Antennas for 902-928 MHz.
Your application will determine what type of antenna you
will need. Should it be horizontally
You might consider the
|V Omnidirectional||* The 'J'|
|V Omnidirectional||* Quarter wave (GP)|
|V Omnidirectional||* 1/2 wave Ring Radiator (GP)|
|H Omnidirectional||* 1/2 wave Halo|
|C Omnidirectional||* Lindenblad|
|C Omnidirectional||* Skew-Planar Wheel|
|H or V Directional||Yagi or Quagi Beam|
|C Directional||Axial-mode Helix|
Signifies a 'POGATOE' antenna.
The Helical Antenna for 904 MHz
This web site by John Watrous, K6PZB, describes helical antennas for 904 and 1265 MHz, plus designs for simple bandpass filters for these bands which are constructed from copper water pipe parts. These bands are in use in California for a high-speed digital network.
Helical antennas have many
advantages including circular polarization,
Yagi Beam for 902-928 MHz.
Here's a 10-element yagi design by Kent Brittain, WA5VJB. It uses a trombone feed for a direct match to 50 ohm coaxial cable. I used this feed many years ago on a 450 MHz antenna, and it worked well.
This antenna is constructed on a wood boom 3/4 inch square, about 3 feet long. You can mount the antenna for either horizontal or vertical polarization. Other non-conductive material than wood could be used for the boom. Do not mount these elements on a metal boom, or the lengths will not be correct.
This antenna is small and light enough to end-mount, so you should leave enough boom material behind the reflector to attach your U-bolt or other mounting hardware. Mounting at the end is desirable for horizontal polarization, and necessary for vertical if you have a metal mast.
Elements are 1/8 inch diameter, and must be cut precisely to length. You could use aluminum for the Reflector (R) and all the Directors (D1, etc.). Use copper, brass or bronze rod for the Driven Element (DE) so you can solder the coaxial feedline directly to it. Lengths in the table are in inches. Spacing of the elements on the boom are measured from the Reflector.
Drill holes in the boom for the elements, preferably using a drill press. Fasten all the elements in the boom with cyanoacrylate adhesive (Crazy Glue, if you must) or a little silicone sealer. Alternatively and probably better, you can hold the elements in place by running pointy wood or sheet metal screws in from the top of the boom to lightly spear the elements. Stainless steel screws would be best. The boom must be varnished, painted or otherwise treated so it will not absorb moisture.
The Driven Element is 1/8 inch diameter. It is pictured below. As mentioned above, copper, brass or bronze is best so you can solder the coaxial cable directly to it. Note that the center conductor of the coax goes to the bottom of the folded part. The coax shield goes to the center of the long portion. The length of the driven element is 5.7 inches. The length of the bottom portion is 5.7/2, or 2.85 inches. The spacing of the top to bottom element portions is 1/2 inch.
Route the coaxial cable along the boom to the rear of the antenna. Be sure to seal the antenna end of the cable to keep out moisture.
1/2 Wave Ring Radiator
This antenna is small at 900
MHz. It has a low profile
It is constructed over a groundplane. Tune the
antenna to resonance
Dual Rhomboid Antenna for UHF
If you are serious about gain, this may be the antenna for you. It is small enough to be readily rotated at 902 MHz and higher frequencies. If it is not rotated, it had better be lined up carefully with your intended target. These antennas can have very high gain, with a corresponding narrow beam width.
The lines at points G and H are terminated in 600 ohm resistors with ratings equal to half the power input from the feedline. The feedpoint is balanced in the range of about 300 to 800 ohms. Where the lines cross over one another they must be separated so they do not short out.
The details and measurements of this antenna are from the Rhombic Antennas web site of Ian Cummings, KB1SG. Unfortunately, his rhombic web site has disappeared - temporarily, I hope.
You can click on the image for a printable version of the diagram with a table of dimensions for 435, 910 and 1255 MHz.