An "OFF-CENTER-FED DIPOLE" with a very modest feedpoint split.

Tested Here: 

GENERAL TEST CONFIGURATIONS:

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SPECIFIC TEST CONFIGURATIONS:

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BASIC ANT             COAX λ/2 (el.)            COAX λ/2 (cmc)       COAX λ/2 (cmc)WC

NOTE:

BASIC CHARACTERISTICS:

SWR Curves measured with a RigExpert AA-54:

NOTE:  THERE WERE 128 MEASUREMENTS MADE IN THIS SECTION.  

ANALYZER DATA:

Here are the original 128 data files, as measured on a RigExpert AA-54.

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TEST CONFIGURATIONS:

(click on picture)

ANTENNA              ANTENNA ROTATED     WORST CASE  (WC)         WC ROTATED

The MEASURED CMC RESULTS while running 100W into this antenna are shown in the file below:

NOTE:  The 16 basic test configurations used here are the same as used all the way through the test.  However the "Test Numbers" for testing these OCFD antennas differ from the test numbers used when testing the dipoles.  That is because the dipoles were monoband antennas.  These OCFD antennas work on 4 bands. 

REVERSED = ROTATED

In my tests, sometimes I refer to this as "reversing the antenna" and sometimes as "rotated antenna".  Please assume these mean the same thing through the test.

In all of the drawings, the long leg of the antenna is shownin RED and is connected to the hot terminal of the balun.  The short leg is shown in BLACK.

The 88 different test results shown in the files  ABOVE  were tested with the antenna's:

Normally if the antenna were erected in an empty field, free and clear of all objects, and with the coax running at 90 degrees away from the antenna, it would make no difference how the antenna is oriented

The 44 different test results shown in the files  BELOW  were tested with the antenna's:

The difference in CMC between "normal" and "rotated" when measured at the antenna pole was not expected to be significant, so I did not measure the CMC of the rotated antenna at the pole.

Therefore there are only 44 measurements in this section of the test.

HOWEVER, when measureing CMC at the Worst Case position with the coax run nearly parallel to the antenna, I expected there might be significant difference with the antenna rotated.  I took these 44 measurements very carefully.

COMPARING "NORMAL" TO "ROTATED"

To make it easier to compare normal tests to rotated or reversed tests, I began numbering the 44 reversed tests with "49" (instead of "01").  This enables us to easily compare tests between normal and reversed.

This was the first Off-Center-Fed Dipole that I tested.

Many of the results were as I had expected but there were a few surprises.  I also have a few preliminary conclusions which I hope to confirm when I test ANT-3 and ANT-4.

PRELIMINARY CONCLUSION #1:

Whenever more than just a tiny bit of Common Mode Current is present on the coax, it causes skewing of the antenna characteristics.  This of course was suspected.  The higher the CMC, the more it skewed. Whenever I incountered a large amount of common mode current on the coax (>100mA), the SWR curve of the antenna was skewed dramatically.  The analyzer's measurements were sensitive to touching of the coax.

PRELIMINARY CONCLUSION #2:

PRELIMINARY CONCLUSION #3:

The W2DU (Maxwell) RF choke is quite effective as long as the CMC on the line is not too strong.  When the coax is run diagonally, quite a ways out of perpindicular to the antenna, this choke is insufficient for reducing the CMC to an acceptable level.  A better solution is required. The Reisert-type (Guanella) or GM3SEK-type choke (loops of coax through hunks of ferrite) both reduce the CMC to an acceptable level (0 or very near to 0).

PRELIMINARY CONCLUSION #4:

Although even harmonic frequencies exhibit far less CMC than the that found on the fundamental frequency, the level of the CMC on the odd harmonic frequencies was similar to that of the fundamental frequency.  This was totally unexpected.  NOTE: I have never seen this reported before, probably because 99% of the OCFD antennas in the world do not function on their odd harmonic bands.  

(Mine do!).

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