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L-Band Helix Antenna Array                

Design by                                       

Clare Fowler, VE3NPC                             

[email protected]                               

                

        
Abstract
For 23 cm (1269 MHz) SSB operation on AO-40 a nominal 1500 watts ERP of uplink power is required. This can be obtained with a low gain antenna driven by a power amplifier or alternatively, with lower power into a higher gain antenna. Following classical standard textbook design and utilizing available hardware and shop tools, the author has constructed four helical antennas in a square array, driven through a four-way power divider. Chain link fence fiberglass tension bars are used for the antenna and bracing frames. Aluminum ground wire is used for the helix conductor. This design provides an inexpensive durable lightweight array with relatively low wind loading. With a maximum input power of 10 watts into a feed line with about 2dB total loss, this antenna array has performed well, both operationally and mechanically, for over two years. A novel implementaton of the quarter wave matching section makes for simplified adjustment of the SWR. Construction material, dimensions, assembly and testing are covered.

Introduction
The helix is a relatively broadband antenna and small dimensional irregularities have little effect on performance. This makes it an ideal antenna for home construction. An antenna array comprised of four identical 27-turn right hand polarity helix antennas provides sufficient gain for L band operation into AO-40 with 10 watts of RF drive. Aluminum ground wire, carred by the local electronic chain store, is used for the helix conductor. Chain link fence fiberglass tension bars are used for the helix support boom and antenna cross bracing. These bars may be obtained in the fencing department of hardware and building stores. Aluminum sheet is used for the reflector. Sections cut from aluminum angle stock are used for support brackets. Stainless steel machine screws and nuts are used to prevent corrosion. To minimize losses, type N connectors are used on each helix, the four-way power divider and interconnecting calbes.

Helix Antenna Match
The feed point impedance of a helix antenna is on the order of 140 ohms. A quarter wave matching section may be used to transform this to 50 ohms. To match 140 ohms to 50 ohms the quarter wave matching section must have an impedance of a circular conductor above a ground plane is defined by the formula: Z=138 log 4h/d; where h is the height of the concuctor about the ground plane, and d is the diameter of the conductor. Placing a conducting metal ground plane under the first quarter turn of the helix conductor provides a suitable quarter wave matching section. The width is mot critical but should be several times the width of the helix conductor.

Quarter Wave Matching Section Construction
The ground plane for the quarter wave matching section is cut from a piece of hobby shop brass sheet. See Figure 1. With a compass draw two concentric 90-degree arcs on the brass, the first with a radius of 27 mm and the second with a radius of 52 mm. From the compass center mark draw two lines across the concentric arcs with a 90-degree angle between them. These four lines define a 59 mm length quarter circle ground plane that will be mounted directly under the helix. Cut or file a semi-circular notch in one end to clear the pin base of the N connector. Drill holes on each side of the N connector. Drill holes on each side of the notch to match the N connector mounting holes. The off-axis N connector locantion and orientation on the reflector plate is required for installation of the ground pane. For the ground plane to function, both ends need to be shorted to the reflector plate. A short loop of brass sheet or a piece of coax braid is soldered to the end of the ground plane. The other end of the loop is fastened to the reflector plate with the machine screw that holds the boom angel bracket.

Figure 1

Helix Antenna
The helix dimensions are determined using the formula by Kraus in The Satellite Experimenter's Handbook. The circumference is equal to one wavelength and the pitch angle is 12.5 degrees. Construction details are shown in Figures 1 and 2.
Two five-foot length fiberglass tension bars are used for the antenna boom frame held apart with cross braces cut from another bar. See Table 1 for positioning. The ends of the boom frame rails are fitted through cuts in the aluminum reflector plate. Each boom rail is held in place by two angle brackets, one on each side, cut from 3/4" aluminum angle stock. A chain saw file is used to file angled notches in the boom rail bars into which the aluminum helix wire is fitted. To avoid cumulative errors, mark the position of each notch measuring from the reflector plate as indicated in Table 2. Size the aluminum coil diameter by close wrapping it around a 65 mm diamater form. Wind the aluminum wire off the coil directly onto the form, without straightening it first. This way it stays pre-stressed, and will later spring out to the required 75 mm diameter when sstretched to the correct helix length. Using a hammer, flatten the first 1 cm of the helix wire and drill a hole in the center of the flattened end to clear a 4-40 screw.

Figure 2

Trim off the half lip from the female chassis type N connector solder contact pin. Cut the head from a 4-40 brass screw. Dress one end to fit into the N connector pin by putting the screw into a power drill chuck and holding the edge of a flat file against it while it is turning. Solder the screw into the N connector contact being careful to not get excess solder up into the screw threads.
Bolt the N connector and the quarter wave ground plane section to the reflector plate. Extra screw nuts are used as spacers on the N connector so the first quarter turn of the helix is close to the ground plane and the reflector. Using the brackets, mount the rails to the reflector plate. Stretch the helix to its approximate length. With the rails adjacent at the outer end, slip the helix coil over the rails and bolt the end of the helix to the serew that is soldered into the end of the N connector. While fitting the helix loops into the notches bolt the cross braces into place starting at the feed end. Omit the second cross brace. Initialy the spacing between the hilix first quarter turn and the ground plane should be adjusted to about 3 mm by bending the ground plane.

Table 1

Cross Brace Spacing from Reflector


Both

 Rails

1

 18.8 cm
2 39.8 cm
3 60.7 cm
4 81.7 cm
5 102.7 cm
6 123.6 cm
7 144.6 cm


 Acknowledgments
Thanks are due to Gordon Grant, VE3DY, for all of the drafting.


**WARNINGS**

Working with Fiberglass
Some people are adversely affected by fiberglass. In any case, precautions
should be taken when cutting drilling or filing fiberglass to protect against
the dust. Wear glasses, a mask and gloves. Wash areas where the dust
comes in contact with the skin.
************************************************************

23 cm Helix Antenna

 Specifications

Frequency = 1269MHz

 Theoretical Array Gain = 25 dB
Circumference = 1 wavelength Probable Gain = 23 dB
Number of Turns = 27 Theoretical Beam Width = 11 degrees
Pitch Angle = 12.5 degrees Antenna Gain = 19 dB
Spacing Between Turns = 52.4 mm Helix Diameter = 75 mm
Beam Width = 21 degrees Reflector Side = 200 mm

Sources for Chain Link Fence Fiberglass Tension Bars

Manufacturer: Master-Halco, Inc.
The Home Depot
Lowe's
ACE Hardware
True Value Hardware
Local Fence Contractor
Bottom Rail Top Rail
1 4.4 cm 1.8 cm
2 9.7 cm 7.0 cm
3 14.9 cm 12.3 cm
4 20.1 cm 17.5 cm
5 25.4 cm 22.8 cm
6 30.6 cm 28.3 cm
7 35.9 cm 38.5 cm
8 41.1 cm 43.7 cm
9 46.3 cm 49.0 cm
10 51.6 cm 54.2 cm
11 56.8 cm 59.4 cm
12 62.1 cm 64.7 cm
13 67.3 cm 69.9 cm
14 72.5 cm 75.2 cm
15 77.8 cm 80.4 cm
16 83.0 cm 85.6 cm
17 88.3 cm 90.0 cm
18 93.5 cm 96.1 cm
19 98.7 cm 101.4 cm
20 104.0 cm 107.0 cm
21 109.2 cm 111.8 cm
22 114.5 cm 117.1 cm
23 119.7 cm 122.3 cm
24 124.9 cm 127.6 cm
25 130.2 cm 132.8 cm
26 135.4 cm 138.1 cm
27 140.7 cm 143.3 cm
Table 2

Notch Spacing from Reflector

Four-Way 23 cm Power Divider
Theoretical and structural details for quaterwave power dividers are covered in many of the microwave handbooks. Construction details are shown in Figure 3. Four type N female chassis connectors are fastened with #4 x 1/4" self-tapping screws, one on each side of a 1-inch square piece of aluminum tubing cut 207 mm long. A 177 mm length of 1/2" copper water pipe is used for the center conductor. File four equally spaced notches into the ends of the four N connector center pins to accept the end of the copper pipe. The notches should be just deep enough so that the end of the pipe reaches to the center of the N connector pins. A fifth N connector is screwed to one side of the square tubing with the center pin spaced 177 mm from the center pins of the other four. A notch in the copper pipe fits halfway over the center pin. Center the copper pipe and solder the five pins to the pipe. Square plastic plugs can be used to cover the open ends of the power divider.

4 way power divider

Joining Cables
To minimize the effects of any impedance mismatch, the lengths of the four cables and connectors that connect each helix antenna to the center conductor of the power divider should be a multiple of an electrical half wavelength. RG-8 or RG-213 coax cable has a velocity factor of 0.06. A cable of four wavelengths at 1269 MHz (23.64 x 0.66 x 4). Allowing for the female chassis connectors at the helix and the power divider, the interconnection cables should be 600 mm long overall, measured from the open end of each male cable connector.

Helix Array Structure
The helix antennas are arranged in a square 640 mm apart as shown in Figure 4. The four antennas are all oriented in the same way with the position of the N connectors all being in the same relative position so that the output from the antennas will combine in phase. Two sections of tension bars, bolted one on each side of the helix antenna booms, support two helix antennas one above the other. By replacing the third cross brace of each helix antenna with the support bars, the array will be suspended near the balancing point. An H frame using two more sections of tension bars is used to join the double helix antenna sections. A mounting plate made from 3/8" Lexan plastic is used to attach the array to a non-conductive cross boom with muffler clamps. Additional support is provided by bolting sections of tension bars to the rear reflectors. The power divider is fastened to sections of tension bars going between the rear support and the middle H frame.

Figure 4

SWR Adjustment
The quarter wave matching section on each antenna should be adjusted individually for 1:1 SWR with an SWR meter connected directly to the antenna, using a coaxial barrel adapter. With a plastic tool, adjust the ground plane spacing. As the spacing is pushed from close to the wire to farther away a position should be found where the SWR decreases to 1:1.
If an SWR meter for 23 cm is not available an alternative method is to use a field strength meter and adjust for a maximum reading. The construction of a very simple FS meter is shown in Figure 5. The dimensions of the 2 1/2" to 3 turn helix are non-critical, but keep the leads of the 1k resistor, the signal diode and the bypass capacitor as short as possible. A 20 kilohms per volt analog multimeter set to the 50 microamp position may be used for the meter. Position the FS meter helix two or three meters in front of, and facing the 27 turn helix being adjusted. Apply a 1296 MHz signal from the transsceiver to the 27 turn helix. Only a small amount of power should give a good meter reading. Adjust the ground plane spacing for maximum field strength meter reading. To facilitate reading the meter while making adjustments the multimeter connection wires can be extended. Adjust each antenna matching section individually and make no further adjustments.

Feed Line
For this antenna array to provide good L band operation on AO-40 with a drive of 10 watts from the transmitter, the feed line must have a total loss of no more than 2dB, preferably less. This requires a relatively short, low loss feed line, e.g., 10 meters or less of hard line with 1.5 meters or less of low loss flexible coax at the antenna and in the shack.

Variations on a Theme
Assuming a low loss feed line, 20 watts of power driving an array of two helix antennas would provide equivalent performance. A two-way power divider will be required in place of the four-way divider. A two-way divider requires a center conductor with an OD of 9.8 mm (use 13/32 in. brass tube) with the length remaining the same.
With a drive of 40 watts or more a single 27 turn elix will give good performance with 2 dB or less feed line loss. If the feed line has a total loss of more than 2 dB, performance can be maintained by increasing the driving power using an amplifier in the shack or at the antenna.

Matching Section                           Field Strength Meter