VHF Optimized Yagi Antenna for the 6-meterband (50 Mhz)
By Guy, de ON6MU
Parts list for the long yagi antenna RE-A50Y3
2 meter long
alu/copper pipe/tube of +/- 20...28mm (square or round)
or if using portable, use 2 x 1 meter pipes/tubes fitted together in the middle of the boom with a larger piece that slides over. Same goes for the centre of the elements.
6 pieces of 1
meter alu or copper tubing:
- 12...16 mm diameter
3 pieces of 2
meter long alu or copper tubing:
- 15...20 mm diameter
Gamma match: alu or copper tube of 250 mm of 8mm diameter
some cul wire (isolated wire, installation wire, etc.) of 4 mm copper diameter (+/- 6...7mm diameter with isolation)
some alu/copper plates to construct the strap holder for the gamma-match
1 female PL 259 chassis
some silicon, grease... to make the construction weather resistant.
several lengths of innox bolts or other non oxidating bolts
a bracket to mount the yagi to a boom
and a few
innox hose clamps
Note: there are many ways to
build your antenna and I'm sure some can come up with better
mechanical designs then described here although the design and
material used here is cheap and easy to find. Also, the diameters
of the tubing described here is not too critical.
Links of interest:
When the dipole is
the driven element of a Yagi parasitic array, the impedance that
appears at the center of the driven element will usually be quite
different from that of the isolated dipole. The reason for this
is that the Yagi antenna is equivalent to a number of resonant
circuits tuned to different frequencies and coupled together. The
self-impedance of the parasitic elements and mutual impedance
between parasitic elements and driven element cause resistance
and reactance to be coupled into the driven element.
A gamma-match tube act's like a capacitor. The "capacitor's" sole purpose, is to oppose unwanted inductive reactance. It is a series-tuned LC circuit between the coaxial cable's center conductor and the gamma's connection point at the driven element hence getting a pure Ohms resistance. So, a gamma match taps a point on one side of the center of the driven element and connects the feedline to that point through a capacitor. In this design a tube with a copper wire inside acts as air-spaced capacitor. Another advantage is to allow the center of the driven element to be directly grounded to the antenna's boom, making driven element assembly easier and no static build-up on the driver.
To tune the gamma-match to a desired frequency for best SWR you will need to experiment with the placement of the strap on the driver (feedpoint) and the position of the strap relative to the feedpoint.
If you build the yagi antenna according to above specs, then you should be pretty close (if not spot-on) to a ideal match.
How Hernan, LW5DTZ did the Gamma-match
Specifications ON6MU 50 Mc wide-spaced yagi antenna RE-A50Y10
Total length 2 meters
Max width 2,95 meter
Peak centre frequency: 50,5 Mhz
Maximum bandwidth: 2...2,5 Mhz
Gain: 6,1 dBd
F/B ration: 25dB
Horizontal opening angle: 45°
Vertical opening angle: 55°
Maximum tunable frequency range: 49...54 MHz+-
impedance: 50 Ohms
Maximum power using the components described: 300 watt
DC grounded (no static buildup do to all elements being electrical/mechanical connect at the center. Of course the boom/tower/mast itself needs to be grounded too)
Solid air-spaced gamma-match design
If needed, it
can be disassembled into a very small bundle no longer
than the longest element being 1 meter.
Be sure to seal everything up to avoid moisture, corrosion etc...
.Radioamateur related projects
.ON6MU Ham mods
.Modifications of transceivers
..................................... .Please take a look at my 50mc vertical antenna
is how Greg, SP5LGN made my 50mc long-yagi:
please click the images to enlarge
Take a visit to Greg, SP5LGN his website: http://www.sp5lgn.ampus.pl
Thanks Greg for the photo's!!
is how Hernan,LW5TDZ made it:
Thanks Hernan foir the pictures!