- Carmile, LeBlanc wrote on the 9th July, Andrew, I was intregued by the way you calculated the impedance. What formula did you use. I would like toMessage 1 of 5 , Jul 9, 2013View SourceCarmile, LeBlanc wrote on the 9th July,"Andrew, I was intregued by the way you calculated the impedance. What formula did you use. I would like to recalculate my new design at various frequencies. I was also planning on rolling my own impedance transformer on a 100mm ferrite rod but from what I've read it is recommended to isolate both windings and not tap a single winding. (perhaps I misunderstood what you were telling me). I presume the smaller circumference would raise the feedpoint impedance from the 220 ohms you quoted me."
Carmile, generally speaking, 1m of thin wire has a measured inductance of 1.7uH. The Impedance is 2Pi F x L where 2Pi is 6.28, F MHz and L uH. Hence at 1MHz a 1uH inductor has a reactance of 6.28 Ohms. I would recommend that the transformer should be constructed using 10 turns turns trifilar 36 SWG (0.2mm) enamelled copper wound an a Fair-Rite twin hole core 2873000302. The antenna side is 2 x 10 turns in series, so you will have a centre tap. The receiver side is the other winding. Note; type 73 ferrite is conductive so you will need use 3mm nylon insulation sleeving in the core. Alternatively you could use Fair-Rite 28430000302 core, this has a much lower AL value ( 2uH per turn based on 10 turns ) but it is non conductive. If you do end up using a Ferrite rod, then use isolated windings close to each other. Also the ferrite rod needs to housed in screened enclosure. It must be large enough so that the rod is several diameters from the metal work.
A smaller loop circumfrence will reduce the impedance.
Don mentioned the Wellbrook ALA100, this has been replaced by the ALA100LN. It has 8 Fets configured using Gate Source feedback, thus dynamically reducing the Fet Channel resistance and noise figure to a fraction of a dB with low IMD and high gain.