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Re: [Antennaware] Traps

To: "K9AY" <>, "Dallas and Lucy" <>,<>
Subject: Re: [Antennaware] Traps
From: "Mauri" <>
Date: Wed, 7 Dec 2005 12:42:22 +0100
List-post: <>
> * 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).

I want to add some detail and considerations about the above.
Although extra losses in a dipole/vertical (of reasonable size) are more or 
less coincidentd with trap losses, traps and coils in parasitic elements of 
yagis have quite a bad effects over gain, much more than is normally 
The usual equivalence that trap/coil losses is comaparable with its heating 
is not true in parasitic elements.
A small heat can also be the sign of a small current flow, which means a low 
contribution of the element in object to the antenna gain. If the main 
problem of yagi is that parasitic elements are reactive (longer or shorter) 
and the radiated power quickly decrease if elements are made shorter or 
longer than a 3-5% compared with operating frequency, a trap/coil further 
contributes to reduce current flow, and consequently gain.
It's not an absolute, but along 30 years of tests I never found a single 
calssic tribander with traps on all elements showing any sort of real gain 
on 20m if compared to a simple full size dipole.
My conclusion is that tribander manufacturers are extremely optimistic 
individuals dealing with a no way job of pulling out gain where it's not 
possible to have.
Frequency, inductor material and size togheter dimension and shape and 
capacitor Q are factors to be modeled, the latter has to consider also 
dielectric losses.
May be it's Dallas interest to visit and dowload something from

Mauri I4JMY 

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