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From: "Hans-Joachim Brandt" <hajo.brandt.dj1zb@t-online.de>
To: rsgb_lf_group@blacksheep.org
Date: 08 Nov 2003 01:32 GMT
Message-ID: <1AIHyv-25VU5x0@fwd05.sul.t-online.com>
Subject: LF: Re: Dummy loads for LF
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Dear all,

for the time being I am using a dummy load made of ten low inductivity DALE resistors Type NH-50, 499 ohms each, in parallel on a large aluminum cooler (the fins of which could even be immersed into water when necessary), which could be used up to 30 MHz with a coil/capacitor matching arrangement at the input resulting in an VSWR of about 1,2. The matching arrangement has been designed about ten years ago in my qrl by measuring R and X of the parallel resistor arrangement over the frequency range and trying to find a suitable match by employing the old Supercompact software or the ARRL Radio Designer.

Therefore I guess (I have not tried it yet, no need to do so) for such a small band like 136 kHz it should be possible to build a dummy load using ordinary wirewound resistors (preferably DALE or similars because of their easy mounting on a cooler surface) because their inductance could be cancelled by a suitable capacitance in parallel, or by several distributed capacitors within the parallel resistor arrangement. It should be rather simple to determine the capacitance needed, a VSWR meter designed for LF should do it. 

HW?


73 Ha-Jo, DJ1ZB


"James Moritz" <J.R.Moritz@herts.ac.uk> schrieb:
> 
> Whilst on the subject of dummy loads, are Metal Oxide resistors suitable
> for
> dummy loads ? because much higher ratings (5 watts) are available for
> same
> physical size as the 2 watt carbon film resistors in my home brew dummy
> load.
> 
> 73
> 
> Gary - G4WGT
> 
> 
> Dear Gary, LF Group,
> 
> The metal oxide types would be fine, as are metal film, etc. In fact,
> even some types of wirewound resistors are reasonably non-reactive at
> 136kHz. Several people have resorted to electric heating elements like
> toasters and fan heaters to make satisfactory LF dummy loads. But
> obviously, it is a safer bet to stick to known low inductance types like
> carbon, metal oxide and metal film unless you have the equipment to
> evaluate the actual RF impedance.
> 
> The Vishay RCH50 50W film resistors sold by RS components are pretty
> good value at about 2-3 pounds each, and work OK up to HF - but
> obviously you need a hefty heatsink as well as the resistors. 
> 
> Cheers, Jim Moritz
> 73 de M0BMU
> 
> 
>