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
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