Small Loop Antennas for FM / VHF / UHF Radio Receivers - Antenna Briefs #9

A fairly in-depth look at using highly frequency-selective, small-loop antennas to create improved radio receivers. Prototypes and demonstrations focus on the FM broadcast band, but the techniques are general. Topics include design motivations, the evolution of the "FM Tiny Loop" antenna from a ham-radio / shortwave HF small loop, and a brief overview of the theory of operation. Several references to other videos on this channel are made to reduce the amount of explanation detail and therefore video length - perhaps at the expense of clarity in some areas. But the main goal is to present a broad and relatively complete explanation of the technology. See comments section for some additional specific videos cited as references.

[@vrjb100]

Very interesting, finally found the physical equations to understand why this works. Thanks!

[@MegawattKS]

Glad it was helpful ! 73's

[@loretti1039]

Hello. Congratulations on your channel. I'm 50 years old and I'm just starting out in the field of electronics, especially in the RF (Radio Frequency) area. There aren't many specialized RF channels out there. I wanted to offer my encouragement and express how important this kind of content is to me and many others. I'll be eagerly following your channel and look forward to more content in the future. Thank you for sharing this with us!

[@MegawattKS]

Thanks for the encouragement. It helps. I've backed off on making new videos, but have at least one in the works. This will help! Glad you found the channel. There's also an associated website with the slides used in the videos, plus some additional materials if you are interested. It's not fancy, but its the same kind of content: ecefiles.org/

[@electronics.unmessed]

Nice and comprehensive video. Love your experiment. Thx for sharing. BR Klaus

[@MegawattKS]

Thanks! I watched your video on your loop antennas too. Beautifully made.

[@electronics.unmessed]

@@MegawattKS Glad you like it!

[@TheElectronicDilettante]

There’s a video on Electronics unmessed channel that is presenting an antenna design, that in my opinion, is just an artistic representation of your small loop. It was just released.

[@MegawattKS]

Thanks. I think I found it and am watching it now - trying to figure out if/how frequency tuning is done. The design seems to be a little different in not having a resonating cap - but it's hard to tell. I'm also a little confused about the bandwidth. It talks about the narrow bandwidth in the early part, but then later they talk about 442 MHz BW at 2.45 GHz - which is not super narrowband... Anyway it looks cool 🙂

[@raulgongora5288]

Very interesting sr twaskyou for the antenas radio. Blessings

[@MegawattKS]

Glad you enjoyed it. Thanks for the note.

[@hubercats]

Very helpful discussion. Thank you!

[@MegawattKS]

Thanks - glad it helped !

[@TheGmr140]

Nice little talk, good information 😊😊

[@MegawattKS]

Thank you 🙂

[@ernestb.2377]

Thanks yet again for some High Q content 😀 It really motivates to go and experiment yourself. It is quite complex sometimes to understand, especially when you combine the components, like with the L-match stub on the antenna. Lower the number of components higher the complexity of the design 🙃

[@MegawattKS]

Thanks for the insightful comments. It's quite nice to know it came across as "High-Q" :-) Apologies for the hard-to-follow parts. I tried to cover a lot of material and just had to reference a number of existing videos for the detailed theory/methods. You're spot-on about the L-match. I definitely didn't go into that well enough. For the underlying theory on the tapped-L method, see maybe Episode 2, Part 2 of the Radio Design 101 series. Thanks again for the comments !

[@ernestb.2377]

@@MegawattKS RF electronics is a different "animal" all together, so it takes (of course) some time to comprehend. I can follow the topic very well, but of course I am in the "learning curve" now. I have zero experience in RF, but have background in electronics. But once again I really appreciate you taking the time to educate and to motivate. Very nice lessons 👍

[@TheRcEngineer]

Love your channel, thank you vy73

[@MegawattKS]

Thanks ! 73

[@abraheom]

Thanks for the video!!!😍😍

[@MegawattKS]

You're welcome 😊

[@coffeecuppepsi]

This is so cool

[@MegawattKS]

Thanks !

[@johnwest7993]

A small, very high Q tunable loop works great for receive on the 1750 meter LowFer band.

[@MegawattKS]

Nice. (Electrically) small loops are amazing at any frequency :-)

[@johnwest7993]

@@MegawattKS, I commented on at least 2 KZbin ham channels to explain to the KZbinrs that narrow-band antennas were not 'junk'. They thought they were, because the antennas wouldn't cover an entire ham band with a low SWR without the hassle of retuning the antenna. I explained to them the great value of a high Q antenna, and how it is very often that having a high Q antenna outweighs their convenience from having a low Q antenna. I just shake my head at KZbinrs who only memorized the answers for the Tech test and now think that it's OK to get on KZbin, act like they are experts, and tell thousands of hams things that are not true. There was also a channel that sold video lessons to help people get their ham licenses. The guy doing the promotion for the class popped up a squashed lop antenna and called it a Yagi. I commented and explained that it was not a Yagi. He replied that yes it was. KZbin is indeed a double-edged sword. Thanks again for providing a high standard of knowledge and clarity for hams with your You Tube channel.

[@jonathanhendry9759]

Say I'm interested in only two stations at 88.5 (weak) and 90.5 (stronger but could be better). Listening through an 80s Proton receiver (stereo component). Would it be worthwhile building two of these, each tuned to one of the stations, and connecting them to the receiver through a diversity antenna circuit (from Electronics Now, November 1993 issue) that selects the antenna with the strongest 19kHz carrier signal (via an audio feed from the receiver)? Or are the two stations close enough that there wouldn't be much difference unless I used very expensive components and test gear I don't have to tune each very precisely?

[@MegawattKS]

I think your second idea is probably the best - just use a single one and assume the bandwidth will be wide enough to get both. With the Q I have, the bandwidth seems to be 200 kHz, but this implies that a station 2 MHz away will be attenuated by about 26 dB. But if the resonating cap is lower Q, or different gauge wire is used for the loop to lower the Q, it can widen up. My intuition says it should work, as you said. Maybe tune to favor the weak one and let the stronger one take a bit of an attenuation hit.

[@jonathanhendry9759]

@@MegawattKS Excellent! Thanks!

[@DJMotavirust]

This is just I'm looking for! I have some questions. In order to simplify the adjustement, can we use a fixed capacitor and a fixed tap point for the matching part, for the whole FM band? Could we use cheap varactors instead, How? Are they enough Q for this application?. Is it possible to make a kind of yagi, adding a reflector and directors to increase the gain of the loop antenna? Good video, and thanks for share the information!

[@MegawattKS]

Good to hear. On question 1, yes. The matching network is pretty forgiving. I used the piston cap to make the main loop tune across the whole band and it worked fine with the fixed matching network. The varactor use is an open question. They will have much lower Q, but the overall performance specs are not set in stone for the antenna. So it might still be useful. However, note that varactors tend to have highest Q at lower frequencies. They're not bad at 100 MHz, but probably significantly worse at 440 MHz for example. The Yagi-Uda antenna question is interesting. Let me think about it and answer in another reply...

[@MegawattKS]

Very interesting idea on the Yagi-Uda ! Honestly, this might make a good Master's degree topic. But here are my initial thoughts: First, I think the answer is yes. BUT - the boom length will probably still need to be commensurate with the wavelength. Close to the same length as for a regular design. The other BUT, is the tuning. Every element would have to be retuned if the frequency changed. Not a problem for a repeater setup which uses a fixed-frequency, but for general use, it's a handicap. My final thought is related. The phasing for the reflector and director is going to be very sensitive to the resonance tuning of each element. Maybe the best way forward is a fixed-tuned design and an EM simulator to explore the design-space. Love the idea. Let me know if you decide to do it !

[@DJMotavirust]

Have piston caps the best Q? Are there another alternative instead them? What value do I have to use to cover from 88 to 108 mhz?. Another thing. Let me understand, if you use fixed elements in a yagi antenna, the Q won't be the same in all FM frequencies. Isn't it? On the other hand, I don't have a enough knowledge to desing and simulate an antenna. It's just an idea. I'm only be able to built antennas designed by people with more experience than me. I remember watching a page where someone changed the length of the elements of an homemade FM antenna, by adding capacitance using varactors, long time ago. But I'm unable to find it now.

[@DJMotavirust]

I found it! pa1bj.blogspot.com/2021/07/afstembare-en-omkeerbare-yagi-voor-2.html . I hope It`ll be useful for you.

[@MegawattKS]

@@DJMotavirust Yes - the piston caps are probably the highest Q one can get. And they also allow for multiple-turns - which is important when the Q is very high, as the bandwidth decreases with high-Q. A single or half-turn tuning cap would be too sensitive to adjust well. On the other hand, if a lower Q capacitor type is used, like in the example loops shown at 40:00 in the video, then a half or 3/4 turn might be OK. I actually bought that antenna set after making the video. Their Q was much lower and they didn't clean up the spectrum nearly as well as the loop design discussed in the video, but they did work, and they had the advantage of having a nice dial 🙂 Back to the Yagi, I'm sure Q will change with frequency, but it's the resonant frequency that is the critical thing that would have to be retuned a new frequencies. It's probably a very difficult thing to tune all the elements at once if small loops were used in place of traditional dipole-like elements. For Yagi antennas with traditional element design, the bandwidth is probably good enough for FM broadcast use, even if not perfect. And there's always the option to go to a log-periodic.

[@victoryfirst2878]

Would using braided wire used for lightning rod cable make the gain greater since it has so many inner connected wires ?? Thanks

[@MegawattKS]

That's an interesting question. My gut feeling is no. Not for this single-turn loop antenna. There is a type of wire made for transformers called Litz wire that tries to address skin-effect and current crowding. I'm not that familiar with it, but have studied these physics issues in general (in spiral inductors used in ICs). I think it's mainly applicable to multi-turn loops. The only way to know for sure is probably to do experiments. In the end though, it may come down to structural issues and definitions of wire mean diameter. Definitely larger diameter wire (solid or stranded) should help. And solid core is structurally better - at least in this particular example. Might be different for other frequencies / etc...

[@victoryfirst2878]

What you said has merit. Years ago I seen a guy make antenna using braided wire with crazy results. Very similar to fractal antenna. His idea worked. He just dropped off the face of the map. Could not ever locate him. Would love to know the whole story but that did not happen fella. @@MegawattKS

[@brianclements1014]

I would be grateful if you could advise the values of the capacitors in your basic loop setup. I have tried to replicate the loop but have been unsuccessful in getting any meaningful results from my nanoVNA that show any resonance across the bands of interest. I’ve tried mica capacitor 33pf as the loop resonator, and various coupling capacitor values and spacing, but can only seem to achieve a virtual circle around the rim of the smith chart. I am expecting a resonant result that I can tune with the value of the resonating capacitor, and would like to experiment with frequencies in the 2meter and 70cm ham bands.

[@MegawattKS]

Hmm... 33 pF sounds too high if you're using a diameter of 7 cm. Also, have you 'hunted' for the resonance? It is so sharp, that without narrowing NanoVNA span, and then trying different regions inside and outside the target band, it can be missed due to the frequency step-size of the instrument. The resonating cap I used is 15 pF. That value was very close to calculations (X_L for the 22 cm circumference loop is 100 Ohms at 100 MHz, so 15 pF gave X_c = 100 Ohms at 100 MHz). I ended up with 30 pF (15pF in parallel with 15 pF) for the matching network, tapped-in 2.8 cm from the location where the shield is soldered to the loop. I estimate the effective C in the MN is about 22 pF after accounting for the inductive parasitcs of the long leads. That got me the 8 to 10'ish dB return loss, so I stopped at that point. I was surprised I had to go up to 30 pF in the build, as my rough calculations suggested less C in the MN. Not sure of I should go to a little more or a little less to get a better match. But regardless, - make sure to hunt for the resonance too...

[@brianclements1014]

@@MegawattKS Thanks for the response. My frequency range was quite broad as I looked across the band, and so, with your advice, I will take much smaller chunks during my search. At least my initial choice of 33pf for resonance is not out of the ball park. I’ll use 30pf for the coupling capacitor. I’m unsure why you didn’t use the ‘traditional’ method of a 1/5 diameter loop (or similar) for matching? (In a former life I’ve used pairs of high Q antennas suitably phased to cancel out the affects of high amplitude signals on much lower level signals snuggled close by - but never vhf loops)

[@MegawattKS]

@@brianclements1014 Looks like 33 pF should give about 69 MHz for the resonant frequency if using a 22cm circumference loop. Maybe test with a 20 MHz span centered on 60 or 70 MHz? I think I didn't go with the transformer / coupled-loop approach because I hadn't done the theory for that - so I wasn't sure what the effect might be on the bandwidth. It would be interesting to investigate that !

[@brianclements1014]

Thank you for your advice. By limiting the search width, I was indeed able to find the resonance. For the values I had started with it resulted in 71MHz. With a Johanson 0.8 -6pF trimmer, I can increase the resonant frequency into the 2metre amateur band no problem. I have also started playing with the 1/5 loop matching method which works surprisingly well. Still room to refine things, but getting a return loss of about 14dB, with a 3dB bandwidth of 180kHz. VK4BAC

[@MegawattKS]

@@brianclements1014 Excellent news. Thanks for letting me know !