By using monobanders the out of band signal is reduced by the antenna, for
example, the 15m beam does not receive a 20m TX well. Even closely spaced
monobanders have this "pre-filter". Using tribanders probably requires
better bandpass filtering and you need especially good filtering if you
are using a single tribander.
John KK9A
jimlux wrote:
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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