I also think you would have problems. Some years ago we tried a 5el 15M about
8ft above a 2el 40 (full length elements). The 40 worked fine but the 5el 15
was almost useless - higher VSWR, gain & FB both severely degraded. Replacing
the 15 with a 5el 10M yagi and both antennas performed well.
The harmonic relationship between 40 & 15 has always been a problem if the
antennas are too close. The ratio between 40/15 (7mhz X 3 = 21mhz) and 80/20 is
the same (3.5mhz X 3 = 14mhz).
I think that the 20M will have real problems in this case.
73, Stew K3ND
--- On Wed, 10/6/10, TexasRF@aol.com <TexasRF@aol.com> wrote:
> From: TexasRF@aol.com <TexasRF@aol.com>
> Subject: Re: [TowerTalk] Min spacing to 80m yagi ?
> To: email@example.com, firstname.lastname@example.org
> Date: Wednesday, October 6, 2010, 8:48 AM
> Jim, wouldn't you think that the 20m
> yagi would suffer pattern degradation
> while the 80m yagi would be ok?
> Gerald K5GW
> In a message dated 10/6/2010 7:35:00 A.M. Central Daylight
> How close can you get a 20m yagi, below a 2-el
> 80m yagi ?
> I have an option to use a F-12 Sigma
> 280-S shortened 80m yagi
> at the top of the mast, with the F-12
> 340N at the bottom of the mast.
> Spacing between the two of them would be 10.5'
> If possible, I
> would like to sandwich my F-12
> 520 [20m yagi] between the
> 80 and 40m yagi. Apparently, you can
> mount any other F-12 HF
> yagi within 36" of the 3-el 340N
> yagi. If I put the 20m yagi
> 4' above the 40m yagi... that would leave
> 6.5' between the 20m yagi
> and the 80m yagi. Will this work
> ?? If it doesn't, one option is
> to remove the 20m yagi. Another option
> is to place the 80 and 40m
> yagi's at right angles to each other [ no 20m yagi at
> all] . I don't
> think this
> is required, but would minimize the total wind load.
> Plan B would be to replace the 2-el 80m yagi
> with a
> 80m rotary dipole...with dipole inline with 20m/40m
> booms below it. IE:
> is then at right angles to both yagi's below
> it. Due to day time height
> the above options are strictly an after dark
> scenario. I don't have any
> software on hand
> to model any of the above.
> Thanks.... Jim VE7RF
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