Ferrite rod antenna for AM radio - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: April 08, 2023, 10:32:25 am
content: Found in the scrap boxes a piece from a former AM radio. No enclosure, only a PCB from around the 80's with the AM ferrite fallen out. The IC in the pic has pins at 0.1'', it's there for scale only.The main coil has had about 60-65 turns, single layer, with a strand of 9 twisted wires of 0.04mm or 0.0015'' each (probable AWG 46), and it looks pretty bad. The end coming out from underneath the coil has only a few strands left, so I guess I'll have to redo the coil.I want to reuse the LC tank to build a very small AM transmitter for indoor use (just an oscillator, no Tx amplifier). It is not important to cover the entire AM band, so no problem if a few turns are missing.Should I redo the coil? Can I use candle wax to fix the turns, or nails lacquer? Any advice about how to fix or rewire, please?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 08, 2023, 11:43:10 am
content: Forgot to ask, must the coil be slide-able along the ferrite bar?I remember we used to adjust the position of the AM coil for maximum reception, but the coil I have is fixed on its position, not sure if it stuck itself with time, or if it was glued like that from the factory. My guess is the exact position might not matter much for an oscillator, by I don't have enough hands on experience, thought I might ask first, since those wires are very delicate. This is what I want to use the LC tank for:Fromhttps://www.eevblog.com/forum/rf-microwave/how-can-i-improve-this-cheap-am-transmitter/msg4785965/#msg4785965

author: Xena E
author_rank: Frequent Contributor
timestamp: April 08, 2023, 04:29:50 pm
content: Is there a specification for the coil in the 'transmitter' design? There is no reason why your coil as it is won't function, (it really doesn't need to be litz wire either...)Give it a go! It was made for MW BCB so that's what it should hoot at...

author: RoGeorge
author_rank: Super Contributor
timestamp: April 08, 2023, 05:02:00 pm
content: Indeed, but one end of the coil is coming from underneath all the 60 turns, it's a little too short to use, and the other end is loose and tend to unwind itself. Thus the idea to rewind it all, then to fix the wire in place with either candle wax or nails lacquer, not sure which one would be better.Meanwhile I've searched about the expected Q of an AM rod antenna and it's in the range of 1000.https://www.electronics-notes.com/articles/antennas-propagation/ferrite-rod-bar-antenna/parameters-specifications.phpThat makes me think I might not care to adjust at all the position of the coil along the ferrite bar (and thus to search for the position with the highest Q), the Q is much bigger than than the minimum Q needed for that kind of oscillator.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 08, 2023, 07:23:32 pm
content: If you have an lcr meter then rewind with confidence.Personally I would start again if the coil is damaged . Aim for 180uH going by your comments about the original turns count, if you can't measure the original. put more turns on than you think you will need and then its just an easy job to to adjust coverage by removing turns as necessary.As for wire, enamelled copper or even fine (0.25 ø) ptfe insulated hook up will do if the original is scrap. The Q in circuit won't need to be very high.To fix the windings pass a piece of thread back and forth along the length of the rod three times and temporarily hold down at the ends of the rod with tape, wind your coil over the centre and pass the ends of the wire through the thread loops and pull the ends of the thread to hold the coil in position. If you use adhesive it will preclude modification.Good luck.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 08, 2023, 09:08:01 pm
content: Thanks!Eventually got a breadboard-friendly AM rod antenna, and now the whole lab smells as fried aspirin. Used aspirin to solder the ends through the wire isolation.I don't have any decent LCR-meter, but I have an AWG generator and an oscilloscope. Might make more measurements tomorrow. Until then, I've used this universal T4 transistor/LCR/ESR tester thingy, that autodetects whatever was connected to it and can measure almost anything:The restored AM rod antenna measures 2.7\$\Omega\$, 0.20mH for the big coil, but the T4 can not measure the small coil. It thinks it's a 0.22\$\Omega\$ resistor.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 08, 2023, 10:08:59 pm
content: Nice job... 0.2 mH seems about right, I'm sure it will work fine.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 09, 2023, 11:18:44 am
content: Hope so, if I got the time to build it today.Meanwhile measured Q, out of curiosity.It is nowhere near the Q=1000 as stated in that link from electronic-notes. Q=1000 would be way too high. Only later realized that if it were to be 1000, then the bandwidth of the antenna would be too small for an AM radio. I've read the space between AM channels is 9kHz, thus the useful audio bandwidth of about 5kHz. At Q=1000 and, say 1MHz, the antenna bandwidth alone would be only 1KHz, too narrow for an AM broadcast.The measured Q is about 55 (measured 14kHz RF bandwidth at f=762kHz, with a parallel polystyrene film C=180pF).The setup was very sloppy: clip together alligators from the AWG, and that's it, Q-meter!- the red and black wires from the generator were clipped together, thus making a 1 turn coil- then the LC tank to measure was placed in the the middle- then clip the oscilloscope probe on the parallel LC tank (probe loop placed perpendicular on the generator loop)- find the resonant frequency, note the amplitude, then sweep left and right until the voltage drop to 0.707*Vresonance- calculate Q as 762kHz/14kHz ~55 after averaging a few more measurements

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 09, 2023, 02:17:38 pm
content: Quote from: RoGeorge on April 08, 2023, 05:02:00 pmI think positioning the coil on the bar is to tune it to the correct frequency (think about all those small coils with a ferrite tuning 'screw') and not to find the highest Q. Yes, there are small trimmer capacitors on the tuning capacitor but they have a small range.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 09, 2023, 06:37:39 pm
content: Thank you for pointing that, changing the resonant frequency for a better tuning makes more sense, indeed.I was not sure about the effect of shifting the coil, and assumed it might increase Q only because we used to slide the coils when we were kids until we get maximum volume.

author: RoV
author_rank: Regular Contributor
timestamp: April 09, 2023, 08:52:33 pm
content: Quote from: PA0PBZ on April 09, 2023, 02:17:38 pmI confirm. Actually, the trimmer cap had a much larger effect on the top of the band (where the variable cap is at the minimum capacitance), while moving the coil had effect on all the band. So their adjustment was combined, by iteration, to optimize both the low and the high sides of the AM band.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 10, 2023, 09:20:23 pm
content: He, he, guess I'm an AM pirate now! On a radius of only half a meter or so, but still.Side note, that schematic works horrible as an AM modulator.

author: radiolistener
author_rank: Super Contributor
timestamp: April 15, 2023, 10:13:31 am
content: Quote from: RoGeorge on April 10, 2023, 09:20:23 pmAdding two RD16 mosfets with driver and RF transformers can extend the distance

author: A.Z.
author_rank: Super Contributor
timestamp: April 15, 2023, 03:06:50 pm
content: Quote from: RoGeorge on April 10, 2023, 09:20:23 pmhave a look herehttp://techlib.com/electronics/amxmit.htm

author: RoGeorge
author_rank: Super Contributor
timestamp: April 15, 2023, 03:25:48 pm
content: Quote from: radiolistener on April 15, 2023, 10:13:31 amThanks, the plan was to sit the mp3 player+modulator near a boombox that has AM radio, so to listen to audiobooks in the kitchen. Half a meter Tx distance would be enough. The problem is that it works very bad. The carrier is assimetric, and indeed the radio can receive it at the second harmonic, too, but much lower in amplitude. At main frequency is distorted, and very finicky with the power voltage.I've posted some screenshots in the thread that made me dig for that antenna:https://www.eevblog.com/forum/rf-microwave/how-can-i-improve-this-cheap-am-transmitter/msg4807103/#msg4807103Those waveforms were captured from the second coil, of only 6 turns. Later I've tried to change the bias, then I've stripped away everything and kept only the Peltz oscillator, tried many types of transistors, and still not happy with the results.I'm thinking of replacing the schematic entirely, and pick something else that works properly. Took a peak at what hams are using for their AM modulators at ~1MHz.

author: A.Z.
author_rank: Super Contributor
timestamp: April 15, 2023, 09:28:50 pm
content: did you ever look at the schematic at the link I posted ?!?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 15, 2023, 10:09:31 pm
content: When I've wrote to abandon "the schematic" I meant to abandon the schematic with 3 transistors posted at the beginning of the topic, not the one you linked.Seen yours, thank you. I'm considering it. I don't have any quart in the MW band, so I'll have to use something else for an oscillator.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 15, 2023, 10:32:26 pm
content: I've read and found the LC osc example to replace the Quartz osc.Did I understood correctly the regulations, that a max 100mW home Tx is allowed in the same range with the MW broadcast stations? I thought this band would be forbidden for any Tx power. I'm not a ham and don't have a call sign either.Is it legal to build a 100mW AM Tx at home, in the MW commercial band?

author: radiolistener
author_rank: Super Contributor
timestamp: April 16, 2023, 07:31:59 am
content: Quote from: RoGeorge on April 15, 2023, 10:32:26 pmIt depends on the country and frequency band and bandwidth, usually from 10-25 to 200 mW is allowed, but for MW band this is nothing due to high power looses, since it needs full size antenna and there is a lot of noise. But on the other hand MW broadcast band is not used for commercial and government anymore in many countries and doesn't have so strict regulations. I think if output has a good quality with good out of band suppression, output with 5-10 W will not be a problem. But again, it depends on the country, some countries have more strict regulations.Usually power limit is specified for isotropic radiator with no loss which is close to a full size half wavelength dipole which has a gain about 2.15 dBi. But such wavelength (200 meters) have significant wire resistance and as result it have some power loss. If you're use shortened antenna, you will have a lot more power loss (due to power loss multiplied by Q factor which is very high for a shortened antenna), so the actual transmitting power will be much smaller.

author: A.Z.
author_rank: Super Contributor
timestamp: April 16, 2023, 08:20:06 am
content: Quote from: RoGeorge on April 15, 2023, 10:32:26 pmsince you're at it, have a look at the AM modulator used herehttp://qrp.gr/classic40/it's pretty slick imHo

author: RoGeorge
author_rank: Super Contributor
timestamp: April 16, 2023, 05:08:53 pm
content: Yes, nice one.Meanwhile I've modified a little the initial schematic, like this:It modulates well, but the carrier is far from sinusoidal. In practice it radiates very weak. Can barely receive it at half a meter away. No external antenna so far, only the ferrite rod.Then I've wired the coil alone to the AWG, set the frequency as in the radio, and still can't hear it farther than 1 meter, not even with 10Vpp from the AWG. However, if I remove everything and use myself as an antenna, I can hear the AWG signal strong enough in certain positions.I think it will never work farther than 1 meter without an external Tx antenna.

author: A.Z.
author_rank: Super Contributor
timestamp: April 16, 2023, 05:25:41 pm
content: take the time to read the notes about antenna coupling at the techlib link, and also ensure to read the notes regarding the modulation level adjustment

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2023, 09:37:05 am
content: The modulation in the LTspice plots is bigger than normal, I think. It is so because the other schematics were having stability problems for the oscillator when modulation goes near 100%, so the simulation was for a bigger modulation factor.I've read AM broadcast uses 20...40% modulation factor, depending on the type of the program, voice or music:https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BS.2433-2018-PDF-E.pdfIn practice I've looked with the oscilloscope at the carrier, while injecting 1kHz audio with the generator. Tried all, from 10-20% to overmodulation. The carrier was stable and there were no audible distortions unless when going to more than 70% or so.The problem is the ferrite antenna alone does not radiate strong enough to cover the RF polution. Along the entire MW broadcast band, on each channel, the radio receives much stronger humm and vrrr and SMPS noises than the 2 national stations it can still receive. The audio from the broadcast is probably 20dB lower than the noises, though when tuned on the exact radio channel, I can hear only the broadcast, not the noises.While typing the above, I now realize maybe the noises I hear is not coming through the RF, but are induced somewhere else in the reception path of the radio. Maybe the interference is induced in the 455kHz intermediary frequency.That is if it has an intermediary frequency, because the radio is digital, it can only jump from one channel to the other, in steps of 9kHz between 531-1602kHz (the radio has a frequency display, and the displayed frequency is correct - it matches with the signal from the AWG). I've noticed the position of the telescopic antenna of the radio has a big influence. Being digital and getting the MW signal from the telescopic antenna makes me think the receiver might not have an internal ferrite rod antenna at all. I guess if it were to have an internal ferrite rod, that would easily couple (magnetically) with the ferrite rod from the AM modulator, but I didn't observe that in practice. And if I rotate the radio relative to the AM modulator, the difference is not as big as I remember from analog AM radios with only a ferrite antenna.About antennas, I'll have to add one (electrical, not magnetic), like I've seen in your box. I've plugged random wires to the modulator output, but didn't make much of a difference. I guess it has to be a matched antenna.That would be my first Tx antenna.I know most of the theoretical aspects, reflections, impedance matching, transmission lines, S parameters and such, just that I have zero hands-on experience. Well, at least the frequency is low enough that I can see the carrier on the oscilloscope, and have a dual channel AWG to generate good signals. No spectrum analyzer and no VNA, but I have a home lab with enough electronic scrap to improvise a reflectometer for MW.I don't even know if tuning a MW antenna is really needed to be done by the books, or it can be done without measuring instruments. I remember in the magazines from when I was a kid, they were tuning the 27.12MHz Tx antenna with a miniature light bulb in series with the antenna (for remote controlls, in modelism). Will see, sounds like fun.