diversity is to provide the best possible throughput by reducing the number of packets that are missed or retried.

If two diversity antennas are installed, Cisco recommends that you configure Diversity for the antenna receive and antenna transmit settings.

If only one antenna is installed, Cisco recommends that you connect it to the right RP-TNC antenna connector and that you configure Right (Primary) for the antenna receive and antenna transmit settings.

Also, here is an excellent thread where NetPro ScottMac explains diversity in a very insightful manner!

http://forum.cisco.com/eforum/servlet/NetProf;jsessionid=4818C15620758A61F20BAFEB84E01E7A.SJ1B?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1dd7905b

Hope this helps!

Rob

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[scottmac]

Electrically, you can connect any kind of antenna with a 75 ohm impedence to either RP-TNC connector. All the transmitter cares about is the electrical load.

Practically, it wouldn't make much sense to have to different kinds of antennas to the same AP (or bridge).

The two connectors are there for "diversity"; both antennas must look at the same area, but from slightly different positions (~5 inches - 36 inches apart). At any given time, only one antenna is active; if you try to use both antennas to "relay" from one antenna to the other, you will get extremely poor performance from both sides.

You could theoretically use a dual-band AP (802.11a and 802.11g) and have the 802.11g pointing to the East and have it relay the signal via 802.11a to the West (and vice-versa).

So, the short answer is: Yes, you can have two different antennas, but practically it'd be a bad thing, and NO, you cannot use the same AP to cover two different areas.

Good Luck

Scott