Kevin Normoyle wrote:
> Don K5AQ to me mentioned the 50 ohm yagis in the ARRL antenna book. . I
> have 20th ed so. let's see.
>
> oh yeah Don's right..I guess I misremembered stuff..
>
> But let's look at the performance:
>
> using the ARRL/N6BV Yagi for Windows, the 2 element 19.5 ft boom
> (240-20W.YW) has 2.0 SWR from just 7.025Mhz to 7.2Mhz. (at 60')
>
> Maybe when I was playing with simulations, I was thinking "50 ohm swr <
> 2:1 for the full 7-7.3Mhz...with good gain and reasonable f/b --> not
> possible). So while I said one thing in my prior post, I guess I was
> thinking in the context of wanting <2:1 50 ohm swr over the full 7-7.3Mhz.
> (which is one of the advantages of a moxon-like design)
>
> Yes the 50 ohm goal is met in the ARRL design (20 ft boom, full length
> elements) ... but not over the full band, right. Tweaking it to a lower
> impedance, and matching it, can give better swr bandwidth?
>
> I think my bent tip yagi shows that?. It's full length elements, bent,
> with a hairpin (coil).
>
> Maybe that was my point..tweaking a 40M 2L to 50 ohms, makes you give up
> stuff....i.e. it's a peaky 50ohms.
Or, make a slightly more complex matching network. 7.0-7.3 MHz is 5%
bandwidth, which is fairly broadband for a High Q (low loss) matching
network.
But nothing says you have to be limited to a single L and single C in
the network. Start looking at more complex networks, and you can
probably come up with something that gives you low loss AND low SWR over
the band.
historically, this hasn't been done because of the math complexity
(you're not going to be doing multiply tuned networks with a ruler and
Smith chart by hand), but these days, you can let a computer grind the
math and come up with something.
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