Topband: sloper puzzler

[Bill Tippett]

SP5EWY wrote:
 >My tower is not too high,only
18m with TH6DXX and above it is 2 meters tube with A3WS on the top.

         Be careful with conclusions.  There are multiple issues at
work simultaneously.

1.  Nobody knows for sure the patterns for slopers....some seem to
work and some do not.  This means the patterns are very variable
and probably impossible to model.  Do you really know what the
take-off angles are for your sloper on 80m and 160m?

2.  Propagation can be very variable, especially for optimum
TOA's at any given time.  One day a vertical will be better and the
next a low inverted-V will be better.  I have seen this many times
having simultaneously had both in both Colorado, and now in NC for
the past year.  For TOA's of 40 degrees and above, a simple inverted-V
at 30m is better than an 3 element vertical!  Sometimes there can be
10 dB difference (either way).  Does that mean one is better than the
other?  No...only different TOA's.  In general, the vertical is
better for DX and the inverted-V is better for local, but under some
conditions the inverted-V is better by nearly 10 dB for DX.

3.  The electrical length of your shunt-fed tower on 80m could be too
long making it a high-angle radiator there versus a low-angle radiator
on 160m.  I tried shunt-feeding my bare 55m tower on 80m and it was a
terrible DX antenna.  Perhaps the sloper on 80m has a lower take-off
angle than the tower, which may be the inverse situation on 160m.

         Bottom line to me is "vive la difference".  There may be cases
when one is better than another so keep both and choose the one that
works best for particular conditions.  After awhile, I suspect you
will learn the "general rules" as to when one will be better, but
always check to make sure the propagation is not playing tricks on
you.

                                                 73,  Bill  W4ZV

P.S.  Below is an elevation plot showing TOA's for my 30m high inverted-V
(in blue) versus my 3 element vertical parasitic array (in black).  TOA's
of about 40 degrees are the "breakeven point" between these 2 antennas.

http://users.vnet.net/btippett/new_page_10.htm (top plot)