16 turn Helix Antenna for 70 cm Band

Introduction
Ask anyone with a helix. It really is the best and simplest antenna for the job. Selected by the University of Surrey (SSTL) for the UoSat ground control station, and for their many international customers; used by hundreds of satisfied radio amateurs worldwide.

These stations demand assured circular polarisation and honest gain, and use this 70 cm helix with outstanding results on all satellites - UoSAT, PACSAT, AO-10, FO-20, AO-40 and especially terrestrial. Size for size it outperforms everything! Fully described in the June 1985 issue of Wireless World (GB), pages 43-46 and cover. A4 reprint available.

Description
The 16 turn antenna, which is available in either left hand or right-hand circular polarisations (LHCP or RHCP), is designed for a maximum gain of 15.5 dbi at 450 MHz. Gain falls pro-rata at lower frequencies, 350 MHz being an approximate limit.

Matching is adjusted by means of a broad radial feed strap. An SWR of less than 1.1 is easy to achieve at the operational frequency by careful adjustment of the spacing. A small inductor is placed at the feed to prevent electrostatic charge build-up. (It has no effect on tuning).

The coaxial feed is via an N-type socket, forward facing. The antenna will handle 1 kW transmit power comfortably, more if the feed wire is enlarged.

The antenna uses HD40 aluminium for the boom, ultra-violet resistant black nylon for the spacers, and the helix structure is 10mm copper tube.

16 turn kits are available and made-up/tested antennas for collection only. A kit comprises reflector support and Expamet mesh; N­type connector, feed strap, protective sleeve and anti-static coil; drilled boom and end plugs; pre-wound copper tube helix; spacers; boom clamp; all nuts, screws and washers. RHCP assumed, LHCP optional. Parts available separately.

Summary

  Frequency       430-450    MHz
  Turns                16
  Impedance            50     ohm
  SWR         typ.  1.1:1
  3dB beamwidth        ±18°
  Front-to-back       ~15     dB
  Rating                1     kW
  Reflector       600x600     mm
  Helix diameter      240     mm
  Gain               15.5     dBic
  Length              2.8     m
  Weight              4.0     kg

Update: 2015 May 13

No longer in production. This antenna design was introduced in 1985. The last was shipped in 2015. In those 30 years 170 antennas were sold, 105 as kits and 65 built. There were 100 distinct customers, 30 of them overseas. I plan to place the production drawings on this page for historical reference. Thank you to everyone!

Back to Product List

Last updated: 2020 Jul 05