> * The next-best performance is the method used by most tribander
> manufacturers, using the capacitance between the metal cover and the coil.
> 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
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
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