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

[Andre Kesteloot]

James Moritz wrote:

> At 13:49 04/11/2002 +0100, you wrote:
>
> >Wonder why seems to be a habit use a series coil to tune out the
> >capacitance of the 136kHz aerial, instead of more normal sort of matching
> >- using T- or L-network
>
> Dear Jan-Martin, LF Group,
>
> I would agree with everything Andy says - but there are also practical
> difficulties in making the T-network scheme work:
>
> The components in the tuning network have to be highly rated - for example,
> a 500W TX driving a 50R load has an output current of about 3.2A. The 142p
> input capacitor has a reactance of about -j8.2kohms, so with 3.2 amps
> flowing in it, the voltage across it's terminals will be 26kV! The
> capacitor plates would probably need at least 50mm spacing... The inductor
> will have to withstand this voltage, as well as a higher current - about
> 8.3A instead of 5.1A, and since it also has to have a higher Q and about 3
> times the inductance of the simple series coil, it will be a much more
> massive component. The output 295pF capacitor will have around 20kV across
> it. The capacitive reactance of the antenna can be fairly accurately
> predicted, but the loss resistance is more unpredictable, so at least 2 of
> the 3 components must be variable to be sure of getting a reasonable match.
>
> So as well as improving efficiency, the series coil is much easier to build
> and adjust - The resistive component can be matched using a small ferrite
> cored auto-transformer, with several taps to allow a wide range of antenna
> resistance - an ETD49 core is more than adequate up to 1kW or more, and all
> the tuning is done by small variations to the series coil of about 1.3mH.
> The maximum voltage (now the antenna voltage) will be under 6kV with 500W
> TX, so much more manageable.
>
> 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!
>
> PS...Once you have got your TX antenna working, the PS3 in conjunction with
> a calibrated antenna, is the perfect tool for measuring field strength,
> etc. It is also extremely useful in conjunction with a bridge for impedance
> measurements. The selectivity is more than adequate with about 100-200Hz BW
> for use as an RX, which it will do fairly well when the sensitivity is
> increased with a suitable preselector. I would not be without mine. But the
> 1.5kHz "IF" output is really a bit uncomfortable to listen to, so
> down-conversion of some sort would be a good idea.
>
> Cheers, Jim Moritz
> 73 de M0BMU