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Author Topic: K40 antenna conversion?  (Read 1075 times)
KB1IUI
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Posts: 48




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« on: February 15, 2013, 09:45:05 AM »

I bought a used K40 mobile antenna for CB. I know it would be a simple matter to  tune it for the 10 Meter, but I was wonder if I could change it somehow to work on 20 meters? The antenna comes off the mag base with a twist and I was wondering if there is a coil in the antenna or base section? If there wasn't wouldn't I be able to tune in for twenty meters a antenna about twice as long? I know it could work with an antenna tuner, but I know it is better to have the antenna tuned than do that. could I add a coil to the 11 meter antenna sectin to convert it to 20 meters?

Mike
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W5WSS
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Posts: 1656




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« Reply #1 on: February 15, 2013, 10:05:47 AM »

Yes there is a coil inside the K-40 housing.

Try this.

At the end tip of the far end on the whip add some wire long enough to accomplish a match on 20m.
Yes it is hard to get the wire to stay attached but should give you an idea on the length that you will need when turning to a whip.
And yes it is easier to use a more rigid conductor to hold vertically from the point of connection upwards.

I have done this using a heavy duty wire to wire connector for Hill topping and worked good from the mobile using such a base loading coil in conjunction with a longer wire added to bring to resonance on 20m.

The question as to whether or not the resulting field strength is equivalent to an actual 20m  mobile antenna design and an optimized loading coil could be relevant.
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WB6BYU
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Posts: 13041




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« Reply #2 on: February 15, 2013, 11:46:54 AM »

Yes, there is a coil inside the housing.  You can extend the whip or increase
the size of the coil (or both) to get it to resonate on 20m.

HOWEVER, it will require more than doubling the element length or the
coil to get it to work on 20m.  To scale the design as-is, you would have to
double both of them.  That way you would have the same effective
electrical length of the antenna, and the same loading reactance (since
the reactance of a coil varies with frequency.)
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N6AJR
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Posts: 9892




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« Reply #3 on: February 18, 2013, 10:17:39 AM »

just buy a hamstick for 20
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W4OP
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Posts: 393


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« Reply #4 on: February 19, 2013, 11:48:01 AM »

The K-40 base is a series coil and capacitor to ground. The coax is tapped across the cap and ground.

Dale W4OP
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