On 5/29/18 10:39 AM, jimlux wrote:
On 5/29/18 10:06 AM, Ed Sawyer wrote:
I have 2 towers separated by 140 ft. Monobanders. The towers are
running
roughly N/S so I am not fully pointing at one when beaming EU but its not
"tip to tip" either.
Keeping this "antenna" related - 140 ft is 2 wavelengths on 20m. On the
lower bands, you're substantially less than wavelength apart, so the
interaction is probably predominantly "near field" - that might make the
pointing direction irrelevant. The distribution and direction of the
electric and magnetic fields (esp if there's significant directive gain)
may be substantially different from what's in the far field.
Running a quick NEC model using L.B. Cebik's 3 element 20m Yagi,
separated by 140 ft..
Here's the relative current induced in the victim antenna by the
transmit antenna, as a function of the angle (0 is parallel, 90 is
victim pointing at Tx)
angle dB
-90 -59.3 (pointing away)
0 -54.8 (pointing parallel)
15 -50.3
30 -46.9
45 -43.7 (pointing 45 crossing boresight of tx antenna)
60 -40.2
75 -37.2
90 -35.2 (pointing right at tx antenna)
this means that if you have 1 Amp flowing in the transmit antenna, at 90
degrees, you'll have 35.2 dB less (0.0174 Amps)..
Assuming constant impedances,etc.. if you put a kilowatt into one
antenna, you'll get a about 300mW into the receiver (25 dBm).
If they're pointing parallel, the coupling is a lot less (-55 dB) - For
a kilowatt Tx (60dBm), you get +5dBm into the victim receiver.
Interestingly, pointing away only knocks it down by 3-4 dB relative to
pointing parallel.
Note that if these were 40m antennas, keeping the 140 foot spacing, the
coupling would be a LOT more.
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