“Q” – G3JKX

Q, ‘the goodness factor’

Here is the promised article on Q, ‘the goodness factor’ of an inductance. One formula for Q is the resonant frequency divided by the bandwidth, i.e. fres / bw
which means that the narrower the bandwidth the higher the Q must be. A quartz crystal has a high Q. It’s resonant frequency depends mainly on the size of the quartz crystal and circuit capacitances. The high Q is partly due to it also having an extremely high DC resistance. The Q can be up in the thousands. It must be kept dry though, as quartz is hydroscopic and, if it gets damp, the Q deteriorates and the frequency changes. This is why they are enclosed in metal cans with the connections coming out through glass seals.! This means that when a crystal is used in an oscillator the frequency stays put ! A resonant tuned circuit using a coil and a capacitance must have lower Q because the coil and the circuit wiring has some resistance. (VLF coils which need a huge number of turns must use
Litz wire to keep R very low or the Q suffers) The coil and capacitors are subject to heat and vibration so, when used in an oscillator, the frequency is less stable.
An example of where a low Q is needed is the TX output stage tuned circuit of your rig. Firstly, the circuit is being loaded at one end by the impedance of the output valves, transistors or chips and secondly by the 50ohms impedance of the coaxial cable taking the RF output into the matching unit and then up to your aerial. If the Q of the PA is too high, then you would have to retune the PA output every time you shifted frequency, even by a little. A typical figure here is a Q of
only 12.

Having too low a Q tuned circuit in a receiver RF amplifier causes problems. If very strong out-of band signals appear, even some way LF or HF of the frequency you are tuned to, then cross modulation can occur and basically chop up the wanted signal. Therefore, if you want the very best Rx performance, get a ham bands only set, which will have only sufficient bandwidth to cover our bands, without the need for variable RF tuning. If you really want to listen to broadcast stations, then buy a SW broadcast Rx. There used to be a gadget called a ‘Q5er’ which was basically an add-on RF amplifier, sometimes with reaction, with a set of high Q coils. This went between the aerial and your receiver. This reduced the bandwidth and could amplify the signal if required. Reducing bandwidth reduced
much cross-modulation. The important thing is that, with less bandwidth, there was also less noise for the rest of the Rx to amplify. It did mean having another tuning control and an RF gain control to twiddle though! You could then keep the main Rx RF gain low, reducing noise level even more. Receiver RF tuning was common in expensive wartime military valve sets, (such as the HRO, AR88s & Eddystones) Some had 2 or even 3 RF stages, with their variable tuning capacitors
ganged together, giving superb low noise narrow bandwidth performance. There really is a case for an amateur bands only rig and proper narrow band Rx front end tuning, like the FT101D. Listening to this Rx today is still a pleasure after 30 years and is matched by very few rigs available today.