On 11/6/14, 5:26 AM, David Gallatin via TowerTalk wrote:
Hello group and once again I thank everyone who helped with my previous post
about the concrete.
My set up: Mosley TA 33 JR tri-bander that will be at about 31 feet. The
question: I've been debating ways to get at 40-80 meters. I have a very small
lot so cant run the usual wire suspects and was thinking about making a mag
loop.
But I've also thought about an end fed Chameleon or 33' Ultimax at about 28
feet as I do have room to run one of those horizontal to a tree. These are
multi-banded antennas but I've never given considered using them on 20,15,10 of
course.
But then a thought occurred to me. Might the take off angles of the wire
antenna be different enough as to be beneficial for transmitting on those bands
and hitting different spots of the globe that the Mosley is skipping over?
73,
David, AA9G
ex W5DCG and KC9EEV
HFTA (in the ARRL antenna book) answers these questions really well..
A 3 el Yagi with about 8dBi gain has a 3dB beamwidth on the order of
120-140 degrees.. in the horizontal plane, where you really have the
pattern of the underlying dipoles helping you. In the vertical plane,
it's even wider.
But, let's just assume 140 degrees for now.
here's the "angle of maximum pattern" for a few heights (in wavelengths)
ht dipole only Yagi
0.1 90 deg 60 deg
0.2 90 54
0.25 90 48
0.3 60 45
0.4 39 36
0.5 30 30
0.75 18,90 18
1 15,48 15, 48(-1.3 dB)
A more interesting question might be "what's the gain effect at 12
degrees above the horizon" ( I assumed 8dBi for the Yagi)
ht dipole Yagi
0.1 -15.6 -9.8
0.2 -9.6 -3.8
0.25 -7.7 -2.0
0.3 -6.2 -0.4
0.4 -3.9 -1.9
0.5 -2.2 +3.6
0.75 0.5 +6.3
1 1.8 7.6
No matter what height, the Yagi has a gain advantage of 5.8 dB (because
that's what the Yagi's gain is at 12 degrees off boresight (essentially
the original 8dBi - the 2.15 dB you get from a dipole).
Take home message:
an 8dBi yagi is like a 6dB power increase over a dipole, no matter
what height
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