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Author Topic: 20 Meter Dipole Construction  (Read 27101 times)
K9BAY
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Posts: 144




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« on: April 21, 2010, 06:22:28 AM »

I am constructing a 20M dipole.  I cut the wire to 35 (17.5' off of each side of the balun) feet so I have enough length to trim to make the antenna close to resonant.  I am using insulators and a 1:1 Balun.

My question is, do I include the length of the 14 gauge wire that is twisted around the insulator as part of the overall length?  I have an insulator in the center (center fed) near the 1:1 balun so it can relieve tension off of the balun, it is twisted around the center insluator as well.  When I strung this antenna and attempted to tune with an auto-tunner, the SWR was WAY too high.  I would think even at 35 feet, the auto tuner should be able compensate for the extra 1 1/2 feet appprox length.

I am debating whether to start over, and need some ideas on construting this 20 M dipole properly. Left Insulator > 14G Wire > Center Insulator > Balun 1:1 < 14 G wire < Right Insulator (facing front).

Thanks!

K9BAY
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K1WJ
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Posts: 464




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« Reply #1 on: April 21, 2010, 06:38:55 AM »


468 / Freq in Mhz. 

468 / 14.2 = 32.9ft so call it 33ft ( tad extra )

33ft / 2 = 16.5ft each leg of dipole.

You will only be using a 2inches +- for end insulators & so on. No big deal.

At 30-35ft is ideal height for 20m dipole.

If inverted may need to shorten orginal calculation by 5%.

Bottom line: if formula is used & dipole made correctly SWR & tuning should not be an issue what so ever.

A proper coax choke close to feedpoint of dipole - not a bad idea.

Keep clear of other metal objects near antenna if possible.

When ever I have made a dipole ( too many times ) I have never had an issue with SWR - always in the ball park 1.5-1 +-.

73 K1WJ
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W9XAN
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Posts: 43




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« Reply #2 on: April 21, 2010, 06:41:11 AM »

Quick answer: no
measure the length from end-of-loop to end-of-loop
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N1LO
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« Reply #3 on: April 21, 2010, 06:46:02 AM »

How high is 'way higher'? Your lengths are only just a little long. If your SWR is higher than 3:1 or so, and is *NOT* lower at the lower end of the band than the higher end, look for a shorted connector.

All of the wire in each path contributes to the length, but you shouldn't have to worry about it as long as both sides are the same length. Sounds like you have a good start, the antenna is long, and it's time to start pruning 'n' tuning.

The formulas usually deliver lengths that are a little long to allow for pruning.  If you used insulated wire, you will have to prune off more wire.

--...MARK_N1LO...--
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KI4SDY
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Posts: 1452




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« Reply #4 on: April 21, 2010, 06:47:00 AM »

If you thread the wire through the insulator, loop it back and add an adjustable cable clamp you are shorting the additional wire out. The total length of each leg of the dipole would be measured from the end of the wire (as it goes through the hole of the insulator) to the other end of the wire (as it goes through the hole of the other end of the insulator). At low HF frequencies, the measurements should not be too critical for a good SWR reading, but you should try to get as close as possible. With the adjustable clamps, you can easily lengthen or shorten the wire for proper resonance or adjust it as the wire stretches over time. If your measurements are close and the SWR is still high check your contacts at the dipole, feedline and connectors. Be sure to waterproof the dipole contacts and feedline where they connect.
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W5DXP
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« Reply #5 on: April 21, 2010, 10:56:40 AM »

Murphy's Law: It will not be resonant where you want it no matter what formula you use.  Smiley  Hams who have tried this before usually cut the dipole a little too long so it can be trimmed to resonance at the desired frequency. 492/f is based on the speed of light in free space so it will always be too long in the real world, usually by about 5%.
--
73, Cecil, w5dxp.com
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AD6KA
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Posts: 2238




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« Reply #6 on: April 21, 2010, 07:36:23 PM »

"Murphy's Law: It will not be resonant where you want it no matter what formula you use."

And it will work better if you put it up in lousy weather. (This is a physical law)  Grin

You've got good advice here, make it long then wrap or clamp the extra wire around
the ends of the dipole. (Tight wraps back towards the center, it wont add to electrical
length) Don't solder the ends till you've got it right. Hang it up, measure, bring down,
prune, re-wrap the ends, hang back up, measure, bring down, prune, hang antenna
: rinse and repeat as needed.   Smiley

Keep written records or make a graph. If you shorten both sides,
by 4 inches, write down what the change in SWR or FWD power resulted.
You may just be able to see a pattern developing that you and use.

We've all been where you are. This is part of ham radio. Don't fret about getting a "perfect" 1:1 SWR.  1:5 to 1 is more than fine.

And BULLY FOR YOU for building your own antenna!
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WX7G
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« Reply #7 on: April 21, 2010, 07:40:10 PM »

It is way too long for the autotuner in your radio to tune. Trim it for minimum VSWR in the band and the autotuner will do the rest.
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W7ETA
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« Reply #8 on: April 21, 2010, 08:07:29 PM »

I make a chart of of SWR readings, bottom of the band top of the band and two points inbetween.  I can usually tell from that if I need to lengthen of shorten the dipole.  The ends on my dipole go into an insulator and then wrap around the dipole end back towards the center of the dipole.

I then adjust for lowest SWR either for the center between where I operate on phone and the bottom of where I operate CW, or for lowest SWR where I operate the most, like 40 CW and use a tuner if I need it for phone operation.

Once you get here up and running, its like driving a car you just washed on a sunny day--it runs better.

73
Bob
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W0FM
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Posts: 2057




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« Reply #9 on: April 22, 2010, 01:59:53 PM »

I'm a little confused by your comment that the wire is "twisted around the insulator".  Can you be a little more clear on just how the wire is connected to the insulator.  As stated, it should go through the hole in one end of the insulator (one time) and twist upon itself back toward the center insulator.

Can we also assume correctly that your balun "bridges" the center insulator?  I know it's hard to draw diagrams with words only, but your "diagram" appears that your balun might be installed between the center insulator and the "right" half of the dipole, which would not be good.

73,
Terry, WØFM
« Last Edit: April 22, 2010, 02:02:16 PM by Terry Schieler » Logged
K9BAY
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Posts: 144




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« Reply #10 on: April 24, 2010, 07:15:43 PM »

Thanks everyone for the advise.  I just want to re-emphasize that I am also using and LDG 1:1 balun.  

I did some playing around with some short wire lengths (scraps), and it takes about 3 inches of wire (secure) to wrap (through the insulator hole and twist) the wire around the insolator.  Using an insolator in the middle, to relieve tension for the balun, I would need 4 inches x 2 for the center insulator.  So that is approximately 14 inches total for just in wrapping wire around the insulator and feeding the balun.  So do I just cut each leg 16 foot 7 inches?  

Just to confirm, the wire wrapped around the insulators and wire to the balun feed is included in the 16' 7" wire measurement?  Clamps were mentioned, what kind metal? (please provide a link if possible)?

Thanks again for the help and advice.

K9BAY


« Last Edit: April 24, 2010, 07:21:27 PM by Barry Young » Logged
AD6KA
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Posts: 2238




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« Reply #11 on: April 24, 2010, 11:32:53 PM »

Quote
I just want to re-emphasize that I am also using and LDG 1:1 balun

Um, actually you said 1:1 Balun, but not an LDG 1:1 balun.

Barry, the balun you have was designed to interface coax
with ladderline (another form of feedline as you probably know).
Or to feed a long random length of wire as an antenna.
It was was NOT designed to be hung up in the air as the
the middle center support of a dipole. It's just wasn't
designed for what you want to do with it.

You want a balun also designed as a center insulator, which
is physically strong enough to support an antenna.
I know, 20m is a short dipole, but I think you are going to
run into construction difficulties  trying to make the LDG balun
into a center fed dipole balun.

This is a photo of what you want.
It is NOT a good quality balun, don't buy it.
I AM ONLY USING THIS LINK FOR THE PHOTO!
I want to go watch something on TV starting soon Smiley

http://www.mfjenterprises.com/Product.php?productid=MFJ-918

The other guys here will tell you (and argue) over which balun to buy.

73, Ken  AD6KA
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K9BAY
Member

Posts: 144




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« Reply #12 on: April 25, 2010, 06:04:18 AM »

Thanks for the info. Interesting, when I first posted using this Balun, it didn't seem to be an issue.  Some responded, center feed should not be an issue with this balun.  I even prived a link to the documentation in the thread. I have a balun similiar to the MFJ one that I will try.  Amazing the difference of opinion in this hobby.  Was I mis-directed to use this balun center fed?  Maybe others can commnet...

Are the measurements of 16 feet 3 inches for each segment correct (subtracting 4 inches on each for the feed to the balun and no center insulator, which is a direct connect?

Thanks!


Barry
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W3ML
Member

Posts: 169




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« Reply #13 on: April 25, 2010, 07:21:52 AM »

There is nothing wrong with making it a little longer to wrap the wire.  The thing to remember is you can always shorten it easily, but it is harder to add to it.

You can also go without a balun in the middle, if you want to do so.

73
John, W3ML
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W4VR
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Posts: 1198


WWW

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« Reply #14 on: April 25, 2010, 08:36:54 AM »

468/f (MHz) works well for a typical back yard dipole (BYD).  The capacitive end effect of the looping through the end insulators in taken into account in the formula.  If you use bare wire the formula should be on the money.  If you use insulated wire the antenna may turn out a few inches shorter for resonance than what the formula gives you because the velocity factor is reduced slightly by the insulation on the wire. 
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