Sorry again about using bandwidth, but the "bouncers" don't want to play
fair!! So I am trying to post again, using a message that I had sent
previously that might have been "lost".
73,
Geoff
GM4ESD
----------------------------------------------------------------------
Vertical full sized dipoles have worked very well for me while "DXing" on
40m, both as an omnidirectional antenna or two dipoles phased to produce
a unidirectional pattern that can be reversed. At this QTH the ground is
usually damp with a number of underground springs around, one being below
one of my 40m verticals. The bottrom ends of the dipoles are about 10ft
above ground level, although I have had had them 10ft or so higher but
could not over a period of time detect any real improvement in
performance when the dipoles were higher. But that is at this QTH and at
another the increased height might make for an improvement in performance
provided that the vertical pattern does not develop secondary lobes at
high take off angles.
Although the vertical dipoles are centre fed, I do not use separate coax
feeders that would have to come away at right angles to the dipoles and
be well supported. To avoid this complication the outside of the feeder's
braid forms the lower half of the dipole up to its centre, where the
feeder's centre conductor is connected to the top half of the dipole.
Returning to the bottom of the dipole, something has to be added so that
the outside of the feeder's braid knows when to stop being part of the
dipole. So far I have used a simple tuned trap, winding a few spaced
turns of the RG-59 feeder onto a former with the tuning capacitor
connected to the outside of the feeder's braid at each end of the coil.
When the position of the "coil" is right and the capacitor's value is
known (I use a small air variable during set-up), I button the thing up
using a piece of RG-213 as the capacitor wired so that it is not acting
as a stub. I have no reason to believe that this method of feed affects
the vertical pattern, and as a bonus it is possible to "fine tune" the
antenna.
The main problem that I have had here with verticals on 40m is the effect
of very tall trees, such as 80ft Beech or Chestnut, although tall (120ft)
Scotch Pines do not appear to have much affect. In one experiment with a
single 40m vertical dipole I found that it had to be about 40ft away from
a Beech's trunk before its drive point impedance "settled". I had also
found that the verticals performed much better on long path into JA / ZL
when firing across open fields than on the short path, firing through a
belt of large trees nearby. So I put up a 40m horizontal dipole a half
wavelength up with its lobes favouring the same directions to make some
real time comparisons, instant switching, and have found after several
months that across the fields the verticals are "better" but through the
trees the horizontal has the edge. The very real problem with my
horizontal dipole is that it also hears high angle signals well (read QRM
and noise) especially off its ends, whereas the verticals do not.
The Half-Square I found to be a good performer on 40m, which I fed at a
corner with the outside of the feeder forming one leg. I took it down
only because it was using up a lot of space that I needed for other
things. Strangely on the higher bands the trees here appear to have less
effect on the performance of verticals.
73,
Geoff
GM4ESD
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