On Mon, 03 Mar 1997 20:10:56 -0900 "Jim writes:
>... a 2 element 40 meter beam... at 40 feet
>what can I expect ?
Hi Jim, Nancy,
I made a quickie beam on EZNEC: 1" conductor, driven = 64',
reflector = 70', spacing = 25'.
Ht dbi angle (-3db) F/B Imp
30 8.40 48 22, 85 8 45.2-j3.3
35 8.86 44 21, 79 9.8 49.8-j1.7
40 9.22 41 19, 73 10.7 54.2-j1.7
45 9.54 38 18, 67 11.8 58-j2.9
50 9.84 35 17, 61 12.9 61-j2.9
55 10.13 33 16, 56 13.9 63-j8
60 10.4 30 15, 51 14.4 64-j12
65 10.68 28 14, 47 13.9 63-j16
So you are right, the primary effect is on the take-off angle.
Almost as significant is the effect on the F/B ratio. Now you DO know
the real purpose of a beam, don't you? It is NOT so's you'll be heard
better. An inverted V, apex at 40', gives 4.6 dbi at a 40 degree
take-off angle, which is only 4.2 db down from your 2 element beam, or
2/3 of an S unit. That is not worth the effort (IMHO). But the beam
improves your "ears". The noise is 2 S units down in the undesired
directions. THAT is what a beam is for. I have to credit Moxen for
first pointing that out to me (no, I do not talk to the gods; I only
read his book). I only sorta believed it then. But I keep bumping
into that same concept again and again, and it makes sense.
OK, so the purpose of your beam is to HEAR better. That means a
better F/B ratio. I have two thoughts on that. First, look at the
current thread about the ZL-Special. It is a two element phased beam.
Both elements are driven. It's advantage is that the F/B ratio is
higher than can be achieved with a yagi. Building one for 40 that can
withstand an Alaskan winter I leave "as an exercise for the student."
The second thought was this. I bet you only really care about a
few directions. So I modeled a third element for your beam, but it is
ground-mounted. I put up a 63' element of 16 gauge wire.
15' 9.51 38 18, 65 12.93 47.4-j11.8
20' 9.81 dbi 38 deg 18,
63 14.4 45.3-j16.2
It works! It lowers your radiation pattern by as much as if you
had stuck your beam up another few feet, and it increases your F/B ratio
by 3db. All by just sticking a wire up on some long 2 x 4s. I also
tried rotating the antenna 45 degrees, with this director stuck where it
was. The F/B ratio remained about the same, and the forward gain fell a
few tenths of a db. The beam pattern was largely undisturbed. I
betcha 3 or four of these gound-mounted directors would be as good as a
3 element beam up there! (Doesn't everyone live on 160 acres???)
Good Luck.
73, ; D DWink@Juno.com Dan Winkler N7IVR Seattle, WA
----------whom the gods would destroy, they first make proud ----------
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