You can easily model this situation. These distances are VERY close. And if
your
SWR is affected by adding another antenna, you can bet that your pattern is
distorted to beat the band. I've done lots of modeling and if you have the SWR
shift you described in your posting, you will not be pleased with your
operation.
SWR distortion like that says that the 4L20 and/or 6M beams are severely
distorting your 40 M pattern If you don't have the capability to model it, I'd
remove the 6M beam and measure it again, but I suspect your problem is with the
20M beam. Model it.
Bill, N3RR
AA4NC@aol.com wrote:
> As I reported yesterday, I was extremely pleased overall with the Cushcraft
> XM240, especially the SWR curves. Today we mounted a 5L 6m beam 5' below and a
> 4L20m beam 11' below the XM240. Even at these distances, there is some
> interaction between antennas, although it actually improved the SWR on the
> XM240.
> As measured with the MFJ 259 which was the most conservative meter that I
> have, the 2:1 points went from 7.007/ 7.260 to 6.990 / 7.280 with the addition
> of the other antennas. The FT990 and WM-1 meters also confirmed this shift.
> I would assume that the pattern and gain were also affected, but I have no way
> of measuring this.
> This is something to keep in mind when stacking antennas. Even though 9-10'
> stacking distance is recommended, there is still some interaction there,
> although I doubt it is too detrimental. Moral: If you have a big mast - use it
> (but not if it's aluminum !! :>)
>
> 73,
>
> Will
>
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