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 resulting impedance.

To feed our antenna without acrobatics, I think we've got three choices:

  1. obtain a 50 Ohm resistance and feed direct
  2. obtain one around 100 Ohm and feed thru a 75 Ohm 1/4 wl transformer
  3. obtain a 200 Ohm resistance and use a 4/1 balun
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.
Anywhy here the first one. Take note of the 20mm diameter of the radiator, to simulate the folding.

reflector at 0.000 m.diameter 02mm5.42m
driver at 1.000 m.diameter 20mm5.20m
director at 2.710 m.diameter 02mm5.10m

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 02mm5.60m
driver at 1.360 m.diameter 20mm5.26m
director at 2.710 m.diameter 02mm5.05m

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