LF: <more> Dummy Loads for LF

["G3WCB" <ganymede444@btopenworld.com>]

Hi Gary and LF,

 

I've been using "Dale" 50W metal-cased resistors for a dummy load. They're wire-wound but it doesn't seem to matter, and they work just fine at LF. They've got a gold anodised metal jacket with two lugs. RS Components sell them in UK. Look at VE7TIL's "Goggle-watt" dummy load, and you'll get the picture ;-)

 

http://www.scottboxx.com/radio/lf/tx/part1/

 

The 50W models have a 1250 volt rating, and the 100W ones are rated at 1900 volts. I found it useful to make up my dummy load as a 50-ohm 6dB power attenuator rather than a terminating load. This was because I wanted to use my existing 25W powermeter with a 100W tx. It's also handy if you are doing "dodgy" antenna experiments, and don't want to expose your PA Fets to undue stress. Feed the PA to the antenna via the 6dB attenuator until you are happy about the match.

One final cautionary tale.... if you are using white ceramic-bodied high-wattage resistors salvaged from SMPSU's etc, be very careful as some of these contain a thermal fuse which opens at a certain temperature...so look carefully for labelling that contains references to temperature...eg "135'C" etc.

 

Cheers and good luck with the project.

 

Dave G3WCB.

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org]On Behalf Of Gary - G4WGT
Sent: 28 January 2005 22:55
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: G0MRF 300w Tx PA mods

Hi Markus & LF,

 

Thanks for the ideas, I had thought about using 2 x 100r 100 watt wire wound resistors one of my electronic suppliers has.

 

Regards

Gary - G4WGT

Web: http://myweb.tiscali.co.uk/wgtaylor

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org]On Behalf Of MarkusVester@aol.com
Sent: 28 January 2005 22:24
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: G0MRF 300w Tx PA mods

Gary and LF Group,

types of dummy-loads tested successfully at LF here

- a toaster (1000W @ 230V = 53 Ohms),
- a coffee maker (750W = 70.5 Ohms; needs some water, otherwise it'll turn itself off).

Before connecting them, I checked that there were no internal RF-bypass capacitors before the power switches.

73 de Markus, DF6NM

In einer eMail vom 28.01.2005 22:39:48 Westeuropäische Normalzeit schreibt g4wgt@tiscali.co.uk:

Next is to borrow a better dummy load &carry out some prolonged tests.