Topband: Antennas

[Dinsterdog@aol.com]

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