[Lf] [Fwd: LF: VS: Feeding an inverted L-antenna]

[Andre Kesteloot]

Vernall wrote:

> Jan-Martin, Jim and others,
>
> I also agree with comments that a series inductor (the traditional "loading
> coil") is the best approach for compensating capacitive reactance in amateur
> top-loaded verticals, and then either an autotransformer or L match to
> convert the R value to being reasonably close to the desired load
> resistance.
>
> > All this begs the question "why do people use T networks at HF?" The
> answer
> > is, I suppose, that at HF the degree of mismatch is usually smaller,
> > requiring a much lower loaded-Q network which is more practical to
> realise.
> > Also, being able to twiddle all 3 component values gives you a reasonable
> > chance of getting a good match, without having the faintest idea what the
> > antenna impedance actually is!
>
> At HF the antennas can present the full range of capacitive to inductive
> reactance, in combination with high to low resistance.  This includes the
> transformer action of transmission lines that can "invert impedances" every
> quarter of a wavelength, so a multiband antenna and feeder can actually have
> a wide range of impedance presented at the shack end of a feeder.  A high
> pass T tuner can address such a wide range of incidental impedances, in an
> effective manner.  At LF, a top-loaded vertical has capacitance and low to
> medium series resistance, and transmission lines are of negligible length,
> the net result being that impedance is fairly restricted in limits, so the
> versalitity of a T network is not needed.  Jim's calculations show why it
> should be avoided.
>
> 73, Bob ZL2CA