Q For Simpletons - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 11th May 2022, 2:13 pm

Ribbed formers - so most of the winding is actually in free-air rather than contacting the coil-former,, are good for minimising dielectric losses which compromise Q; I've got several Eddystone-type ceramic ones here that I've used in transmitter projects [though there you tend to aim for a working 'Q' of somewhere between 10 and 20 or the circulating currents/voltages can rapidly become unmanageable].

You used to be able to buy polystyrene 'combs' - lengths of square rod with slits cut into one side - a set of these would be fitted around a suitably-cylindrical thing and the coil then wound into the slots, after which the turns were held in place using "Q-dope" [polystyrene cement] and the cylinder up the middle was then pulled-out to leave just the combs and the winding. The result was a nicely low-loss coil suitable for receiver front-end or transmit-PA service.

"CODAR" down on the South Coast used to sell pre-made versions of these coils, and the US B&W company still offers its "Air-dux" coils to professional customers.... https://www.bwantennas.com/Airdux.html

Radio Wrangler · 11th May 2022, 2:36 pm

'Working Q" being the critical phrase. You want the Q of the in-circuit resonance to be controlled by the transformed antenna impedance, not by coil losses. You want your power to wind up in the antenna, not in warming an inductor. So very high Q coils are welcome in even circuits running at low Q, if you want good efficiency. The load on the circuit damps the Q of the resonator by taking the output power from it.

So you want the Q of each resonator component in isolation to be significantly better than the resonator in circuit and driving the load.

David

kalee20 · 11th May 2022, 5:47 pm

Thanks David!

That's one of the points I was making in my post #35.

11th May 2022, 2:13 pm	#41
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Q For Simpletons Ribbed formers - so most of the winding is actually in free-air rather than contacting the coil-former,, are good for minimising dielectric losses which compromise Q; I've got several Eddystone-type ceramic ones here that I've used in transmitter projects [though there you tend to aim for a working 'Q' of somewhere between 10 and 20 or the circulating currents/voltages can rapidly become unmanageable]. You used to be able to buy polystyrene 'combs' - lengths of square rod with slits cut into one side - a set of these would be fitted around a suitably-cylindrical thing and the coil then wound into the slots, after which the turns were held in place using "Q-dope" [polystyrene cement] and the cylinder up the middle was then pulled-out to leave just the combs and the winding. The result was a nicely low-loss coil suitable for receiver front-end or transmit-PA service. "CODAR" down on the South Coast used to sell pre-made versions of these coils, and the US B&W company still offers its "Air-dux" coils to professional customers.... https://www.bwantennas.com/Airdux.html __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

11th May 2022, 2:36 pm	#42
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Q For Simpletons 'Working Q" being the critical phrase. You want the Q of the in-circuit resonance to be controlled by the transformed antenna impedance, not by coil losses. You want your power to wind up in the antenna, not in warming an inductor. So very high Q coils are welcome in even circuits running at low Q, if you want good efficiency. The load on the circuit damps the Q of the resonator by taking the output power from it. So you want the Q of each resonator component in isolation to be significantly better than the resonator in circuit and driving the load. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

11th May 2022, 5:47 pm	#43
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Q For Simpletons Thanks David! That's one of the points I was making in my post #35.