Inductors and Tuned Circuits

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1. Making high Q inductors is generally thought to be a daunting task but in practice there are some fairly basic rules to follow. I will show a number of inductor examples on this page with some measured results. Generally the most difficult problem is getting a high Q in a small space.

The first inductor assembly shown below is an early prototype for a lifeboat emergency transceiver from the 1970s. The former is made of fibreglass and the two sets of windings shown are for 500KHz on the left and 2182KHz in the centre (the 8MHz inductor that was on the right is missing). Approximately 110 strand enamelled and cotton covered Litz wire was used and the 500KHz winding used a distributed wave winding technique to minimise the self capacitance and maximise the Q - typically around 300 with the dust iron core in place. Each inductor was use to resonate a short external whip aerial on up to three emergency/distress frequencies and at 500KHz this aerial looked like a 10pF capacitor in series with a resistor of about 0.1 ohms excluding any earth connection resistance. With a 3.5W AM/CW transmitter the voltage at the aerial terminal with a 10pF load could easily exceed 3KV.

2. Photo 2 shows three Wearite air cored inductor assemblies, type PA1 (long wave), PA2 (medium wave) And PA4 (a short wave band). Each assembly consists of a tuned winding and an aerial coupling winding. Note that with the two lower frequency types where the aerial is usually very short compared to a quarter wavelength and therefore exhibits a capacitive impedance the aerial windings have a relatively high inductance. The short wave assembly has an aerial coupling winding that is small compared to the tuned winding. Measurements were made on a Marconi TF1247A Q Meter.

Type

Tuning Capacitance

Resonant Frequency

Q

PA1

300pF

148KHz

48

PA2

300pF

668KHz

82

PA4

100pF

21.72MHz

58

PA4

300pF

12.7MHz

40

These were made primarily for the home construction market for a frequency range of 100KHz to 30MHz and were available for TRF receivers and with matching oscillator inductor assemblies for 455KHz IF superhet receivers.

3. Standard Resonators

Many years ago I developed a range of standard resonators that may be used in a wide range of applications covering 77KHz to 205MHz. The assemblies from which they are made were manufactured by Neosid UK and were in common use in mobile radio communications equipment in the 1960s and 1970s. They are still available, usually mounted on scrap PC boards or similar at radio rallies.

An attached document here shows their construction and test results.


4. Example Inductor Assemblies

The following assemblies are in regular use in my home construction activities:

From left to right:

  1. Single assembly manufactured by Neosid in the 1960s and 70s and available with 5 or 6 pin base, threaded bakelite former, aluminium can, ferrite or dust iron cup core and ferrite or dust iron core. Still available on assemblies from old mobile radio equipment
  2. Double height version of item 1 with space for two tuned circuits
  3. Double width version of item 1 with a 6 pin base
  4. 3/8inch diameter threaded bakerlite former with dust iron core and wiring support
  5. 1/4inch diameter threaded bakelite former with dust iron core and wiring support


Items 2 and 3 were purchased from J Birkett in 2016/17

Items 4 and 5 were available from Denco (Clacton) Ltd but they closed some years ago. Seen occasionally at radio rallies.


Cores are now retained with silicon grease so that they may be retuned without breakage.