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[VE4AAZ]

What is the  minium distance  away from a  20  inch self support  tower  face to  mount a  5/8 wave  ground plane  6 meter vertical  with three radials,  antenna vertical propossed  height begins  at approx.  30 feet  AGL.  What effect on  rx and tx  signal pattern will  occur from the tower proximity.
73
Bill
VE4AAZ

[RFRY]

In general, the net radiation pattern of the 6m ground plane + tower will be a cardioid with its null behind the tower.

[KM1H]

Which means from HF to UHF. Spacing can be set for a deep cardioid or two phased, one on each side of tower for a more conventional pattern, even switchable directions if desired.

Carl

[N8EKT]

The further away the better.
1 wavelength or more is desirable but difficult.
1\4 wave spacing will be directional
1\2 wave spacing will be bi-directional
And all patterns will have a deep null in the direction of the tower.

[WB6BYU]

And there really is no advantage of using a 5/8 wave vertical in that situation.  A simple dipole offset from the
tower with the top at the same height will work just as well, and likely will be simpler to mount and maintain
spacing from the tower.

The 5/8 wave antenna only has an advantage when:
1) it is mounted over an infinite, perfectly conducting ground plane, and
2) the feedpoint must be on the ground plane.

Since neither case is true on a tower, it is worthwhile to do further analysis before assuming anything about
the relative performance of the antenna.

[KL7CW]

Bill,
   I agree completely with BYU.  I always try to keep it simple.  If you build a simple half wave dipole you can predict its performance at various distances from the tower leg.  With the 5/8 wave vertical, it is unknown how the short radials will interact with the tower.  With a half wave dipole, you can simply go on the internet and view various patterns you can obtain with various distances from tower of various width's. Just scale any of these for your frequency.  A dipole is simple to build, is sturdy, and will probably have adequate bandwidth.  Just build it a few inches long, then shorten it a bit if the resonant frequency is too low.  I have been able to take advantage of a mast or tower, by choosing a distance which gives me maximum radiation (and even possibly some gain) in a favored direction.  The antenna Z may not be exactly what a free space dipole is, but probably will be close enough to use, and you can investigate matching schemes later if necessary.  I would construct one dipole, install it, and get it working.  Later you could consider additional identical dipoles on other tower legs, either to switch or feed together.  However this is not a trivial exercise...lots of calculations and matching sections.
    If you want vertical polarization for contacts with local vertically polarized stations great.  If you want to try for DX probably a horizontal dipole mounted on the tower would be just as easy, and probably better in many situations and most directions other than nulls off the dipole ends.  You could always go vertical and horizontal with one coax run, just put a remote antenna switch and choose V or H radiation with the push of a button.
            Rick   KL7CW

[KL7CW]

If you build a half wave vertical dipole, be sure to route the coax back directly to the tower, and hopefully if possible run the coax inside the tower and taped to a leg.  If the antenna is going to be a "permanent" fixture, some folks like to use a coax choke at the dipole feed point...not necessary, but sometimes helps.  If you do use the 5/8 wave vertical, be sure to continue the coax down vertically from the antenna for some distance before routing it over to the tower....just my GUESS at least 5 feet. 
              KL7CW

[RFRY]

For a general idea, below is a clip showing the measured h-pol and v-pol radiation patterns of a 2-bay, c-pol antenna used for FM broadcast applications, when side mounted on a triangular, 24" face tower section.

Vertical polarization is more affected than horizontal polarization regardless of the distance of the antenna from the tower structure, and its location along the faces/legs of the tower.