On 1/2/16 8:43 AM, StellarCAT wrote:
ok – sorry – stub.
Specifically adding something to a 4 element 40 to eliminate 15 meter interaction? I
believe Optibeam does this – not sure how they do it. Is this just using the
element with a parallel line tuned through a cap for max current on 15? If so any
suggestions on how long to make it and where specifically to put it on each element
1/2?
Or is there a way to use a relay, open normally, closed when on 15 that would
add some impedance to the antenna causing it to be detuned at 15?
You could just open the feedpoint of the driven element, and that
effectively takes it "out of the circuit", but I think your problem will
be the 3 other elements. They're going to be "close to resonant" on 40m
(how close depends on your antenna design), and hence, they will be
"close to resonant" on 15m as well.
Optibeam may have some sort of tuned network on each element to kill the
resonances out of band (which would be quite clever, but I don't see any
reason why you couldn't do it.. it's just another kind of "trap", after all)
one online calculator would give element lengths for 7.2 MHz as 20.60m
(Refl), 19.98m (Driven), and 18.49m (2 directors).
say the elements are 1" in diameter and 60 feet long, that's a l/d of
720:1. The 3dB bandwidth of a dipole with that l/d is around 9% (call
it +/- 5%)
Doing a quick calculation to see how close any of the elements are to
(resonant) on 15m (and bearing in mind that "resonant" is usually about
3-5% lower than 1.5 wavelengths, I used 1.42 here), I think you'd have a
real problem.
MHz 21 21. 21. 21.45
lambda (m) 14.28 14.1 14.08 13.98
length (m) length in wavelengths
20.6 1.44 1.46 1.46 1.47
19.98 1.40 1.41 1.42 1.43
18.49 1.30 1.31 1.31 1.32
resonant length 1.42
percentage from resonant length
2% 3% 3% 4%
-1% 0% 0% 1%
-9% -8% -7% -7%
Alternatively I can rotate the antenna 90 degrees but boy it would be so much
better not to have to do that to avoid interaction on 15. thanks in advance,
Gary K9RX
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