Author Topic: Interesting mod to off center fed dipole  (Read 1176 times)

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Lamewolf

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Interesting mod to off center fed dipole
« on: November 30, 2016, 09:13:16 AM »
My homebrew off center fed dipole has always needed a tuner on 80 meters, but I discovered that by adding a 80pf to 130pf high voltage capacitor between the balun and the longer wire it lowers the swr and moves the lower swr up the band somewhat.  My best swr on 80 was 2.4:1 at 3.5 MHz and quickly rose as I tuned up the band.  I operate around 3.810 to 3.830 where the swr was around 3:1.  I found a 120pf 5KV disc cap in my junkbox and decided to try it and it moved the useable swr up to 3.850 and dropped it to 1.5:1 with a <2:1 swr from 3.750 to around 3.950.  The value of the cap determines the resonant point, so by varying the value, you can move the resonant point around within the band.  Also, put a 1 megohm 3 watt non inductive resistor across the cap to protect it from static charges.  Anyway, I can now operate 80 meters on some of my favorite frequencies without a tuner.
« Last Edit: December 01, 2016, 07:45:43 AM by Lamewolf »

Flight-ER-Doc

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Re: Interesting mod to off center fed dipole
« Reply #1 on: November 30, 2016, 11:10:15 AM »
The cap is doing what a tuner would do....good and bad.  It's just not adjustable.

What about the other bands?

Lamewolf

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Re: Interesting mod to off center fed dipole
« Reply #2 on: November 30, 2016, 11:55:09 AM »
The cap is doing what a tuner would do....good and bad.  It's just not adjustable.

What about the other bands?

Quite true, it is cancelling out some inductive reactance.  But its nice not to need a tuner and still have the other bands un-affected !

ghrit

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Re: Interesting mod to off center fed dipole
« Reply #3 on: November 30, 2016, 12:19:08 PM »
Would it be useful to use a variable cap in place of the fixed cap?
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Lamewolf

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Re: Interesting mod to off center fed dipole
« Reply #4 on: November 30, 2016, 12:40:06 PM »
A variable cap would work if it was rated for high voltage but would require a much bigger balun enclosue.

Tevin

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Re: Interesting mod to off center fed dipole
« Reply #5 on: November 30, 2016, 06:25:46 PM »
Flight-ER is 100% correct. You're not exactly operating 80 meters "without a tuner". All you did was trade one type of tuner for another and the antenna still has the same properties it had when you were using a standard tuner.

Very clever solution. Good job.



Lamewolf

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Re: Interesting mod to off center fed dipole
« Reply #6 on: December 05, 2016, 02:28:29 PM »
Well, I took the cap back off my windom this weekend because it actually raised the swr on 40 meters  - back to the drawing board !

Lamewolf

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Re: Interesting mod to off center fed dipole
« Reply #7 on: December 13, 2016, 07:09:29 AM »
UPDATE !  When I first read about adding the capacitor, I thought it said to add it between the longer wire and the balun which did work well for 80 meters but raised the 40 meter swr.  The cap is supposed to go in the middle of the antenna, so if your antenna has a total length of 134' the cap would go 67' from the end of the longer wire.  Value of the cap can be anywhere from 100pf to 230pf depending on where you want resonance to fall on the 80 meter band - the higher the cap value, the higher the resonance is and most folks are getting good results using 200 to 220 pf.

Lamewolf

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Re: Interesting mod to off center fed dipole
« Reply #8 on: April 26, 2017, 07:41:38 AM »
Finally got this thing working fairly well and here is an update in diagram form
« Last Edit: May 23, 2017, 01:29:11 PM by Lamewolf »

Tevin

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Re: Interesting mod to off center fed dipole
« Reply #9 on: April 29, 2017, 07:31:13 AM »
Wow, I'm impressed with your charts and diagrams! You should put that in a blog or something, or submit it for publication.

Lamewolf

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Re: Interesting mod to off center fed dipole
« Reply #10 on: April 29, 2017, 09:15:41 AM »
Wow, I'm impressed with your charts and diagrams! You should put that in a blog or something, or submit it for publication.

Thanks Tevin, but I'm not that good yet !  Not in it to make money or anything, just trying to help out our fellow hams is all.

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Re: Interesting mod to off center fed dipole
« Reply #11 on: May 02, 2017, 09:08:54 AM »
Wow, I'm impressed with your charts and diagrams! You should put that in a blog or something, or submit it for publication.

If you want to wite an article for the blog page www.preparedham.com I would be happy to post this.

I have been tied up with a new real estate career and have not published anything for a while.

I have storm tracking videos/ lighting material for videos in the can but have not made time tO edit and oost.

Let me know if you or any one on this forum what to post on the blog page.

Write it in word or another platform and send images and I will do the rest. You get the credit as the author
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