Extra dipole on the same balun

For some time I've been switching between a 40/80m trap dipole and a 20m dipole but it's a nuisance taking one down to put the other up.

After talking with Sam VK2BVS and Binu, VK2BNG I thought I'd try simply connecting both antennas to the same balun.

First contact today on 20m PSK31 got an excellent report from Adrian, VK4RV so clearly 20m works fine.

I can see that the 40m antenna has been somewhat de-tuned and now resonates below the band and is SWR 2 where I need to use it. 80m seems relatively unaffected.

This nifty graph was drawn with gnuplot and the following commands:

set terminal png
set title "20m and 40/80 dipoles"
set output "out.png"
set xlabel "Frequency MHz"
set ylabel "SWR"
plot "swr.dat" using 1:2 title 'SWR' with linespoints

Binu also pointed me to an interesting article about Off-centre-fed (OCF) dipoles by Ron Bertrand, VK2DQ that looks worth a go too.