A remotely switched 40m-80m matching
transformer set for a 10m circumference transmitting loop.
40m (Tr2) and 80m (Tr1+Tr2) matching transformer set for a 10m circumference loop.
According to by G0CWT developed way, my 10m circumference loop on 7.1 MHz is fed and matched to 50 Ohms by a low coupling capacitance ring core transformer with separated primary and secondary windings (Tr2). Its secondary winding is inserted between one end of the loop radiator and the accompanying side of the loop tuning capacitor.
On this place, the feed point impedance at 7.1 MHz is 22.5 Ohms. With a winding ratio of 9t : 6t , Tr2 secondary impedance is (50 /(9/6)2) = 22.22 Ohms.
From 7.1 MHz to 3.65 MHz the feed point impedance of the loop drops with a factor 9 to only 2.5 Ohms. Tr1 is switched in cascade, and with a winding ratio of 15t : 5t pre-matches the impedance with a ratio of 9:1.
ratio on 3.65 MHz is : (9x15) : (6x5) = 135 : 30 = 4.5 : 1 .
Total impedance transformation ratio is (4.52) : (12) = 20.25 : 1 = 50 Ohms : 2.47 Ohms.
Good to know :
Tr2 MUST have low coupling
capacitance between its primary and secondary windings. This is accomplished by separated primary and secondary windings.
The 40m (9t : 6t) matching transformer MUST have an as low as possible (<= 5pF) stray coupling capacitance between the primary and secondary windings.
A to large primary to secondary coupling capacitance in the 40m matching transformer causes :
- Capacitive UN-balance
- Non-pure-magnetic behavior wile receiving or transmitting causing
- higher chances for BCI and TVI
- higher noise level received by E-field (man made) noises
- notice able lower loop-Q
- The used Fairite or Amidon FT240-61 (=61mm) ring cores are chosen for their :
- very low losses, no core heating
- very high curie temperature. No vanishing self inductance due to warming-up
- flat temperature / initial permeability characteristic.
- flat frequency / initial permeability characteristic, and constant self inductance
- On 3.65 MHz and 100 W power applied, in the secondary winding of Tr2 carries 6.3A RF current. As wound on a ferrite core, the RF current mainly runs on the INSIDE of each turn. The temperature rise is there the largest, but the air cooling the worst.
Therefore wind both primary and secondary windings of Tr2 using 2 wires in parallel, loosely and well spaced..
using lacquered copper wire :
- has a far bigger copper surface than PVC insulated wire of the same total thickness
- has a heath resisting thin insulation and therefore will be better air cooled than PVC insulated wire
- therefore will generate much lower temperatures
- is solder able after removing some insulation and using a HOT iron (435C).
- The secondary turns
of Tr2 also can carry up to 1.1kVpp RF voltages.
Prevent flash over.
- Wind around the whole ring core an eight times thick insulating layer of Teflon tape (gas tape).
- Be sure to keep at least 2mm free air space between primary and secondary windings.
When a wire goes 1x through the hole of a ring core, it counts for ONE COMPLETED winding.
the 40m 9:6 low capacitance ring core transformer.
Insulate the ring core with a 1mm thick layer of Teflon tape on the half where the secondary winding comes.
Wind two 2mm thick lacquered copper wire in parallel and loosely, in order not to damage the Teflon insulation, and to improve air cooling of the wire.
* Wind one wire of the secondary winding 6 times through the hole first. Then wind the 6 turns of the second wire in between the windings of the first. Wind loosely with airspace between the core and the turns. Divide them evenly.
wind the two wires of the secondary winding in the same way 9 times through the
* For best cooling, divide all windings equally around the core
* Be sure to have at least 2mm air space between the primary and the secondary windings (see photo).
15:5 80m ring core transformer.
* Wind one 2mm lacquered copper wire loosely 15 times through the hole of the ring core.
* Divide windings fully and evenly around the core to improve air cooling
After the wire has passed 5 times through the hole, remove the insulation of the wire at the outside of the core. Use a sharp knife. Solder the tap wire here.
The used FT185-61B (pink Philips / white Fairrite 4C65 36mm) ring core haves very low losses and will not become hot, has constant properties between 3 MHz and 25 MHz, and to over 200C.
The use of 50 Ohms Teflon coax is recommended for its high temperature and voltage properties. Thin 2.7mm dia RG188U is specified from -40C to 200C and 160W @ 1GHz. It can handle With 100W the peak voltage is only 100V.
Thicker coax makes less turns and self inductance possible on the same core. Per turn 2.7mm coax is 1.5cm coax needed. Per turn 5.1mm coax 8cm is needed.
Fix the coil ends to the ring core with Ty-wraps.
Wind 2,7mm RG188U Teflon coax 14x through the hole of the ring core.
Wind 5,1mm RG142U Teflon or RG58U coax 7x through the hole of the ring core.
Fix the coil ends to the ring core with tywraps.
Important : balancing the loop.
In my situation, two TV sets and a hi-fi radio / amplifier are situated only a few meters under the antenna. They are wired to separated hi-fi loudspeakers, a blue-ray player and a cable tuner. After installing the low capacitance 40m transformer, and balancing the loop, i had NO BCI nor TVI.
cabinet photo) :
1. Connect a 5pF / 4kV capacitor (5cm RG214 coax) over the opposite half of the split stator tuning capacitor. This balances-out the 5pF stray capacitance of the 40m matching transformer.
2. FORCE the loop to capacitive balance in respect to all surroundings, by connecting the split stator rotor shaft to the screening of the coax feeder.
3. A big Hi-Mu ringcore (Ui=5.000 to 10.000) with a few windings feeder coax through it, blocs feeder coax mantle currents best. Use the thinnest coax which can handle the used power for more windings through the core.
The loop itself is fully insulated from ground. Static charges on it could cause flash-over in the split stator tuning capacitor, or even between the secondary and primary windings of Tr2.
Static charges are flowing to ground via a chain of ten resistors 220kOhms 0.25W in series. Using 100W power, each resistor is safely loaded only with max 100 Vrms and 26 mW. The chain can be floating mounted, or could be on a piece of copp
er less hard paper or a piece of plexy glass.