Feeding and matching a 1/4 lambda circumference loop antenna.
Wrong and correct loop calculations.
pa0nhc  201061201

Magnetic loop calculation software presumes that the RF-current has a constant value everywhere around the radiator.
This assumption gives useable results for very small loop antennas, with a circumference smaller than 1/20 lambda.

What if the circumference is larger?

        A tuned 1/4 lambda circumference loop antenna :
- 7.1 MHz
- 100W
- Circumference 10m
- Radiator 80mm (!!) thick:

        Calculation example using 66pacific calculations :
- The "Radiation resistance" is only 0,566 Ohm.
- The "Resonant circulating current" is up to 9,2A
        The with R=P/I2 calculated feedpoint impedance is only 1,2 Ohm.
- The loop bandwidth on 7.1 MHz should only be 16 kHz => Q=444 !!
- The voltage at the tuning capacitor should be up to 5kVrms (equals 7kVpeak).

G0CWT found empirically that the current around the circumference of a 1/4 lambda loop is NOT constant, but varies largely.
This has large consequences for the real loop properties.

        Calculation example using practical impedance measurements by G0CWT.
    The loop current recalculated.
G0CWT measured a feed point impedance of 5.5 Ohms
in the current maximum of a resonant 1/4 lambda loop.
                    Five times larger than calculated from data of 66pacific.

From this and the 100W transmitter power we can calculate ( I2 = P / R) the loop current as 4.26A.
                    Half the value calculated by 66pacific.

G0CWT measured a feedpoint impedance of 22.5 Ohms at the tuning capacitor.
The current at the tuning capacitor can be calculated as 2.13A.
                    4x smaller than calculated by 66pacific.

        The capacitor voltage re-calculated.
The 63pF tuning capacitor shows at 7.1MHz a reactance Xc=365 Ohms.
With a loop current of 2.13A,
the voltage over the capacitor is (365 Ohm x 2,13 A) = 777 Vrms or only 1100Vp.
                    6x smaller than calculated by 66pacific.

In practice my loop has on 40m an SWR < 1.5 bandwidth of 75kHz => Q = 95.
                    Nearly 5x better.

Still not convinced ? Compare these results with calculations after DL4CKJ.


    Matching a 1/4 lambda circumference loop antenna.
G0CWT uses a broadband ferrite core transformer to match the 50 Ohms coax feeder to the loop. It has separated primary and secondary windings.

A suitable transformer can be inserted in any point into the loop :
- in the current maximum
- in the current minimum or
- in-between.

Low capacitance 7MHz matching transformer for a
10m circumference loop antenna.
FT240-61. Sec : Pri = 8t : 12t  (13uH).
Ample pri-sec separation ensures low coupling capacitance.
As it the secondary caries 1.1kVp RF voltage, it is wound over
8 layers Teflon tape insulation.
Ample turns separation ensures air cooling.

The transforming ratios will differ, as the feed point impedances differ with different locations. G0CWT measured an impedance of 22.5 Ohms near the tuning capacitor. I connected my transformer there, as it is conveniently installed inside the capacitor tuning box. The windings calculation :

        Zpri : Zsec = 50 : 22.5 = 2.25 : 1 => turns = 1.5 : 1

To avoid capacitive coupling and capacitive unbalance to the loop, and minimize chances of BICI and TVI,  i constructed a transformer with very low coupling capacitance between primary and secondary windings.


After experiments with several transformers i found :
To small winding inductance has negative influences to the tuning behavior, selectivity and performance of the loop.
A to high secondary winding inductance introduces, in parallel with the total tuning capacitance of the loop, a blocking parallel resonance.

#61 material is chosen for its low losses, and flat frequency- and temperature responses.

An FT240-61 ring core with sec. 8t and pri. 12t is working well.

In my case, i use this 7MHz transformer also for 5.4MHz, as the VSWR only rises to 1:1.2.

Matching the antenna system on 80m is accomplished by a second matching transformer connected between the TRX and the transmission line coax.