Here are the details of some measurements I’ve made on my antenna:

I have my antenna mounted on an 8m mast (this is a bit less than the recommended height). 4nec2 calculates the feed point impedance of a 7m antenna mounted on such a mast as follows:

 3.6 Mhz   231 - j695
 7.1 Mhz   290 - j115
10.1 Mhz   580 + j20
14.2 Mhz   379 - j204
18.1 Mhz   604 + j337
21.2 Mhz   991 - j702
24.9 Mhz   255 - j243
28.5 Mhz   615 - j262
51.0 Mhz   158 - j300


The VSWR measured at the feed line in the shack is (measured with MFJ-259B SWR Analyser):

Freq       SWR     R     X
 3.6 Mhz   2.1    24     7
 7.1 Mhz   2.5    21    16
10.1 Mhz   2.5    28    29
14.2 Mhz   1.2    57     8
18.1 Mhz   1.1    46     5
21.2 Mhz   1.2    52    10
24.9 Mhz   1.1    51     5
28.5 Mhz   1.2    49    11
51.0 Mhz   1.3    41     8


The VSWR never goes higher than 1.2:1 between 13.6 MHz and 35 MHz.

28 March 2008 – More Tests

I have now perfomed some more tests to try and determine the losses in the matching unit (balun).

The first test uses my MFJ-259B antenna analyser as a signal source. The matching unit was terminated with a 333 ohm resistor in place of the antenna. I measured the RF voltage at the input and output of the matching unit and calculated the power at each point. The voltages were measured using a Tektronics oscilloscope.

      MFJ-259B -->--[Vin]-->-- Matching Unit -->-- [Vout]-->-- 333 ohm load.

The results of this test are shown in the following table. The R and X values are from the MFJ-259B antenna analyser. The input power (Pin) is calculated using the R value (P = E2/R). The reactance values are mostly low enough (except, perhaps, at 3.6MHz) to have little effect on the results.

Frequency  R   X   Vin   Pin(mW)  Vout   Pout(mW)  Loss(db)
 3.60      43  27  0.601  8.4     1.110  3.7       -3.6
 7.10      57  17  0.626  6.9     1.070  3.4       -3.0
10.10      60  10  0.628  6.6     0.986  2.9       -3.5
14.00      61   5  0.648  6.9     0.870  2.3       -4.8
18.10      60   6  0.670  7.5     0.727  1.6       -6.7
21.20      56  10  0.677  8.2     0.646  1.3       -8.1
24.95      50  11  0.688  9.5     0.573  1.0       -9.8
28.50      45  11  0.680 10.3     0.525  0.8       -10.9
51.00      53  20  0.671  8.5     0.485  0.7       -10.8


The second test I ran the matching unit backwards using an LDG AT-897 antenna tuner to match the unit's impedance to 50 ohms. i.e. the power was fed to the antenna end of the matching unit. I measured the power going in and out to determine the loss.

      Transmitter -->--[Pin]-->-- ATU -->-->-- Matching Unit (backwards) -->-- [Pout]-->-- 50 ohm load.


The results of this test are shown in the following table. For convenience the input power (Pin) was set to 100w.

Frequency  Pin  Pout  Loss(db)
 3.60      100  45    -3.5
 7.10      100  38    -4.2
10.10      100  30    -5.2
14.00      100  23    -6.4
18.10      100  20    -7.0
21.20      100  21    -6.8
24.95      100  22    -6.6
28.50      100  22    -6.6
51.00      100  25    -6.0

I also considered that some of the losses from this test would be due to the antenna tuner. To check this, I conducted a third test using an LDG 4:1 balun in place of the Comet matching unit.

      Transmitter -->--[Pin]-->-- ATU -->-->-- 4:1 Balun -->-- [Pout]-->-- 50 ohm load.


The results of this test are shown in the following table. Again, for convenience, the input power (Pin) was set to 100w. Note that the LDG balun does not work at 51 MHz so this frequency is missing from this test.

Frequency  Pin  Pout  Loss(db)
 3.60      100  91    -0.4
 7.10      100  72    -1.4
10.10      100  75    -1.2
14.00      100  78    -1.1
18.10      100  62    -2.1
21.20      100  60    -2.2
24.95      100  75    -1.2
28.50      100  58    -2.4

I actually suspect that most of these losses are in the balun but, if we assume these losses are split 50/50 between the ATU and the balun then you could deduct half these amounts from the test results using the Comet matching unit. I need (but don't have) a high power 200-300 ohm load and a means to measure the power it's receiving to check the ATU without the balun. This will have to wait for another day.

You will note that these two tests produce slightly conflicting results, especially for the 12m, 10m and 6m frequencies. I intend to do further research to find out why. Either way, the matching unit is not a very efficient device!

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