Extra 60M/80M matching transformer for the 10m circumference pa0nhc loop.
The feed point impedance of every loop antenna varies with the used frequency. Below the optimal working frequency, the feed point impedance of a resonating 1/4 lambda circumference loop decreases. Although such a 7MHz loop can be tuned with a VSWR <= 1:1.5 over a fairly wide frequency range, on 80m the SWR is higher than 1:1.5 .
Matching the loop on 80m using an antenna tuner :
- without the use of a tuner, firts tune the loop for resonance (to lowest VSWR value)
- then (without retuning the loop) adjust the antenna matcher for SWR 1:1
A simple and effective 3.65MHz matching solution for my 7.1MHz loop
is an extra matching transformer.
This 80m-transformer is inserted between the transceiver and the 50 Ohms coax transmission line. You then do not need an antenna matcher. Just tune the loop for min. VSWR.
Winding the transformer.
Use 2mm dia lacquered copper wire. This wire has a thin polyurethane insulation and greater copper cross section than plastic insulated copper wire with the same outer diameter. It generates less heath and is better cooled by air. After 10min. 100w power, nearly no temperature rise was noticeable at the coils windings.
- Wind 38 turns over a FT240-61 ring core,.
- Bend the windings for equal space between them.
- At the outside of turns 38 to 19, remove the insulating lacquer (using a sharp knife and/or a sanding block).
- Pre-tin the blank copper surfaces with a very hot soldering iron (435C).
Finding the optimal position of the tap. It is easiest done using a simple handy tool : "Antenna SWR Sweeper".
Test at 3.65MHz, and start at 3/4 from the GND connection.
Without an antenna sweeper or analyser :
Connect the transformer between the transceiver and the coax feeder.
- Transmit on 3.65MHz with low power, and do not change the transmitters frequency.
- Tune the antenna for lowest SWR reported by your transmitter
- If the SWR is not 1:1.0, change the position of the tap 1 turn up or down.
Repeat the above until SWR = 1:1.0.
Check the result while transmitting with max. power. Move the tap one turn if needed.