57cm Bicycle Wheel Mag Loop
57cm Bicycle Wheel Mag Loop
Embedded Files
This MagLoop was was made using a 57cm aluminium bicycle wheel and the following parts:
A 100pf variable capacitor with wide spaced plates able to cope with 100 watts
A short length of fiberglass rod
Coax for faraday loop
Old music stand
Small block of plastic (possibly Nylon?)
Small strip of 8mm thick perspex
A dismantled connector block
Cable ties, nuts, bolts and self taping screws.
The plastic block I happened to have on hand, measured 7 x 7 x 2 cm and was used to mount the wheel firmly on top of the music stand using self taping screws and a finger bolt as shown here.
The wheal was cut through and using the short strip of perspex, the two ends were secured with a gap of 10mm.
The body of the capacitor was bolted to the wheel with threads taped into the back plate, making the body electrically in contact one side of the gap.
The other capacitor terminal was connected with a short length of wire to the opposite side of the gap. The bolts securing the perspex were used to make this fixing.
A short length of fiberglass rod was passed through some of the spoke holes in the lower part of the wheel in order to support the faraday coupling loop. Cable ties passed through holes in the rim and wraped around the loop and fiberglass rod held the loop firm.
The terminals from a connector block were removed and cut in half. These were then used to secure the ends of the fiberglass rod and the cable ties passing through the rim.
The Faraday coupling loop
Trying out the magloop with the FT818 and 6 watts in the garden.
When trying out the magloop in the garden for the first time on Thursday 16 April 2020 on twenty meters, I was very surprised and encouraged by the sharpness of the tuning of the loop. As can be seen by the spots on the Reverse Beacon Network, My CW QRP at just 5 Watts was being heard in the Czech Republic, Hungary and Finland.
My CW CQ calls unfortunately did not initiate any contacts, therefore I thought I would take a listen on the SSB end of the band.
In came a strong 59 signal from EW2W (Belarus) I took up the microphone and gave him a call. Amazingly, he came back to my call and gave me a 55, I could not have been more surprised. Never before had this bicycle wheel covered so much distance and only 5 watts of power, I would have been pretty puffed out peddling it that far!
My first tests with QRP had proved how surprisingly well the loop tuned on 20 meters and bands above, however, it would not tune bands below 20 meters. This was as I expected for the loops 57 cm was much smaller than what I had seen constructed for the lower bands when looking on the internet.
Having tried the loop with QRP, I thought it would be interesting to see if the loop could handle a full 100 watts from my FT991. I was fairly confident that it would - as the capacitor used was a high voltage type.
Therefore, a week later I arranged with a fellow amateur located 11 miles away to be on hand should the 20 meter band be quite during my test transmission.
Unfortunately at the time I put out a few CQ calls the band was indeed quiet, with only one contact into Russia (R3MD) and two local fellow amateurs. Never the less, the loop had proved it could cope with a 100 watts and that contacts could be made at much lower power levels.
Considering its size, it was astonishing that it worked as well as it did. An antenna this small is never going to be as good as a full sized resonant dipole. But at least you could get on air with limited space and power and make a few contacts that you otherwise would not be able to make. Amateurs living in flats for example, would find this advantageous. Even if only used as a receiving antenna, it would prove very practical for limited space and means.
These two pictures show some improvements added later to improve the ease of tuning.
Slow motion drive added.
Page updated
Google Sites
Report abuse