MW antenna to fit in a small box - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2023, 06:24:49 pm
content: What MW antenna should I use for a low power (in the same room) AM transmitter (~360m band)?The Tx frequency is fixed, 819kHz, AM radio, channels are 9kHz apart.The wooden box inside which I would like to fit all (transmitter + power supply + antenna) has 30x8x8 cm, or ~12x3x3 inch. The box exists already, it's a former showcase from a bottle of wine. Also, the transmitter is right near the antenna, so there is no long transmission line between them.I've seen indoor MW antennas, but usually they are on a wooden frame about the size of a TV screen or so. Since I'm not interested in best antenna gain, but only to transmit good enough to cover the same room, can an MW antenna be made smaller? I have close to zero practice with antennas.-1. Is there any chance to fit an MW antenna inside that small wooden box?-2. Can it be made omnidirectional, or at least with a wide enough lobe? Would be nice to cover the entire room without reorienting the Tx antenna.-3. Do I have to design both the transmitter and antenna for 50 ohms, or do I start from the antenna properties, then use a matching network just right for the given transmitter?

author: MarkT
author_rank: Frequent Contributor
timestamp: November 11, 2023, 06:55:54 pm
content: Small MW antennas are ferrite rod antennas. Nearly every MW receiver built since 1960 uses such an antenna. Basically a magnetic loop antenna with a core - the antenna sensitivity scales with the length of the rod IIRC.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: November 11, 2023, 06:57:50 pm
content: Hi RoGeorge,1.: Yes. Do you remember the japanese transistor-radios (AM only)?You could carry them in the pocket.2.: Probably yes. You'll have to experiment with the orientation of the antenna-coil (ferrite-rod).3.: Sorry no idea, i'm an audio-guy. But i know that the impedance is 120 or even 240 Ohm for AM-radios.Good luck

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2023, 07:17:36 pm
content: The problem with the ferrite rod antenna is its directivity. I guess a ferrite rod antenna would be too directional (but I didn't try).From what I remember from the AM radios, the ferrite rod antenna has to be pointed in the right direction. I would like to cover with signal the entire room without the need to change the orientation of the transmitter antenna, no matter in which corner of the room is the receiver.Would it be possible to make an MW antenna small enough for that box, but without ferrite?

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 07:27:20 pm
content: well the best you could do is a 12 inch square loop with alot of turns right? It's still gonna be directional cuz its a coil.

author: MarkT
author_rank: Frequent Contributor
timestamp: November 12, 2023, 10:50:12 am
content: Quote from: RoGeorge on November 11, 2023, 07:17:36 pmVertical polarization?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 12, 2023, 11:31:27 am
content: Vertical would give you an omnidirectional pattern but 3 inch is not that much (assuming the box will not be vertical?) and you would have to do the same for the receiver.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2023, 12:10:32 pm
content: The receiver is supposed to be any consumer radio, the ones used to receive national AM broadcasted programs. Most I've seen only have an internal ferrite rod (sited horizontal) as a receiving antenna.The only radio I have for receiving tests is a boombox radio. All my back envelope calculations give very bad values for a small antenna, so I've just made some practical tests this morning. I've used the AWG to generate a 400Hz 70% AM signal at 819kHz, then arbitrarily attached various wires, and/or wire loops as Tx antennas. The results were disappointing.The best Tx antenna was when I've accidentally touched a wire to a 2 meters vertical metal pole (a leg of the shelves in the lab).With only one wire to it and 0.5Vpp RF, it covers the room, at more than 4Vpp the boombox can receive with a decent S/N ratio anywhere in the house.I guess I should give up the idea of fitting the antenna inside the box. Maybe I should just add a banana connector for an external Tx wire/antenna. Or use a ferrite Tx antenna.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 12, 2023, 03:15:17 pm
content: Not knowing what your setup will look like but if you could just hang a several meters long wire from the transmitter that might give you enough range.

author: vu2nan
author_rank: Contributor
timestamp: November 13, 2023, 06:43:12 am
content: Such low power medium wave broadcast band AM transmitters fall under the FCC Part 15 rules in the USA that limit output stage power to 100 mW and antenna length to 3 metres.These constraints ensure that such transmitters will have an effective range of no more than about 30 metres.You may use a small length of wire as an antenna that limits the range to the extent of your room only.Nandu.

author: A.Z.
author_rank: Super Contributor
timestamp: November 13, 2023, 06:50:18 am
content: just for "fun", try the following; pick a 1KpF/2KV capacitor, connect one end of the cap to a wire going to your TX antenna port and the other end of the cap to the "neutral" of an AC plug, insert the plug into a nearby AC socket and transmityou may also try putting together a T match, by adding a variable in series with the fixed cap and a tapped coil connected between the two caps and ground, but keepnin mind that it's the AC POWER LINE, so be careful !

author: jonovid
author_rank: Super Contributor
timestamp: November 13, 2023, 10:13:31 am
content: fit anlow power20 or 50mW MW transmitter antenna inside that small wooden boxyes A halo loop type antenna 100feet or 30 meters of wire in a loop as a tuned circuit. rap the wire around the box .will radiate 1MHz RF omnidirectional to the horizon. set up high off the ground. on a wooden table or up on the ceiling of the room.as a loop radius anything from 20cm to 1 meter. try it

author: A.Z.
author_rank: Super Contributor
timestamp: November 13, 2023, 04:49:41 pm
content: Quote from: A.Z. on November 13, 2023, 06:50:18 amjust to be clear, check the attached sample schematic, the capacitor value may be adjusted if desired, the formula is XC(ohm)=159000/(Hz*uF) and with 1KpF it's ok for 1MHz and up, willing to get lower, say 450KHz, the capacitor may have a value of 7KpF (if my calc is right), the neon lamp is optional, but helps fitting the plug so that the neutral will be connected to the capacitor (the lamp will then light on)The use of AC lines as antennas is known since the early '900, nowadays using it as an RX antenna isn't a good idea anymore, due to the noise generated by a bunch of "domestic appliances", but for TX that isn't an issue and you'll have a pretty long TX antenna; notice that it's IMPORTANT to use anY2 class capacitor for safety reasons, also notice that the use of such an antenna to transmit is probably against any existing law/rule/commandment"Thou shalt NOT..." and all thatjust to add a bit of infos, one of the specs which an Y2 cap must meet is that it must "fail open", see why I'm insisting on those ?

author: iMo
author_rank: Super Contributor
timestamp: November 13, 2023, 06:59:16 pm
content: The AC antenna above would work best, sure, on the other hand it could easily happen you would switch off or on a gigawatt oven in the nearby steel company

author: A.Z.
author_rank: Super Contributor
timestamp: November 13, 2023, 08:01:45 pm
content: Quote from: iMo on November 13, 2023, 06:59:16 pmwell, it all depends from the TX power you'll be using, at any rate, didn't you read my note about "laws/rules/commandments"?Oh and btw, let's hope the "CBers" won't find what they could do using such an antenna with an ATU to TX and some separate one to RX... that would really be a hell

author: RoGeorge
author_rank: Super Contributor
timestamp: November 13, 2023, 08:04:23 pm
content: The mains AC antenna is very tempting, just that I'm afraid it will broadcast much farther than intended.

author: A.Z.
author_rank: Super Contributor
timestamp: November 13, 2023, 08:06:16 pm
content: Quote from: RoGeorge on November 13, 2023, 08:04:23 pmwell... you may give it a try and see...

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2023, 05:19:41 am
content: fcc partyvanmore likely then you think if you interrupt some home wiring comms or a greater party van can come if you upset a feeder MW control channel.in the USA they have feeder towers that communicate with LF carried by wire (up to 500KHz). Supposedly its being replaced by fiber lines but yeah... i doubt it would get that far but you really should not try. Like they do exactly what you wanna do but on 100's of kv lines to control switchgear. The power company hates changing anything so I bet there is strong demand to use existing wiring. Especially in recession.and it says they even are using/investigating it for communicating to customers meter for low frequency uplink instead of putting WiFI on it. (smart grid).. possibly in italy.now will it get that far? IDK. not a fucking clue really. I bet it won't go far at low power levels. And its probobly filtered from the mains that gets to you. But if there happen to be some kind of meters that use it, that might get you a problem...

author: A.Z.
author_rank: Super Contributor
timestamp: November 14, 2023, 09:44:42 am
content: @copperconemaybe you missed my note about "rules/laws/commandments"that said, I doubt that a short test transmission with a few mW would trigger the intervention of SWAT

author: iMo
author_rank: Super Contributor
timestamp: November 14, 2023, 01:15:46 pm
content: Yep, my comment above - the electricity distributors/producers here in Europe are using their wires for communication with their assets, and with their so called "megawatt customers" as well, and the distributors can switch on/off customer's large loads via commands when necessary.. I think it runs in XXX kHz band

author: A.Z.
author_rank: Super Contributor
timestamp: November 14, 2023, 04:44:06 pm
content: Quote from: iMo on November 14, 2023, 01:15:46 pmRight so, they use power lines signaling to read and control the current meters (and then some other stuff) remotely, but they use a digital protocol with error correction (now don't ask me how I know that) basically it's an IP stack using powerlines as the transmission media

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2023, 06:04:19 pm
content: The thing I feel like is that they might try. To use you as a scapegoat for problems. Rf is well defined but the power co is kind of like a mafia running it’s own thing . If something happens and your signal was there it might be really easy to pass the buck. It might really stand out

author: A.Z.
author_rank: Super Contributor
timestamp: November 14, 2023, 07:15:10 pm
content: well... before getting to the "SWAT part" let's deal with some mW stuff being xmitted for (say) 15 minutes... pls get down to real life(otherwise someone may get caught after hacking a "devolo" unit to "sniff and decode" some "stuff")

author: radiolistener
author_rank: Super Contributor
timestamp: November 14, 2023, 07:55:35 pm
content: Quote from: RoGeorge on November 11, 2023, 06:24:49 pmYou can put wire coil around the room (1-2 winding), so your receiver will be inside coil and you will receive signal through near field coupling which is much stronger than receiving signal in far field zone.