[TowerTalk] SO2R or Multiop Contesting without

john at kk9a.com john at kk9a.com
Tue May 29 16:50:30 EDT 2018

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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