Feeding and matching a 1/4 lambda circumference loop antenna.
Magnetic loop calculation software presumes that the RF-current has a constant value everywhere around the
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
- Circumference 10m
- Radiator 80mm (!!) thick:
Calculation example using 66pacific calculations :
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
From this and the 100W transmitter power we can calculate ( I2 = P / R) the loop current as
a feedpoint impedance of 22.5 Ohms at the tuning capacitor.
The capacitor voltage re-calculated.
In practice my loop has on 40m an SWR < 1.5
bandwidth of 75kHz => Q = 95.
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
Low capacitance 7MHz matching transformer for a
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.