Francesco L. wrote:

I was and still I'm conscious of this but my question was different
and maybe not so well expressed, so excuse me and read this please:
my intention is to use the two antennas also to radiate in the 7 mhz
band (spacing in this case would be 1/8 and the radiation lobe almost
omnidirectional) and 28 mhz (spacing would be 1/2 wavelength and the
radiation pattern should be end-fire). This is because the system
would be designed for the 20 meters band with 1/4 wave spacing between
the two elements, but as you know, that spacing corresponds to 1/8 and
1/2 respectively in the 40 and 10 meters band. I know the sensitivity
should not be the best but using an antenna tuner I think the
radiation patterns should be as predicted before, what do you think?

It's easy to get the correct phasing with two identical elements which
are in phase or 180 degrees out of phase -- you simply feed them with
equal lengths of coax (in phase) or lengths which differ by 1/2
wavelength (out of phase). Alternatively, you can use a reversing
transformer for the out of phase case. You can also use other odd
multiples of 1/2 wavelength although the beam and SWR bandwidth gets
narrower as you use longer lengths. So as long as you create a feed
system which does this on every band, you'll be fine.

The problem is in making the 90 degree phased array on 20, which I
recall you wanted to do. (If not, disregard the rest.) The feed system
for that pattern will work only on the one band, so you'll have to
switch it out on the other bands. The L network feed systems described
in the _Antenna Book_ use equal length lines plus an L network for doing
90 degree feeds. So you could use the L network on 20, and switch it out
but use the same feedlines for the other bands to get two in-phase
elements on those other bands. You would need a tuner, though, since the
impedance at the common feed point will be different on each band.

Roy Lewallen, W7EL