[TowerTalk] Why isn't end to end the best method for least interference between two 20 m dipoles?

[jimlux]

On 3/26/18 6:21 PM, Kipton Moravec wrote:
> Below are the results we did with 2 tuned 20m inverted V dipoles at 35 
> feet, in a test for interference during Field Day.
> The results for Orthogonal was the same, independent of which side was 
> transmitting.
> 
> -------------------------------
> 
> With the antennas at ≈ 350 ft apart. An experiment was undertaken to
> determine the greatest isolation.
> • The source was an HF radio operating at about 5 watts.
> • The receiver was a spectrum analyzer.
> Antenna 1   Element North/South
> Antenna 2   Element North/South
> Orientation   Broadside
> Received Signal   -5 dBm
> 
> Antenna 1    Element East/West
> Antenna 2    Element East/West
> Orientation    End to End
> Received Signal    -10 dBm

Doing a quick NEC model, assuming a droop of 30 degrees from horizontal, 
and putting 5W into the antenna, I get -28 dBm for the other antenna.

Drooping at 45 degrees (and adjusting the antenna for resonance), I get 
-19 dBm.

I'm sure, playing around with "not perfectly in line" or not exactly 350 
feet, or the soil properties, I could get it.

An easy way to get more coupling than expected is from the feedline.

I also didn't model the crossing dipole.





> 
> Antenna 1     Element East/West
> Antenna 2    Element North/South
> Orientation    Orthogonal / Perpendicular
> Received Signal    -22 dBm
> 
> • This as not the expected results. But did show that the optimum
> orientation was orthogonal and it did provide about 17 dB of isolation.
> • Further testing will be done to determine if these results are
> repeatable.
> 

I would expect more isolation from the orthogonal orientation - my NEC 
model, with 45 degree droop, shows huge isolation for both crossed cases 
(>100 dB for the one 350 ft away).

The fact that you're only getting 59-60 dB means you're getting some 
other coupling.

For instance, your transmitter might be radiating, or, more likely, the 
feedline(s), or, if the antenna is slightly asymmetrical, so the Vpol 
component is bigger

As noted elsewhere, free space loss for 350 ft is on the order of 35 dB.
If your transmitter leaks 10mW (+10dBm) you'll see -25 dBm at the 
receiving antenna.  The modeled pattern for a 45 degree droop is about 
-3.5 dBi at 6 degree elevation, and that's where your receive antenna is 
(atan(35/350) is about 6 degrees)

Try doing the test with a dummy load at the end of the coax next to the 
feedpoint.


Excellent practical test.. And good questions... And why making good 
antenna measurements in the sidelobes is hard.



> ------------------------------
> 
> We expected the end to end orientation to have the most attenuation.
> 
> Can someone explain why we got what we got?  Is there reference material 
> out there that explains this?
> 
> Kip
> AE5IB
> 
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