Wednesday, January 20, 2021

Rebuilding the WARC Trap Inverted-V

Inverted V just below beam.

Fifteen years ago, I put up a trap Inverted-V antenna for 30/17m roughly the 12.5m level of the tower -- right above the rotator where the tower necks down to a single tube. A couple of years latter I added 12m traps to have  coverage of all the WARC bands. 

It was my first experiences building trap antennas. Naturally, I did everything wrong.

I originally created the traps using series-wound coax cable traps. They seemed easy enough to make, and pretty weather-proof. I also made the traps resonant in the band.

This article by Tom Rauch, W8JI showed me the error of my ways. 

Coaxial cable traps actually have a rather low Q. And any trap that is resonant in the band in use is going to induce more loss. Tom's advice is to make traps with discrete components, and put the trap resonance outside the band. 

Old coaxial Traps. The 17m traps are only 5
years old, but they are badly weathered.
From the modeling work I've done, a trap antenna works with the trap resonance just about anywhere between the two bands. I tend to push the resonant frequency close to, but just below the frequency - usually by 3-5%. They key is to make pairs of traps identical, so their resonant frequencies are  the same. 

I used a rather small value for the capacitor in the trap. The actual value isn't critical, anything will work. A smaller capacitor tends to couple less energy out past the trap above resonance. You'll always get some current in the entire antenna (unless you operate at trap resonance - but then you'll get the most trap loss, too). 

A smaller capacitor means a larger coil, and on the lower frequencies, this inductive loading shortens the antenna a bit. Pick a coil diameter so that the length / diameter ratio is somewhere between 1/2 and 1, as this generally results in higher Q. 

New traps, 17m on left, 12m on right.
The replacement traps are constructed on 1 1/2" schedule 20 PVC, which is roughly 1 5/8" OD as follows:
  • 12m - 6 turns 14 gauge THHN and a 22 pF 6 kV capacitor, resonating about 24.4 MHz
  • 17m - 9 turns 14 gauge THHN and a 27 pF 6 kV capacitor, resonating about 17 MHz
I used the existing length of 14 gauge THHN wire for the 12m segment, which, unfortunately, I did not measure. I'd start with 112" each side. the 17m segment is 18 1/2" of 12 gauge THHN each side, and the 30m segment is 93" 12 gauge THHN each side. The 30m segment is doubled back about 8" each side, twisted to connect to the end insulators. The total length of the antenna is about 37 feet.

I trimmed the Inverted-V by running it up the tower to about the 35 foot level using a rope and pulley. I learned this technique from the last time I repaired this antenna. This round, trimming only took four attempts. 

The antenna SWR curves fit perfectly on 30m and 12m, where the SWR is less than 1.5 and 1.2 across the entire band, respectively. 17m the SWR is closer to 2:1, with the lowest SWR at 18.010 at 1.58:1. In retrospect, perhaps I should have trimmed another 1/2" off the 17m segment. 

The apex of the Inverted-V is about 12.5 m high on the tower, about 8 feet below the A3S/A743. I used my RigExpert AA-55 to measure the SWR curves of the A3S/A743 before and after installing the WARC Inverted-V. There was no appreciable change in these curves on any band, which indicates there is little interaction between these antennas.