A 32 foot vertical will need a pretty large top hat to be resonate on 80m
and I don't think that your tubing is of adequate size to support it. If
you come down 8 feet as you suggested you will need an even larger top hat
as I don't think having your vertical go above the top hat does much. You
can easily model this to come up with the top hat sizes. A vertical with a
top hat will have a lower impedance than a full sized one so you'll likely
need some type of matching system. Another effective method of top
loading is to run a wire (30 - 40' long) from the top of your vertical to a
tree or back down to the ground on an angle.
To: TowerTalk Reflector
Subject: [TowerTalk] 40 meter vertical to 80 meters.
From: Lee Buller <firstname.lastname@example.org>
Date: Sat, 17 Mar 2007 10:18:28 -0700 (PDT)
Ladies and Gents,,,
Last year, I build a homebrew 1/4 wave verticle and mounted it on the
with 16 radials underneath it. I am in the process of adding 16 more
The antenna is made out of aluminum tubing starting with and 1" 1/4 and
down to 3/4". The length is 32 feet and with 16 radials under it...it
resonates at 7.100...at least it is flat there...and the MFJ 259 says it has
imdedance of 55 ohms...R=53-55, X=3-6, Theta is 10 degress, Works good too.
What I want to figure out is how to had a "top-hat" to the antenna to
it down to 80 meters. I know that the MFJ brand of verticals uses a top hat
loading scheme and I wonder how you calculate or figure out the capacitance
would need to build something like this? Or, is it trial and error?
In addition, the top of the antenna is fairly small tubing (I am afraid I
will bend it with to much weight or load) and I need to come down on the
antenna about 8 feet to install the top hat. I guess one would have to use
trial and error to find the right combination of "spokes and Lenghts" to
the capacitance hat. Looking at the Handbook, it does not really have a
"formula" for such things.
Anyone have any insite? Give me a starting point?
Lee - K0WA
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