Components behave differently at RF
Every component is only approximately the thing its symbol says. At HF the approximations start to matter.
Why should wire-wound resistors not be used in RF circuits?
The resistor’s inductance could make circuit performance unpredictable.
A wire-wound resistor is a coil. At DC it is a resistor; at 14 MHz it is a resistor in series with an inductor whose reactance may exceed the resistance. This is also why a dummy load must be non-inductive — the same problem, at a hundred watts.
What happens when an inductor is operated above its self-resonant frequency?
It becomes capacitive.
Every inductor has stray capacitance between its turns, so it is a parallel resonant circuit with itself. Below self-resonance it behaves inductively; above it, the stray capacitance dominates and the component behaves as a capacitor. An RF choke used above its self-resonance does the opposite of its job.
Choosing capacitors
| Type | Characteristic |
|---|---|
| Electrolytic | high capacitance for a given volume |
| Low-voltage ceramic | comparatively low cost |
Electrolytics are polarised, lossy at RF, and limited in life — but nothing else gives microfarads in a small can, so they are the power-supply filter choice. Ceramics are cheap, small, and good at RF, which is why bypass capacitors are ceramic.
Diodes
Two forward threshold voltages worth memorising:
Silicon junction diode: approximately 0.7 volts.
Germanium diode: approximately 0.3 volts.
The lower germanium drop is why germanium diodes persist in low-level detectors, where 0.4 V of difference is the whole signal. The Technician semiconductors lesson noted that drop varies by type; these are the two numbers behind that.
Transistors
What are the operating points for a bipolar transistor used as a switch?
Saturation and cutoff.
Cutoff is fully off — no collector current. Saturation is fully on — as much current as the circuit allows, with minimal voltage across the device. A switch lives at the two extremes and passes through the middle as fast as it can, because the middle is where the power is dissipated.
Which of the following describes MOSFET construction?
The gate is separated from the channel by a thin insulating layer.
That insulating layer is why a MOSFET gate draws essentially no current — and why it is fragile. A few tens of volts of static across a layer that thin punches through it permanently, which is why MOSFETs ship with their leads shorted together.
Vacuum tubes
Still present in HF amplifiers, and the pool still asks about them.
Which element of a vacuum tube regulates the flow of electrons between cathode and plate?
Control grid.
Electrons flow from the heated cathode to the positive plate; a small voltage on the control grid in between modulates that flow. Same function as a transistor’s base or gate.
What is the primary purpose of a screen grid in a vacuum tube?
To reduce grid-to-plate capacitance.
A second grid between control grid and plate, held at a fixed positive voltage. It screens the control grid from the plate, and the capacitance it removes is exactly what would otherwise feed output back to the input and cause self-oscillation.
That is the connection to the next lesson’s neutralisation: the screen grid solves in construction what neutralisation solves in circuit design.
Batteries at HF power
What is the minimum allowable discharge voltage for maximum life of a standard 12-volt lead-acid battery?
10.5 volts.
Below 10.5 V a lead-acid cell begins sulphating irreversibly. It is the number built into every low-voltage cut-off, and the reason a “dead” battery left flat often never recovers.
What is an advantage of batteries with low internal resistance?
High discharge current.
Internal resistance limits current and drops voltage under load — which is the same voltage-sag problem as undersized DC wiring, from inside the battery. A radio drawing 20 A on transmit needs a battery whose internal resistance is low enough that the terminal voltage holds up.
Check yourself
- Why must a dummy load resistor be non-inductive?
- Your RF choke works on 80 metres and seems to do nothing on 10. Why?
- A silicon and a germanium diode in the same detector circuit. Which passes a smaller signal, and why?
Answers
- A wire-wound resistor is a coil; its inductance makes the load reactive rather than 50 ohms resistive at RF.
- You are probably above its self-resonant frequency, where stray winding capacitance dominates and the choke behaves capacitively.
- Germanium, with about a 0.3 V forward threshold against silicon’s 0.7 V.