Hello Core People,
I am building the Silicon Chip Magazine Super-9 FM Radio Receiver (Nov. & Dec. 2019), and the instructions call for some inductor coils using 0.125mm diameter enamelled wire wrapped around plastic formers. I haven’t been able to source wire of that diameter, so I am wondering if 0.12mm or 0.13mm diameter wire may work just as well, because that size is readily available.
Any ideas?
Hey @Chris74411, welcome to the forums 
In lieu of somebody more skilled in RF weighing in - my intuition tells me either diameter would be fine. Since we’re probably working with very small currents, the diameter of the wire is probably non-critical.
Thanks Michael,
You may well be right, but I will put the question to the designer of the project, after they respond to my previous email. The parts list for the project specified a wire diameter of 0.25mm, but in the detailed construction plans it was referred to as being 0.125mm, so I emailed them to clarify the issue.
With power transformers, current is the big issue, but with RF signals, as you suggest, it is not an issue. The number of turns is the critical factor. I have a number of PCBs from old analog TVs, and I am thinking of sourcing the wire from their tunable inductors. Problem is in measuring the diameter of the wire with my vernier micrometer.
Cheers…
i Chris
Oh yes it is.
Firstly it has to fit on the bobbin. This may not be an issue in this case.
Secondly it will affect the “Q” factor of the tuned circuit. Depending on the end use and the frequency involved this may not be an issue either. The capacitance of the coil may be an issue when you go to tune it (yes a coil has capacitance as well as inductance and will actually be self resonant at some point).
There again if the frequency is low none of this may be a problem.
I think if it was designed locally by Silicon Chip staff I think you will find that the wire will be readily obtainable.
Be careful also of the type of plastic the coil is wound on. If a type is specified use it.
Cheers Bob
You guys are correct regarding your advice re the diameter of wire for winding RF
transformers and inductors. The Silicon Chip Online has an errata section which states:
“The parts list on p36 of the November 2019 issue called for 1m of 0.25mm diameter ECW for winding T1 & L6 while the winding instructions on p63 of the December issue say 0.125mm diameter. You can use either diameter; if using 0.25mm diameter, wind the first layer on T1 & L6 in two layers. If using 0.125mm diameter, you should be able to fit the turns in one layer. (06/20)”
In response to my email to SC about substitution of wire diameter, they responded:
“Yes, 0.12mm or 0.13mm are within manufacturing tolerance of 0.125mm diameter so would be expected to give identical results.
The diameter has a small effect as it determines how thick the windings become but you are right that the number of turns is the primary concern, along with the resistance and current-carrying capacity of the wire (neither being terribly important in this case).
I agree that you can get away with using a range of different wire diameters. Keep in mind that the errata doesn’t say that you need to change the number of turns depending on whether you are using 0.125mm and 0.25mm diameter wire, so you can probably use anything in between those values with satisfactory results.”
Kind regards
Silicon Chip Admin
Hi All.
There is another important resistance to consider when dealing with Radio Frequency circuits. It is known as HF (High Frequency) resistance. These currents travel at or near the surface of the conductor so as the surface area decreases so the HF resistance increases. Particularly important in high “Q” circuits where the Q is a measure of the bandwidth of a tuned circuit. The higher the Q the narrower the bandwidth. Important in notch filters and the like. Simply the Q is the coil reactance (Ω) divided by the coil resistance (Ω) taking into account the HF resistance. Simple math will tell you that the lower the resistance the higher the Q.
There are a couple of ways to achieve a low resistance. The most noticeable being high frequency earth connections. They are usually flat copper straps. This will have much less inductance and a vastly greater surface area than an equivalent size copper cable. Any wiring point to point or around switches will be much larger diameter also.
Another method used in higher power antenna matching networks and other places is to use copper tubing, this almost doubles the surface area compared to similar size solid.
I have not heard much of this one over the later years but I am sure it is still used. That is LITZ wire, used in very high Q circuits. It is wire made up of very many strands of fine wire, each strand is insulated from the others by being enamel covered. When you think about it you get a MASSIVE increase in surface area for a given wire size. A fair dog to work with and terminate though. Each strand has to be carefully cleaned and tinned and you have to make sure each strand is properly soldered at the joint or the whole principle fails. Maybe in recent times the invention of solderable enamel may have made this a bit easier. Still have to take care. The largest LITZ wire I have ever seen was on an antenna matching coil for a 200kW VLF TX and it was something more than 50mm diameter. I never did find out how many strands in that one.
Just something more to think about when dealing with high frequency issues where the diameter of a wire can make the difference between working properly or not.
Cheers Bob
Good point Bob! Fortunately, the designers of the subject FM radio project assure me that the alternative wire sizes I was contemplating will not be causing any problems in the performance of the unit…
Hi Chris
Yes it is true that the slight difference in wire size will have no practical effect in your situation.
I was pointing out that broad statements re current and resistance have a bit more going on at radio frequencies that the DC or ohmic resistance. Stray unwanted inductance can be another nightmare. A straight piece of wire has inductance and the smaller the diameter of the wire the larger the inductance. That is why you will find connections at these frequencies made with copper strap or what seems to be oversize wire. All in the name of reducing stray inductance over which you have little control. This occurs particularly where there is some power (kWs) involved.
Cheers Bob
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