Wire gauge for air coil - Page 1
Forum section: electronics / beginners

author: tony3d
author_rank: Frequent Contributor
timestamp: March 06, 2017, 05:55:00 pm
content: Hi all I'm building a small two transistor FM transmitter, and was wondering what gauge wire to use for the small air wound coil? I'm thinking 18 gauge.

author: orolo
author_rank: Frequent Contributor
timestamp: March 06, 2017, 06:30:57 pm
content: The gauge is one of many choices you have, maybe not the most relevant. At very high frequencies, the skin effect is so prominent that significant variations in gauge make not a great difference, except in the rigidity of the resulting coil, ease of winding, and possible winding pitch. Take a look at thismagnific page: for a 15mm diameter coil, with 5 turns and 8mm length, using 18AWG gives 339nH with Q=687 at 100MHz, while the same coil with 22AWG gives 368nH with Q=699. The series resistance of the 22AWG coil is just 9% higher, while the wire diameter is 47% less. The resulting Q and inductance are in fact higher. I spent a lot of time winding my own air coils for a similar project some years ago, and I wound up favoring thegreatestlowest (greater diameter) gauge possible for rigidity and control, using big screws as formers, and approaching the desired inductance with the page linked above. It worked out rather well, considering my huge inexperience.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 06, 2017, 06:43:47 pm
content: If you are making a legal unlicensed transmitter the wire gauge doesn't matter as far as current carrying ability. The only real consideration is how stiff the wire is so that it will maintain its shape over time, whether you are able to form the wire into the size you want, whether you can solder the ends into or onto your circuit and the Q which will be slightly improved by larger gauges. Only you can say how important Q is for your application. You will also find the approximation formulas generally published are less accurate as the wire diameter becomes significant relative to coil dimensions. This last effect isn't usually a problem since you normally need to tweak your coil dimensions/parameters a bit if you need an exact value anyway.18 gauge is good, you might even want to try the next two sizes thicker.

author: tony3d
author_rank: Frequent Contributor
timestamp: March 06, 2017, 08:33:30 pm
content: Thanks everyone. Appreciate it!

author: MrAl
author_rank: Super Contributor
timestamp: March 07, 2017, 02:15:23 pm
content: Quote from: tony3d on March 06, 2017, 05:55:00 pmHi,The thicker the wire the higher the Q of the coil, all other things equal. That's because the skin effect increases the AC resistance (and this is not the reactance) which effectively makes the coil look less efficient, and the increased surface area of thicker wire reduces the AC resistance (it becomes all about surface area).At 100MHz (near broadcast band FM frequencies) the effect could be noticeable but 18 gauge is probably ok. As the wire gets thicker it gets harder to wind into a small coil also.Usually good schematics for these transmitters show a way to wind the coil and what wire to use and what diameter for the coil form and how long the coil should be and if the turns should be close or separated. If you find that you should probably follow that.

author: innkeeper
author_rank: Unknown
timestamp: March 07, 2017, 09:53:13 pm
content: The smaller the gauge, the less stiff the air wound inductors will be, and the ability for the coil to introduce microphonic sounds will increase. So anything you can do to keep them from vibrating will be a plus.Also keep the coil(s) as closely wound as you can, no wide spacing. between loops.20 to 22ga is very common in these type of low power unlicensed transmitters.building one is a fun project .. enjoy!