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| To: | <[email protected]> |
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| Subject: | Re: LF: Re: Ferrite Loops |
| From: | "James Moritz" <[email protected]> |
| Date: | Tue, 16 Aug 2011 15:45:15 +0100 |
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Dear Roger, LF Group, As long as the ferrite doesn't saturate am I right in thinking that the useof ferrite rods as coil formers for 137 and 500kHz is basically "a good idea"? I think it would be practically quite difficult to get such a ferrite rod to saturate, but that isn't the limiting factor. At 136k and 500k, the core would be drastically overheated long before the flux reached saturation levels. This is due to a combination of losses including hysteresis, eddy currents, dielectric loss, winding losses. Ferrite manufacturers' data sheets give graphs of core loss vs. flux and frequency, but calculating the resulting temperature rise would need to be done on a case-by-case basis and is quite complicated. If you have some ferrite rods and a reasonably robust TX, trial and error is probably the easiest way to find out ;-) I'm sure this can be done, but a much simpler and more flexible approach is an air-cored solenoid coil. For example, for 136k some hundreds of turns of 0.5 - 1mm enamelled wire on a 100mm dia. plastic tube is quite compact, and will usually have relatively low losses compared to the rest of the antenna system. For 500k, the coil will need much less wire. This sort of coil is adequate up to a couple of hundred watts, and can withstand antenna voltages of several kV, which is often an issue at 136k. Using a ferrite rod as a "tuning slug" to obtain 5 - 10% inductance variation with such a coil works well. An early 136k loading coil I used was mounted horizontally, and fine tuning was achieved by having an old SMPS "E" core attached to a plastic ruler, that could be slid in and out, and just left resting in position when the correct inductance was reached. The ruler provided a convenient tuning scale... Cheers, Jim Moritz73 de M0BMU |
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