Examples of varactor diode or gyrator based antenna tuners / ATUs? - Page 1
Forum section: electronics / rf-microwave

author: Thomas Ballantine
author_rank: Contributor
timestamp: July 25, 2025, 09:53:01 am
content: I've been looking around for examples of all solid state antenna tuners for HF, and haven't come across any, and have been puzzled as to why it doesn't seem to have been even tried, outside of a few research papers I found on using varactors to tune UHF antennas, which means that they are suitable for RF usage (haven't seen anything on RF gyrators). Am I looking in the wrong place, or is it just not something that's done? I understand that the fixed minimum capacitance is a possible problem that needs designed around, and that high duty cycles might cause unusual variations in capacitance.I'm especially interested in adapting theArmy Looptuner to such techniques, as I think it would make a great portable/urban antenna if the bulky tuner could be shrunk.

author: mawyatt
author_rank: Super Contributor
timestamp: July 25, 2025, 01:13:07 pm
content: Check Non-Foster Networks and NIC (Negative Impedance Converter) and Dr Stephen Sussman-Fort.https://www.ieee.li/pdf/viewgraphs/matching_network_design_non_foster_impedances.pdfDr Sussman-Fort became a distant colleague when EDO was acquired long ago, we did some experimental work with SiGe BiCMOS way back.Additional noise was always the limiting factor, contributed by the active devices.https://apps.dtic.mil/sti/trecms/pdf/AD1197544.pdfhttp://web.eng.ucsd.edu/ece/groups/electromagnetics/Theses/MinuJacob.pdfhttps://etheses.bham.ac.uk/id/eprint/8417/Best

author: shabaz
author_rank: Frequent Contributor
timestamp: July 25, 2025, 02:35:42 pm
content: Hi,A few points:1. Solid state tuner designs do exist, e.g. see thisvaractor tuned loop antenna PDF doc.If you wanted that to be an ATU, you'd need to implement your own algorithm with say a microcontroller.2. Obviously, the above is only good for receive. If you plan on transmit, you'd need to specify what voltage do you expect the solid state circuitry to experience? It may be thousands of volts..3. The reasoning that you mention about the solid state circuitry being smaller, may not be too important, because the size is more determined by the physical size of inductors/capacitors more than the solid-stateness of the switching circuitry (if we're talking about a conventional ATU). I doubt using mechanical relays would be much of a limiting issue for the desired size/form-factor, at least for some typical power level ranges one might use for HF (I don't know what you're targeting).

author: LaserSteve
author_rank: Super Contributor
timestamp: July 25, 2025, 03:13:23 pm
content: There are MEMS modules made for this.Problem is they are MARKET optimized and low power. Cellular and Domestic Video Receive are markets, so designs go where the money is.Here is two examples from memory:https://psemi.com/products/rf-switches/antenna-tuning-switcheshttps://wispry.com/solutions/tunable-digital-capacitor-arrays/Looking at that might give you some areas to look at and keywords to find..PIN diode based switches used to be the way to go, instead of varacters.My fellow amateur radio operators do much of the world's research on HF ATUs, might just need to change your search parameters. Again, noise is a factor.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: July 25, 2025, 07:51:40 pm
content: sure, for low frequencyhttps://www.aavso.org/improved-gyrator-tuned-vlf-receiverand they are hard to get working IMO . I had a number of ideas that did not work because I did not know enough yet

author: coppercone2
author_rank: Super Contributor
timestamp: July 25, 2025, 08:06:17 pm
content: as to why, I mean the main reason why is because it depends on if you have a vendetta against coils.Its kind of insane because its not that hard to make or get a coil, and then you are considering replacing a coil with a very complex piece of ceramic tile.Even the most complicated tuner, the motorized catspan ,is not really that complicated a device.There are engineers that have a phobia of inductor based technologies and magnetic fields. This is what some people see

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 25, 2025, 09:33:02 pm
content: Variable capacitance tuners are used in NFC devices to adjust for metal close to the antennas.Example:https://www.st.com/resource/en/application_note/an5322-automatic-antenna-tuning-aat-for-st25r391616b-and-st25r392020b-devices-stmicroelectronics.pdf

author: mark03
author_rank: Frequent Contributor
timestamp: July 27, 2025, 10:02:25 pm
content: Thanks@mawyattfor that fascinating reference (EDO).For me, the coolest thing about negative capacitance/inductance is that it appears easy (?) to make an antenna broadband, whereas traditional L/C matching is a bandwidth reducer. If this could be done at modest voltages on transmit (even ~ 10V could be useful for QRP ham operation), that would be amazing. I suspect it's not easy or it would be out there already.The only other way I've seen to extend dipole bandwidth (not counting physically large conductors, i.e. pipe sections instead of wire) are tricks with transmission line matching sections which cause extra loops on the Smith chart, effectively extending the SWR < 1.5:1 or 2:1 BW. That approach is interesting too and I've wondered if there are free tools to optimize such things computationally.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 28, 2025, 04:58:14 am
content: Quote from: mark03 on July 27, 2025, 10:02:25 pmhttps://www.mathworks.com/help/rf/ug/designing-broadband-matching-networks-part-1-antenna.htmlHere is an example of people doing a wideband match. The idea is to generate a filter where the load + a reactive component have the same impedance as your antenna. For example, if an antenna can be modeled as a 100 ohm + 100nH inductor at your frequency of interest, then this network could be used. If you mess around with the filter design tool you'll see that if the series inductance is high, you will need a higher passband ripple to match the inductance, which means your match won't be as good.

author: richard.cs
author_rank: Super Contributor
timestamp: July 28, 2025, 02:20:13 pm
content: I've used SiC Schottkys as high voltage varactors in the HF and low VHF region before, they work surprisingly well.

author: mawyatt
author_rank: Super Contributor
timestamp: July 28, 2025, 11:39:02 pm
content: Quote from: EggertEnjoyer123 on July 28, 2025, 04:58:14 amBartlett's Bisection Theorem is a means to allow classic ladder type filters to also become impedance matching networks as well as filters. Very powerful theorem that seems long forgotten.Way back in grad school we had to prove this theorem as a take home exam (long before the internet) over the weekend (ruined our entire weekend, however we learned something very valuable which was the intent).Best