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 4to11to1
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 4to11to1
DiffDiffDiffDiff
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
--
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