Tuning a Keleman Trap Dipole 30/17/12 - Length Chart?

[N0JVP]

I have a Keleman 30/17/12 meter trap dipole installed in my attic. The 17 and 12 meter bands tune up just fine. However, SWR of 3:1 or lower exists between 9.536 MHz and 9.953 MHz, rather than the objective sweet spot of 10.10 MHz to 10.15 MHz, on the 30 meter band side of the antenna.

Question: about how many inches do I have to take off each side of the center fed trap dipole, at the ends, to achieve resonance at the 30 meter sweet spot? Since this is a trap antenna, using a dipole length calculator didn’t help. I also own a Diamond 8010 80/40/20/15/10 trap dipole. In the manual, Diamond provides a handy table that shows how many kHz in frequency the antenna will move up with each inch of trimming. However, Keleman‘s manual has no such table.

I own an antenna analyzer. My brute force attempt to figure it out (which requires some serious attic crawl) was unavailing. So, I need some better guidance, and an idea of likely trim length, before I attempt the next attic visit. Any ideas?

Eric
N0JVP

 

[prcguy]

30m would be the end most lengths of wire after the last traps and that is the easiest to adjust without affecting the other bands. You can still use a dipole calculator to estimate how much wire to remove to go from 9.536 to 10.10MHz and that would be cutting off about 1ft 4in each side of the dipole. I would make a couple of smaller cuts like 6" each side to see where it resonates then proceed in small steps.

 

[N0JVP]

Thanks much. All good advice. I used the dipole calculator the calculate dipole leg length based on the center frequency of the analyzer range above (basically the resonate frequency before trimming) and then used the dipole calculator to calculate the dipole leg length for the correct frequency. The latter minus the former gave the length to fold back onto the antenna (length to trim). That basically got me to exactly where I wanted to be. Not a ten minute project, but I finished it this morning. Made an FT8 contact and used the spot map on Gridtracker. Looks like everything is working perfectly. Thanks for the help.