moving slugs in a FM tuner design - Page 1
Forum section: electronics / rf-microwave

author: Martin.M
author_rank: Frequent Contributor
timestamp: December 23, 2019, 10:38:14 am
content: I am planing to build a fm tuner with tubes and mechanical inductive tuning.Question:What core material is better, higher Q ,a) Aluminium cores, the frequency get higher when they drive in the coil, orb) Ferrite made for around 100MHz, the frequency get lower when they drive in the coil?Examples for b): Amidon, material F100thank you.

author: szoftveres
author_rank: Regular Contributor
timestamp: December 23, 2019, 03:01:27 pm
content: I think the solid aluminum core works on the principle of eddy current, and hence probably lowers the Q of the coil. Not sure though if it makes any notable difference on the performance of an FM tuner.

author: Ian.M
author_rank: Super Contributor
timestamp: December 23, 2019, 03:42:06 pm
content: It may be worth experimenting with stretching a coil spring made of fairly thin phosphor bronze wire to make an air core inductor with variable turn spacing. Silver plate it to improve the Q.

author: Martin.M
author_rank: Frequent Contributor
timestamp: December 23, 2019, 03:58:50 pm
content: interesting idea, never seen so.My problem is, I have to tune 3 coils syncron,the input,the anode coil from input stage,the oscillator coil.I have no knowledge if stretching of coils moves the inductance linear or log or otherwise.The moving of slugs is near related to log.

author: radiolistener
author_rank: Super Contributor
timestamp: December 23, 2019, 06:15:02 pm
content: if you want the best Q, then use air core and silver plated thick copper wire.In order to get the best Q performance, the length of inductor should be equal to the diameter

author: coppercone2
author_rank: Super Contributor
timestamp: December 23, 2019, 06:38:21 pm
content: lol, is a stretching coil something that is ever used ?I want to make that. You can buy brass wire in the hardware store that might work for prototyping, its good enough, but its not spring brass wire, which you need to buy specifically, because brass is brought to optimal spring form through extrusion, not heat treatment (like spring steel).. so you just bend spring brass and its done. If you mess with temperature I am pretty sure you mess with optimal spring behavior.however, good enough for battery contacts IMO if you don't care too much.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 23, 2019, 07:28:17 pm
content: A permeability tuner, nice, I think the Quad FM and FM2 were permeability tuned. I don't recall seeing aluminium or brass tuning slugs in FM radios and ferrite will give you a higher Q. If you're lucky and find some low permeability ferrite available in rod or tube form then that will make constrution easier if you intend to move the core to do the tuning, ferrite beads might be an option. Ferroxcube used to have a good selection and they were good with free samples. Epcos used to be good but now they're TDK and crap.Another way of tuning inductors with ferrite cores is to add an additional winding with a lot more turns to DC bias the core and move the operating point on the B/H loop to change the inductance.It might be worth looking at how Quad did things in the FM and FM2 receivers. Good luck.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 23, 2019, 07:36:43 pm
content: Helical resonators might be another approach and I don't see any reason why you can't permeability tune them. They're somewhat difficult to build but will give you a high Q.

author: coppercone2
author_rank: Super Contributor
timestamp: December 23, 2019, 07:47:02 pm
content: https://hackaday.com/2017/10/07/radio-tuning-the-quicksilver-way/http://webshed.org/wiki/Mercury_PTOPerhaps run some DC through it to heat the coil up and use gallium as not to be at a dinosaur safety level. You can put 2 syringes end to end with some inert gas to make it not oxidize.however when you start mixing conductive liquid metal, radio frequencies and high voltage vacuum tubes I expect bad shit to happen. the failure mode is t-1000. Stick with low voltage regulated circuits on medical power supplies if you mess with this IMO.

author: Martin.M
author_rank: Frequent Contributor
timestamp: December 24, 2019, 05:28:52 am
content: thank you for the tips.I will order cores F100 from Amidon.The friends in the TEA have seen my style at the amplifier, this radio project will also take some time to finish.Very interesting will be for me the mechanical construction bec. the inductance will move -log, not lin, so there must be another good idea to have a linear run on the frequency scale.have all a fine and blessed christmas time.... and now I will warm up the VLF radio to receive the SAQ this morning, at 17,2kHz cw

author: Unknown
author_rank: Unknown
timestamp: December 25, 2019, 01:30:33 am
content: Quote from: Martin.M on December 24, 2019, 05:28:52 am2 methods used in vintage radios:A linear screw, turned by an arm running on a log cam operated by the tuning knob shaftA coarse log-threaded screw, running on a single-turn nut.Merry Christmas