A max. performance 40m - (60m) - 80m transmitting loop antenna.
This 10m circumference loop is at 7.1MHz matched to 50 Ohms coax, using a ring core transformer. When tuned over a range of 3.3MHz, the VSWR <= 1:1.5. For use on 60m or 80m you could use an (automatic) antenna matcher. But when using an extra ring core transformer, good to perfect match can be achieved on the whole 80m band (and wider).
In the lowest point of the loop the ring core matching transformer is inserted, together with the split
stator air tuning capacitor, DC tuning motor, 14.400 : 1 gearing and a 5W anti-condensation heather installed in an ABS box under a Trespa roof.
|Sense and nonsense
|Properties of a 1/4 lambda
aluminum radiator ?
or square radiator ?
|Matching a 1/4lambda loop.||Making the 7.1MHz transformer at the loop.||Making an extra 80m transformer at th TRX.||Making a 40m+80m
transformer set for at the loop.
|Construction.||Installation.||Other interesting loop
Properties of this loop :
- Small, only 2.5m square
- Omni-directional radiator for 40 (60) 80m.
- No radials nor ground needed.
- Installed at low height (loop center at 2.5m up) it radiates predominantly vertical (NVIS), optimal for regional traffic.
- If installed at 7m height => also very useable for DX (see radiation patterns below)
- Transformer coupling. Immediately perfect match on two bands.
- Closed, balanced system.
pure magnetic radiation
While receiving: insensitive for E-filed man-made-noises
While transmitting: Minimal chances for BCI/TVI
- No tuner needed. Tune the loop => ready for transmitting.
- Large range for tuned SWR < 1:1.5 (depending on the length of the coax feeder en the position of the tap on the 80m transformer)
Extension possibilities for easy 80m use (see the link buttons above) :
a. (preferred) : use the extra 3:1+3:2 transformer duo as 40/80m matcher at the loop
b. Place an extra auto transformer at the transceiver as wideband 80m matcher.
Use on 60m :
In practice the performances on 60m were comparable with those on 40m. I had VSWR 1:1.5, but this depends on the length of your coax feeder
- Tune the antenna for min. VSWR
- If the VSWR is between 1:1.5 and 1:3, use the ATU for optimal matching.
Measurements done with abt. 16m coax feeder:
Measured using an extra 80m auto transformer connected to the transmitter.
Radiation patterns, OMNI DIRECTIONAL.
Calculated with MMANA-GAL_basic for specified height of loop center (The tuningC is 1.25m lower).