1. 222 MHz Yagis with Direct 50-ohm Feed
Steve Kavanagh, VE3SMA

2. 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 !)

3. 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
Not adjustable
“Half-folded dipole” (WA5VJB)
Simple to build

4. 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 ?

5. 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
Computer model scaled to 222 MHz by W4RNL (

6. Wideband Yagi Example

7. Wideband 2304 Yagi Simulation
Modeled with 4NEC2

8. 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

9. 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

10. 432 MHz Yagi Feedpoint
Works well, weatherproof, but heavy

11. WA5VJB Cheap Yagi Feedpoint

12. 222 MHz Yagi Feed Approach

13. 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

14. Predicted Performance – 4 el.
Parameter
Frequency
(MHz)
YA Predictions
4NEC2 Predictions
Gain (dBi)
222
8.90
8.54
SWR
(limits over which I can measure)
< 1.35
< 1.16

15. Predicted Performance – 7 el.
Parameter
Frequency (MHz)
YA Predictions
4NEC2 Predictions
Gain (dBi)
222
11.91
11.58
SWR
< 1.21
< 1.39

16. 4 Element Yagi

17. 7 Element Yagi

18. Measured SWR

19. Measured SWR

20. Measured Gain At 222.1 MHz Parameter 4 el. 50 ohm feed
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
11.6 dBi
Measured

21. 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 )
Parasitic elements: 1/8” 5356 aluminum alloy rod (from Maple Leaf Communications , 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

22. Distance from reflector (mm)
Dimensions – 4 el. Yagi
Element
Length (mm)
Distance from reflector (mm)
Reflector
665.6
Driven Element
645.0 (overall)
333.5 (each half)
177
Director 1
623.2
271
Director 2
600.4
514

23. Distance from reflector (mm)
Dimensions – 7 el. Yagi
Element
Length (mm)
Distance from reflector (mm)
Reflector
680.0
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