Balanced Antenna Feedline Current Measurements

[John W2XS]

I used an MFJ-835 Balanced Antenna current meter to measure the currents in each side of the ladder line connected to my Cobra Ultralite antenna. It has two 0 to 100 scales on it.  One is for Side1 and the other for Side2 of the ladder line. I compared the Johnson Matchbox, the KAT3/BL2 balun (4 to 1 ratio), the KAT3/BL2 balun (1 to 1 ratio), and the KAT3 directly connected to the ladder line with no balun. In each case there was a 1 to 1 SWR on the K3 SWR meter and the K3 was set to 100W. The antenna current meter was in the same spot for all tests which is right after a DPDT knife switch that can switch between the Matchbox output and the direct (or balun) connections. (I also switch between the MB input and the balun.ladder line input).

I wrote down the scale reading for each of the 4 matching circuits.  Then I added Side1 plus Side2 (to show the total current) and subtracted Side2 from Side1 (to show the balance).

I am still digesting the data, but here is what I think it is telling me:

1. The balance with the Johnson Matchbox is pretty good (within 10%) from 80 to 10, including WARC bands. My antenna itself slopes down on one end so the balance isn't perfect to begin with.

2. With a direct connection form the K3 (and KAT3) right to the antenna, the match is OK but the balance is poor to horrible from band to band.  On 30 meters, the reading on the 0 to 100 scale was 70 on one side and 5 on the other! Only on 160m and 6m does the balance look decent (within 10 to 20%).  The other bands are very unbalanced compared to the JMB.

3. Except for a band or two, the balance is reasonable with the BL2 balun in the line. I find that both the K3 and K2 can find perfect matches in the 4 to 1 position, but the K2 has trouble on some bands in the 1 to 1 position. But the balun heated up significantly on 40 and 20m indicating some kind of loss. I have not yet tried other bands.

4. The built-in tuner finds a perfect match with or without the balun, but the balance is way different from band to band.

5. This is very subjective, but I think the bands sound quieter with the Matchbox in line and noisiest with the direct connection.

6. Due to the balun heating, the use of a balun on the output of an unbalanced antenna tuner is not the best idea - although it will work and contacts will be made. The best situation would be to have a built-in BALANCED antenna tuner that remembers its settings from band to band connected right to the ladder line but this does not (yet?) exist.

Here's a summary of the sums of the scale readings (side1 plus side 2).  Keep in mind that I may have switched scales from band to band but it is the relative numbers in each row are all on the same scale.

        JMB Direct 4to1 1to1
        Sum Sum Sum Sum
160 84 71 75
80 73 62 67 65
40 124 87 108 84
30 93 75 101 88
20 73 50 61 42
17 140 117 135 117
15 75 80 80 80
12 50 49 57 55
10 70 65 73 67
6 44 44 45

Here's a summary of the differences of the scale readings.  The smaller the better. The direct case is pretty poor except for 160m and 6m.

        JMB Direct 4to1 1to1
        Diff Diff Diff Diff
160 6 7 5
80 3 20 3 1
40 6 23 12 18
30 -3 65 3 8
20 3 4 3 2
17 2 33 17 7
15 1 16 4 4
12 0 11 1 1
10 0 25 3 3
6 0 2 3

In summary, for me, I think that the use of the Johnson Matchbox is the best choice for 80 to 10. It has the hoghest totqal power output and best balance. For 160 and 6 (and general coverage receive), direct seems the best choice except that the meter arced on 10m in the direct connection case.  It didn't do that in any other case.  I haven't opened it up to look inside.

For the no-balun case, if the antenna tuner is remotely located, then it is possible that the unbalance can be reduced with a choke on the matched coax line.  If the antenna tuner is built into the rig, then it's hard to do that.

73,

John W2XS

[John W2XS]

Let's try spaces instead of tabs for the tables.

Here's a summary of the sums of the scale readings (side1 plus side 2).  Keep in mind that I may have switched scales from band to band but it is the relative numbers in each row are all on the same scale.

Band    JMB    Direct    4to1    1to1
          Sum   Sum      Sum     Sum
160      X        84         71        75
80       73       62         67        65
40       124     87        108       84
30       93       75        101       88
20       73       149       61        42
17       140      117      135       117
15       75       80        80        80
12       50       49        57        55
10       70       65        73        67
6         X         44        44        45
 
Here's a summary of the differences of the scale readings.  The smaller the better. The direct case is pretty poor except for 160m and 6m.

             JMB       Direct    4to1      1to1
Band       Diff       Diff        Diff        Diff
160        X          6           7           5
80         3          20          3           1
40         6          23          12         18
30        -3          65          3           8
20         3          11          3           2
17         2          33          17          7
15         1          16          4           4
12         0          11          1           1
10         0          25          3           3
6           X          0           2           3


73,

John W2XS