[Antennaware] Traps

[K9AY]

Dallas,

It's difficult to get accurate measurements of trap performance, but a few 
general points are:

* High-Q traps with inductors and fixed capacitors are the lowest loss, but 
voltage breakdown in the capacitors is a constant problem at high power.

* The next-best performance is the method used by most tribander 
manufacturers, using the capacitance between the metal cover and the coil. I 
have heard reliable estimates that these traps are roughly 0.1 dB loss per 
trap. This is not necessarily total gain loss per trap, but loss in that 
element, which may not have a big effect if the current is low (e.g. a 
director has lower current than the reflector or driven element in a Yagi).

* Coaxial cable coils plus the inner-to-outer conductor capacitance are 
convenient traps, and can have reasonable Q, but will be lower on the 
performance list than the others. The dielectric losses in the capacitor are 
greater than other types, and the braided shield of the outer conductor can 
be a reliability problem as oxidation begins to insulate the strands and it 
ceases to function as a single large conductor. At high power, the coaxial 
capacitor has had many reported voltage breakdown problems, so if KW power 
levels are anticipated, use high quality PTFE dielectric cable and protect 
the ends as much as possible, since that where the voltage is usualy 
highest, as well as where a path can develop with salt, dirt and water. The 
3 dB loss number is way too much -- the trap would melt rather quickly if 
dissipating half the power! A well-built coaxial trap is probably some 
fraction of a dB loss and well worth a try.

Someone else needs to answer the question about modeling traps, since that 
is not an area where I have done much work. My own work has emphasized 
trapless designs.

73, Gary
K9AY
___________________

> Some questions about traps:
>
>  With regard to coaxial vs. discrete component traps, some have said that 
> coaxial traps exhibit about 3 dB loss over discrete types.  Where is this 
> loss manifested?  How does it affect antenna radiation?
>
>   In the case of a trapped inverted L for 80/160, would this loss have 
> much affect on 160, or would it tend to reduce the horizontal component?
>
>  Also, how does one determine series R when modeling a trap?
>
> 73  Dallas W3PP
> _______________________________________________