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Author Topic: Interesting mod for off center fed dipole  (Read 2742 times)

Lamewolf

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Interesting mod for off center fed dipole
« on: December 01, 2016, 08:19:28 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, 08:44:59 AM by Lamewolf »

gil

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Re: Interesting mod for off center fed dipole
« Reply #1 on: December 01, 2016, 08:37:09 AM »
Excellent thanks. Good trick to know about..

Gil

Lamewolf

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Re: Interesting mod for off center fed dipole
« Reply #2 on: December 05, 2016, 03:27:47 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 for off center fed dipole
« Reply #3 on: December 13, 2016, 08:08:53 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.

gil

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Re: Interesting mod for off center fed dipole
« Reply #4 on: December 13, 2016, 09:17:19 AM »
Do you mean in parallel with the BALUN?

Gil

Lamewolf

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Re: Interesting mod for off center fed dipole
« Reply #5 on: December 13, 2016, 05:57:12 PM »
Do you mean in parallel with the BALUN?

Gil

No, in series with the longer wire at the exact center of the antenna.  If the total length of the antenna is 134' then the cap and resistor goes 67' from the long end.

Rick

gil

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Re: Interesting mod for off center fed dipole
« Reply #6 on: December 13, 2016, 06:01:50 PM »
Interesting, I have never heard of this... Mine is working better now that I slipped ten ferrites over the coax, close to the middle, about 11m from the BALUN. I had to change the BNC connector so it was the perfect opportunity. Now the SWR is lower on all bands.

Gil

Lamewolf

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Re: Interesting mod for off center fed dipole
« Reply #7 on: December 17, 2016, 09:37:17 AM »
Interesting, I have never heard of this... Mine is working better now that I slipped ten ferrites over the coax, close to the middle, about 11m from the BALUN. I had to change the BNC connector so it was the perfect opportunity. Now the SWR is lower on all bands.

Gil

Thinking of experimenting with this myself.  I have a jumper with ferrite beads on it between the balun and feed line on mine, but been thinking of turning it into a Carolina Windom by moving it down the feedline enough to give me 15 meters.

madsb

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Re: Interesting mod for off center fed dipole
« Reply #8 on: December 17, 2016, 10:02:41 AM »
Remember that a low SWR does not indicate an efficient antenna. Otherwise I would immediately swap my dipole for a 50 ohms resistor since the resistor is both compact, lightweight , easy to set up and has a consistent low SWR across all the HF bands :)

Sendt fra min S60 med Tapatalk


cockpitbob

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Re: Interesting mod for off center fed dipole
« Reply #9 on: December 17, 2016, 02:53:42 PM »
Remember that a low SWR does not indicate an efficient antenna. Otherwise I would immediately swap my dipole for a 50 ohms resistor since the resistor is both compact, lightweight , easy to set up and has a consistent low SWR across all the HF bands :)

Sendt fra min S60 med Tapatalk
Very true.
For the EFHW couplers I've made I'll string the antenna, use a short feedline, find a very empty portion of the CW band and go key down for maybe 2 minutes.  I did this with a 100W for my big coupler.  I know the coupler is effecient because it didn't get warm at all.  Since the SWR was good and feed line short I know it's all going into the wire.  This is a little harder to do with dipole baluns since it takes several minutes to drop them down and feel if they heated up.  I've also used candle wax as a tell-tale.  When I lower the antenna if the chip of wax I taped in place was was melted I know things got hot.

gil

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Re: Interesting mod for off center fed dipole
« Reply #10 on: December 18, 2016, 06:37:50 AM »
Interesting tricks Bob, I wouldn't have thought of that!

Gil

Lamewolf

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Re: Interesting mod for off center fed dipole
« Reply #11 on: December 29, 2016, 03:12:08 PM »
Remember that a low SWR does not indicate an efficient antenna. Otherwise I would immediately swap my dipole for a 50 ohms resistor since the resistor is both compact, lightweight , easy to set up and has a consistent low SWR across all the HF bands :)

Sendt fra min S60 med Tapatalk

True, but the resistor in  this application is not for any loading purpose since it is one million ohms, it only purpose is to protect the capacitor from static damage.

Lamewolf

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Re: Interesting mod for off center fed dipole
« Reply #12 on: April 26, 2017, 08:18:49 AM »
Finally got this thing working and here is the results in diagram form.