Low capacitance matching transformer for a 10m circumference loop.
According to findings of G0CWT, on 40m and 60m my loop is broad band matched by a low capacitance transformer. In my case, with 100W at 5.4MHz, SWR is still 1:1.0.
Between 4.8MHz and 8.9MHz, the tuned SWR is better than 1:1.5.
Note: this can be influenced by the length of the coax feed line.
IMPORTANT (1): minimal transformer coupling capacitance.
This transformer is inserted between one side of the loop, and one side of the split stator tuning capacitor. A to large stray capacitance between the transformers pri. and sec. windings causes unwanted capacitive coupling between transformers primary winding and one side of the loop.
Several transformers were tested, showing different capacitances and heath generation in the secondary winding.
To big coupling capacitance results in :
- Capacitive unbalance
- a NOT pure magnetic behavior :
more chance for BCI/TVI
more chance for reception of E-field related noises
- lower loop-Q, hence less selectivity
- a noticeable difference between the "min-SWR" frequency and the "max.-field-strength" frequency, causing lower efficiency.
Winding the 9:6 40m transformer.
One time through the hole of the ring core = one complete turn.
- Lacquered copper wire
- has a thin, heath resisting insulation
- has larger copper area than that of plastic insulated wire
of the same outer diameter.
- develops less heath and is better cooled by air
- is solder able with a HOT iron (435C).
- The used type 61 (Ui=125)
ring core is chosen for :
low core losses
flat frequency characteristic
flat temperature characteristic
high curie temperature
* First wind the whole circumference of the core with a 1mm thick layer of Teflon (R) tape.
* Wind 2mm lacquered copper wire loosely around the core to prevent damaging of the Teflon (R) insulation, and to improve air cooling
* Adjust for at least 2mm free spaces between the primary and secondary windings for low coupling capacitance and high voltage insulation.
Although a 36mm ring core could be used, on a 61mm ring core gives more space for the windings, enabling better cooling, better insulation and lower stray capacitance.
* The secondary winding goes 6 times through the hole of the FT240-61 (61mm) or FT185-61B (36mm) ring
* The primary winding goes 9 times through the hole of the core.
* Connect the split-stator-rotor (the neutral middle of the tuning capacitor) to the coax screening (mass).
* Connect a 5pF/4kV capacitor over the opposite splitstator-half (5cm screening of RG214 coax, see photo at the right side of the split stator tuning capacitor).
A hi-U ring core (Ui= 2.500 - 10.000) with a few turns feeder coax through it blocks mantle currents and noises.
- RF-detection (TVI+BCI) is completely gone in my LEDtv and plasma TV and audio amplifier with separated speakers directly under the loop.
- Both frequencies for min.SWR and max. field strength were now nicely coinciding.
Static charges on the not-grounded loop can cause flash-over in the split stator capacitor or the transformer. Charges are routed to mass using a 2M2 resistor 2.5kV/2.5W, consisting of a floating chain of ten small 220k 0.25W metalfilm resistors in series. With 100Wrf power, each little metal film resistor is loaded with only 100Vp and 26mW.