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[Towertalk] Cuscraft A3 Tribander

To: <>
Subject: [Towertalk] Cuscraft A3 Tribander
From: (Dave Hachadorian)
Date: Thu, 13 Jun 2002 19:43:06 -0000
----- Original Message -----
From: "Gary D Elliott" <>
To: "TowerTalk" <>
Sent: Friday, June 14, 2002 12:55 AM
Subject: [Towertalk] Cuscraft A3 Tribander

> Hello TT
> I am rebuilding a A3 trap tribander and have
properly identified all the
> traps TA,TB, and TC.  My question is on the TA
Trap (10 Meter) which end
> points toward the boom ? The trap has one end
that the aluminum shield is
> bend down and has a self taping screw into the
tubing.  The other end does
> not. Does it matter which end of this TA trap
faces the boom ? Appreciate
> some help. Thanks in advance.
> 73 Gary

>From the archives. The answer (contact screw
toward the boom), and a lot of other
stuff, is in there. I remember going through the
same drill with the grid dip meter:
Dave Hachadorian, K6LL

> Can anyone help in distinguishing between the
15m traps (TB vs. TC) and
> 10m traps (TD vs. TE)??  My e-mail request to
Cushcraft stated that "we do
> not offer Trap diagnostics or identification as
part of our Technical
> Support Service" and that I should use the book
"Troubleshooting Antennas
> and Feedline", MFJ part number 3301 to help
identify the trap.  Any advice
> or experience in how to do this?  Unfortunately
Cushcraft does not mark
> their traps...  nor was the technical advice
particularly insightful.
> -- Rich K1CC
Rich, the text below is the response I got from
Cushcraft when I asked the
same question of the A3.   Cushcraft states that
there is only one way to
determine which is which, and that is to use a
grid dip meter.  Actually,
the best way is to put them on a HP network
analyzer which is what I did.
The network analyzer revealed that I had chosen
the wrong traps using the
grid dip method.  The problem I had with the grid
dipper was that it was too
coarse to separate the minor differences in trap
resonance frequency.  Just
remember that the director traps will resonate at
a slightly (I mean
slightly!) higher frequency that the other ones.
Now, you have one more
element that I do, but just remember to put the
higest resonating trap up
front, and work your way back.  To check the trap
resonance, put the trap on
a non-conductive table, and place the grid dip
meter against but not
touching the trap tube.  Don't be suprised if the
traps seem to resonate at
a frequency other than what you expect them to,
My guess is that they must
interact with the rest of the antenna when it's
assembled and on the tower.
BTW, according to a former Cushcraft engineer,
they use to have a test
fixture which was used to find the correct traps
for antenna owners who were
in the same predicament as you and I.  He told me
that he suggested kapton
labels be used instead of the paper stickers, abut
was voted down due to
costs.  I wish you luck in finding the right ones
and hope you have access
to a network analyzer instead of resorting to the
grid dip meter.  I can't
tell you how many time I have seen this question
appear on the different
reflectors and news lists!  A simple plastic
sticker or engraving mark at
the factory would easily solve the problem.

Tom K3GM

Cushcraft's "official" response:

The traps of the A3 and A3S can only be separated
one way. First of all,
the 10 meter traps are the same and you can use
them in any position. Be
sure the end with the contact screw for the body
to element junction is
nearest the boom. The 15 meter traps can be
identified by the fact that
they are "swagged" to a smaller tubing size on the
outboard side. There
are 2 different types of 15 meter traps. 4 of the
traps are used in the
reflector and driver elements. There are two 15
meter traps that are used
in the director and they have a higher frequency
of resonance. That is the
only way to tell the difference. Check the traps
with a dip meter and that
should tell you which are which.

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