TopBand: Testing beverage xfmr cores for 160

[John Rogers]

With all this talk about various types of cores and their merits, you
may be interested in an easy way I have found of checking on the
effectiveness of cores that does not require test equipment.
Two identical trifilar wound matching xfmrs with thin insulated wire
(almost anything works 0.5mm upwards, the 3 wires loosely twisted
together) are connected together top and bottom, ie across the high Z
ends. The bottom low Z winding on one is connected to a 50 ohm dummy
load and the other low Z winding (bottom/outside to earth) is connected
to the transmitter via a SWR meter. If the transformers are working
properly with low loss they can pass at least 50w with SWR of around 1.2
to 1. You soon know if they are inefficient by the smoke (and high VSWR)
HI. I have used 0.5mm plastic coated telephone wire - not ideal, but a
5/8in ferrite core of unknown origin passed 100w and was only barely
warm!
The number of turns can be adjusted to the core mix, normally between 6
and 20 (ie total 18 - 60 for trifilar) are needed. Minimum turns for
best HF response. 
You can rewind with separate windings for isolating earth currents - a
lot has been written about this on the reflector before.
Obviously the method is only approximate, and needs caution when
increasing power on test, but is a way of sorting out old cores with a
good chance of success.
73's
John
G3PQA


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