Construction of the 40m TX loop
All wood is painted with two layers outdoor wood paint.
In the first constructed 8-sized loop, the silver plated double screening (only 1cm diameter) of RG214 was used as radiator. This sloppy coax was supported by 32mm PVC pipe frame. Compared with a 1/2 lambda dipole at low height, this relative thin radiator did perform well on both 40m and 60m.
But on 80m, due to the short circumference (1/8 lambda) and the relative thin radiator, 80% of the RFpower was dissipated as heath. The signal strength on 80m was abt. 1Spoint weaker than on 40m. A thicker radiator improves that now.
This older 8-sized RG214 radiator was replaced by a square loop, made of 35/28/22mm copper pipe.
- Signal should be abt. 4dB stronger.
- Much higher power capacity, as
50% less heath is generated
the surface of the radiator is greater
air cooling is better.
The bare copper is weather-proofed by two layers metal-primer and one layer blank metal paint. The copper knees are electronic tin-lead soldered, this should not cause any noticeable signal loss.
New version loop.
Square thick copper radiator : Top 35mm, sides 28mm, bottom 22mm.
Soldered copper knees with reinforcing stainless steel bolts drilled through.
Trespa plates serve as insulators. Best use plexy glass here.
The wooden mast (5.4 x 7.2cm2x 3.3m) has 3 guy wires connected at the top.
Best use garden composite material.
The mast is bolted to a satellite dish stand, which is secured by 4 concrete tiles.
If possible, use a longer mast.
Use composite garden materials.
Three horizontal bars which support the upper, lower and middle of the loop
should make the whole stiffer.
In windy circumstances, i advise to connect a few guy wires to the ends of the middle
horizontal bar, to prevent "no shaking" and wear to the joints.
Tune box with transformer set.
(2z 600pF @ 180deg) tuning capacitor has no end stops, air insulation of 2mm,
At the right side of the splitstator tuning capacitor is a 5pF/4kV
The tooth wheels (2x 10t and 1x 48t) and tooth belt
can be ordered at "stappenmotor.nl".
The loop is insulated from mass. A serial string of 10 resistors 220 k / 0.25W, connected between one side of the tuning capacitor and the coax screening (ground), prevent flash-over in the split stator and transformer. For higher transmitting powers than 100W, use 15 resistors in series.
The lid of the ABS cabinet is stiffened by a Trespa plate, so it closes tighter onto the cabinets packing. All hardware is waterproofed using waterproof glue. The Trespa roofing is screwed and glued together (waterproof glue). Total weight is 9kg.
At 40m a 9t : 6t low capacitance transformer (green) perfectly matches the 50 Ohm coax transmission line. It is wound with 2 wires in parallel for lower wire temperature.
At 80m a
15t with 5t tap auto-transformer (black) pre-reduces the feeding impedance by a
factor 9 before the 40m transformer reduces it further. This proved to be a good
It will be modified to 30t : 10t an possibly wound on a FT240-43 core, in order to prevent SWR changes with changing RF power.
A cheap OMRON 12V DPDP relay with 10A / 250V~contacts switches between 80m and 40m.
The small 5pF stray capacitance of the matching transformer
still causes a little capacitive UN-balanc in, and capacitive coupling to the loop.
Unbalance can be fully compensated by connecting a (5pF) 4kV capacitor in parallel with the other half of the split stator (seen at the right). I used 5cm
RG214 screening length.
The result is :
- nearly ONLY H-field (magnetic) radiation.
- minimal chance for BCI and TVI
- maximal immune for reception of E-field "man-made-noises".
MAGNETIC radiating loop.