Hello Roger,
in the early days of 136kHz I did use a "ferrite rod variometer" for fine tuning the antenna.
It was a 700uH coil (abt. 100 winding of 1mm CuL on a 10cm diameter former if I remind well), with a ferritte rod that could be inserted at the center of the coil. Inserting the rod completely increased the inductance to abt.
1mH.
I did test my complete supply of ferrite rods (abt. 50) and with almost all of them inserting the rod caused a nefast loss and the rods got very hot with 100W RF. But a couple of rods got only lukewarm and the losses were neglectable
compared to ground loss.
Interesting was that these low loss rods also introduced the smallest variation of L (abt. 50% increase) while with some of the lossy rods I could increase the L up to 2.5mH.
I did use this variometer for a while and it worked fine, but after increasing power to 450W even the low loww rods got too hot and I had to use a traditional variometer.
73, Rik ON7YD
Hi Jim (et al)
Ferrite rods as 5-20W TX loading coils?
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"?
>From personal experience with 5W this worked well at 500kHz so I assume that the idea could be translated to 136kHz if using separate rods for each 500uH of inductance so the cores of each do not saturate. Am I right in thinking that if you bundle x cores together
(in parallel) the core will saturate at x times the power? Making a ferrite rod based variometer would be straightforward - PVC tube with cores sliding together lengthwise for example.
Engineering large air-spaced loading coils is quite a feat whereas making up, for example, 8-10 separate ferrite coils with a range of taps on each is quite easy (and small). Less wire would be needed so the losses in the coils would be lower compared with
the air-spaced equivalent.
Is there mileage in this, say up to 15-20W RF?
73s
Roger G3XBM
On 16 August 2011 11:53, James Moritz
<[email protected]> wrote:
Dear Tom, LF Group,
what do you think about an array of many parallel mounted ferrite rods, each of them carrying only a few windigs, all windings connected in series (and then perhaps tuned) and the rods arraged in such a way that the individual apertures dont touch? Or will
this lead to the dimensions of a comparable air loop ;-) ?
I am sure this would work, but I think you have also identified the limitation ;-) Fundamentally, if the signal has a particular power density at the receive site, the antenna must intercept the signal from a certain aperture area in order to deliver a certain
power to the receiver. So there is a limit to how small it can practically be, although the actual shape can vary to obtain the same aperture - one could make a rough comparison between the short, wide loop vs. the long, thin ferrite rod, and a long yagi vs.
a broadside array of dipoles.
I think an array of ferrite rods might be attractive in some circumstances - for instance, you could have numerous small rods stacked vertically, to produce a "ferrite rod vertical" with a small turning circle but a relatively large effective area.
Cheers, Jim Moritz
73 de M0BMU
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