April 25, 2021:
Revised: v1.0

Make Small Nano Henry RF Inductors Accurately

If you design and build your own VHF and UHF circuits then it's almost certain you will need to make small nano Henry (nH) inductors. The problem is to make these reasonably accurately. Here is my approach to making these little inductors.


I’ve been designing and testing a number of VHF power amplifiers recently. Initially, I built a 10W VHF Class C RF power amplifier for single frequency operation using what I thought to be a well-proven design. It proved to be unstable and very difficult to align. That led to an extensive set of designs and tests using different semiconductors, output power levels and matching methods to identify a more reliable and stable power amplifier design. 
As part of this process, I needed to make lots of low value inductors ranging in value from 5nH to 100nH. The obvious question: How could I make such low value inductors accurately? How could I be confident those inductors had the correct inductance?

Internet Sources

Looking on the web, there are a number of web pages that claim to be able to calculate the length of wire and/or number of turns necessary for inductors in this nH range. I began by making a few of these inductors. However, when I fitted them into my power amplifier designs, this quickly led to considerable head-scratching. The results were far from those expected.

This presented a challenge. I clearly had to verify the value of these small inductors. But how could I do this  reasonably accurately with such small inductances?

Methods to Measure Small Inductors

I have an LC Meter on my bench (Figure 1). It's actually one I designed and built some years ago, loosely based on the very popular LM311-based designs described everywhere. Actually, my version uses a 74HC04-based oscillator, an ATtiny85 processor, and an I2C LCD to minimise the size of the meter and the current drain. I like my batteries to last a long time!

I can measure inductors down to the 5 - 50nH limits with this meter. It'salso  reasonably stable, unlike many I have tried. Regardless, I was very doubtful as to its accuracy down at that low value nH range.

Figure 1 : My homemade LC meter 

Looking around my workshop bench, I also had a reasonably accurate gate dip oscillator (GDO) which allows the resonant frequency of parallal LC circuits to be measured up to 250MHz, and an inexpensive Nano-VNA (50kHz to 900MHz).

Figure 2: My commercially made gate dip meter covers from 700kHz up to 250MHz with impressive accuracy for such an ancient, oops, I mean  "legacy" instrument

Figure 3 : The Nano-VNA covers up to 900MHz and can provide moderately accurate impedance measurements 

As the saying goes, a man with two watches is never certain of the
exact time. If I included my LC meter, I potentially had three ways to measure these small nH inductors, each with its own advantages and disadvantages. With care, arguably the most accurate is the Nano-VNA. With similar care, the GDO is also sufficiently accurate to verify the Nano-VNA results. In turn, these would allow me to determine the accuracy of my LC meter.

Test Inductors

I began by making a further series of inductors across a range of different sizes. These were made from 24SWG, 28SWG and 34SWG enameled copper wire, some bent into hairpin shapes, others into small coils, as well as several larger coils of three to five turns, some using bare copper wire, and even one made with plastic coated solid copper wire. These were each measured using the GDO and the VNA.

80 - 110nH14nH15nH12nH30nH20nH55nH
Figure 4 : A selection of the test inductors showning wire length and measured inductance

The GDO measured the resonance of each inductor when connected in parallel with a suitably sized  capacitor, typically 15pF, 33pF or 47pF depending on the inductor size. The capacitors were measured on my LC meter. I was confident that the measurements were within 5% based on past results confirmed by measuring small value silver mica
1% tolerance capacitors.

I also used my Nano-VNA to measure the inductance of each inductor directly by mounted each one onto a male SMA connector and measuring the inductance across the 50 – 500MHz range.

Test Results

Here’s what I discovered:

The GDO and VNA gave results within 10% of each other, more than adequate accuracy for the inductor measurements I was wanting to make for my RF power amplifiers. My LC meter measurements were within about 25%, a better result than I initially expected. For example, if an inductor measured 30nH with the GDO resonance method, the VNA typically reported values somewhere from 27 to 33nH, and results were often much closer.

Predicting Inductance

To simplify the presentation of these results and their future use in VHF/UHF designs, I plotted the measured values on this graph. Only a selection of the tested inductors are plotted on this graph.

Figure 2 : nH Inductor Wire Length

I’m summarizing my results here (so I can find them more easily when I need them!) It’s also possible this information may be useful for others, too. Just be aware - Your results may differ from mine.

My VHF power amplifier tests have definitely improved now I can install inductors of the correct value into each trial design!

Want to go back to the main page? Click here to return directly.