Here are the details of some measurements I made on my antenna:
I had my antenna mounted on an 8m mast (this is a bit less than the recommended height).
Using 4nec2, I calculated the feed point impedance of a 7m antenna mounted on such a mast and the results were 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 was (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 = E
2/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 intended to do further research to find out
why. Either way, the matching unit is not a very efficient device!
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