1/4 vertical vs 1/4 dipole

[KF4URO]

I am wanting to build a few general pupose antennas for 802.11 and ham experimentation in the 2.4 ghz band. I was going to build a simple antenna out of a chassis rf connector, one radiator on each corner and a 1/4 verticle attached to the pin in the center of the connector. However I have seen cisco use an antenna in their fixed cofiguration 1100 ap that appears to be a 1/4 dipole with a ground " plate " if you will placed behind it. Anyone seen one of these? Its very small and has excellent coverage even at 1mw. I would really like to build something similar out of a chassis connector and some copper tube or wire. I can post pictures of the cisco unit if need be.
Thanks

 

[Higgler]

A quarter wave monopole at that frequency is only 1.3 inches long [1/4 * (11.803/2.4ghz)= length with dimension of inches]. If you have a connector already, it only requires a wire attached to the center conductor of your connector.
Most simple omni antennas are very easy to build. To work over wider frequency ranges, make the wire thicker. Or the widest frequency range is with the wire shaped like a funnel. With taper angle at 30-60 degrees.

kch

 

[KF4URO]

i took pics of the antenna i am trying to mimic. It looks like a horizontal dipole with two 1/4 wave elements and i suppose there is some type of balun feeding them. Is there an easy way to mimic this design with a so-239 and some wire?

The pictures are at

http://photos.yahoo.com/ciscomanvann/antenna

Any help is appreciated.

 

[VEBill]

Re:

http://photos.yahoo.com/ciscomanvann/antenna

FYI - "Sorry! This page is unavailable."

As of 7:37AM EST today.

 

[KF4URO]

try just photos.yahoo.com/ciscomanvann and then clicking on the album named antenna . i dont know why the above link is not working , sorry.

 

[VEBill]

Man! That first picture is so out of focus that it can give you a headache... ;-)

So, looking at the second photo ('ant_top') it looks to be two dipoles fed separately. I assume that this is for spacial diversity.

Each dipole has an unusual feed in that the right hand side of the dipole is fed via a half-wave delay line. I presume that this is done to provide a good match and because it is so easy to do with PCBs. If you're not making a PCB antenna, then I wouldn't bother trying to mimic that system.

One important thing to keep in mind is that you need to decide if you want high gain with directionality, or low gain with (close to) isotropic coverage. In other words, some applications demand low gain in order to achieve (nearly) isotropic coverage.

If you're making a 'ground plane antenna' using the connector, then the four horizontal radials are typically about 5% longer than quarter lambda and can be bent slightly downward to increase the feed point impedance. The primary disadvantage of a ground plane antenna is the large volume (X, Y and Z) that it occupies.

A dipole can also be assembled onto a connector. Just a quarter lambda wire going each way. There's also the sleeve dipole where the feed goes into the far end of one element.

High gain antennas for 2.4GHz are well covered on the

http://WWW (Pringles

cans for example, but there are better choices...).

Pay attention to polarity (H vs V). There can be a big (20-30dB) cross pol. loss if you not matched.

 

[KF4URO]

So do i need a feed for the antenna or just solder each 1/4 piece of wire to the center conductor of the conector to make a small T ? I will definitly throw in the ground plain wires on each corner. This is for in home use but I have been VERY impressed with this antenna the problem is its on a 802.11b ap and i want to do something like it on a G ap. THere is two seperate feeds and antennae because of the seperate antenna connectors on teh radio for diversity that is correct. I tried a 5db mag mount and stuck it on a large sheet of metal but the performance is lack luster in comparison to this antenna.
Thanks for the feedback, i will try to roll this puppy soon and post my results and pictures for your critique if you dont mind.

 

[KF4URO]

Pay attention to polarity (H vs V). There can be a big (20-30dB) cross pol. loss if you not matched.


Can you elaborate on this? Also, can you all recommend any good books on microwave antennae , delay lines etc?

 

[VEBill]

Feed:

You don't necessarily need to bother with complications (precise matching) between the cable and the antenna. In some cases something might be required, but it is worth trying the easy way first. The cable is probably 50 ohms, and thus would reasonably closely match either a 35-50 ohm monopole (ground plane), or a 70-75 ohm dipole. In other words, try a simple connection first.

H vs V:

The antenna shown on your picture - how are the pair of dipoles oriented when in use? Does the circuit card stand on edge so that the two dipoles are vertical, or is the circuit card laying flat so that the two dipoles are oriented horizontally?

The antennas at each end of the link need to be oriented with the same polarization (H or V).

Books:

http://www.ARRL.org

- The American Radio Relay League has some great antenna books. Also the UK's RSGB has some great books. Browse their websites or Google for vendors.

 

[KF4URO]

If you dont mind please let me pose another question. Should i choose a vertical 1/4 wave and wanted to put some type of reflector behind it to increase gain and focus horizontal beamwidth how do i properly calculate a solid metal or screen reflector? Is there a basic formula for this? Does it work with other antenna types such as a 5/8 or colinear?
Happy New Year.

 

[VEBill]

Investigate "corner reflector" using Google.

You gain achieve moderately high gain (about +13dBi) using two flat plates or screens.

Obviously, high gain antennas need to be aimed.

 

[KF4URO]

I didnt find much on the theory behind them. I did find one article that stated the reflector should be about .3 wavelegnth behind the antenna. I was hoping to see how diff reflector types and sizes would affect the coverage patern of the antenna.

 

[VEBill]

The usual variables are the spacing (to the driven element) and the corner angle. The gain increases as the length of the corner reflector is increased (roughly along the boresight).

The material and design of the reflector must be appropriate for the wavelength - at UHF you can use a few metal rods. For your application, cardboard lined with tinfoil would be the ideal prototype material.

Keep in mind that gain makes the antenna more directional. Directionality can be a Good Thing or a Bad Thing depending on your application.