25-07-2024, 04:07 PM
(This post was last modified: 25-07-2024, 04:08 PM by Mike Watterson.)
The tuned loop is actually an L C as long as the loop diameter is a small fraction of a 1/2 wave. At 1/2 wave diameter it's obviously a kind of folded dipole, but at even 30 MHz that's 5 m!
The inductance is the formula for an air cored solenoid, with 1 (HF above 7 MHz) to 50 turns (long wave and 138 kHz).
The screen can be a wire stood off slightly and wound loose (as if the loop is a toriod core), but must only be connected at one end or the middle, or at both ends and cut open at the middle, for LW/MW.
The Q depends on the wire diameter more than resistance, as at higher frequencies the current is more limited to the surface.
For two bands having four loops (one turn or several) that are identical and side by side like one coil will be x4 Q in parallel and in series will be 16x inductance (approximately) and 1/4 Q than one loop.
Frequency is the normal equation f= 1/(2 π SQRT(L x C))
The capacitance of the screen to the loop is included in C, which is a pair of varicaps (one alone will cause FM intermode with a strong signal) or a well spaced variable capacitor (or vacuum cap) driven by a motor. With good Q and 100W transmit the voltage /current in the circuit is amazing.
For lower frequencies and in the attic, a wooden + frame can be used. The loop in my avatar is LW/MW using two sets of windings interspersed and switched by a 4 pole switch, thus x4 inductance on LW. The diamond shape is enamelled copper wire supported by a wooden +.
At HF a good mechanical solution is microbore copper pipe as a single loop, but two to four can be close for a higher inductance.
Signal level on a loop is roughly related to area. Maybe the square of rhe area, as indeed is the case with most aerials except ferrite (where the length affects sensitivity and inductance). A ferrite rod aerial is simply a very compact loop aerial. Can't transmit on it and it's not much use above 2 to 3 MHz.
A self-supporting loop can be a loop of Heliax cable which is huge diameter very low loss coax for UHF & SHF masts. The coax inner looks like microbore water/gas pipe. You cut the outer conductor leaving a 1cm gap at the top of the loop (and tape with amalgamating tape to seal) and earth the outer with two clamps either side of the loop driving connections (the inner pipe-like coax).
You can calculate the capacitance to screen by the pf per metre spec of the cable, or measure it. A 1m to 3m diameter loop depending on frequency. The bandwidth depends on the variable capacitor range in parallel with shield capacitance.
The next issue is that for best interference rejection the loop needs not just screened but a balanced connection. A balun, depending on design, has a particular bandwidth.
Here is simple solution:
It's not actually very good.
There is a way to tune the back to back varicaps (RX only!) via a voltage on the coax. The simplest is to connect the centre of coax to the midpoint of the varicaps and feed at radio end with 0.5V to 30V via 100K resistor (decoupled at supply side).
Any balun solution suitable for the band can be used.
You can use diamond, square, round, oval, hexagon or octagon shaped loops. It's not critical as the loop is a small fraction of a 1/4 wave.
A 1920s LW loop.
The inductance is the formula for an air cored solenoid, with 1 (HF above 7 MHz) to 50 turns (long wave and 138 kHz).
The screen can be a wire stood off slightly and wound loose (as if the loop is a toriod core), but must only be connected at one end or the middle, or at both ends and cut open at the middle, for LW/MW.
The Q depends on the wire diameter more than resistance, as at higher frequencies the current is more limited to the surface.
For two bands having four loops (one turn or several) that are identical and side by side like one coil will be x4 Q in parallel and in series will be 16x inductance (approximately) and 1/4 Q than one loop.
Frequency is the normal equation f= 1/(2 π SQRT(L x C))
The capacitance of the screen to the loop is included in C, which is a pair of varicaps (one alone will cause FM intermode with a strong signal) or a well spaced variable capacitor (or vacuum cap) driven by a motor. With good Q and 100W transmit the voltage /current in the circuit is amazing.
For lower frequencies and in the attic, a wooden + frame can be used. The loop in my avatar is LW/MW using two sets of windings interspersed and switched by a 4 pole switch, thus x4 inductance on LW. The diamond shape is enamelled copper wire supported by a wooden +.
At HF a good mechanical solution is microbore copper pipe as a single loop, but two to four can be close for a higher inductance.
Signal level on a loop is roughly related to area. Maybe the square of rhe area, as indeed is the case with most aerials except ferrite (where the length affects sensitivity and inductance). A ferrite rod aerial is simply a very compact loop aerial. Can't transmit on it and it's not much use above 2 to 3 MHz.
A self-supporting loop can be a loop of Heliax cable which is huge diameter very low loss coax for UHF & SHF masts. The coax inner looks like microbore water/gas pipe. You cut the outer conductor leaving a 1cm gap at the top of the loop (and tape with amalgamating tape to seal) and earth the outer with two clamps either side of the loop driving connections (the inner pipe-like coax).
You can calculate the capacitance to screen by the pf per metre spec of the cable, or measure it. A 1m to 3m diameter loop depending on frequency. The bandwidth depends on the variable capacitor range in parallel with shield capacitance.
The next issue is that for best interference rejection the loop needs not just screened but a balanced connection. A balun, depending on design, has a particular bandwidth.
Here is simple solution:
It's not actually very good.
There is a way to tune the back to back varicaps (RX only!) via a voltage on the coax. The simplest is to connect the centre of coax to the midpoint of the varicaps and feed at radio end with 0.5V to 30V via 100K resistor (decoupled at supply side).
Any balun solution suitable for the band can be used.
You can use diamond, square, round, oval, hexagon or octagon shaped loops. It's not critical as the loop is a small fraction of a 1/4 wave.
A 1920s LW loop.
