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Author Topic: Strange observation on magnetic loop  (Read 1237 times)
ON4CKM
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Posts: 40




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« on: April 20, 2011, 01:27:34 PM »

I've build a magnetic loop and have some strange observations when testing it with the antenna analyser.

The design is the usual. The capacitor on top has a small motor attached to it to tune it. The cable to drive the motor has banana plugs so I can easily connect and disconnect it. If the motor wire is not connected, the lowest SWR I get is 1.6. When I connect the motor cable, SWR on that same frequency is 11 and I can't find another frequency with low SWR. And the other side of the cable is not connected yet, so no current on the wire. The motor wire goes straight down from the capacitor to the ground and hangs in front of the loop.

Does anybody have an idea why a just a cable hanging there can change the antenna so drastically?

Cedric
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N3OX
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« Reply #1 on: April 20, 2011, 02:02:12 PM »

Cedric,

Anything that electrically couples to the capacitor and unbalances it will change things a great deal.

How is the motor shaft insulated from the capacitor?  What type of capacitor is it?

It is very easy to drive RF currents on the motor wires because the electric field is very strong around the capacitor  and you might need a LONG insulating shaft coupling to minimize problems.

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5DXP
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« Reply #2 on: April 21, 2011, 05:27:36 AM »

Does anybody have an idea why a just a cable hanging there can change the antenna so drastically?

Do you have any husky RF chokes installed in the power/control wiring path?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
ON4CKM
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Posts: 40




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« Reply #3 on: April 21, 2011, 11:51:06 AM »

I disconnected the motor completely and I can now measure an SWR of 1.0 flat. I guess N3OX is right. I will re-build the system with an insulating shaft.

Thank you for helping out.

Cedric
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