A set remotely switched 40m-80m matching transformers for a 10m circumference transmitting loop.
Combined matching transformers for a 10m circumference loop for use on 40m (Tr2) and 80m (Tr1+Tr2).
Both ground return paths (Tr1 to coax feeder) and (Tr2 to Tr1) will thus be separated, and run on top and bottom surfaces..
My loop is on 40m matched to the 50 Ohms feeder impedance, using a low coupling capacitance ring core transformer Tr2. This way of matching is developed by G0CWT.
If on 80m you should use an antenna matcher (tuner) to achieve an SWR of 1:1.0, you cannot be sure that the loop itself is resonating at the same frequency on which the transmitter now "sees" a good SWR. The coaxial feeder cable carries standing waves, and therefore WILL act as a frequency dependant RF transformer. Which also depends on the physical length of the coax feeder. In fact, you could force a good SWR several tenths of kHz lower or higher.
The feed point impedance of a tuned 1/4 lambda circumference loop decreases with decreasing tuning frequency. Decreasing the resonanating frequency of a 10m circumference loop from 7.1MHz to 3.65MHz, causes the feed point impedance to decrease by a factor of 9.
To be able to match this loop to 50 Ohms on 80m, an extra 3:1 (impedance 9:1) transformer is connected in series with the 40m transformer. The transformer is switched at a distance using a 5V or 12V - 10A/250V~DPDT relay. The result is a 50 Ohms coax feeder, which does not carry standing waves. Its length is not of any influence to the "optimal-SWR-frequency" seen by the transmitter. The transmitter detects the real loop resonance frequency without using the antenna tuner.
Good to know :
If a wire goes 1x through the hole of a ring core, it counts for ONE WHOLE winding.
You must count the number of turns inside the hole of the ring core.
- The loop circuit carries large RF currents. Do not wind the wire to stiff around the ring core. This
improves air cooling of the transformer windings.
- The used FT184-61B (=36mm) of FT240-61 (=61mm) ring cores are chosen for their :
- very low losses
- flat frequency / Ui characteristic
- flat temperature / Ui characteristic
- high curie temperature
- The secondary turns can carry up to 1.1kVp RF voltages.
- Wind around the whole ring core a layer of Teflon tape, at least 1mm thick.
- Be sure to keep at least 2mm free air space between primary and secondary windings.
- Laquered 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).
Winding the 40m 9:6 ring core transformer.
A big FT240 ring core has more space for winding turns. It enables thicker copper wire and bigger air space but costs more..
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
- higher chances for BCI and TVI
- higher noise level received by E-field (man made) noises
- notice able lower loop-Q
The 40m (9t : 6t) matching transformer MUST
have a low as possible (<= 5pF) coupling capacitance between the primary and secondary windings.
* Insulate the Fairrite FT185-61B (36mm) or FT240-61 (61mm) Ui=125 ring core with a 1mm thick layer of Teflon tape.
* Wind the 2mm thick lacquered copper wire loosely in order not to damage the Teflon insulation and to improve air cooling
* Wind the secondary first, 6 times through the hole of the ring core
* Then wind the primary 9 times through the hole of the ring core
* 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 phopto).
It is wise to insulate the primary turns from the base plate by an extra piece of single sided PCB or
Winding the extra 80m 15:5 ring core transformer.
* Wind the 2mm lacquered copper wire loosely in order to improve air cooling.
* Wind the wire 15 times through the hole of the ring core.
* Divide windings fully and evenly around the core to improve air cooling
* Remove the insulation at the outside of the 5th winding seen from the ground connection
* Remove some insulation, and tin solder it using a HOT (435C) soldering iron.
Important : balancing the loop.
Two TV sets and a hi-hi radio/amplifier are situated only a few meters under the antenna. They are wired to separate hi-fi loudspeakers, a blue-ray player and a cable tuner. Using the low capacitance 40m transformer, and after balancing the loop, i had NO BCI nor TVI.
Balancing (see 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. An extra hi-MU ringcore (Ui=10.000) with a few windings feeder coax through it, blocks mantle currents on
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 copper less hard paper or a piece of plexy glass.