Dan,
Glad my memory wasn't "totally" gone. K8CC told me 25 years ago that if I put
up two towers about a hundred feet apart and installed 20m antennas on both
that I could cover JA and EU (the two directions of major interest for a
contester) and of course North and South) Actually I did put up those towers -
just never got to the point of putting a couple of 20m antennas on each one to
have an H frame good for all the right directions.
73...Stan, K5GO
Sent from my iPad
On Mar 17, 2013, at 4:36 PM, Dan Maguire <djm2150@yahoo.com> wrote:
> Although this topic has been beaten into the ground I'd like to post a
> clarification. Stan Stockton and I have exchanged a few private emails and I
> now see how I misinterpreted his remarks.
>
> Stan proposed two towers separated by 1.5 WL. He then suggested looking at
> the pattern when both antennas were rotated 45 deg off broadside. Turns out
> he was thinking of *only* that particular rotation angle. *My mistake* was
> making the assumption that he was interested in *all* rotation angles.
>
> If the two towers are on a North-South line, with both antennas at 1.5 WL
> above real/average ground (which puts the TOA at 9 deg elevation) and the
> towers are separated by 1.5 WL, here's a comparison of the azimuth patterns
> at 9 deg elevation. The blue trace shows both antennas facing East and the
> red trace shows both antennas facing NE.
> http://ac6la.com/adhoc/TwoTower0vs45.gif
>
> Stan was correct in asserting that the NE facing antennas produce a pattern
> which is comparable to the broadside pattern. One might even say that the
> pattern is better. The max gain is down by only an insignificant 0.20 dB and
> there is only a single large sidelobe.
>
> To satisfy my own curiosity I then wanted to see what would happen if the
> spacing between the towers was changed. In this animation the towers are
> still on a North-South line, both antennas are pointed NE, but the spacing
> between the towers is varied from 0.75 WL to 4.0 WL. Look in the lower right
> corner to see the spacing (variable "S") in WL. With most browsers you can
> use Esc to stop the animation and F5 to restart.
> http://ac6la.com/adhoc/Pattern_vs_Spacing.gif
>
> Another way to look at things is to plot the max gain vs the tower spacing.
> In the first frame of the animation above the tower spacing is 0.75 WL (S =
> 0.75). The outer ring is frozen at 18.42 dBi and with S = 0.75 the max gain
> is down 3.56 dB from the outer ring, or 14.86 dBi. In the chart below, Max
> Gain vs Spacing, the first data point is for S = 0.75 and the dBi value is
> 14.86. Other data points show the max gain at different tower spacings.
> http://ac6la.com/adhoc/Gain_vs_Spacing.gif
>
> The S = 1.5 WL spacing is a good compromise between max gain and clean
> pattern. I stand corrected.
>
> Dan, AC6LA
> http://ac6la.com/
> _______________________________________________
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