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Author Topic: Trimming a trap dipole? How much?  (Read 1779 times)
N4NYY
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« on: February 28, 2012, 05:05:27 PM »

I built a trap dipole for 20-40-80 (phone portion). The 20M and 80M sections are right where I want them. The 40M section appears resonant at 7.130, which is the bottom of the band. I would like to move it to 7.175 or 7.2. Since this was a trap dipole, the 468/f is not totally appropriate. But according to the formula, one leg would need to be shortened .32 ft (about 3.75 inches).

Should I just trim it 1" or 2" at a time? This is my first wire antenna.
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N4UM
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« Reply #1 on: February 28, 2012, 06:35:27 PM »

Try one inch and see where that moves it to.  Then you'll have a good idea how much more to trim to get it where you want it.  You might have to lengthen the 80 meter portion of the antenna a small amount to get it back where you want it.
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N4NYY
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« Reply #2 on: February 28, 2012, 06:38:51 PM »

Quote
Try one inch and see where that moves it to.  Then you'll have a good idea how much more to trim to get it where you want it.  You might have to lengthen the 80 meter portion of the antenna a small amount to get it back where you want it.

Actually, if it raises 80M, I don't mind. That is a little low, to.
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W5DQ
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« Reply #3 on: February 29, 2012, 12:11:54 PM »

The 40M section appears resonant at 7.130, which is the bottom of the band ......

That's strange? My copy of the US band plan shows 40M goes all the way down to 7.000  Smiley
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
N4NYY
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« Reply #4 on: February 29, 2012, 12:59:56 PM »

Quote
That's strange? My copy of the US band plan shows 40M goes all the way down to 7.000

For phone? I thought it was 7.125
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W5DQ
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« Reply #5 on: February 29, 2012, 01:36:21 PM »

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That's strange? My copy of the US band plan shows 40M goes all the way down to 7.000

For phone? I thought it was 7.125

It is.

I was pulling your leg about the CW portion Smiley Sorry for the confusion.
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
N4NYY
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« Reply #6 on: February 29, 2012, 01:44:11 PM »

Quote
It is.

I was pulling your leg about the CW portion Smiley Sorry for the confusion.

You had me running for the bandplan! LOL
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K3WEC
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« Reply #7 on: February 29, 2012, 07:31:40 PM »

Did you make your 20m portion 468/f ? Just wondering as I'm building another trap dipole myself.
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N4NYY
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« Reply #8 on: March 01, 2012, 05:53:11 AM »

Quote
Did you make your 20m portion 468/f ? Just wondering as I'm building another trap dipole myself.

I did not. The 468/f appears to be for no traps, as the traps shorten the length. I used the Unadilla plans. The good thing is if you use the 468/f, you will likely be long, and can trim.
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N4NYY
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« Reply #9 on: March 03, 2012, 05:39:46 PM »

I ended up trimming 4" off the 40M wire, and here are the numbers. I think this is good and I am going to leave it this way. I don't care much about 7.200 - 7.300.

Freq     SWR     Z
3.600    3.9      149
3.700    1.5        71
3.750    1.2        37       
3.800    2.0        36
3.900    4.1        95
4.000    5.8      205

7.100    1.8      35
7.150    1.1      41
7.200    1.7      36
7.250    2.5      24
7.300    3.3      15

14.150  1.5      73
14.200  1.2      62
14.250  1.1      56
14.300  1.2      56
14.250  1.4      62
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K0IZ
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« Reply #10 on: March 04, 2012, 07:24:34 AM »

Although you have finished your tuning, just a suggestion for future tuning.  Rather than cutting wire, I double back the excess wire and use small split-bolt clamps to hold the wire.  Split-bolts are available in electrical dept of hardware/Home Depot, etc.  That way you can continue to adjust without permanently making the wire too short!
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WA9YSD
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Posts: 138




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« Reply #11 on: March 04, 2012, 09:22:32 AM »

492 you can cause it gives you more wire to play with.  You can use your own number cause every ones situation is different.

Relax, it is the name of the game with pruning.  This is why you have tuners. You can all ways add on wire. Being 1 to 3 inch off SWR shift at the frequency you want, the traps have more loss. You will drive your self crazy if you want perfection.

If you cut off too much, add wire just before the trap and let it hang down. Adding wire and re-attaching it to the trap is a pain in the butt each and every time is not worth the trouble.

If you trim one band the other next to it will shift a little bit.

It will be ok and you will like the way it works.

Jim K9TF
« Last Edit: March 04, 2012, 09:43:03 AM by WA9YSD » Logged
N4NYY
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Posts: 4799




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« Reply #12 on: March 04, 2012, 09:40:26 AM »

Since this is a trap dipole and the wire between traps is not as long as a fan, if I overdo it, I cam simply replace the wire section if I had to. I am glad I went this route.
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