Inductor Coil Question - Page 1
Forum section: electronics / beginners

author: Electro Fan
author_rank: Super Contributor
timestamp: April 02, 2015, 12:35:39 am
content: How important is symmetry for a copper wire coil in an FM transmitter circuit? I understand the number of turns and total length of the coil can effect transmission frequency but does the symmetry of the turns impact the quality of the audio transmission? Thx

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2015, 11:57:36 am
content: Any of the geometrical factors (shape, wire length, number of turns, area, etc.) affects the inductance of the coil, which will affect the frequency of the circuit. To avoid "incidental frequency modulation", i.e. unwanted garbage on the signal, you need to make sure that the coil will not vibrate (in any of those factors), both within and with respect to external metal and dielectrics. The usual method is to cement the wires to a high-quality cylinder (coil form), such as ceramic or polystyrene.

author: jlmoon
author_rank: Unknown
timestamp: April 02, 2015, 05:28:34 pm
content: Another way to look at this is:RF does not like to travel around sharp edges such as bent coils of wire, jagged microstrips and such.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 02, 2015, 08:13:26 pm
content: Quote from: TimFox on April 02, 2015, 11:57:36 amThanks Tim and jl,I have a coil that is firmly soldered into place on a circuit board. I found that by compressing the length of the coil I could reduce the transmission wavelength by about 10MHz (on about 100 MHz total). The compressing process (squeezing with a pliers) somewhat degrades the uniformity of the coil. It might not degrade the quality of the signal (or it might), but it isn't a very flexible way of experimenting with coil spacing, overall coil length, etc. (It would be easier and better I think to just make various different coil configurations - it's pretty quick to make a new coil from scratch and then they are uniform in spacing and made to spec. Bending and re-bending a coil in place isn't nearly as precise.)So, is there a recommended way to make the coil "replaceable" on a small (cramped) circuit board? I could de-solder the coil from it's connection points to the board and then solder a new coil onto the board, or I could cut the coil so as to leave the vertical risers of the initial coil in place and then try to solder a new coil onto the old coil riser remnants but this all might require applying enough solder heat to the board that I run the risk of overheating a transistor, etc. So... is there any good technique for making the coil "interchangeable"?For example, could I leave a small vertical riser attached to the circuit board at each end of the coil and then buy/make/insert "junction collars" to connect the new coil to the "risers" at each end of the coil? Or I could use risors with alligator clips and some wire and more alligator clips - but I'm guessing that wouldn't be optimum given the advice to make sure to avoid vibration.Mostly looking for any practical ideas on how to experiment with different coils short of de-soldering and re-soldering.Thanks, EF

author: jlmoon
author_rank: Unknown
timestamp: April 02, 2015, 09:07:19 pm
content: Depends on the frequency range you're interested in. If I recall E.F. Johnson used to make a plug in assembly for coils. I can't remember much about them but they were used in various pieces of equipment back in the 60's. I barely can remember those parts but knew they existed. If you're working with just a few MHZ then you might get by with a dip header type arrangement that you could solder different coils on top of a dip to solder pin block, then you could just unplug one and insert another one (with different coil) into your dip socket.

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2015, 09:20:30 pm
content: The traditional way to adjust a coil in-circuit was to use a coil form with an iron-powder core that moved in and out on a screw. Sometimes a brass slug was used instead of iron: iron increases inductance but eddy currents in the brass reduce the inductance.If squishing the coil suffices for your adjustment range, you could use a plastic screw and nut to do the compression and leave the screw inside the coil. I wouldn't worry about the non-uniform pitch that might result, so long as you don't actually touch turns. There might be a small loss of coil Q, but that shouldn't matter for an oscillator.A good way to build such a coil is like the conical helix used in old-fashioned flashlights to contact the battery.I'm afraid the EF Johnson, Millen, and National plug-in coil forms are long gone, but you might want to use the DIP header idea (see above), especially if you find a header with 0.6" row-spacing. If your coil wire is small enough, you might be able to use a SIP socket with round socket pins and insert the wires directly for interchange.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 02, 2015, 10:57:18 pm
content: Thanks for the DIP socket suggestion.The coil I'm using is 18 gauge which I think is just a tad more than 1mm in diameter. Most DIP sockets seem designed for chips with narrower pins but I found this:http://www.advanced.com/pdf/AIC_DIP_Sockets_Molded_16A_rev0.pdfLooks like Type-237 (on page 2) might possibly accommodate 18 gauge (~1mm) wire.Thanks again for the info and ideas.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 03, 2015, 02:33:40 am
content: Yeah, you can use sockets or headers, or you can get Mill-Max style machined pins (individual or in rows).http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=ed5009There's a variety of styles, from "socket on a pin" to "sits flush with the board" style.As for coils, as long as the Q is sufficient for the purpose, it doesn't really matter. The main thing that costs you is Q (quality factor / losses) -- for an oscillator, not a big deal, unless it gets really bad and the oscillation halts. For tuning coils and filters, this affects bandwidth and sharpness, which may also have a pretty wide margin, but will eventually get screwy.At high power levels, the localized losses of a kink in the wire *will* cost you, because, say, if you're pumping 50kW into the thing, and even if it's water cooled... it only takes a few thousand watts more to burn a hole in the thing, and then it's all over! But, such coils are usually quite large (and made of rigid tube), so the power density isn't all that bad.Tim

author: Rupunzell
author_rank: Frequent Contributor
timestamp: April 03, 2015, 03:54:03 am
content: Be aware coils have a temperature coefficient or change in value over a range of temperature. Temperature coefficient of copper wire coils are often positive, to compensate for this negative temperature coefficient ceramic capacitors are often used to help bring the resonator frequency drift over a range of temperatures to within required limits.This where ceramic capacitor designation like NPO, N1800 and others came from and why they were made.Bernice

author: Ian.M
author_rank: Super Contributor
timestamp: April 03, 2015, 04:13:22 am
content: Many budget commercial VHF receivers of years gone by, when discrete transistor superhet circuits were common, had coils that were trimmed by bending - compress the turns to increase the inductance and open them out to decrease it. Alternatively, if a larger range was required the coll might have a moderately sharp bend at a kink in it and the inductance would increase if the bend was reduced so the two halves of the coil were closer to being on the same axis and decrease if the bend was increased so the halves were closer to being at right angles.Sometimes the coils were then stabilised after adjustment (if they had too many turns or were too thin to be entirely self supporting) by inserting a slightly compressed foam plug in them and applying a few drops of superglue.

author: jlmoon
author_rank: Unknown
timestamp: April 03, 2015, 03:02:18 pm
content: Quote from: Ian.M on April 03, 2015, 04:13:22 amWay back when, I used to physically bend the L.O. and Front end coils on my portable FM radio and listen to Aircraft (~118 - 135 MHz) all the time. Loved to do that!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 03, 2015, 04:04:56 pm
content: Quote from: jlmoon on April 03, 2015, 03:02:18 pmThat doesn't make sense: aircraft use AM. Was it that awful of an FM receiver??Tim

author: Ian.M
author_rank: Super Contributor
timestamp: April 03, 2015, 04:52:06 pm
content: It may have been really crappy and used a slope detector, or more likely jlmoon has forgotten about shorting out the earthy side diode in a Foster-Seeley discriminator to convert it to an AM detector.http://www.tpub.com/neets/book12/51c.htm

author: SeanB
author_rank: Super Contributor
timestamp: April 03, 2015, 05:07:31 pm
content: Aircraft AM radios will do FM demodulation as well, you can tune them down in most cases to cover the top end of the FM band for the non digital versions. not the best linearity, but still does sound.Of course having the DC bus on all night so the sentries can listen to the radio does mean the next morning the battery will be flat, and the Alouette then needs to have a session with the generator to start it, along with a 15 minute idle to check the battery is not getting ready to go thermonuclear on you as well. Pilots were not pleased with the sentries, and they had a "really bad day" from their base RSM. I was just there and knew how to do it, having had a few flat batteries on aircraft in the service line where the switch had been left on for long periods, though there we just used a charger, as the motor was in many cases sitting next to the plane in a service frame.Incidentally you need power to close the fuel valves on larger aircraft, as they use a motor to both open and close them. Thankfully it was not me that knocked the switch and dumped 6kl of fuel all over the inside space where there used to be an engine as the tank drained out...........

author: jlmoon
author_rank: Unknown
timestamp: April 03, 2015, 08:02:35 pm
content: Quote from: T3sl4co1l on April 03, 2015, 04:04:56 pmback in 1966 or so.. yes pretty crappy FM radio.. think they made an attempt at superhetrodyne technology using a tradename such as "SuperHet"

author: jlmoon
author_rank: Unknown
timestamp: April 03, 2015, 08:04:26 pm
content: Quote from: Ian.M on April 03, 2015, 04:52:06 pmsounds like some kind of mattress

author: Rupunzell
author_rank: Frequent Contributor
timestamp: April 04, 2015, 02:37:39 am
content: Does "Foster-Seeley" deliver a good night's sleep?Quote from: jlmoon on April 03, 2015, 08:04:26 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2015, 05:50:29 am
content: Foster-Seeley: Australian for Beer\\\\Radio