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Author Topic: Dipole swr  (Read 1778 times)
KD2BHY
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Posts: 14




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« on: August 04, 2012, 06:38:26 PM »

Hello, I wanted to operate portable on vacation so I home brewed a 20 meter dipole. I strung it up outside and measured swr. 3:1 . So I went out and trimmed it. I trimmed it about a foot and still the swr didn't move. Not really sure what to do next

 Thanks so much and 73 s
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AD5ZC
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Posts: 69




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« Reply #1 on: August 04, 2012, 06:55:58 PM »

Same advice will be given that can be found in your thread about the 10 meter dipole that you were able to get to a 1:1 match.
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KB4QAA
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Posts: 2236




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« Reply #2 on: August 04, 2012, 08:51:56 PM »

You can't take one measurement and have any idea 1what to do. 

For example, measure the SWR at:  (keep it inside the band tho')

14.000
14.100
14.200
14.300

Once you find where the dip is, then you adjust the antenna longer or  shorter.
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W5DXP
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Posts: 3528


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« Reply #3 on: August 04, 2012, 09:40:42 PM »

Not really sure what to do next

Instead of cutting the wires, just shorten the antenna by bending each end back on itself until the SWR goes up or down.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 12979




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« Reply #4 on: August 05, 2012, 05:07:03 AM »

The 20m dipole has to be higher above the ground to avoid interactions than was needed
for the 10m dipole.  That's because the distances are relative to the wavelength.
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KU3X
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Posts: 140




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« Reply #5 on: August 05, 2012, 01:48:40 PM »

I've mounted a 20 meter dipole only 5 feet above the ground, to prove a point to some local hams, and I worked into Europe, from PA, with only 100 watts and 100 feet of RG-8X connecting the antenna to the rig.

What you need to do is borrow an antenna analyzer and see what you are working with.
1. Check your coax and make sure there are no issues with it. Put a 50 ohm carbon resistor on one end and connect the other end to the antenna analyzer. You should see a flat SWR. If not, you have an issue with your coax.
2. Cut two lengths of wire 17 feet  5 inches long each. Make a dipole out of those wires and they will be a tad too long. Use the antenna analyzer and trim as needed.
3. This should be number two but.....make sure the antenna is somewhat in the clear. Use rope to tie off the ends of the dipole to whatever you end tie points are. DO NOT use extra wire that you have laying around to tie from the end insulators to a tree. The RF will see that wire and really screw things up.

Good Luck with your project,
Barry, KU3X
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KI4SDY
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Posts: 1452




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« Reply #6 on: August 05, 2012, 08:17:37 PM »

Check your feedline and connectors and make sure they are installed correctly, especially of you soldered the PL259 connector on. Check for shorts or bad connections. Many antenna problems are with the feedline, not the antenna. HF is very forgiving in minor measurement errors on dipole legs.  Grin
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AD4U
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Posts: 2150




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« Reply #7 on: August 06, 2012, 05:44:35 AM »

Something here just is not right.  If you cut a HF dipole by the formula 468/f, use good end and center insulators, use good coax, and install your coax and connector(s) properly, you WILL get a very good SWR (without a "tuner" and without a balun) if the dipole is over 30 feet high.

Check your connections and the dipole length again.

Dick  AD4U
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W5DXP
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« Reply #8 on: August 06, 2012, 06:44:10 AM »

Common-mode problems can cause erroneous SWR readings by "un-grounding" the SWR meter chassis. Is there a good 1:1 choke at the dipole feedpoint and a good RF ground at the SWR meter chassis?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KC4MOP
Member

Posts: 728




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« Reply #9 on: August 07, 2012, 05:44:24 PM »

The magic word is MFJ 259 or 269 antenna analyzer. Saves a lot of grief.
If you are cutting long lengths of antenna on each side, then you may have gone through the dip or resonance of the antenna you were trying to design. Get a "feel" for the amount to cut. 20 Meter and up is about an inch at a time.
40M and down can be as much as 5 inches at a time before you see a change.
Always start out longer than needed.

fred
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