[TowerTalk] Re: Yagi's
Mon, 5 Feb 2001 18:21:21 -0500
As for your antenna, on ten meters you have you have:
> Yagi, < 1 WL boom,13 elements 3 of which are reflectors at 65 feet.
My three element 40 meter yagi over the same percentage
bandwidth with only three elements on a boom shorter than 1/2 wl:
13.8 to 14.4 dBi gain.
F/B better than 20 dB
SWR under 1.35:1
In contrast your antenna has more than twice the boom length and
over four times the elements, yet only has 2 dB more gain and 5
dB more F/B at bandwidth edges!
If I consider front-to-null, since the null is placed at a useful angle,
my common less-than-half-wave boom antenna has more than 45
dB rear rejection across 3% bandwidth.
I can't find a 1 wl boom Hygain antenna, but an optimized 24 foot
105BA over earth has:
16-16.2 dBi gain
23-27 dB F/R
It sure doesn't look like anything remarkable is going on.
Your antenna works about like any 1 wl yagi should work, even one
with 5 elements instead of 13 elements.
I'm sure you will want to shoot me for saying this, but it looks like
much ado about nothing to me. Gain is proportional to boom
length, and F/B is mostly a matter of element placement and
Yagi's have been investigated pretty well over the years. They have
evolved into single reflector yagis with multiple directors because
that is what gives the best performance for a given size and cost.
People can natter all they want about gain and stuff, but the truth is
there is very little (if any) difference between the magic antennas
and any other antenna the same physical size, assuming no one
did anything wrong in the design.
There is no free lunch.
73, Tom W8JI
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