Re: LF: Re: Ferrite Loops

["James Moritz" <james.moritz@btopenworld.com>]

Dear Roger, LF Group,

> As long as the ferrite doesn't saturate am I right in thinking that the 
> use
> of 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 Moritz
73 de M0BMU