Transformer turns ratio question - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 15, 2015, 06:27:25 pm
content: I am told I need to wind a toroidal low RF (136kHz) matching transformer, for aerial impedance matching. I am told it will need a 1.7 to 1 turns ratio.Firstly, is there a convention in quoting a turns ratio as to which is the primary and secondary? Can I wind it with ten turns on one winding and seventeen on the other, as obviously I can't put 0.7 of a turn through a toroid?Thanks.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 15, 2015, 06:38:48 pm
content: Quote from: Chris Wilson on August 15, 2015, 06:27:25 pmWhich way does the impedance go? More turns = higher impedance.QuoteYes, any number of turns that preserves the ratio will be fine, but you also need to take account of the power handling of the core (I'm assuming this is going to be output rather than input matching?)

author: Zero999
author_rank: Super Contributor
timestamp: August 15, 2015, 06:44:48 pm
content: Do you need any galvanic isolation?If not, you could have one winding of 17 turns with a tap after 10 turns.

author: kg4arn
author_rank: Unknown
timestamp: August 15, 2015, 07:57:38 pm
content: Also, as a general guideline, the winding inductive reactance should be 5-10x the impedance level seen by the transformer at the frequency of operation.So if the primary sees 50 ohms for example, then the number of turns on the primary should give you at least 250 ohms of reactance. Then the turns ratio determines the number of secondary turns.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 15, 2015, 08:42:47 pm
content: Thanks for the replies.Ed: The TX output is 50 Ohms impedance, and I want to wind on a T200-2 toroid. Is there an on line calculator to see how many turns on the primary are needs to get about 250 Ohms reactance please?

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 15, 2015, 08:55:28 pm
content: The Micrometals has a software (not online script, just windows desktop UI) with calculator.For Al value 12 NH, toroid 2 inch of material number 2 with wire #22 AWG and frequency 136 KHz the "AC resistance" 250 Ohm quantity of turns is 3185.I possibly misuse the software, because for impedance of 250 Ohm at this frequency L should be around 29uH. Not 0.122 H. But you asked for software.Edit: Used wrong tab in their UI. It is 49 turns for 28.9 uH.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2015, 07:10:06 am
content: You don't want #2 powdered iron, not at 136kHz. You hardly want it until 20MHz, even...A regular ferrite, say #43 or even #77/78 will do just fine. Or cores pulled from most any switching power supply (common mode chokes or ungapped transformers, not gapped cores or powdered irons).Minimum turns is determined jointly by impedance AND saturation, so signal level matters. How much voltage or power does this need to carry? Is the antenna tuning reactance (capacitance or inductance, or a combination of several) before or after this transformer?Tim

author: kg4arn
author_rank: Unknown
timestamp: August 16, 2015, 10:24:35 am
content: The formula for number of turns isN = sqrt [ desired L(nH) / AL] where XL = 2*pi*L for 250 ohms at 138 kHz, L is about 300uHALis taken from the databook on the material of the core and its size/geometry. For T200-2 it is 12.From micrometals,http://www.micrometals.com/downloads/RF%20Catalog%20Issue%20H.pdfon page 1 you can see that material 2 is generally used for high Q applications between 250KHz and 10MHz.You can use material 2 at 138KHz but the Q will probably be low. Look at the Q curves on the Micrometals web site as they give typically wound inductors performance.You would want higher Q if you are building a tuned transformer or a filter.Ferrites give a low Q and are often used for broadband transformers. As T3sl4co1l stated, may be a better choice for you.Can you show the schematic and give an estimate of the power levels involved?Did you pick the T200-2 because you had one in the junk box? Material 3 is probably a better choice for a high Q at 138KHz.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 17, 2015, 09:31:51 am
content: OK, many thanks for the replies. The transformer will go after the low pass filter on a 1kW, 136kHz amp for LF usage on the amateur band here in the UK.I am trying to set up my ladder line fed horizontal loop as a 136kHz transmitting aerial. As it stands I use it for HF where it works well, and is much quieter than a dipole, although noise around here is not a big issue. Here is what I have done to get a few scans to see how to load it for 136kHz TX.I have dropped the ladder line feeder from my upstairs shack window,and cut it to a length where it just touches the ground. I strappedthe two conductors at ground level. Up aloft they still connect tothe two legs of the horizontal loop, which is about 380 feet incircumference, at about 25 feet high. Higher in 3 corners, lowest atthe bungalow end where it's fed at the fourth corner.My AIM 4170 analyzer was calibrated with a PL259 to dual bananasocket adapter fitted. The banana sockets connected the centre pinof the PL259 connector to the strapped feeder, the other socketconnected to a laid on the ground hefty cable, going along about 10feet or so to my ground rod, which is driven in about 6 feet. Iobtained several scans, and link to a typical one below, all weresimilar with no sudden abnormalities.http://www.gatesgarth.com/scan5.jpgThe schematic, below, uses iron dust core toroids T240-2 for the LPF, and I had some spares. I was told via this post that a matching transformer would be required:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Hi Chris Ah that is more recognisable though the capacity does seem a bit onthe high side, I would expect for 120m total length about 700 to 800pF. Thismay be due to the lower height or the ladder line. You will need about1.35mH coil in series with the strapped ladderline to resonate that againstground at 136kHz.The series resistance which is mainly ground loss is rather high. You canuse this form of measurement to evaluate any attempts you make to reduce theground loss.....extra stakes, a wire grounded at the remote end and rununder the top-wire to your ground at the feed point. For test you don't needto get into heavy engineering until you have evaluated the best strategy.....just lay a length of wire on the ground and see if you can get areduction in Rs. I think about half this resistance is due to the groundloss between the remote corner of the "loop" and your ground stake. Theremote corner will be the high voltage point and thus will force mostcurrent into the ground at this point, and this has to return to the feedpoint.To use the loaded strapped loop as it is you will need a transformer with a1:1.7 turns ratio. As a quick guide the inductance of the "1" winding,secondary open, should provide a reactance of about 200 to 250 ohms if youtransmitter wants to feed 50ohms. If you manage to reduce Rs then the turnsratio of the transformer will reduce. A good, probably achievable, target toaim for is an Rs of about 50ohms .....you can then dispense with thetransformer! Any improvement will probably be of some benefit on HF too!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I will, in due course, try and improve the grounding, but for now want to try it as is, with a need to match to 200 to 250 Ohms from the amp output expected impedance of 50 Ohms.Subsequent to posts here I did some calculations on on line formulae sites and saw I would indeed need a circa 3mH loading coil and a circa 0.3mH matching transformer.It was then I realised that a T240-2 toroid would need a LOT of turns, whereas a T240-43 would need just 16 turns to give 0.3mHNow, I believe I can make it as a an auto transformer and wind it with 17 turns, tapped at turn 10. Does this sound right? It would leave one end of the tapped, single winding floating.... Can;t quite get my head around that! Is that OK? Thanks, if you have got this far!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2015, 10:49:05 am
content: At 1kW and 50 ohms, the primary (or tap) must handle:sqrt(1000 * 50) = 223V (rms sine)The FT-240-43 has Ae = 158 mm^2 and Bmax = 0.3T.The number of turns required at saturation is:N = Vrms / (4.44 * Bmax * Ae * F)= 223 / (4.44 * 0.3 * 158 * 0.136)(using mm^2 and MHz so the multipliers cancel out)= 7.8 turns.So 10 turns will be reasonably safe, having a Bmax of 0.23T. Less might still be desirable (more turns --> less core loss), but this should still operate safely. (The next most accurate turns numbers include 34:20 (exact), 24:14 (0.8% high), 27:16 (0.7% low) and so on.)Wind the primary as a single layer, then wind the secondary (or extended winding) back over top of it, to minimize the distance between windings. Don't wind the entire winding as one single layer, or put the primary and secondary on opposite sides.Tim

author: Andy Watson
author_rank: Super Contributor
timestamp: August 17, 2015, 12:40:00 pm
content: Quote from: Chris Wilson on August 17, 2015, 09:31:51 amI don't think any end will be floating - especially with an auto transformer.I would do the extra work and make it as two separate windings. This allows you to use a balanced output and ground isolation. It is easy to change from balanced to unbalanced, and to connect the grounds together (should you wish to do so at some later point in time), but it's not so easy to go from unbalanced to balanced, or isolate connected grounds.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 27, 2015, 10:34:06 am
content: Some great info, my apologies for them late reply, work took me away againI now have a much better understanding of this and your replies have been very helpful, I appreciate all your trouble, thanks.