HOMEBREWING RF COILS FOR YOUR RADIO PROJECT
A BIT OF BACKGROUND:
If you are anything like I was here, back not so many years ago --- as I gamely ventured-forth into the RF aspects of my home-brewing activities --- the notion of constructing air core coils-inductors is one that instantly casts most any home-contrived exercise in this regard deep within the realm of the darkest of voodoo black magic...! And that includes antenna coupling coils for receivers, plug-in coils for RF pre-selectors, tickler-feedback coils for oscillators, fixed coils for that transmitter that might catch your fancy, etc. etc. etc.
Well, fear not, intrepid builder: the design & construction of any such coil(s) should be perhaps the least of your personal concerns --- provided, of course, that you might follow a few logical, common sense steps first in the execution thereof...
GETTING STARTED:
Perhaps the best way to demonstrate what I am attempting to show here, is to use an actual “...do-it-yourself” example --- and to this end, strictly for the sake of argument-example --- let's assume that you have might have need of a 70.0-uh.-something inductor for a device that will tune from just below the lower end of 160-meters, to just above 75-meters...and what you might have on hand to resonate such a coil is an air-variable variable capacitor with a maximum value of some 140-pfd. The most expedient “quik-n-e-z” solution would be to place an order with Barker & Williamson to-day for one of their #3016 miniductor pre-fabricated coils --- at a mere $100.00 per copy(!) --- that, or, you could do the alternative, and --- GULP...! --- homebrew your coil of choice.
You already know your required actual inductance of around 70-uh., so an easy enough thing to do at this time in formulating its design is to dust-off your old A.R.R.L. cardboard slide rule “Lightning Calculator,” or elect, simply, to “...plug-n-play” the critical dimensions on one of the many on-line website URL's for just such an application --- and to that specific end, https://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml is probably about the most user-friendly of any of them...
HANDS-ON TIME:
So what might come next...? This is where some enamelled wire, and a few surplus ordinary plastic pill holders from up in your medicine cabinet, will come to your rescue. Let's assume that you have a spool of #26 enamel wire handy, & an impromptu pill-holder-cum-air-core-coil-form of 1.0” in diameter. How many close-wound turns might be required to achieve your calculated 70-uh. for the project at hand...? Again, look into most any of the older A.R.R.L. HANDBOOKS, specifically that page near the end of the book that features a wire page summary chart. There, you'll see that #26 wire winds to 58 close-wound turns per inch. Consulting your ever-trusty “Lightning Calculator” reveals that at 58 T.P.I., a 1.0” diameter coil that is 1-1/8” long would fit your needs. And that translates to this in terms of actual final turns: 58 (close-wound turns per inch) x 1.125 (final coil length in inches) = 65.25 total turns (with 65 turns being close enough).

Run the numbers through the URL posted earlier, & you'll see that the two are in agreement within some 2.0-uh. --- a “...plus-minus” factor that lead lengths, etc., would doubtlessly have a play in, one way or the other, anyway...
Now, let us suppose --- and again, strictly for the sake of example in this tutorial --- that this 70-uh. inductor is to be used at the front end of your proposed homebrewed RF receiver preselector. With your pill box core in hand, measure down less than 2/3 of the way from the very top, and mark that spot with something like the fine-tip end of a magic marker --- this will be the starting point of the bottom of your coil. You will need to make a very small hole there, one that might just accommodate the diameter of your #26 wire to pass through it, but no more. Now, locate --- and mark again --- a secondpoint up toward the top of the coil form, that is the exact distance of the calculated length of your coil. This will be the end point of your coil...
So --- who makes a drill so very small that it might barely allow a #26 enamelled wire to pass through it...? Nobody that I know of --- and I really don't care, either. Because you'll form these small holes with an improvised ad hoc “...MacGuyver-style” heated device that consists of nothing more complicated than an ordinary sewing needle, and a masking tape protective heat-proof “handle” folded back on itself at the eye-thread end, as shown in the accompanying photograph. With the steady open flame of a candle, or a lighter, grasp this handle while placing the pointed end of the needle into the flame, until the metal appears red hot --- next, very carefully just touch the marker point on the plastic pill box with the hot needle, until it makes a small hole through it...and it will not take long to do so, either, so make it brief! You do not want an overly large yawning cavern --- only a pinhole that will just accommodate your wire's size passing through it...
ON WINDING THE ACTUAL COIL(S):
Now you can begin the process of adding your wire turns onto the pill box such that it might start to take actual coil form. Slip about 6”, or so, of the end of the wire through this hole, then wind your turns tightly onto its outside surface --- slowly --- by gripping the form in your left hand, slowly rotating it, while simultaneously pressing down upon each new turn of wire, and firmly against the preceding turn already on the coil form, with your RH thumb.
Once you've traversed the length between the two holes, counting the number of turns you've made (which, hopefully, will be the same as your pre-determined requirement), you then tightly secure the final turn with your LH thumb against the form, cut another 6”, or so, of wire more than needed, then thread this excess through the second hole, into the case. This will secure all of the turns of the coil that you've just wound, making them ready for an initial sealing-securing by way of one coat of the very cheapest “Dollar Store”-brand of clear nail polish that you can find. Brush this nail polish onto all of the turns, including the outside of the outermost turns of the wire onto the plastic coil form. After 24-hours, this coating actually shrinks upon drying, imparting a nice, tight protective permanent finish onto your coil.


What if circumstances are such that, perhaps you came up one coil turn short, or one turn extra...? Is that any reason to throw yourself into convolutions of worry...? Hardly. Remember --- your coil will be just a part of a tuned L/C combination, one half of which is a variable capacitor for continuous adjustment...! And that reality will more than compensate for any “...plus-minus” factor that may have been introduced into the process...so put your mind at ease.
GENERAL RULES-OF-THUMB:
Now, what of the link-tickler feedback coil that is required in your circuit and is magnetically coupled to this main coil? Well, it is wound below your main coil, so as to ensure that this latter inductor is kept reasonably far enough away from the proximity of the metal of your chassis-rig foundation, that could possibly affect its final inductance value (that's why you started winding the bottom of your first coil about 1/3 of the way up from the bottom of the coil form, remember...?). Should you be overly concerned at all about the inductance value of this specific coil...? In a simple answer, “No,” you needn't especially fret about it, not at all. Just follow these very simple, basic guidelines, and you'll do just fine:
RULE #1 --- No matter the actual inductance of the main coil, if it has 10 or more turns, the link-tickler feedback coil should have 1/3 of these turns;
RULE #2 --- If the main coil consists of 5 to 9 turns, the link-tickler feedback coil should have 1/2 as many turns, and,
RULE #3 --- If the main coil has less than 5 turns, the link-tickler feedback coil should have the same amount of turns as the main coil.
Oh yes, one more very important point of consideration here that we'll call RULE #4 --- The primary and secondary windings of each coil must be wound in the same direction...!!!
The bulk of these recommendations all come courtesy of C.F. Rockey's most excellent book of some years ago,SECRETS OF REGENERATIVE RECEIVERS. But they've been validated --- and on more than just one occasion, too! --- courtesy of “The School Of Hard Knocks” at VE3CUI.
OK, that's all simple enough. But what about the matter of physical proximity between the two coils...? How much might be “...too much coupling,” and how much is “...not enough coupling”? Just remember this easy guideline: the spacing between the primary and secondary winding of each coil assembly (i.e. each respective main tuning coil vis-a-vis its associated link-tickler feedback coil) should be no more than 3/8”. Personally, I utilize 1/4” (or, 2/8”) spacing between coils here as a good compromise between “...over coupling,” and “...just barely coupling.” And it works well enough for me.

Now, with these rules in mind, you can readily commence the winding of your smaller link-tickler feedback coil, with the end of its winding spaced to the prescribed 3/8”, or less, below the bottom of the main coil, basically replicating those very same steps that you initially took in making that first main coil. Once wound utilizing these very same techniques, apply a coating of clear nail polish to this smaller coil, set it aside to dry for a day, and then perhaps brush a second andfinal light coating onto both coils for good measure.
FINISHING-UP:
Once dry and secure, drill 4 small holes through the base of the plastic pill box coil form --- these will serve as exit points for the 4 wires of your 2 coils, allowing you to attach the assembly within a standard larger-diameter coil form, or a simple 4-pin plug. I had an ample supply of salvaged 4-pin tube bases from a quantity of spent 811A transmitting tubes that I used here: I scraped the enamel off of each wire where it exited through the base of the coil-form, then took very careful note of which pin was to retain which wire, and from which coil, in keeping with the needs and the schematic of the project at hand. I then slipped each wire through the appropriate pre-cleared solderless pin of the salvaged 811A base (just heat each pin with the tip of your soldering iron first, press your mouth against the cool end of the base, then blow your breath through it in order to extricate the original solder)...but before actually soldering the coil wires within each pin, I applied an ample dab of 2-stage epoxy glue onto the base of the pill cap coil form, then pressed it into its final permanent position within the 811A base. Once the glue dried, I soldered the leads, then “dressed” each of the pins by a combination of both trimming and filing, as appropriate.
You may chose to mount your coils differently, in order to suit your project at hand. “Whatever might work best for you,” is the order of the day here --- and why is that...? Because the otherwise onerous task of the actual coil manufacture is now well behind you...!
FOR ALL YOU RF POWER AMPLIFIER FANS:
Have you ever wondered just what, exactly, the optimum value might be --- in a transmitter RF power amplifier stage --- for the final plate tuning capacitor used in a parallel L/C network that is link-coupled to an outside antenna? This design was “...de riguer” for the 1930's home brew crowd in those halcyon days preceding the universal use of pi-tank tuning networks in modern-day transmitters...
Well, if you are fortunate enough to have in your library a 1938 edition of the book R.C.A. AIR-COOLED TRANSMITTING TUBES (TECHNICAL MANUAL TT-3), you will find, buried within its pages, a calculating formula, specifically:
C (capacitance pfd.) = 300 x “Q” (12 to 15) x PLATE CURRENT (milliamperes)
FREQUENCY (Mhz) x PLATE VOLTAGE (Volts)

As I did some years ago in The Antique Wireless Association's OLD-TIMER'S BULLETIN, let's use the example of a twosome of paralleled 245 tubes running a combined 400-volts at 100 ma. on 3.5-MHz, with a “Q” factor of 12:
C = 300 x 12 x 100 = 300 x 1200 = 360000 = 257 pfd.
3.5 x 400 1400 1400
You can readily increase the “Q” in this equation, and in so doing, sharpen the selectivity of the circuit (detected by the “sharpness” of its tuning) --- but the value of the capacitance required will also rise.
Opting for a “Q” factor of 15 in this example, mandates a capacitance of 321 pfd., or, a 20% increase in the value of final “C,” with a “Q” of just 12. Consult the L/C resonance chart in any of the HANDBOOKS, or your ever-trusty “Lightning Calculator,” to determine the value of “L” needed to achieve resonance on your frequency of choice...with 257-pfd., a coil of some 8-uh. would be the ticket needed --- conversely, with a “Q” factor of 15, you'll require 6.25-uh.
So there --- at long last --- you might have it, everyone. The long-awaited answer, that finally served to alleviate all of your fears and apprehensions as to RF coil design, and construction, maybe...? I should be so very bold and arrogant...! But if even just one, single reader managed to glean enough information from this tome of mine so as to inject him-herself with the confidence of “...boldly going forth to where that particular home-brewer has not gone before,” then good on me --- and good, certainly, onto that very same intrepid soul who may have so been inspired to press-on, and perhaps to fulfill a dream, as small though it may be...
Any general questions, or anything specific that you might want to know...? Well, you certainly weren't too shy about flattering me by reading this tome of mine thus far, now, were you...? So feel free to just ask away...!!!
Edward “Eddy” Peter Swynar
VE3CUI - VE3XZ
e-mail: trenear3773@gmail.com
PS: I took the liberty of attaching some jpeg photos of the different examples of the coils that I've wound --- and successfully used here --- over the years. And even this is but a small sampling of what your creativity and resources will limit you to, believe me...


| W2EAF | 2021-07-22 | |
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| Re: Homebrewing RF Coils for Your Radio Project | ||
| Ve3CUI Don't despair! There are plenty of people into this type of homebrewing, myself included. Just check out the homebrew forum here. I'm knee deep into building a receiver from a 1959 QST article, and started a thread in that forum about it (substitute a ceramic filter for a mechanical filter). I also highly recommend you check out the Mikeowave1 YouTube channel by WU2D. It is full of fascinating projects and techniques, including one on how to wind your own rf chokes. He also has a Facebook page Mikrowave1's Radio Project Resources. It is very educational and entertaining. Excellent article! Keep them coming. Reply to a comment by : K9FV on 2021-06-03 Good article and well written. Thank you for taking the time to write the article. Homebrewing has always been one of my favorite parts of ham radio. | ||
| WA5VGO | 2021-06-06 | |
|---|---|---|
| Homebrewing RF Coils for Your Radio Project | ||
| I find your rules of thumb for tickler windings to be extremely useful. Thanks for the fine article! Darrell | ||
| VE3CUI | 2021-06-05 | |
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| Re: Homebrewing RF Coils for Your Radio Project | ||
| Good Day Bill, And thank-you for YOUR comments, as well... But WOW! --- toroid-magnetic coil forms, you say...?! Yet MORE "...voodoo black magic" to hold the wannabe home brewer in rapt spell-boundness, determined to adhere to each & every step of the "...coil recipe" without exception...! Hi Hi I have yet to come across any E-Z user guide for the crafting of things like that --- it took ME long enough just to come up with all of the machinations required for air-core solenoid-type coils. :o) Maybe I was probably just "...looking for love in all the wrong places." Still, I wish somebody might draft an essay-guide in THAT specific regard for at least MY benefit, if nobody else's. How about it, Bill...? Seriously. I would love to read something along those lines, & learn a much-needed thing, or two, from it... Reply to a comment by : VE1YY on 2021-06-05 Thank you, Eddy, for the article. When toroidal coil forms with ferrite or iron cores are the order of the day, it is refreshing to recall building inductors on solenoidal forms. I most appreciate that you encouraged experimenters to calculate using the many aids, ancient and modern, in order to get values and additionally to treat them as approximations (e.g.: non-ideal inductance can be tuned out by complementary capacitance). You comment about a lack of replies. Is that so often the case: people take and do not reciprocate. 73, Bill | ||
| VE1YY | 2021-06-05 | |
|---|---|---|
| Homebrewing RF Coils for Your Radio Project | ||
| Thank you, Eddy, for the article. When toroidal coil forms with ferrite or iron cores are the order of the day, it is refreshing to recall building inductors on solenoidal forms. I most appreciate that you encouraged experimenters to calculate using the many aids, ancient and modern, in order to get values and additionally to treat them as approximations (e.g.: non-ideal inductance can be tuned out by complementary capacitance). You comment about a lack of replies. Is that so often the case: people take and do not reciprocate. 73, Bill | ||
| VE3CUI | 2021-06-05 | |
|---|---|---|
| Re: Homebrewing RF Coils for Your Radio Project | ||
| K9FV, many thanks for your kind words, & observances... Quite frankly, it surprises me that absolutely nobody beyond just yourself had anything to comment re. the article...well, it surprises me, & yet it does not surprise me, both. In retrospect, I'm sure that the vast majority of readers' eyes simply glazed-over right after they may have first waded-into the piece. After all, who is left in Ham radio such as it exists to-day who might "...home brew" anymore, anyway...? If something can't be purchased & delivered within 48 hours online through amazon.com, then it's not worth bothering with...or so it seems. And that's a pity, as well as a great personal loss to those in the ranks, too, as I am sure there are many undiscovered & un-mined talents within the ranks of 2021's "Consumer Amateurs"... But how will we ever know...? How would Wayne Gretzky have ever known that he'd evolve into being crowned "The Greatest" in the hockey world, if he hadn't strapped-on a pair of ice skates in his infancy...?! Oh well, as the saying has it, "That which I don't know, can't HURT me." But it can't HELP me, either... Reply to a comment by : K9FV on 2021-06-03 Good article and well written. Thank you for taking the time to write the article. Homebrewing has always been one of my favorite parts of ham radio. | ||
| K9FV | 2021-06-03 | |
|---|---|---|
| Homebrewing RF Coils for Your Radio Project | ||
| Good article and well written. Thank you for taking the time to write the article. Homebrewing has always been one of my favorite parts of ham radio. | ||