The loop tuning capacitor
The quality of the loop tuning capacitor can be the difference between a great loop and a loop that transmits like a wet noodle.
The loss introduced into the loop by a low Q capacitor must never be underestimated.
The reported Q of an air variable appears to be around 1000.
The Q of a vacuum capacitor can be as high as 4000 to 5000.
Transmitting doorknob capacitors can have a Q as low as 50 but are usually between 200 to 2000 at 80m dropping rapidly to around 800 or less at 40m and decrease above this frequency. You cannot use this type of capacitor across a variable capacitor to resonate the loop on a lower frequency; furthermore paralleling capacitors substantially lowers the total Q.
When using a butterfly capacitor it is important to ensure that the assembly is not made of iron or other lossy material, plates should be bonded to the rails and not held with spacers. The loop current flows through the capacitor, therefore RF resistance in the capacitor will cause a similar effect to placing a resistor in this position.
The construction and loss to RF currents approaching 90 amperes must be taken into account when selecting a suitable capacitor.
The calculated values for my loop on 160m:
Antenna bandwidth: 1.73 kHz
Tuning Capacitance: 648 pF
Capacitor voltage: 1KW = 16,900 volts Peak, 400W = 10,000 volts peak, 100W = 5,400 volts peak.
Resonant circulating current: 1KW = 89.9A, 400W = 56.8A, 100W = 28.4A
Radiation resistance: 0.005 ohms
Loss Resistance: 0.057 ohms
Inductance: 11.4 microhenrys
Inductive Reactance: 133 ohms
Quality Factor (Q): 1,072
Distributed capacity: 31 pF
Never use copper braid; coax braid is a shield medium not a high current RF conductor.
Heavy braid may be ok carrying DC currents but it is not ok at RF frequencies.
Think about RF skin effect when connecting the capacitor to the loop.
I have seen some beautifully built loops with a vacuum capacitor connected by a single screw into the copper pipe and a thin piece of braid to the cap - The loss at RF frequencies would ruin the attainable efficiency of these loops.
A problem with vacuum capacitors is the current rating, this rating, unless otherwise stated is usually quoted for full mesh at 60Hz.
The current capacity drops off with frequency and partial mesh.
A capacitor rated at 70 amperes my handle as little as 2 amperes at 5% mesh and even less as the frequency rises.
The current rating is usually tied to the temperature the "vacuum seal" can handle before failure.
Bolting the capacitor to thick copper plates attached to the loop ends will increase the handling capacity. With low duty cycle modes like SSB this becomes less of a problem however it is something to keep in mind with small capacitors and when choosing the minimum capacity for loop resonance.
In other words, make sure the capacitor is not near its minimum value when designing a loop.
The loss introduced into the loop by a low Q capacitor must never be underestimated.
The reported Q of an air variable appears to be around 1000.
The Q of a vacuum capacitor can be as high as 4000 to 5000.
Transmitting doorknob capacitors can have a Q as low as 50 but are usually between 200 to 2000 at 80m dropping rapidly to around 800 or less at 40m and decrease above this frequency. You cannot use this type of capacitor across a variable capacitor to resonate the loop on a lower frequency; furthermore paralleling capacitors substantially lowers the total Q.
When using a butterfly capacitor it is important to ensure that the assembly is not made of iron or other lossy material, plates should be bonded to the rails and not held with spacers. The loop current flows through the capacitor, therefore RF resistance in the capacitor will cause a similar effect to placing a resistor in this position.
The construction and loss to RF currents approaching 90 amperes must be taken into account when selecting a suitable capacitor.
The calculated values for my loop on 160m:
Antenna bandwidth: 1.73 kHz
Tuning Capacitance: 648 pF
Capacitor voltage: 1KW = 16,900 volts Peak, 400W = 10,000 volts peak, 100W = 5,400 volts peak.
Resonant circulating current: 1KW = 89.9A, 400W = 56.8A, 100W = 28.4A
Radiation resistance: 0.005 ohms
Loss Resistance: 0.057 ohms
Inductance: 11.4 microhenrys
Inductive Reactance: 133 ohms
Quality Factor (Q): 1,072
Distributed capacity: 31 pF
Never use copper braid; coax braid is a shield medium not a high current RF conductor.
Heavy braid may be ok carrying DC currents but it is not ok at RF frequencies.
Think about RF skin effect when connecting the capacitor to the loop.
I have seen some beautifully built loops with a vacuum capacitor connected by a single screw into the copper pipe and a thin piece of braid to the cap - The loss at RF frequencies would ruin the attainable efficiency of these loops.
A problem with vacuum capacitors is the current rating, this rating, unless otherwise stated is usually quoted for full mesh at 60Hz.
The current capacity drops off with frequency and partial mesh.
A capacitor rated at 70 amperes my handle as little as 2 amperes at 5% mesh and even less as the frequency rises.
The current rating is usually tied to the temperature the "vacuum seal" can handle before failure.
Bolting the capacitor to thick copper plates attached to the loop ends will increase the handling capacity. With low duty cycle modes like SSB this becomes less of a problem however it is something to keep in mind with small capacitors and when choosing the minimum capacity for loop resonance.
In other words, make sure the capacitor is not near its minimum value when designing a loop.
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