Thanks for reading along. This should just about wrap it up:
I haven’t said much about the two-element Yagi configuration of this antenna. The students were only able to get to 1.5 SWR. I don’t blame them, end fed antenna impedance cannot be modeled or measured without the matching network, as far as I can tell, so the antenna impedance is unknown but very high. The single fat ski pole dipole driven at the end is about 1500 to 2000 Ohms. We only know this after actual construction, matching and then tearing it down and measuring component values.
A center fed dipole has 72 Ohms impedance. If you add a parasitic radiator next to the driven dipole, and optimize for directional gain, you get a Yagi with a feedpoint impedance of about 2 Ohms. This is impossible to match over any reasonable bandwidth. Typically, designers do a couple of things. 1) They spread the elements apart to raise the two ohms closer to 50 Ohms, thereby sacrificing directional gain. 2) They use a folded dipole which by itself has 300 Ohm impedance and then drops to near 50 Ohms rather than 2 Ohms when the parasitic element is added.
What we have here, with the ski poles clipped together is an end fed Yagi. I’ve searched and this appears to be the first instance of this. In the same way a folded dipole has high impedance and makes it easier to design Yagis, so does an end fed dipole. Here is the matching network and the antenna circuit model. As I said, the students could only get to 1.5 SWR. Their C2 was the same as C1, 0.8 to 10 pF. I changed out their C2 for a 6-30 pF Arco trimmer,it will be replaced with a good piston trimmer. Both C1 and C2 were nearly maxxed out to get a match. Modeling this, we get that an end fed ski pole is about 1500 Ohms, but clip on the other pole (trimmed for best gain) and you get the input impedance of the end fed Yagi dropping to about 300 Ohms. The clips separate the elements by 1.5 inches. This is half the distance needed for best directional gain but we only sacrifice 1 dB of gain by using them. This matching network will still match the single pole.
The SWR was surprisingly flat. At first I thought I had constructed a 50 Ohm dummy load. I rebuilt the whole thing and got the same result.
NEC2 models predict 6.05 dBi, 15 dB front to back (F/B) gain ratio. The actual antenna is 5.7 dBi, 15.9 dB F/B ratio. This antenna will throw a signal a little over 1.5 times as far with the second element clipped in place.