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Author Topic: 80 meter Full loop Calculations  (Read 2957 times)
K8POS
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« Reply #15 on: October 14, 2012, 08:15:28 PM »

Checked the analyzer into both a 500 Watt dummy load and my 2m antennas.
Reads as it should thru those.
Been reading on baluns, numbers could be correct if my balun is bad.
Won't be able to take it down till next week to check for sure.

Bob
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WX7G
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« Reply #16 on: October 15, 2012, 11:29:04 AM »

No, the VSWR numbers do not match the impedance numbers at all.

Two examples:

At 14.025 MHz you list the VSWR as 5.2:1 with the impedance being 30 j16 ohms. The VSWR should be 1.9:1.

At 52.500 you list the VSWR as 3.2:1 with the impedance being 52 j33 ohms. The VSWR should be 1.9:1.

Perhaps you have a nearby AM broadcast station that is bothering the MFJ-269. Until the analyzer is sorted out we might be dealing with random data.

« Last Edit: October 15, 2012, 11:40:00 AM by WX7G » Logged
WB6BYU
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« Reply #17 on: October 15, 2012, 12:57:04 PM »

Except that the R and X columns are given in the reverse order from what we'd expect:
that's 16 +/- j30 rather than 30 +/- j16.  (I made the same mistake when I first looked
at the numbers.)

16 +/- j30 is an SWR of about 4.3 : 1.  While this doesn't match the reported 5.2 : 1,
it isn't quite as far off as it might appear othewise.

I checked a couple numbers from other bands and the error was of the same magnitude.

If you need a quick way to do this, you can use VK1OD's transmission line calculator:

http://vk1od.net/calc/tl/tllc.php

Enter the load impedance as Zload (it doesn't allow spaces in the impedance string, so
use something like "16+j30"), choose a 50 ohm coax type and use a very short length,
such as .01 feet.
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WX7G
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« Reply #18 on: October 15, 2012, 01:02:13 PM »

You're right, X and R are listed backwards from the standard way!

The impedances still look wrong. Is the balun mounted at the antenna or at the shack? And is it connected correctly? That is with the high impedance end to the antenna.
« Last Edit: October 15, 2012, 01:32:14 PM by WX7G » Logged
K8POS
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« Reply #19 on: October 15, 2012, 03:02:30 PM »

Balun is at the antenna.
It is a store bought 4:1 but after reading the reviews on it, there is a good chance that it is also junk.
Going to replace it with a 5kw Balun designs one.
Will check the calibration on the MFJ269 later this week and try it again.
I appreciate everyone's help so far.

Bob
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K8POS
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« Reply #20 on: October 21, 2012, 09:26:23 AM »

Swapped out the balun today.  Took apart the original balun, what a piece of junk.  Found one connection hanging on by 2 strands of wire.  Will post a pic later today.  Installed new 4:1 Balun rated for 5KW.  Here are my old readings and the readings with the new balun, same wire length and the wire just off the ground.

Measured with the MFJ 269

               OLD                                    NEW

80M       SWR      Xs     Rs               80M       SWR      Xs     Rs   Ohms
3.5         5.9      150   106                3.5         6.4      44    13     48
3.8         8.3       44     10                3.8         2.6      22    23     35
4.0         6.6       15       8                4.0         2.7      44   123   100
   
40M                                              40M
7.025      6.7       83      27              7.025       3.5      30    18      38
7.175      6.6       47      13              7.175       2.7        9    18      25
7.300      6.3       28      10              7.300       2.0        4    24      30

20M                                               20M
14.025    5.2       30       16             14.025      3.7       11    16      22
14.225    5.2        8        11             14.225      3.4        0    14      17
14.350    5.2        0        10             14.350      3.2       12    20      28

17M                                                17M
18.068    5.2       39       17              18.068      3.1       54   50      75
18.110    5.2       46       20              18.110      3.1       59   64      85
18.165    5.2       56       24              18.165      3.0       59   87      98

15M                                                15M
21.025    5.2         2       10              21.025      3.5       0    13      15
21.275    5.1        21      10              21.275       3.5       0    13      18
21.450    5.2        44      15              21.450       3.4      57    35     70

12M                                                 12M
24.890    4.9        58      20              24.890       3.0       62    45     80
24.930    4.9        66      24              24.930       3.0       67    53     82
24.990    4.9        82      34              24.990       3.0       74    75     95 

10M                                                 10M
28.000    4.3        18      15               28.000      2.7       16     22     30
28.300    4.2        60      37               28.300      2.7       52     58     80
29.700    4.4        24      11               29.700      2.0       31     38     52

6M                                                    6M
50.100    2.9        44      37                50.100     1.9       25     38      48
51.500    3.0        18      19                51.500     2.0       16     28      38
52.500    3.2        52      33                52.500     1.4       18     44      50
53.500    2.7        49      43                53.500     1.6       20     41      48   
54.000    3.0        73      74                54.000     1.9       16     95      98

Let me hoist it up where it belongs with 282 feet of wire and see what the numbers say then.
But so far looking better.

Bob

PS: Never buy cheap!!!!
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K8POS
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« Reply #21 on: October 21, 2012, 02:18:48 PM »

This is what you get when you buy a cheap Balun.

http://melvinweather.com/balun.html

Pictures speak for themselves.

Bob
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