2400 MHz Easy-to-Build Helix Antenna

Still Under Construction

Picture of helix antenna

This antenna is easy to build and suitable for broadband work, satellite work and terrestrial work.

Thankyou to Clare Fowler, VE3NPC, for his patience in helping me build my first helix and coaching me with this material. Clare - you are right - they are easy to build.

Before you get too far along, decide if you are building a right hand circular or left hand circular helix. If the other end is a dipole or slot antenna, it does not matter whether you use right and or left hand circular - so you might as well use right hand circular. But if you are working with satellites or other helix, you will have to use the corresponding polarization - if the other end uses right hand - you will too. If you are receiving or transmitting by reflecting off a surface, the polarization is altered.

Use Radio Shack aluminum ground wire for the helix (why they call it ground wire, I'm not sure). You will also need a good quality N-type connector, and a 7cm square of brass plate available from hobby stores. Also pickup 2 grey fiberglass chainlink fence stakes from Home Depot - they're a couple of dollars each.

The secret to success with any helix antenna is symmetry. Keep this in mind during the construction process.

I purchased my aluminum at Ridalco, 1551 Michael Street (near St.Laurent and Belfast). Total aluminum cost was under $5 (incl tax).

Measure, mark, and cut very carefully. Follow the instructions and it will work every time. It's a fun project.

The actual mounting plate is not described on this page but information can be found in other pages of the antenna section on this website. 1/8" thick aluminum is adequate for this mounting plate.

Picture of helix antenna


19 approx
19 approx
Length of Turn
Length of Wire
264cm approx
275cm approx
Antenna Lenth
53cm approx
55cm approx
Beam Width
24 degrees
24 degrees
Coil Backplate (ground plane)

Caution When Working With Fibreglass

NOTE: Some people, like me, are more adversely affected by fibreglass than others. The fibreglass posts must be drilled, and doing so produces a highly irritating dust which can cause your hands to really itch. BE SURE TO WEAR A MASK AND GLASSES WHEN DRILLING. Immediately rinse anything which comes in contact with your skin, like glasses, in running water. Wear an old pair of leather winter gloves when working with this stuff and throw them away when your are done. I tried latex gloves, and that was a disaster - the fibre goes right through them.

Winding the Helix

Carefully close wind the aluminum ground wire onto a convenient form which is 3.7cm in diameter. This should produce a coil with a diameter of 4.0cm. Wind 2 turns or so extra - this allows for the extra wire in the spacing between turns. Flaten one end of the wire with a small hammer and a smooth metal surface like a table saw top or a small anvil. The flatened area should be made so that it can be drilled and will then fasten onto a brass screw which has been soldered into the solder post of the N type connector.

Note: VE3NPC has determined that you can increase the length of the coil upto 100% (or a total 540cm of wire) and you will increase the gain by about 2.5 dB. As an initial project this may be a bit tricky, and the antenna will now become quite long and a bit floppy. Also - the 2 post support may not be practical for 540cm of wire - as it will be difficult to thread that much coil. Going to a single post support system will solve the problem. You can use a fibreglass or white pvc centre post, and offset pegs along the way to hold the coil in place.

Making the N-type to Coil Connection

Obtain a 5-56 brass screw, or threaded brass rod and 2 hex nuts from the hobby shop. Cut the head off carefully with a fine hack saw or pair of side cutters, so that the little brass threaded post is about 1 cm long. Dress up the cut end and insert this end into the solder post for the centre conductor on the N-type connector. Solder the threaded brass rod into the solder post of the connector. Note that the rod will have to be long enough to accomdate 2 brass nuts and the flattened aluminum wire.

Making the Coil Support Frame

Carefully mark the 2 fibre posts with tick marks every 277mm, with a fine lead pencil, for 60cm of the length of the posts. Mark them exactly the same, drill them with a 3/16" drill bit. The holes should be in the centre of the stakes and need to be oversize to allow the 12.5 degree slope of the wire. Once the stakes are drilled, cut one exactly 1.4cm shorter than the other. When these are mounted opposite one another the wire coil will spread the correct amount. Using the picture as an example, fabricate a brace to keep the fibre posts separated at the top end, exactly 4.0cm (centre to centre). You only need one brace at the end of the coil farthest from the connector. Fasten the brace to the posts using pop rivets. Position the brace and pop rivets in such a manner that the rivets will not make contact with the coil.

At this point its a good idea to take the assembly outside or in your laundry tub and give it a wash to get some of the fibreglass particles off.

Picture of helix antenna

Mounting the Coil on Support Frame

Once you have the coil close wound and the frame drilled and assembled, (but not attached to the ground plate) give the coil a bit of a stretch by pulling on the top end (farthest away from the flattened end). Try not to distort the diameter and do not allow it to unwind (turn diameter increases). Then thread the top thru the lowest hole, that is, the hole in the part of post nearest the undrilled end. Force the coil gently around and thread it into the first hole on the second post, then the 2nd hole on the first post, then the 2nd hole of the 2nd post, then the third on the first post, etc. Its quite easy just work slowly and dry not to distort the turns. Watch out for that itchy fibreglass. Keep going until you have the coil all threaded and just a part of a turn, and the flattened area sticking out from the first hole. You may have to adjust slightly as you mount the coil onto the ground plate. If you have more than half a turn of aluminum wire sticking past the last hole (at the top), then clip it off. If you are a little short do not worry about it. As you force the coils on to the posts you use up a little extra wire and when it is all done, you should be pretty close to 19 or 20 turns.

Making the Ground Plate

The ground plate is made from 1/8" aluminum, 10cm square. This plate may be larger but not smaller. Cut the corners off at a 45 degree angle. This is mainly for safety. You will then need to cut 2 slots in the aluminum plate to accomodate the 2 posts that will slide through it - see the pictures. The posts must be exactly 4.0cm (centre to centre)

Fastening the Posts to the Ground Plate

Make 3 "L" brackets by cutting off 13mm wide pieces from 1/8" thick 3/4" aluminum L channel. These will be used to fasten the posts to the ground plate. Pop rivet these L brackets to the aluminum ground plate, and use brass 4-40 screws, nuts and washers to fasten the brakets to the posts. Note that in order to leave room for the N-type connector, you only use an L bracket on one side of the post which is nearest the connector.

Mounting the N-type Connector

The N-type connector is mounted to the ground plate with two 4-40 screws and nuts. Drill the aluminum plate to accomodate the N-type connector as near as possible to one of the posts. The connector should line up with the helix end on the other side of the plate. The connector mounts so that the mounting flange of the connector is on the mounting plate side of the ground plate i.e. only the post projects through the plate. There is a 4-40 nut between the connector and the plate as well as a nut holding the connector on to the plate.This is a little tricky but work carefully and it will work out ok. Note that the capacitor plate fastens to the ground plate using the connector mounting screws.

About the VE3NPC Matching Technique

The helix is approximately 140 ohm impedance and we need to match that to our 50 ohm radio equipment, so we need a 1/4 wave section of 84 ohms. Three things determine the match: the diameter of the conductor, the spacing above the ground plane and it has to be 1/4 wavelength long. A circular conductor above a ground plane has an impedance
Z = 138 log 4h/d

Most helix antennas have a fixed matching plate. However, Clare Fowler, VE3NPC developed a technique to allow easy adjustment of a helix antenna using a movable metal plate. The VE3NPC adjustable match works by effectively moving the ground plane closer or farther away from the helix.

Making the VE3NPC Adjustable Matching Plate

Our matching plate is made from a piece of brass plate and is basically a 1/4 circular plate, which fastens to the ground plane using the connector mounting holes and screws. There is a 4-40 screw which projects through the aluminum ground plate and applies pressure to the back of the matching plate. As you screw in the brass screw, the matching plate moves closer to the helix. Carefully tap the thread for this screw into the aluminum plate, using a tap set.

On the end of the brass plate, solder a piece of braided copper. The purpose of this braid is to provide additional grounding to the plate. I made my braid from a small piece of RG174 with the centre conductor and dielectric removed. Do not be tempted to allow solder to flow in the braid - just solder the very end to the brass plate. The braid is folded in a "v" and the other end will be pop-rivetted to the aluminum ground plate using the smallest possible pop rivet.

Note that the matching plate fastens to the ground plate using the connector mounting screws.

Here is a pattern for the matching plate.

Picture of helix matching plate

More Pictures

Picture of helix antenna Picture of helix antenna Picture of helix antenna

More Detail

Picture of helix antenna Picture of helix antenna Picture of helix antenna Picture of helix antenna

Closing Comments:

This antenna is short and light. Considering its frequency, you must end-mount it. You can use exactly the same mounting plate and clamps as are used for traditional boom-to-mast mounting of yagis. Use aluminum pop rivets to fasten the fibre posts to the aluminum mounting plate. Insert the pop rivet thru the fibre and then thru the aluminum plate. The shoulder on the pop rivet keeps it from pulling thru the fibreglass. Holes should be smallest diameter possible to accomodate the pop rivet.

helix antenna mounting plate

This antenna should look like an electrical open, when you measure across the feedline.

This design has a broad bandwidth. Typically +/- 25MHz or more, of the centre frequency.


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This page was updated on May 29th, 2003