Hello, Jo!!
Good to hear from Indonesia!!
Here you are!
73,
Charlie, K4OTV
-----Original Message-----
From: Charlie Cunningham [mailto:charlie-cunningham@nc.rr.com]
Sent: Sunday, September 08, 2013 2:46 PM
To: charlie-cunningham@nc.rr.com
Cc: Tom W8JI (w8ji@w8ji.com); Mike Armstrong (armstrmj@aol.com); Mike Waters
(mikewate@gmail.com); James Rodenkirch (rodenkirch_llc@msn.com); Carl
(km1h@jeremy.mv.com); ZR (zr@jeremy.mv.com); Lennart M
(lennart.michaelsson@telia.com); Bruce (k1fz@myfairpoint.net); Guy Olinger
K2AV (olinger@bellsouth.net); jim@audiosystemsgroup.com; Shoppa, Tim
(tshoppa@wmata.com); mike l dormann (w7dra@juno.com)
Subject: Verticals - 1/4. 12 and 5/8 wave - Hope this works
All, (Hope this works!)
There has been a lot of "cussin', and discussin' " on this reflector
regarding 5/8 wave and shorter vertical and perceived merits. There have
also been perceived insults, irritation and hurt feelings!
I spent a little time last night and some this morning modeling some of
these cases with EZNEC. I chose 40m, because I had an array stored, that I
could pull out the driver and use it as a quick model, without doing a lot
of construction with EZNEC. In the attached Word document there are
figures showing the antennas and their current distributions along with
elevation plots and data. See above.
All of the radiators were modeled above 4 elevated resonant radials at 10'
above real high-accuracy ground.
Note the following:
. The gain of the 1/2 wave vertical increases approximately 0.3 dB over the
1/4 wave ground plane, and its takeoff angle is reduced about 5 degrees,
from 22 degrees to 17 degrees. (The angles are, of course, height
dependent.)
. Neither the 1/4 wave or half wave verticals have any noticeable high-angle
radiation.
. The 5/8 wave radiator brings the main lobe down another 3 degrees, from 17
degrees to 14 degrees elevation and the main-lobe "gain" is increased
another 0.2 dB., but, in this case, there is a high-angle lobe - that peaks
above 50 degrees elevation, and is only 2 dB below the main low angle lobe.
It is this last point that has led, I expect, to most all of the differing
perceptions of the merits ( or lack of) of 5/8 wave verticals!
When we consider HF propagation via the ionosphere, the high angle lobe can
produce a reflected sky wave that can produce either destructive or
constructive/reinforcing interference with the lower angle main lobe signal!
( If we were discussing VHF/UHF we would call this "multi-path"
interference!)
So the result is that at varying distances from the transmitter we can can
have concentric bands or areas of either constructive, or destructive
interference.
All of this is further complicated by different atmospheric ionization
conditions in different directions from the transmitter and time of day (
where the "terminator" or grey-line is) etc.
So the perception of whether a 5/8 radiator may be superior or inferior to a
shorter radiator, can be very dependent on distance frequency, time of day
etc.
If one has a regular schedule with a fixed end point, or endpoints, that
happen to lie within a region where there tends to be constructive sky-wave
reinforcing interference at that frequency and time-of-day. But this is
unlikely to be true on all paths, I expect.
I hope this helps to shed some light, but this is all the time that I can
put into it at this time. My apology!
Best regards to all!
Charlie, K4OTV
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Topband Reflector
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