Antennas for 902-928 MHz.

Your application will determine what type of antenna you will need.  Should it be horizontally 
or vertically polarized (or Circular), directional or omnidirectional, for short or long range ?

You might consider the following designs.  
V=Vertical, H=Horizontal, C=Circular Polarization. 
 (GP)  indicates that the antenna requires a ground plane.  
For a picture of the antenna and other information, click on the  button.
Polarization, Directivity

Antenna Type
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.  
This is a term originated by Rod Newkirk, VA3ZBB / W9BRD.  
It means 'Plenty Of Gain At The Other End'.

Details of the Skew-Planar Wheel Antenna 
and information on construction for other bands

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, 
relatively non-critical dimensions and broad frequency coverage.  
Have a look at the Quad-Helix Array.  
You can build an Axial-Mode Helix to cover both the 902-928 and 1240-1300 MHz bands.

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.

. R DE D1 D2 D3 D4 D5 D6 D7 D8
Length

6.2

 See text 5.6 5.5 5.5 5.4 5.3 5.2 5.1 5.1
Spacing 0.0 2.4 3.9 5.8 9.0 12.4 17.4 22.4 27.6 33.0

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 
and is vertically polarized with a gain of 1 dBd.  
Compared to a 1/4 wave or a J, it's difficult to 
poke your eye out on this antenna.

It is constructed over a groundplane.  Tune the antenna to resonance 
with the capacitor C, and adjust the feed point FP for 50 ohm coaxial cable.
The groundplane GP should be about 2X the diameter D.
Dimensions in the table are in millimeters (mostly).

Frequency 

General

 147 223.5 445 915
Wavelength  360o 2040.8 1342.3 674.2 327.9
 Diameter D 52o 295 194 97.4 47.4
Circumference 163o 926 609 306 148.8
Height above groundplane h 8o 45.4 29.8 15 7.3
Width d 1-2o 6-12
1/4-1/2''		 4-8
1/4''		 2-4
1/8''		 0.9-1.8
3/32''
50 ohm match Feed Point FP 5o 28.3 18.6 9.4 4.6
Capacitance C pf   2-5 1-4 0.5-2 0.25-1

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.

Graham  VE3BYT

2AE