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Author Topic: adjusting a AV-640 vertical antenna  (Read 2050 times)
AC6IJ
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Posts: 56




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« on: April 19, 2012, 10:46:29 AM »

Is it possible to adjust a AV-640 vertical antenna so that any length of coax cable will not change the SWR reading? This means that all the bands it's capable of will not have a change in reading. Is this asking too much for the antenna to do? Tnx Bill
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W9GB
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Posts: 2623




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« Reply #1 on: April 19, 2012, 11:30:10 AM »

The Hy-Gain AV-640 vertical is a clone of the Cushcraft R7000, R8 ... End-fed 1/2-wave vertical.
you need to check that the matching box and counter poise wires are properly connected.
The length of coax should not effect your SWR substantially.
« Last Edit: April 19, 2012, 11:31:49 AM by W9GB » Logged
AC6IJ
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Posts: 56




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« Reply #2 on: April 19, 2012, 12:42:51 PM »

Greg, substantially is a very troubling word for me regarding this antenna. It seems no matter how much I adjust each element on this antenna for a good SWR, the coax length changes everything quite a bit and I know it shouldn't.  Bill
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W9GB
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« Reply #3 on: April 19, 2012, 12:47:01 PM »

Did you check to see IF your matching box is damaged ....
Cracked or broken toroids will cause erratic behavior.
http://www.qsl.net/ei7ba/r7_vertical.htm

w9gb
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WX7G
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« Reply #4 on: April 19, 2012, 02:53:01 PM »

The length of coaxial cable should make NO difference on any antenna. If it does it means you have a common-mode current issue. That can be minimized by using a 1:1 current balun at the feedpoint.
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W9GB
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« Reply #5 on: April 19, 2012, 05:42:22 PM »

Quote
If it does it means you have a common-mode current issue. That can be minimized by using a 1:1 current balun at the feedpoint.
Bill -

The AV-640 matching box has that 1:1 balun (bottom one of the 2 cores in box)

I have found about 1/2 cracked due to mechanical stress/abuse ...
OR pilot / operator errors (high power with high SWR)
http://www.mrs.bt.co.uk/mrs/r5/

Photo of blown up matching box
http://www.mrs.bt.co.uk/mrs/r5/bust-match-box.JPG

Repaired matching box cores
http://www.mrs.bt.co.uk/mrs/r5/DSCN2479.JPG
« Last Edit: April 19, 2012, 05:45:56 PM by W9GB » Logged
N4CR
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Posts: 1668




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« Reply #6 on: April 20, 2012, 06:32:28 PM »

The length of coaxial cable should make NO difference on any antenna. If it does it means you have a common-mode current issue. That can be minimized by using a 1:1 current balun at the feed point.

If the feed line impedance doesn't match the load impedance than the length of coax will make a difference even if there is no common mode current.

Since this is a 50 ohm feed line feeding a supposed 50 ohm antenna, I'd suggest that the feed point impedance is no longer 50 ohms. It's unlikely that the coax is no longer 50 ohms but it's a remote possibility.

As others have mentioned, the matching box at the base of the antenna is probably compromised.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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