[TowerTalk] SO2R or Multiop Contesting without

[jimlux]

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.