[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
> 
> 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.
> 
> ------------------------------
> 
> 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?
> 


testing at 14 MHz (roughly)?

Inverted V isn't a horizontal dipole.
The sloped element is partly vertical, so "end to end" is sort of like a 
set of verticals (considering the V pol radiation).  Think about it as a 
pair of verticals some short distance apart (maybe 0.1 lambda) fed 180 
degrees out of phase. That's a lot like a W8JK.

I think if you model the V, and look at the Vpol radiation off the end 
of the dipole, it's not zero.

Free space loss (between isotropes) for 100 meters separation and 14 MHz 
would be:
32.4 + 20*log10(0.1) + 20*log10(14) = 32.4 -20 + 23 = 35 dB loss.
With 5 W radiated, that's 37dBm, so you'd expect about -2dBm.

Your parallel V's is in that ballpark (if you get within 3 dB, you're 
doing great)