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Author Topic: High SWR on Gap Antenna  (Read 392 times)
VK2ZXD
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Posts: 15




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« on: February 24, 2006, 10:37:40 PM »

Greetings from VK land.
I've errected a DX V111 Challenger Gap Vertical Antenna at ground level,with the connection of the recommended 3 X 25' insulated radials just 3" above the ground. The instruction book says any SWR, 2 :1 or better is OK. My problem is that on 40M & 80M, the SWR is higher that 2 : 1. Particularly on 80m where the figures are crazy. I checked the figures, using two different SWR meters, and get the same results as shown below
7.060Mhz 2:1  7.160Mhz 2.5:1  7.260Mhz 2.8:1

3.550Mhz 5:1   2,69Nhz 3.5:1  3,70Mhz 1.35:1
Any suggestions would be most welcome.
Many Thanks, Gordon VK2ZXD
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VE3IOS
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Posts: 79




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« Reply #1 on: February 25, 2006, 09:49:21 AM »

I would make the radials a quarter wave long for the 80 meter band.
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WB2WIK
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Posts: 20595




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« Reply #2 on: February 27, 2006, 08:29:53 AM »

Actually, your results are about normal if you use the 3.7 MHz top loading capacitor in the Challenger.

The Challenger SWR bandwidth on 80 meters is about 75 kHz.  So, if you use the 3.7 MHz top loading capacitor, SWR should be about 2:1 at 3.625 and 3.775 MHz, dipping to nearly 1.0 at 3.700, and then rising a lot below 3.625 and above 3.775.  That's the normal response.

If you want to use the antenna more on CW, use the 3.5 MHz top loading capacitor (optional).  This is discussed in the assembly instructions and even in the advertising literature.

This antenna does *NOT* cover the whole 80 meter band with an SWR <2.0, that's impossible.  In fact, it would be pretty impossible even with a full-sized 65' tall vertical.

Your results on 40m indicate a bit of a mismatch issue that could be due to using a length of coaxial cable (feedline) that is resonant and undesirable.  I'd try changing the feedline length 10 feet or so and see if it improves.  They do "warn" you about this in the instructions, also...

WB2WIK/6
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WB2WIK
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« Reply #3 on: February 27, 2006, 08:32:38 AM »

>RE: High SWR on Gap Antenna  Reply  
by VE3IOS on February 25, 2006  Mail this to a friend!  
I would make the radials a quarter wave long for the 80 meter band.<

::That probably wouldn't help, although it's worth a try.

The "radials" on the GAP Challenger are not radials in the conventional sense; the antenna's actually center-fed and the wires at the base are simply end-loading, like a "capacity hat" but at the bottom instead of at the top.  The top is similarly loaded but instead of using wires, they use a doorknob capacitor installed inside the upper element tubing (and accessible under the weather cap).

WB2WIK/6  
 
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VE3WMB
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Posts: 288




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« Reply #4 on: March 09, 2006, 06:46:19 PM »

As the last post said they are not ground radials in the conventional sense. Think capacitive top loading, only
at the bottom. The fact that you are having problems on 40M in particular, means that the issue is with the 3 radials (I speak from 7 years experience as a Gap Challenger owner). I believe that the manual says that they should be on the ground or buried not elevated.
Having them elevated above the ground is likely the issue.

Michael VE3WMB
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