[TowerTalk] Take off angle of yagi vs a wire antenna at about 30'

[Jim Lux]

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