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Weather Satellite Images from Cork, Ireland

The Antenna

The antenna is of quadrifilar helical design and constructed from copper wire and PVC pipe. The conductor is 4.1 mm (6 gauge) solid copper wire. The central post is PVC waste pipe with a 56 mm OD. The support rods are 22 mm PVC pipe.

The excellent on-line calculator at http://jcoppens.com/ant/qfh/calc.en.php was used to calculate the antenna dimensions. I also made heavy use of the technical report TECHNOTE 1999-1: Resonant Quadrafilar Helical Antenna”, R.W. Hollander, 1999, to inform the construction technique and to support interpretation of the output from the calculator. One the observations of this report is that narrow conductor diameters decrease the antenna bandwidth, implying that dimensional accuracy is critical to achieve resonance at the design frequency.

The antenna was designed for a centre frequency of 137.5 MHz, a conductor diameter of 4.1 mm, and bending radius of 13 mm. The bending radius was based on the conductor diameter bent around a former of 22 mm PVC tube. This resulted in an antenna approximately 860 mm height, 320 mm diameter.

Quadrifilar Antenna for 137.5 MHz

Quadrifilar Antenna for 137.5 MHz

The conductor tends to be quite “springy”, it is therefore clamped as it passes through the central post by M12 cable glands, and by friction fit into slots in the ends of the support rods. Each half loop of conductor was manually shaped using a rubbish bin of the approximate diameter with accuracy of the bends being ensured by a custom-built former. The cable connections at the top of the antenna are based on a variation of that shown in Figure 2 of G0HPO’s site at http://www.askrlc.co.uk/, however due to the smaller conductor diameter I tied the conductor to the PCB using copper wire and then soldered everything for maximum strength. At the bottom each pair of half loops are joined by soldering end-to-end with the join then being slid inside the central post.

The isolating choke balun is 4 turns of the feeder cable to eliminate any RF signal flowing on the outside of the cable, and therefore minimise any impact on the antenna radiation pattern. I used FT125 75 ohm low loss satellite cable and accepted the 0.2 dB mismatch loss relative the nominal antenna impedance for the convenience of ‘F’-type connectors and relatively inexpensive cable.

The antenna is mounted on top of a 4 m length of similar 56 mm OD PVC pipe with three guy wires to ground anchors. Although fairly flexible this setup has so far withstood a few Irish gales.

Unfortunately I don’t have the equipment necessary to characterise the antenna, however if worked first time out and the overall system performance seems good even with a 30 m feeder and no LNA.

Antenna and Clouds

Quadrifilar Antenna from below

Antenna Overview

Quadrifilar Antenna mounting


Appendix: Antenna Design Dimensions

For reference the complete antenna design dimensions generated by the John Coppens on-line calculator are presented below.

Antenna Calculator Input

Design frequency 137.5 MHz
Number of turns (twist) 0.5
Length of one turn 1 wavelengths
Bending radius 13 mm
Conductor diameter 4.1 mm
Width/height ratio 0.44

Antenna Calculator Results

Wavelength lambda= 2181.8 mm
Compensated wavelength lambda’= 2330.6 mm
Bending correction c= 5.5 mm
Larger Loop
Total length L1= 2391.2 mm
Vertical separator V1= 886 mm
Total compensated length Lc1= 2413.5 mm
Compensated vertical separation Vc1= 860 mm
Antenna height H1= 728.8 mm
Internal diameter Di1= 316.6 mm
Horizontal separator D1= 320.7 mm
Compensated horiz. separation Dc1= 294.7 mm
Smaller Loop
Total length L2= 2272.3 mm
Vertical separator V2= 842.4 mm
Total compensated length Lc2= 2294.6 mm
Compensated vertical separation Vc2= 816.4 mm
Antenna height H2= 693 mm
Internal diameter Di2= 300.8 mm
Horizontal separator D2= 304.9 mm
Compensated horiz. separation Dc2= 278.9 mm

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