I decided to experiment with Magnetic Loops. First, I tried something simple. A 145MHz 2m loop. Using the software at the bottom of this page, I worked out that a 36cm length of wire, will need a capacitance of around 3.6pf, and around a 50% efficiency. Making it any larger, and more efficient at 2m requires too small of a capacitor (eg 1pf and 70cm long wire loop, for an efficiency of 88%). Making efficient loops at HF is easyer as the capacitances are larger.
So I tried some local contacts on the 2m loop, and was surprised at the results, even with it just poked out of the window. The trimmer is a 2-22pf. Its probably just OK for 5w or so. It will tune from 50MHz to about 200MHz, although tuning is fiddly because of stray capacitance of your hand, and also must be done with an insulated tool. An antenna analyzer is really needed for building this loop.
It is coupled by a gamma match, although with no capacitor involved. It took some fiddling with different lengths to get it right. A faraday loop this small was out of the question.
This inspired me to build a bigger loop for HF!
Here is what I came up with. Its made from eight pieces of 500mm long copper (4m in total). My air spaced capacitor is a dual gang 300pf, and im just using one gang here.
The faraday loop must be one fith the size of the main loop, and is not connected electrically. The main loop is 1.25 metres in diameter, so my faraday loop is 25cm in diameter. It is made of RG58. I did try coupling by a ferrite ring first off, but it didn't work correctly.
Here is my 300+300pf dual gang variable capacitor. It was ok to about 80w, it did flash over once at 100w.
Here is my setup for testing on a nice sunny autumn afternoon! First contact was ON4MI, Michael in Belgium, on 7057KHz
Magnetic loop design software
This is the handy tool that helped me decide what size loop to build in each case. The image below shows the information on my loop above, at 14MHz with 50w RF. You can download this handy tool from G4FGQ below.
|| Transceiving, single-turn, loop aerials of various regular shapes
|| by Reg, G4FGQ
|| Transceiving, single-turn, loop aerials of rectangular shape
|| by Reg, G4FGQ