I have four real world observations about using top loaded vertical antennas
I'd like share regarding W7IUV's questions These are based upon my 4
rebuilds of my 80 meter 4SQR vertical array made out of aluminum tubing
ranging from 2 1/8 inches tappered down to 1 inch diameter in which I think
the same principles would apply towards top band antennas on a bigger scale.
#1 First, the longer the actual vertical, and the shorter the top hats, the
broader bandwidth the antenna will have. For example, my 44 foot verticals
using 20 foot top hats had around 200KHz of usable bandwidth in the array
while my 52 foot verticals with 14 foot top hats had over 400KHz of usable
bandwidth. Seven feet, or just another 11% of the 1/4 wave being in the
vertical and not in the top hat, made a huge difference.
#2 Your minimum SWR reading will be lower using a longer vertical with
shorter top hats vs a shorter vertical with longer top hats. Again, using
the same antennas for comparison, the shorter vertical had a minimum SWR
reading of 2.1:1 vs 1.7:1 of the longer antenna. (MFJ 259 analyzer used for
measurements)
#3 Using a small coil at the base of a top loaded antenna can do some
amazing things to help lower the resonant point without much in the way of
adverse effects. I found this out in my situation by adding a small 14AWG
copper coil with four turns spaced about 1/3 of an inch apart to the base of
my top loaded vertical to bring down the resonant point around 100KHz.
Minimum SWR measurements were the same as was bandwidth with the coil in
place or out. I had a chance to bring the top hats more horizontal to drop
the resonant point but found this to be a time consuming effort requiring a
lot more real estate. The coils were super simple and super effective for
broader and/or finer tuning vs messing with the top hats. Set them at 45
degrees and forget about it- -
#4 Top hats on verticals must be pulled out as tight as possible. If not,
they will blow around in the wind, or sag a lot in with ice etc and your
resonant point will blow and sag with them. They should act as a set of guys
and materials used need to account for this. Structurally, I found that
having a few feet of vertical element above where the top hats are mounted on
the vertical is better then having the top hats attached right at the top of
the vertical. Not much radiation comes off the portion of the antenna above
the top hat but having a few feet above the attachment area seems to reduce
the amount of windloading on the antenna. I use a radio shack guy ring
mounting kit for tying the top hats too then I have a few inches of wire that
does not have tension on it to attach to the vertical. (BTW, I found that by
adding 1 foot of top hat wire length at about a 45 degree angle lowers the
resonant point around 50KHZ on 80 meters... I assume that you would need to
add around four feet of top hat length for every 100KHz of 160 meter
frequency you want the top hats to account for past your vertical's length
....just a guess-)
My advice in making an electrically sound short top loaded vertical is to
make the vertical length as long as possible, keep the length of top hats
for loading to a minimum, and use a coil at the feed point for fine tuning
the antenna. From a weather stand point, keep the 160 meter vertical as
short as possible, tighten the heck out of your top hat wires to help guy the
vertical, avoid guy ropes like parachute cord that loves water, (bailing
twine will work a lot better) and pray you don't get a lot of ice this year-
73 Paul N0AH
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