222 MHz Yagis with Direct 50-ohm Feed

Steve Kavanagh, VE3SMA

Background

Used 6 element W2PV Yagi with gamma match at home and in rover

Lots of QSOs….but…. measured rather poor gain at OVHFA, CSVHFS

Could I get better performance without adding significant weight and wind area ?

Size constraints for transport meant I needed a homebrew antenna (cheaper, too !)

Options

What’s the problem with the existing Yagi ?– Only possibility seems to be the gamma match

Better matching techniques for VHF/UHF Yagis– T-match and balun (K1FO, K1WHS)

Complicated but very adjustable Needs 4:1 coax balun

– Folded dipole (DL6WU) Hard to model accurately on computer Needs 4:1 coax balun Not adjustable

– “Half-folded dipole” (WA5VJB) Hard to model accurately on computer Simple to build

No matching at all ?

It is possible to design a Yagi to have 50 ohm input at the centre of a split-dipole driven element, with no matching device

Not often used – probably mostly due to construction challenges of split

driven element in all metal construction– Usually slightly less gain for a given boom length than if

feedpoint impedance not constrained Easy to model accurately on computer Could it also be simple to build ? Will it work first time without tuning ?

History

DL6WU long yagis (1980’s ?) are really 50 ohm direct feed designs

– With a folded dipole to transform to 200 ohms– And a 4:1 balun to transform back again !

N6BT (Force12 Inc.) started using 50 ohm direct feed in 1993 – Several models currently available

WA3FET/NW3Z “Optimized Wide Band Yagi”– Discovered in 1990s that 50 ohm direct feed and particularly wide-

band performance could be combined– Designed high performance HF yagis– http://www.naic.edu/~angel/kp4ao/ham/owa.html– Computer model scaled to 222 MHz by W4RNL (

http://www.cebik.com/content/a10/vhf/220owa.html)

Wideband Yagi Example

Wideband 2304 Yagi Simulation

Modeled with 4NEC2

More History (closer to home)

I stumbled upon 50 ohm feed Yagi designs while playing with YA.EXE in the 1990’s

First such antenna of my design built was a 903 MHz Yagi by VE3OIK– worked, but never formally tested

I designed (using YA.EXE) and built a 432 MHz 10 element Yagi in late 1990’s which performs well

Simplicity ?

Wood boom rather than aluminum– No boom corrections if boom is thin enough– Just glue elements in holes in boom– Use boom itself as dipole centre insulator– Tradeoffs

Rectangular cross-section has more wind area than round Elements not grounded (but then the driven element isn’t

grounded anyway in direct 50-ohm feed case)

Key for simple construction will be in feedpoint construction

432 MHz Yagi Feedpoint

Works well, weatherproof, but heavy

WA5VJB Cheap Yagi Feedpoint

222 MHz Yagi Feed Approach

Computer Modelling

Used both YA (YO without the optimizer) and 4NEC2 (a free version of NEC2 with decent user interface and optimizer)

Combined manual and automated optimization to come up with design that looked good in both programs, with more-or-less fixed boom length

4NEC2 model included details of feed geometry, YA could only model a straight dipole

Designed two antennas – 4 elements as “proof-of-concept”– 7 elements as “real” design

Predicted Performance – 4 el.

Parameter Frequency

(MHz)

YA Predictions

4NEC2 Predictions

Gain (dBi) 222 8.90 8.54

SWR 222-223.7

(limits over which I can measure)

< 1.35 < 1.16

Predicted Performance – 7 el.

Parameter Frequency (MHz)

YA Predictions

4NEC2 Predictions

Gain (dBi) 222 11.91 11.58

SWR 222.0-223.7 < 1.21 < 1.39

4 Element Yagi

7 Element Yagi

Measured SWR

4 Element Yagi

1.01.11.21.31.41.51.61.71.81.92.0

222 222.5 223 223.5 224

MHz

VS

WR 4 el YA

4 el 4NEC2

4 el Meas.

Measured SWR

7 Element Yagi

1.01.11.21.31.41.51.61.71.81.92.0

222 222.5 223 223.5 224

MHz

VS

WR 7 el YA

7 el 4NEC2

7 el. Meas.

Measured Gain

At 222.1 MHz – measured at OVHFA Do Sept. 27/08

Parameter 4 el. 50 ohm feed

6 el. W2PV gamma

7 el. 50 ohm feed

Boom 21” 43” 60”

YA 8.9 dBi N/A 11.9 dBi

4NEC2 8.5 dBi N/A 11.6 dBi

Measured

Construction Data

Common data– Boom: softwood, 1” nominal thickness (actually about 0.75”),

varnished after assembly– Driven element: 1/8” brass tubing (available from good hobby

shops – mine was from Flite Craft in Kitchener http://www.flitecraft.com/ )

– Parasitic elements: 1/8” 5356 aluminum alloy rod (from Maple Leaf Communications http://www.mapleleafcom.com/ , or welding supply houses)

– Vertical centre-to-centre spacing between driven element halves = 8 mm

– Elements held in boom with 5 minute epoxy– Ferrite choke balun: unknown surplus type

Dimensions – 4 el. Yagi

Element Length (mm) Distance from reflector (mm)

Reflector 665.6 0

Driven Element 645.0 (overall)

333.5 (each half)

177

Director 1 623.2 271

Director 2 600.4 514

Dimensions – 7 el. Yagi

Element Length (mm) Distance from reflector (mm)

Reflector 680.0 0

Driven Element 647.8 (overall)

336.0 (each half)

183

Director 1 619.8 264

Director 2 607.6 472

Director 3 595.8 737

Director 4 601.0 1125

Director 5 589.2 1494