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Author Topic: Good 20m dipole antenna ideas  (Read 540 times)
KE7EOZ
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Posts: 119




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« on: June 25, 2007, 08:43:41 PM »

I am looking for a good dipole antenna for the voice 20m band, if any one can tell me a good lenghth on each side and also what type of balum 1:1 or 4: should I used. TNX
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N3OX
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« Reply #1 on: June 25, 2007, 09:08:30 PM »

The "cutting formula" for wire dipoles is often said to be:

234/(frequency in MHz)

to give the answer in feet (about 16.5ft/side for 14.150)

I always cut a little longer than that to allow trimming rather than having to add wire.

As far as a baluN choice, you should use a 1:1 balun, because the impedance at the feedpoint of a dipole is generally pretty close to 50 ohms, maybe a little more, but using a 4:1 balun will give you a very low impedance and high SWR on coax cable.

I'd also recommend picking up an antenna book of some kind...

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
NA0AA
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Posts: 1042




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« Reply #2 on: June 25, 2007, 10:55:38 PM »

I second the antenna book suggestion.  ARRL has several good books from beginner to advanced.  I regularly look at them for ideas and tips.

Use the formula and leave yourself some adjustment room [i.e. don't solder the ends until you get it tuned and leave some in case it is too short as well] then pull it up and test the match.  with a balanced dipole, you can feed directly with 50 ohm coax no problem - one of the true wonders of resonant antennas.  Once you get it tuned, tighten everything up and go.

When you tune, note where the lowest SWR is - if you need it higher up in the band, then make the legs shorter, if you want the best match lower, then make it a bit longer.  You can estimate by back-calculating from 1:1 and get an idea of how much adjustment to make.

Make sure your coax connection at the antenna is waterproofed - coax will suck up water like a sponge - I suggest liquid electrical tape and shrink tubing - be compulsive about it, coax is too expensive to fill with water, not to mention how dumb you feel when there's a puddle of water on your operating station from the coax....

For antennas you want to keep, use good wire - unless you have to run stealth antennas, bigger wire lasts longer and is easier to repair when it does break.  I have to admit to liking the Van Gordon hard drawn copper which is mechanically robust, simple to solder and is easy to manage.  Lots of guys like copperweld but I don't like how hard it is to bend/twist - that's a personal beef anyway.

The nifty thing about antennas is that there's always something you can try and since wire and insulators can be re-used it's ripe for experimenting until you find an antenna that really works for your station.

Lastly, you might want to check out a local radio amateur club and find someone to help you with the tuning - there are some electronic tools that really can make it fast and easy.
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K4SAV
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Posts: 1850




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« Reply #3 on: June 26, 2007, 05:07:01 AM »

If you decide to use insulated wire, you will need 2 to 3% less length than the formula gives.  It will vary depending on insulation thickness and type of insulation.

If the antenna is erected as an inverted vee, the wire will have to be longer than the formula indicates.  The smaller the angle made by the vee, the longer the wire will need to be.

If the antenna has any bends, like maybe the ends are bent at 90 degrees, the wire will also have to be longer than the formula indicates.

The resonant frequency will also vary a little with height of the antenna, maybe higher, maybe lower, depending on height and ground quality.

That's why the formula close but never exactly right.  Make it a little longer so it can be pruned to the frequency you want.

Jerry, K4SAV
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KB9MZ
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Posts: 124




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« Reply #4 on: June 27, 2007, 07:15:08 PM »

Look up the "constant impedance antenna system" Patent
It shows a dipole good for 10 thru 20M or you can only use it for 20 by not including the capacitor. It will still have a constant impedance across the band.
It is just a hobby for me so I have never written it up. The OWA antenna that came out afterwards is similar but had two critical elements reversed compared to mine which removed the constant impedance workings of the antenna which is what I designed mine for.
Regards
Art Unwin KB9MZ....XG
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