17 March 2013

Moxon Measurements & Adjustments

I wanted to understand how sensitive the Moxon is to wire size and frequency in terms of it's physical elements. I also wanted to adjust my prototype moxon to resonate within the 10m band which it is not currently doing.

I set up a test system in my front yard for this purpose. The antenna was located about 2.3metres off the ground which is roughly a quarter wavelength at 10m. It  i snot a perfect environment since the antenna is about 1.5meters from nearby walls and foliage. However I noted that the resonant frequency was about the same as what I had observed in the field.

The following measurements were obtained using my SWR meter built into the K2 and about 500mW of power:

F (MHz)     SWR
27.6            2.3
27.7            2.0
27.8            1.8
27.9            1.8 
28.0            1.8
28.1            1.9
28.2            2.0
28.3            2.1

As expected at this low height, the SWR is somewhat high but the Fo should be about correct at around 27.9MHz. At this height I have a 2:1 bandwidth of 300KHz. This should increase as I raise the antenna in a field deployment.

 Ok so how do I raise the resonant frequency to the target frequency of 28.2MHz? The design frequency for this moxon was in fact 28.2MHz using dimensions derived from the Moxon calculator by AC6LA. So I have a difference between actual resonant frequency and theoretical resonant frequency of 28.2-27.9 = 300KHz.
So if I input 28.2+0.3 = 28.5 into the calculator as my theoretical frequency than I should see the desired frequency of 28.2MHz in the field?

This yields the following dimensions from the calculator: (28.5MHz, 1mm diameter wire)

A: Rectangle long side long side = 3834mm
B: Driven Element short side = 583.8mm   
C: Between Element gap = 97.7mm  
D: Reflector short side = 712.8mm
E: Rectangle short side = 1394.2mm  

On retest at these new measurements I found that the Fo was about the same which was a big surprise. I did notice that the SWR varied from 1.8 to 1.6 depending upon the direction of the antenna leading me to think that there is more interaction than I had at first thought between the environment and the antenna. Dealing with this solid strand wire is a pain since it breaks all the time. Next I will rebuild the moxin using stranded wire.

The Moxon Antenna Project
Sensitivity analysis
Date: 3/17/2013 
Author: R Hayter, ZS6RSH
Tool: The Moxon Triangle Generator by AC6LA
Wire size sensitivity
Rectangle dimensions (mm)
F  (MHz) Wire size (mm)         A             B           C           D         E
28.3 1.5 3855.9 581.5 104.9 719.7 1406.1
28.3 0.5 3868.9 595.9 89.6 714.6 1400.1
0.5-1.5 = 13 14.4 -15.3 -5.1 -6
% difference 0.3% 2.4% -17.1% -0.7% -0.4%
A: Rectangle long side
B: Driven Element short side
C: Between Element gap
D: Reflector short side
E: Rectangle short side
Frequency Sensitivity
Rectangle dimensions (mm)
F (MHz) Wire size (mm)     A    B   C    D     E
28 0.75 3906.1 598.2 95.1 724.1 1417.5
29.4 0.75 3719.6 569.2 91.2 689.8 1350.2
-1.4 0 186.5 29 3.9 34.3 67.3
-5.0% 0.0% 4.8% 4.8% 4.1% 4.7% 4.7%

Moxon hub using 20mm PVC

Test bed in the garden. 2.3 meters high.

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