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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
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 resistance" is only 0,566 Ohm.- The "Resonant circulating current"
is up to 9,2A The with R=P/I^{2} 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 in the
current maximum of a resonant 1/4 lambda loop.The loop current recalculated. G0CWT a feed point impedance of 5.5 Ohms measuredFive
times larger than calculated from data of 66pacific.From this and the 100W transmitter power we can calculate ( I
at the tuning capacitor.The at the tuning capacitor can be calculated as current2.13A.4x smaller than
calculated by 66pacific.
With a loop current of 2.13A, the over the capacitor is (365 Ohm x 2,13 A) = 777 Vrms or only voltage1100Vp.6x
smaller than calculated by 66pacific.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

- in-between.

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.