[TowerTalk] FW: Trap ID

Matt maflukey at gmail.com
Tue Dec 9 23:24:07 EST 2014

Mosley TA33 trap explanation:

To the best of my knowledge, this is how it works...

Inside the trap there are coils on each end.   The inner element inserts into coil form such that the aluminum trap cover forms a coaxial capacitor along their overlapping length.  This is wired in parallel with the inner coil to form a resonant trap for the 10m band.   On 15m, the 10m trap becomes a net lump-sum inductance, thereby shortening the resonant length of the 15m section.   The length of the aluminum trap cover is cut to resonant the antenna on 15m.   The outboard element tip inserts into the other end of the coil form such that the aluminum trap cover also forms a coaxial capacitor along their overlapping length.  This is also wired in parallel with the outer coal to form the 15m resonant trap.   On 20m, both the 10m trap and the 15m trap both become net lump-sum inductances which shorten the resonant length of the 20m section.   The outer element length is then selected to be resonant on the 20m band.

Stuff that can go wrong with the traps:  

1. Coil form cracks and arcs over to the inner element at high voltages (high power).  Be aware that the coil form is also a structural part of the element - it transfer bending moments from wind loadings or other forces acting on the outer element tip.
2. RF contact quality where the end of the coil connects to the outer trap cover degrades (usually from oxidation - keep it clean and apply some penetrox or similar at the connection).
3. Overheating the coil melts the coil form and physically changes the shape of the coil enough to move the resonance.
4. Coil wire open circuits at the end where it is riveted to the inner element.
5.  Insects, or their residue, become moist with humidity (or infiltrated rain) and short out sections of coil winding, changing the resonance.
6. And my favorite...   the element is mounted upside down such that the trap drains are at the top....  the trap fills up with rain water and....  

BTW, the traps for TA-series are different for each element.   They can be identified (director, driver, reflector) by counting the number of turns on the coils.

Hope this is helpful and good luck on your project.


-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Wilson
Sent: Tuesday, December 09, 2014 9:43 PM
To: towertalk
Subject: [TowerTalk] Trap ID

Someone suggested Mosley, which seems correct.
Their trap design: http://www.mosley-electronics.com/pdf/trapdesigns.pdf

A tribander with only six traps:  http://www.mosley-electronics.com/pdf/amateur/cl33m.pdf

I don’t understand the traps.  There are two coils in each, but I don’t see how that isolates sections for the bands.
The explanation is incomplete, to say the least.

So far, no detailed explanation has appeared from googling.


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