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Sunday, April 15, 2018

Helix antenna 432/435 MHz 14 turns


After my first success with this kind of antenna in 1992 in VE2 land (antenna homemade helix 9 turns) see pic at Blanville, Quebec of my antenna set up, I decided to build another one with 14 turns.
VE2EK Antenna's set-up at Blanville (Quebec)

Wanting to further improve my Satellites antennas after the construction of the portable 2m / 70cm yagi cross, I set about building a 14-turn "Home-made" Helical Antenna for 432/435MHz.

This 50 ohm impedance antenna allows, when fed with 25/50W of SSB RF, to join the 'Molniya' orbiting satellites like AO-40 and AO-10 very easily!
The gain of this antenna is estimated at 16.5 dBd theoretical.
It's all about giving ideas on how to build a sustainable helicoidal antenna, rather than giving you a recipe to follow.

Calculations made with the HelixCalc Software.
Mesured Results with:
- Windfreak SynthNV: 34MHz – 4.4GHz RF Signal Generator/ RF Sweeping/ Power Detector
- Waterbeach Electronics WE-520A 520 Mhz Antenna Analyzer
                           435MHz, VSWR: 1.04/1, Return Loss: 33.17dB
                           433MHz, VSWR: 1.02/1, Return Loss: 40.1dB
                           432MHz, VSWR: 1.01/1, Return Loss: 45.2dB
Bandwith wide and OK for me!!

Note that for the construction of this antenna I chose round metric values;
The diameter of the wire used for the turns and according to ARRL "Antenne Book" should be between 0.006 and 0.05 Lambda.
The following materials were used to build the 14-turn helical antenna:
• Aluminum: 25mm square support, arms reflector supports,
• Teflon PTFE: Spacers
• Galvanized iron: Reflector grid, Bee hive origin !!
• Aluminum: Helical wire (6.35mm (1/4 "dia.) Plumbing pipe
• Stainless steel: All hardware, support.

Notes on the assembly:
• The central support is drilled every 1/2 turns to receive the Teflon PTFE spacers,
• The spacers are drilled for fixing the helical tube at one end. They are also drilled at the other end to be fixed inside the central support.
• Pieces of galvanized wire. are used to fix the turns to the central support in the spacers.
• I painted all steel surfaces with a polyurethane-silicone alkyd paint.
• The coaxial cable used on the antenna is: 10DFB up to the preamplifier. After the preamplifier, Andrew Heliax ½ cable.
• Connector used: Type N, female, panel mount.
• The reflector is designed so that it can be slid on the central support, in order to adjust the TOS. Two screws hold it in place once adjusted.
The final adjustment of the TOS is done by sliding the reflector forwards or backwards. The center pin of connector N, is fixed on the last turn which has a spacing between the back panel ranging from 13mm to 4 mm at the feed point.
**** this antenna can be fixed to the back of the panel with a counter weight or at its equilibrium point on its boom to an isolated support not to disturb the radiation.

See the diagram of the construction: