Birmingham - - - G3UPA - - - IO92dm

Trap construction

Here are some photos of the 40 metre traps that I used in my multiband trapped dipole.
The aerial was at an average height of about 35 ft. The feeder used was 35 feet of RG58 and 20 metres of RG8 to the shack.
The length of the 80 metre ends need to be trimmed to suit individual locations.
You can leave them hanging vertically down from the ends if you are pushed for space.

Click on an image to enlarge. Double click to restore.

40m Trap 40m Traps SWR 2 band layout

Always start with the 80mtr end sections about 5% longer than the length shown i.e about 21ft. and trim to desired frequency.

A simple method of estimating the amount to trim off the end/s of any wire antenna is to multiply the measured resonant frequency,
i.e. minimum SWR, by the actual physical length of the wire and divide the result by the required resonant frequency.
You can use MHz or KHz and feet or metres, it still works, as long as you use the same for both calculations.
For example, my own antenna shows minimum swr at 3.55MHz and the total physical length is 105ft.
To move the resonance to 3.60MHz, the calculation would be 3.55(MHz) times 105(ft) = 372.75 divided by 3.60(MHz) = 103.54,
so trim (105-103.542)=1.46' off the total length or about 8 3/4" off each end. (1.46/2=0.73ft = 8.76in)

I have tried the two band system as a W3DZZ with 75ohm satellite co-ax feeder but I wasn't at all impressed with the HF band operation,
hence the proposed addition of more traps. The W3DZZ layout worked fine on 80/40 but having spent hours calculating and plotting VSWRs,
I get the feeling that it would perform better on the higher bands if the trap frequency was lowered (dramatically to say 6MHz).
Watch this space for results when I have made a pair of traps and run it on-air. (on hold at present while working on new inverted L)