I currently have a hombrew 20m efhw wire vertical which works very well for qrp/dx,I want to put up a 30m wire vertical dipole on same limb about 4 ft. from the efhw.The dipole feedline will be 90 degrees from dipole radial insulater going away from the efhw for a run of about 10 ft. before going down back side of tree away from the antennas.Should I be concerned about any possible interaction between these wires when using either one, or since I'll be using 5 watt max.power level is this a non issue? Any input appreciated,thanks Jim
As WX7G correctly pointed out there will be interaction between these two antennas: both share the same polarization and 4' is pretty close for 20M and 30M. Practically, you'd like to have a minimum of 1/4 wave between both antennas for the lowest frequency; that's about 7.5M or ~23' for 30M.
Also there may be an issue with your current vertical as it is end fed (no ground radials, correct?) half wave. I've heard that end feed antennas don't play well with other close by antennas (such as in a Field Day situation).
You certainly could install the new vertical dipole and see what happens to the loading of your efhw and its performance (can you hear the dx; can you work the same dx as you could before).
BTW, have you considered going to a traditional dual-band trap dipole for these 2 bands? Even configured as an inverted vee and placed with apex above 33' it should be as good or better than the efhw wire vertical on 20M.
GL, 73, Rich K3VAT