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Re: [TowerTalk] Why isn't end to end the best method for least interfere

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Why isn't end to end the best method for least interference between two 20 m dipoles?
From: jimlux <jimlux@earthlink.net>
Date: Tue, 27 Mar 2018 16:46:54 -0700
List-post: <mailto:towertalk@contesting.com>
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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