From: "Hans-Joachim Brandt" <hajo.brandt.dj1zb@t-online.de>
To: rsgb_lf_group@blacksheep.org
References: <65AECDF1F89AD411900400508BFC869F9C0491@pdw-mail-1.dera.gov.uk> <3AFA2E4C.EBB3C4EF@alg.demon.co.uk> <3.0.1.16.20010510132736.2c87c87a@mail.cc.kuleuven.ac.be>
Subject: Re: LF: harmonics / LF antenna
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Date: Mon, 14 May 2001 19:46:01 +0200
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Thanks, dear Rik,

I am also employing a combination of a peaked low-pass filter and a normal 
low-pass filter in the tank circuit. Furthermore I have tried to receive my 
second harmonic in the lf broadcast range of my car radio. In about 80 meter 
distance there is still a beat note to be heard, but in 200 meters absolute 
nothing. 

In Germany we also have discussed that the third harmonic would fall into the 
aeronautical beacon band, and therefore some care to the harmonic problem should 
be taken.

73 Ha-Jo, DJ1ZB


Rik Strobbe schrieb:
> 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
>