Wednesday, January 31, 2018

Tuning characteristics of the Wolf River coil with 102" whip

Today I tested the new mobile antenna on the 10- to 40-meter bands with mixed results.

The antenna is a 102" whip mounted onto a Wolf River Silver Bullet Mini coil which is in turn mounted to one of the stake holes in the bed of my truck.

Bandwidth on the 12 to 20 meter bands is surprisingly wide. As expected, the 12m band requires minimal use of the coil, so the PVC collar containing the tap is all the way to the top.

The 10m position is at the same top-most tap but with the collar turned all the way counter-clockwise. Half a turn clockwise, but still on the same tap, puts the antenna onto 12m.

Clicking the collar down a few turns puts it on the other ham bands. Here are the results of measuring the 2:1 SWR bandwidth on several of the ham bands with the coil adjusted so that the center of each band is minimal SWR (typically 1:1):

  • 10m - 6.0 MHz (24.1 to 30.1 MHz)
  • 12m - 4.4 MHz (23.4 to 27.8 MHz)
  • 15m - 2.9 MHz (20.3 to 23.2 MHz)
  • 17m - 2.1 MHz (17.49 to 19.6 MHz)
  • 20m - 1.2 MHz (13.62 to 14.64 MHz)
Good numbers that easily cover all of each band.

Unfortunately, the minimal SWR achievable on 30 and 40 meters is about 2:1. I blame this on the poor "ground system" a Chevy pick-up represents at those longer wavelengths. Maybe when I'm pulling my trailer...

I believe this will be easy enough to fix with 2 or 3 wire radials laid on the ground while I operate stationary during POTA activations. I'm not the type of ham to have such a large antenna in place while mobile and didn't buy the antenna to use in that type of operation.

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