[Lf] [Fwd: LF: harmonics / LF antenna]

[Andre' Kesteloot]

Rik Strobbe wrote:

> Hello group,
>
> few days ago I got a mail from someone new to LF who asked me how efficient
> a vertical antenna would surpress the harmonic contests from the TX.
> So far I hadn't put much thought on that as my 400W TX has a 3 stage LF
> filter, keeping all harmonics at least 50dB down. But calculations seem te
> be quite simple, above the resonance frequency the antenna becomes
> inductive. The average antenna (few 100pF capacitance) will already at the
> second harmonic have an inductive component of several 1000 Ohms, keeping
> the (harmonic) current low.
> But on the other hand the radiation resistance will increase by the square
> of the frequency, making whateven harmonic current that gets through the
> coil being radiated more efficient.
> Just out of curiosity I calculated the overall efficiency of a loaded
> vertical antenna on the harmonic frequencies and came to the result that
> the harmonic supression is rather constant for all harmonics.
>
> Below you find the result for a vertical monopole with a capacitance of
> 300pF and a loss resistance of 50 Ohm :
>
> FREQUENCY               Impedance       Efficiency (versus fundamental)
> fund. (137kHz)          50 Ohm          0.0dB
> 2nd harm. (274kHz)      5811 Ohm        -29.3dB
> 3rd harm. (411kHz)      10330 Ohm       -30.7dB
> 4rd harm. (548kHz)      14526 Ohm       -31.2dB
> 6th harm. (822kHz)      22596 Ohm       -31.5dB
> 8th harm. (1096kHz)     30505 Ohm       -31.6dB
> 10th harm. (1370kHz)    38349 Ohm       -31.7dB
>
> For the calculations I assumed no stray capacitance in the coil, so in the
> 'real world' the figures might be a few dB different, probably higher (less
> negative).
>
> Further the harmonic surpresssion becomes less is the antenna capacitance
> increases (smaller coil needed) and the loss resistance increases.
> Eg : for a 'big' antenna with 600pF capacitance and 100 Ohm groundloss the
> antenna efficiency on the 2nd harmonic frequency will be only -17.2dB down,
> on the 10th harmonic -19.6dB.
>
> So sending a few 100W square wave into a big antenna is not really
> recommended.
>
> 73, Rik  ON7YD