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Author Topic: EZNec antenna calculation request  (Read 1185 times)
N3BSZ
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Posts: 58




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« on: February 20, 2012, 01:07:09 PM »

I have a dipole fed with 450 ohm ladder line.  Each leg is 64 feet.  I would like to know what the impedance is if fed with LMR-400 or RG-213 or LDF4-50

I am thinking about purchasing an automatic tuner and wondering if it would work.

I know 160 is out unless I feed both legs, so I would like to plan 80-6.  Most tuners will tune up to 10:1, but I think the MFJ will go further.

73,

Tom
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WB6BYU
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Posts: 13353




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« Reply #1 on: February 20, 2012, 01:45:06 PM »

How long is the ladder line?  That makes a huge difference in the impedance.
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N3BSZ
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Posts: 58




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« Reply #2 on: February 20, 2012, 03:41:06 PM »

Forty feet or less
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WX7G
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Posts: 6146




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« Reply #3 on: February 20, 2012, 05:35:33 PM »

Without line loss in the coax an auto tuner will probably not match the impedances this antenna will present.

But with 100' or so of coax the high line loss due to the very high VSWR on the coax will cause the VSWR at the tuner to possibly be within its matching range.

To model this in NEC requires the exact length of the ladder line and the length and type of coax.

Or build or buy a G5RV and it work on the non-WARC bands and possibly on the WARC bands as well.
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WA7RBC
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« Reply #4 on: February 21, 2012, 10:18:49 PM »

Here's my attempt to model your situation:

If the dipole is mounted 40 feet above "average" ground, EZNEC says the impedance is 42-89j ohms when operated at 3.5 MHz.  At 60 feet high the impedance is 65-90j ohms. This is an SWR of about 7:1 with 450-ohm feedline.  With these numbers, a 9 foot section of 450 ohm line will match a 50-ohm coax with a SWR of about 1.3:1.

Hopefully someone will verify my calculations, as I'm an old guy and make a lot of mistakes!

73,  Ron.
« Last Edit: February 22, 2012, 03:16:54 PM by WA7RBC » Logged
WX7G
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Posts: 6146




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« Reply #5 on: February 22, 2012, 03:19:28 AM »

Tom,

now I see what your question is; there is no ladder line involved.

On 40, 20, 15, and 10 meters the VSWR at the load end of the coax will be about 50:1. To get this down to 10:1 for the autotuner line loss can be used.

150' of RG-213 will do the trick and provide a 9:1 VSWR at the autotuner at 7 MHz. You will lose 75% (1 S-unit) of your signal in the form of heating the coax.  On 10 meters you will lose 80% of your signal and the autotuner will be presented with a VSWR of 5:1.
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