On Jan 12, 6:20*am, "Peter" wrote:
I'm proposing to build a new multi-band HF antenna. Previously I used simple
relatively low (8mtr) dipole (28mtr total length) feed with ladder line and
matched with a Z-Match tuner. Worked ok, but was useless for DX.

Shouldn't have been. Maybe not as good as a higher dipole, but
you should have been able to work some DX. I once used a 40m
dipole camping which was about 8 ft off the ground.
I tried it on 15m, and was working JA's left and right.
And 24 ft up is a good bit better than 8 ft.

My proposed antenna is to be vertical (7mtr radiator) to be used on bands
from 10mtr to 30mtr.
I have heard that vertical antennas are more susceptible to noise!

No. There is no difference as far as the antenna. It depends on the
polarity of the noise source. If it's vertical, then yes, the vertical
would pick up more noise.
If the noise source was horizontal, which is often is these days,
then the dipole would receive more noise.
Many man made noise sources are vertical, but that's
no different than having a desired signal being vertical.
Noise is RF the same as any other signal, and follows the
same rules.
Being crossed polarized will generally give you about a
20 db loss in signal. That will apply the same no matter
if it's a desired signal, or noise.
This only applies to a local space wave, which is how
most noise arrives . It gets a lot more complicated for
sky wave, and usually means a good bit less difference
overall.
So don't worry about noise until it possibly becomes an
issue. It's also quite possible for the increase in signals
to be greater than the increase in noise when working
long paths where the vertical has an advantage over the
lower dipole.
They work real well for DX and also local space wave
coverage, which is fairly popular on 10m.
But I recommend an elevated antenna when using the
higher bands. I would not use a ground mount unless
I had no other choice.
The R5 mentioned is a good one for that application.
I'm wondering how you will feed a 21 ft radiator for
all bands using a home brew design. It's not as simple
as it might seem for proper operation if elevated.
The R5 is a "1/2 wave" type design that does not require
radials. It's pretty convenient.
But if you use a trapped 1/4 wave ground plane type of
scheme for all bands, you will need at least one ground
radial for band, and preferably at least two spaced 180
degrees apart. And that's just a minimum.
Most ground planes I use have at least four radials.