Monday, September 21, 2020

80/40/20m Trap Dipole

80/40/20m dipole strung from tower.
I've had good experience with trap antennas. I built an 80/40m trap dipole a decade ago, and it served me well at four different locations. That is, until it met an untimely demise when a rope was cut and the mowers encountered the antenna on the ground.

Wanting to build a three-band dipole, I did some experiments with computer modeling. I found that there was a lot of interaction between the values used for the traps and the wire lengths used. I fussed around with a model for a while. Eventually, I decide it would be simpler to just build one.

I started with the traps. They are made from short sections of 3" schedule 20 PVC. (Having an outer diameter of about 3.25") The exactly frequency of resonance doesn't matter terribly much, just so the traps are identical.

For the 40m traps, I used 9.75 turns of #12 wire with a 56 pF 6 kV disc ceramic cap. This resonates at about 6.88 MHz, just below the band. I started out with 10 turns, but they resonated around 6.5 MHz, which was too low.

On 20m, 6 turns of #12 wire with a 27 pF 6 kV disc ceramic cap. Resonating at about 13.84 MHz, right below the band.

Early version of traps, showing construction.

I then put it together to form a dipole with the following segment lengths on each side:

20m: 17 feet
40m: 17 feet
80m: 20 feet

This made the whole antenna nearly108 feet long. The ends of the 80m segment are looped back on themselves by about a foot and twisted to anchor to the end insulator. I strung it between the tower and some trees in the back yard. It's roughly 10m high. 

The dipole is fed with good quality RG-8X using 16 Fair-Rite #43 1/2" ferrite beads as a current balun. 

Then began the tedious process of trimming to resonance. I started with the 20m segments, then worked out to 40m and finally to 80m. Patience is warranted here, because it is too easy to cut too much off.

I ended up with these segment dimensions, resonance points and SWR:

20m: 16' 4" - 14.125 - 1.45:1
40m: 10' 4 - 7.165 - 1.2: 1
80m: 21' 8" - 3.76 - 1.1:1

And, yes, I added 20" to the end of each 80m segment. The resulting antenna is about 96 feet long. It covers all of 20m and much of 40m with a 2:1 or better SWR. 80m, by comparison is very narrow. I didn't have much of a chance to use the antenna, but it appears to hear very well. 

I've got a second one I've cobbled together for the Fulton County QTH. I'll need to wait until the tree lose their leaves so I can get it up to operating height for final trim.