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6 meter half wave dipole issues




 
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Author Topic: 6 meter half wave dipole issues  (Read 1281 times)
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k7mdo
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« on: April 20, 2022, 05:24:56 PM »

I am embarraed to post my troubles here but I am stumped on a 6 meter dipole construction.  Recent lposts got me interested in 6 meter AM activity so i thought just "whipping" out a dipole would be the first thing to do!?

Well, it has been a real struggle... I probably should not have gotten slightly sophisticated with the idea but since I had a bunch of 5/8" 6061 aluminum tubing I thought it would prove easy to do....  wrong!

I calculated the half wave length to be 55.8" per side and proceeded to cut the tubing a little short and then extend it with other tubing that slides into the 5/8" tubing. Great idea (I thought) I could "fine" tune the thing to exactly 50.4 .....  I slit the 5/8" tube back an inch or so and then put a hose clamp around it to snug up the inserted tube.....

I have been since then spending hours u the ladder trying to find the "sweet spot" for 50.4..... no luck.... in fact I can get a null in the 53 range and a beautiful deep null in the 45 range... but no matter how many different ways of adjustment, I can't get a decent null in SWR at 50.4! 

In addition, I have a 1:1 current balun at the feed point and I have tried bot with and without the balun to no success.

What am I doing wrong?

Here are a few factors....  the antenna is indoors about 10' above the concreter floor and 8 feet below a shingled roof....  it is screwed to the rafters and then the coax leads 25' away to my analyzer and the Icom. No significant conductive materials very near.... 
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Patrick J. / KD5OEI
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« Reply #1 on: April 20, 2022, 11:39:11 PM »

Can you try measuring with a 1:1 balun and the analyzer right there at the top of the ladder at the center of the dipole with least wire length?

If the match is bad, that much cable 25 FT to the analyzer can cause incorrect readings.

Two nulls could be due to the long coax or could be due to some unseen capacitive effect from one side of the antenna to whatever it may be, a piece of wire or flashing or duct.
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k7mdo
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« Reply #2 on: April 21, 2022, 12:00:55 AM »

I'll give that a try this weekend......  One thought is that any building wiring within 3 feet might affect it  ... 
I wondered if corrosion on the tuning might be at fault so I sanded the inserted and outside tubing with no change.... 

The coax has been in service elsewhere....  I'll try substitute cable and length considerations...

I'm not giving up !!! Yet!

I wonder if the value (new) might be faulty?  I'll try removing it... 
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K1JJ
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« Reply #3 on: April 21, 2022, 12:13:50 AM »

It may be a coincidence that 25.7' is close to a 1/2 wavelength multiple on 50.4 MHz  (492/ 50.4 MHz X .66 coax velocity factor)  so the end of the coax may be a  close representation to the antenna feedpoint impedance reading.

But here's what ya gotta try:  As Pat said, take the antenna outside, away from all objects and support it  5-10' off the ground is OK.  Then disconnect the coax and put an MFJ-259B ant analyzer (or equivalent) right at the  dipole feedpoint.  Support the MFJ on a fiberglass or wooden ladder - or rope, whatever.

At one time I built (16) 2el Yagis for 6M and stacked them up a 190' tower.  All fed in phase towards Eu and a second set towards USA.  I learned a lot about antennas.... :-) One thing I learned is getting near objects can have a big effect on 6M.  A tree branch about 4' away can swing the swr. Must be minerals and water in the tree.  So house wiring 3' away can effect things too.

So do the swr test at the feedpoint without coax in the clear and see what it says. That will not take prisoners and you will know... then hook up the coax again and try it in the clear and then back into the house again to eliminate potential issues one by one. Let us know what you find!

While you're at it, you might short the end of the coax and measure for the lowest impedance on the other end, looking and trimming for the 1/2 wavelength multiple point for 50.4 Mhz.. You will now have an accurate swr reading at the end of the coax when connected, assuming the coax is in good condition.

Tom, K1JJ
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Pete, WA2CWA
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« Reply #4 on: April 21, 2022, 01:49:25 AM »

You didn't indicate what impedance cable you are feeding the antenna with. The thickness of the element might play a small part in the result. Don't make a career out of the design and optimum right at 50.4 MHz. If everyone who operated AM were all on 50.4, it would be one mass set of heterodynes. A decent march anywhere from 50.1 MHz to 50.7 MHz is good. It isn't going to make that big of a difference unless you're running mini-watts of power.
You say shingled roof. Is there any metal flashing installed under the roof? Dipole should be free in the air. Under actual operating conditions, dipole should be as high as possible in the air and rotatable.
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WD8BIL
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« Reply #5 on: April 21, 2022, 07:43:19 AM »

Try a 1/4 wave of 75 ohm at the feed and ditch the balun.
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k7mdo
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« Reply #6 on: April 21, 2022, 07:53:52 PM »

Finally I got another Ham to come to my rescue....  I needed someone to go up and down the ladder and make adjustments while I read the MFJ !!  initially we moved the antenna about 5 feet away from a house wiring run.  That didn't bring it in initially but after a good bit of fooling around we got the MFJ down to 1.5 to 1 and 50 ohms (roughly)

It seems that the "simple" dipole (half wave) in the attic of my building is just not going to be as forgiving as is my 1/2 wave 20 meter dipole in the same attic. The 20 was cut to length and installed with no adjustment needed.

As the 6 meter stands right now, one leg is 8" longer than the other but the SWR is around 1.5:1 at 45 ohms.  Good enough for the Icom internal tuner to handle.  I think when the weather breaks I will try to move it outdoors on a post away from the building.
It has certainly been a little frustrating to begin with as after 50 years of making HF dipoles with never a failure, here comes this seemingly easy 6 meter antenna requiring two guys and an MFJ to get it all together!

Thanks all for the encouragement! 

73

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