Collapsible Beam for 6 m

Steve Kavanagh VE3SMA

OVHFA “Do”, Sept. 2009

Idea for Foldable 6 m Beam

Foldable dipole for 6 m used in my VHF contest rover operations for a few years

– takes just a few seconds to unfold or fold up again for transport/storage

Could use similar construction to make a folding framework to support a wire beam

Similar antennas used by others, with bent parasitic elements made from wire

– Penn. State 40 m beam [1]– DJ4SA Spiderbeam (multiband HF antenna) [2]

Spiderbeam

Geometry of 4NEC2 Model

Predicted VSWR (@ 25 )

Predicted Gain

Predicted F/B,F/R Ratios

Modeling Observations

Could optimize element lengths for different combinations of gain and bandwidth

More gain = less bandwidth = lower input impedance

As expected from regular Yagi behaviour

End Loop Compensation

With wire elements held with string, you need loops at ends to tie string to

These loops add capacitance to the element ends To maintain correct resonant frequency need to

shorten elements to compensate I made some 4NEC2 models to predict the required

shortening – not quite sure if correct but included in the actual antenna

Feeding the Antenna

Spiderbeam has 50 ohm input impedance I chose a higher gain design with lower input

impedance as only a narrow slice of 6m needs to be covered

How to feed a balanced antenna with ~25 ohm impedance using 50 ohm unbalanced cable ?

Feed Network

Balun– Two large ferrite beads are slipped over the coax to make a

“choke” or “W2DU-type” balun Matching network

– can be thought of as two quarter-wave transformers in series

Feed Network, cont’d

BUT…you can’t get 70.7 ohm cable ! By adjusting lengths of the two transformer

sections, – can perfectly match a range of impedances – and can use standard 75 ohm cable

Here it is !

On-the-Air Results

My 1st double hop Es QSO to the west from home – DM09 – With about 10 Watts– Using antenna tuner to match before building matching network

1st QSO on Field Day was NP4A on 6m – With 5 Watts– Much better at 12’ than vertical on car on VE3UBL beacon and on

a VE9 on sporadic E– Good F/B ratio observed

Used for CW WW VHF Contest from FN05 (Multi-Op)– Satisfactory results – 41 grids worked with 100 Watts– Balun withstood this power– Significantly better than dipole (much better than vertical on car)

on VE3UBL beacon

References

[1] Nathan A. Miller, NW3Z, and James L. Breakall, WA3FET, “A 40m V-Yagi”, in International Antenna Collection, RSGB/ARRL, 2003, (and in QST,May 1988)

[2] http://www.spiderbeam.com