Two projects for 3 element wire yagi
The below projects came up after an e-mail exchange with Colin,
who had some 300 Ohm stuff to test. So they're run for a folded
dipole radiator. First at all I must explain that the Nec-2 software
is quite reliable when dealing with radiation patterns, but can be
easily deceived about feed-point impedance. And the analysis of the
folded dipole is not a reccomended one. The usual workaround is to
run the analysis with a standard dipole, then multiply by 4 the
To feed our antenna without acrobatics, I think we've got three choices:
Since I mean to do just a test I discarded the balun. Btw I suppose
a balun will be a problem also for real use because of weight.
- obtain a 50 Ohm resistance and feed direct
- obtain one around 100 Ohm and feed thru a 75 Ohm 1/4 wl transformer
- obtain a 200 Ohm resistance and use a 4/1 balun
Anywhy here the first one. Take note of the 20mm diameter of the
radiator, to simulate the folding.
|reflector ||at 0.000 m.||diameter 02mm||5.42m|
|driver ||at 1.000 m.||diameter 20mm||5.20m|
|director ||at 2.710 m.||diameter 02mm||5.10m|
The antenna should have a feed impedance of 12.2 -j.49 ohms.
I already built it with standard electric wire, insulated, so I
shortened the actual length by a -3% factor. When I'll find the
time and the right qth to test it, I'll let you know the results.
But now here the second one.
|reflector ||at 0.000 m.||diameter 02mm||5.60m|
|driver ||at 1.360 m.||diameter 20mm||5.26m|
|director ||at 2.710 m.||diameter 02mm||5.05m|
That should have a feed impedance of 33.5 +j.84 ohms. If the driven
element is a folded dipole, use 1/4 transformer. But if you don't fear a
1/1.5 swr -I don't- then use as driver a simple wire with 5.32m length.
Hi. Paolo 1839