> Coax stubs aren't necessarily lower loss than a lumped L
or C, right at the
> base.
That's actually an understatement!
Coaxial cables used to tune elements are significantly
poorer than almost any inductor or capacitor a person could
come up with from the junk box. Even coils wound with
doorbell wire would likely be lower loss.
Consider 1/8th wave of RG-8X coax on 7MHz.....
The typical impedance of an open stub is .42 -50j, or a Q of
only 119. A typical cheap air variable has a Q in the lower
thousands.
Worse yet consider a reflector. The typical impedance of a
shorted stub is 1.9 50J, or a Q of only 26. That's two ohms
of loss in series with what could be a very low resistance
(high current) element. The 2 ohms also assumes a perfect
short at the far end. A typical air-core inductor of #16
tinned copper would have a Q of 250 or more. A reflector
normally has very low resistance, the additional 2 ohms
could be a substantial portion of element resistance.
Coaxial cables (and even open wire lines) make very poor
inductors. They make moderately poor capacitors at best. In
a parasitic array losses caused by using longer lengths of
feedline (even open wire) to tune the element or using the
feedline to convey power to a remote lumped component can be
much higher than one might expect.
F/B ratio can be excellent but you might wind up with almost
the same gain as a omni antenna if you aren't careful. F/B
ratio does not mean you have gain.
73 Tom
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