6-50M 10W transmitter - Page 1
Forum section: electronics / projects

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 15, 2013, 03:26:34 am
content: Hi.I am working on a meteorological-like balloon thing which contains a raspberry pi and a ton of various sensors.I got it almost all working, and now the only thing missing is the transmitter circuit.The idea is to try to use 3G, but somehow I fear there is no 3G signal in the stratosphere.I was planning to use four crystals for generating 6/12/24/48 MHz carrier frequency, so that it can switch between them if one of the channels has a lot of noise, or if it is in use.So what I know how to build is the carrier frequency oscillator, and some sort of modulator, but then I need a amplifier for the output transmitter stage, and that is what I don't know how to build.I already tried to build a small one, but I used vacuum tubes (EF80), since I don't have any power RF transistors.I already googled for shortwave transmitters, but I usually get a fixed frequency schematic. Also those schematic usually would contain all sorts of inductors, and I am not sure what is their role. So could somebody clarify how such transmitters work, and how could I build one.About the output power, I figured that 10W should be enough to receive the the signal if the transmitter is up to 100km away and more than 10km high.I usually deal with computer related stuff and digital electronics, so that is why I am not an expert in RF related topics.I already know about the radio amateur license thing, so no need to mention that.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 15, 2013, 03:41:04 am
content: Not a lot of loss between the stratosphere & ground,so 3G may work OK!As far as HF Radio (or any unlicenced frequency),you may fall foul of the Regulatory Authorities in your own,& adjacent countries.Maybe your University has its own frequency allocations,but they may not apply over the border in any case--these things can travel a long way!Many balloons use Amateur Radio frequencies---usually in the "2 metre" band,so if there are any Hams in the Uni,or you can link up with some outside,they will probably jump at the chance to help you out.The Hams use a system called APRS to track balloons,cars,people,etc-Google for "APRS & balloons " & see what you find.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 15, 2013, 04:11:14 am
content: QuoteI already payed a visit to the department of wireless communications, and they only use a small transmitter which range is limited to the faculties facilities, so they don't need any licenses.The radio trasmitter is only a backup plan, and not a primary method of communication. It will only be turned on if the 3G link breaks, and it will transmit for a few hours max. I'll try to obtain a proper license by the time the project is done, if not, I'll just go without it, hopefully nobody will notice it.About APRS, I am not sure how exactly APRS works, but I heard about it before. I didn't go for it since our balloon will have lots of telemetry data from the sensors, and it will even stream pictures form the on board camera. The concept of the contraption is that there is the central computer (raspberry pi) which runs a Java program (which I wrote) and it reads from MEMS sensors on the I2C bus, and some other sensors via a PIC. If the 3G link beaks it will switch to streaming the same data to the PIC which will be interface to the transmitter. On the other side there will be a RF preamplifier and a quartz based frequency filter, which will select the band. The signal will the go to a A/D converter which is read by a 8051, and the data will be sent via a FT245 to the client PC which will be running the client software that I made in Java as well.I am also not exactly sure how the receiver side should look like, but I hope that I will figure out something, if not I'll just start a new topic.

author: SeanB
author_rank: Super Contributor
timestamp: June 15, 2013, 08:56:08 am
content: If you are going to stream video you will be stuck with high frequencies. Best is to look at 400MHz or higher amateur TV transmitters, as these are the only things that will have the bandwidth to send video. Lower frequencies will not be able to do it, unless you want to send images every few seconds.In any case you will need an antenna on the ground to receive that will be directional, and the higher the frequency the easier and smaller the transmitter antenna will be ( lighter) and the ground antenna will be smaller as well. If you use 2.4GHz then the antenna is very small, and the transmit power can be up to 500mW so easing the beam width of the receiver antenna and making aiming less critical. The problem with using 3G is the cell stations are designed not to emit up into the sky at all, but only in a narrow vertical beam to maximise the ground footprint. Up over 1000m you will get almost no signal even if you can see the antenna arrays.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 15, 2013, 09:29:27 am
content: QuoteThe video stream will work only when 3G is present, when not, it will slowly stream a picture every few minutes.QuoteIf using short wave, a directional antenna might be huge.Also it should be able to move freely, so that it can track the thing.QuoteDealing with such frequencies is a bit problematic, since I need dedicated components, antennas and expensive equipment. My budget is small and I can't afford such equipment. Short wave has a fairly big range and it is inexpensive to build transmitters, antennas and receivers. I also can't use a directional antenna on the transmitter, since the wind will constantly spin the balloon.QuoteThat is why I need this backup plan.

author: kolbep
author_rank: Frequent Contributor
timestamp: June 15, 2013, 09:30:38 am
content: What about storing The data on the Device, Then You only Need the Telemetry And Triangulation For Recovery

author: Bored@Work
author_rank: Super Contributor
timestamp: June 15, 2013, 11:24:25 am
content: You should probably step back a bit, curb your enthusiasm, and replace it with a bit of thinking and taking responsible actions. That includes to operate the transmitter legally. Ok, I have understood you give a fuck, but your local communication regulator likely doesn't.Further, for lifting a few kg on a balloon in to the stratosphere you need approval of your local authorities overseeing the air space. You won't get approval if they figure out you use an illegal transmitter oe if you can't prove everything is done legally (and safe), and they will question your general competence. Doing it without approval? Well, air space authorities tend to get rather pissed if you mess with them.And even further, your air space authorities will very likely require that you are covered by some insurance. A parachute failure and a few kg heading down quickly to earth is not fun ... You won't get insurance or the insurance company will later refuse to cover you should they learn that some parts of the balloon were operated illegally.So start doing your legal homework.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 15, 2013, 11:52:37 am
content: About the legal things.The project it is not actually a project of mine, I am in a sorta of a club.When I joined them they were already working on this, but then I took over the hardware part, and built it myself, but I left the rest to them, since it is theirs project after all.I am not also responsible for the balloon itself and the parachute. They told me that they will obtain them both, so I don't have to worry about those things.They already mentioned the airspace issue, and they already planned for getting that papers done.About the radio license, there launch is not even scheduled yet, so there is time to get one.So please stop mentioning the license thing, since I already know everything about it, and it is not my job to get the licenses done.About storing data. Data will be stored, but when I joined the project the requirement to have a live telemetry was already set, so there is noting I can do about it.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 15, 2013, 12:35:01 pm
content: Quote from: Dajgoro on June 15, 2013, 11:52:37 amI understand your point, but try to understand what we are saying too: Wrong frequency -> No Licence -> No insurance. So even if it is not your job to get the licensec done it is your job to deliver hardware that will not preventthemto get the proper licences/insurances.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: June 15, 2013, 12:50:27 pm
content: Quote from: Dajgoro on June 15, 2013, 11:52:37 amProject management 101. Step back, take a deep breath and start to see the bigger picture.Projects have failed because people have gone off and done their own thing and then when put together stuff wasn't compatible. And there have been misunderstandings because it was 'not their problem' and there was the assumption that someone else would do it.You say you know all there is to know about licensing. You also say that it can be sorted at the last minute. But by your own admission RF design knowledge is lacking.The thing is that transmitters come in all sorts of frequencies, modes, capabilities, power outputs etc. Plus they may affect other parts of the system, eg power consumption, weight, size, antenna requirements etc.The wrong approach is to build the transmitter first and then look at licensing later. Why? Because the transmitter may be built for different frequencies than are allowed, and modification may be difficult. Everyone else will have their bit done but you might not due to a need to rebuild at short notice (which risks reliability).The right approach is licensing requirements and legalitiesfirst. A 10 watt transmitter high in the air can blot out an entire channel over a whole city or even small European country. Of course legal doesn't necessarily mean it's suitable so there's also a need to investage propagation and signal theory, plus practice from others (link later).Then you'll know exactly what sort of transmitter you need to get or build (most likely 10 watts VHF or UHF rather than HF). Instead of a thousand different designs and configurations you may be down to a few - much more useful. I would strongly suggest extensive experimentation on the ground before launching to ensure suitability and reliability.The very successful Project Horus might give some pointershttp://projecthorus.org/

author: MikeK
author_rank: Super Contributor
timestamp: June 15, 2013, 03:05:58 pm
content: 10 watts is a massive amount of radio power up in the air. Many balloon guys are doing it with 1/2 watt or less.The Alinco DJ-C1 and DJ-C4 transceivers made a good backup. They are 1/3 watt, I think. You can often find them on eBay for around $50 and they are small and lightweight. You'd still to give it a proper antenna, though. A simple dipole is easy to construct.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 15, 2013, 05:40:11 pm
content: Ok, so at this moment I don't know how to build any proper transmitter at any frequency.I can either choose HF or VHF, since anything above that might get a bit expensive.If it turns out that the transmitter can't be used due to license issues, we will just remove it, since the project is modular, and stick with 3G. That means we will have only the beginning and end of the trip, but better that than nothing.About the output power, I don't know how much is necessary, 10W is what some people told me when I asked. Actually it would be better if it would be less, since that would save the battery power.I went for this specific frequency because I can use regular quartz crystals both on the transmitting and receiving side, which would mean that it could be always properly tuned, and the other reason is that when my friend scanned the spectrum with a SDR, this part was unused with little interference, where above that you get radio, tv, and all of the rest.The Alinco DJ-C1 and DJ-C4 are bit hard to find to buy, and they seem a bit expensive.About the experimenting part, we figured to take the transmitter on the top of a mountain near by, and then go to the rooftop of a friends building, and see if we receive something. Also I would need to test how good are various protection codes.For now I don't have any schematic, and I don't even understand how exactly a output stage of a transmitter works, and I don't like building things from schematics that I don't understand.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 16, 2013, 04:34:23 pm
content: Regardless all of the legal stuff and the project, now I am starting to be really curious how the output stage of a proper transmitter works?

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 16, 2013, 07:38:37 pm
content: Quote from: Dajgoro on June 16, 2013, 04:34:23 pmThat is a rather broad question: in short it is a tuned amplifier, and depending on the modulation mostly class A or C. I'm sure there is a lot to read about out there, but if you have any specific questions let us know.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 16, 2013, 10:47:40 pm
content: What is the advantage of being tuned? Why can't it be a wideband amplifier?

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2013, 05:21:53 am
content: Simple answer is regulations, you have to keep to within the allocated band that it is to be used for, and must have below a certain amount of power at any point outside that band. A broadband amplifier will amplify the noise in the devices and transmit that, and that both wastes power and annoys the other users of the shared resource, and if you stomp on a military frequency that is in use they tend to get really annoyed with you, not "please stop" but " we come there and lock you up and smash everything including you" annoyed.A tuned circuit concentrates the input power into the desired signal, and reduces wasted energy in the unwanted bands. Most RF amplifiers are designed to be efficient, so the output stages run in class C, as this is the most efficient. Lower level stages are run in class A, as this enables you to do mixing and modulation properly. Filtering and tuned stages are needed in class C operation, as they generate a lot of harmonics and the filtering is needed to reduce these ( or to select the right ones if you are using it to multiply frequency as well) to a low level.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 17, 2013, 04:50:03 pm
content: How are then made the output stages of transmitters with variable frequencies, like in communication devices, such those used in radio amateurs?

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2013, 05:19:13 pm
content: These are defined bands, and use a series of switched tuned circuits, one set per band. You have a broad tuned circuit for each band and a few switched units covers a large number of bands. They are controlled for each band by either a series of switched controlling relays or by diode switches for lower levels. For some bands the single set of tuned circuits is enough to cover the whole band, especially as the frequency goes up. They are designed to not be terribly selective but to have good attenuation outside the bandpass desired.If you are using a single spot frequency they are adjusted to be a lot more selective, and this then increases the power efficiency at that particular frequency.

author: ve7xen
author_rank: Super Contributor
timestamp: June 17, 2013, 06:48:16 pm
content: Quote from: Dajgoro on June 15, 2013, 04:11:14 amAPRS can handle telemetry as well, though the protocol is a bit archaic. You may not want to use APRS itself, due to protocol limitations and inflexibility, but instead the packet radio protocol it's based on - AX.25. This provides 1200bps packet radio using 2-AFSK or 9600bps using 2-FSK (though this can be trickier to implement properly). You'd want to avoid using the APRS repeater network, at least if you're wanting to stream a lot of realtime telemetry (it's generally considered poor form to broadcast more than a couple APRS reports per minute). What you'd probably do is choose a packet frequency in the UHF ham band and set up a more point-to-point link. You might be able to run it on one of the ISM bands even, though probably with more interference.For radio hardware it probably wouldn't be hard to hack one of the cheap Chinese 2 watt UHF radios for this. On the receiving end you might want something a little better. You'd also want to build yourself some kind of directional antenna. You'll have to experiment regarding antenna gain and range.One-way AX.25 is fairly easy to implement in a microcontroller; I have done it for AVR using the PWM peripheral to output. Add a prebuilt UHF/VHF radio and you've got something legal (assuming you or someone on your team has / can get an amateur radio license) and reasonably easy/inexpensive to implement.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 17, 2013, 07:48:29 pm
content: Thanks for the tip, I'll look into AX.25.

author: MikeK
author_rank: Super Contributor
timestamp: June 17, 2013, 08:31:49 pm
content: I saw there was a Hungarian guy who flew a balloon last week. Perhaps he can give you some pointers. His flight was posted athttps://www.facebook.com/hungariannearspaceflights

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 18, 2013, 12:24:56 am
content: Thanks for the link.Actually that happened less than 200km from my current position.

author: KJDS
author_rank: Super Contributor
timestamp: June 18, 2013, 09:39:43 am
content: It's quite easy to get a broadband output stage, but you'll lose out on power and efficiency compared with a narrow band design.For VHF and above getting good performance is a very specialized subsection of RF design.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 18, 2013, 10:28:35 am
content: QuoteIs there such a schematic that has that sort of configuration that I can take a look at?

author: KJDS
author_rank: Super Contributor
timestamp: June 18, 2013, 11:03:42 am
content: Have a look at the application notes on the NXP website to give you some ideas.

author: bookaboo
author_rank: Frequent Contributor
timestamp: June 18, 2013, 11:42:23 am
content: Have you looked at off the shelf modules?radiometrix, rfsolutions or LPRS (all uk based).I know these guys did something with some form of rocket project but I dont see it on their website anymorehttp://www.radiodata.co.uk/

author: cowana
author_rank: Frequent Contributor
timestamp: June 18, 2013, 12:55:04 pm
content: I've built and flown many balloon payloads, all using the Radiometrix NTX2 transmitter. This is a 10mW transmitter running on theworldwideedit: Region 1licence free (and okay for use in the airedit: with a 10mW power limit) 434.xxxMHz.By using a 1/4 wave aerial on the balloon, a sensitive amateur radio receiver (I use an Icom IC7000), and low data rates (50 baud), you can get amazing range from a 10mW transmitter.The record for one of the NTX2 based payloads I have worked on is 775km radio range (10mW @ 50 baud) and 677km (10mW @ 300 baud). Note those distances were achieved using multi element yagis on the receiving end.Andy

author: bookaboo
author_rank: Frequent Contributor
timestamp: June 18, 2013, 01:58:56 pm
content: 775Km wowThat's incredibly impressive from 10mW, I usually expect 100-200m on the ground from that power level.However with a low baud rate, clear line of sight and well constructed antenna I can imagine a lot more.I'd go with this solution.

author: KJDS
author_rank: Super Contributor
timestamp: June 18, 2013, 03:40:23 pm
content: Quote from: bookaboo on June 18, 2013, 01:58:56 pmI'm not sure that 50 baud video will be quite good enough.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 18, 2013, 07:40:01 pm
content: Quote from: bookaboo on June 18, 2013, 01:58:56 pmIf you do the math:10mW is +10 dBm. Assuming that the transmitter antenna has a gain of 2dB, and the receiver antenna 18 dB the total power is +30 dBm.The free path loss on 434 Mhz for 775 Km is about 143 dB, so you end up with -113 dBm at the receiver which is about 0.5 uV.The Icom 7000 needs about 0.18 uV on 434 Mhz for 12dB SINAD, so it is not impossible at all, you can even have a bit of loss in the cables etc.

author: minibutmany
author_rank: Regular Contributor
timestamp: June 18, 2013, 10:42:22 pm
content: I dont know about radio regulations in croatia, but I would say you should use some sort of unlicensed radio service such as MURS. In the US the MURS frequencies are right next to the 70cm amateur band, so I thought if you had such a service then you could just modify an ATV transmitter made for the 70cm band to work on the MURS band. In terms of an amp, you probably are not going to need more than a few watts if the balloon is still within a clear path of you...just use a good resonant antenna. Generally ATV transmitters already put out about a watt, but if you think you need more power you could try your hands at building an amp, but unless your super pro, UHF amps are pretty tricky especially if you want a clean output which is kinda necessary for video.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 19, 2013, 07:58:39 am
content: Quote from: cowana on June 18, 2013, 12:55:04 pmIt is slap in the middle of the 70cm Amateur band,& in some countries may not be useable at all.In Australia,for instance,unlicenced 434MHz devices are tolerated as "Low Interference Potential Devices"(LIPDs) which have strict power output limitations.As the name implies,they should not cause interference to licenced services.(Unfortunately,they do!)Conversely they must accept any interference from licenced services.

author: MikeK
author_rank: Super Contributor
timestamp: June 19, 2013, 12:38:26 pm
content: It's in the ISM band for Region 1. Australia is Region 3.

author: cowana
author_rank: Frequent Contributor
timestamp: June 19, 2013, 04:02:53 pm
content: Thanks for the correction - as Mike says, it's in the Region 1 ISM band - although worth pointing out that is with a 10mW limit on power for airborne use.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 19, 2013, 10:31:39 pm
content: I guess that some fancy equipment is necessary to receive the signal from a 10mW transmitter.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 20, 2013, 03:45:15 am
content: Quote from: cowana on June 19, 2013, 04:02:53 pmAlso;--Beware of footnotes--a particular country may opt out of the general conditions for a Region.Thus,there may be footnotes which haven't been revisited since WARC'79 or earlier,even though there have been several WRCs since then.

author: MikeK
author_rank: Super Contributor
timestamp: June 20, 2013, 04:18:10 am
content: Quote from: Dajgoro on June 19, 2013, 10:31:39 pmNo, just something high gain. Like a Yagi. You can even make one from a cheap tape measure.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 20, 2013, 07:38:16 pm
content: How precise should be a yagi pointed to the source to get a decent signal. If it is really true that such low power is needed than that makes things much easier.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: June 21, 2013, 11:21:23 am
content: Quote from: Dajgoro on June 20, 2013, 07:38:16 pmThe precision of pointing is directly proportional to the length (or more strictly speaking gain and beamwidth) of the yagi.A 3 or 4 element yagi has a gain of about 7 - 9 dB above a dipole. You could swing it maybe 20 degrees and the signal won't drop much.Whereas a 20 element yagi (a monster on VHF but possible on UHF) will have a gain of maybe 14 - 15 dB but will need to be pointed within a few degrees.A higher gain big dish would be even more harder to point accurately.

author: minibutmany
author_rank: Regular Contributor
timestamp: June 22, 2013, 11:41:04 am
content: If the transmitter is up in the sky somewhere and the only thing between you and it is air, you shouldn't have any trouble receiving it with a directional antenna pointed at it, even if only at 10mw. Hams talk to the ISS on 2 meters with less than a watt all the time

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 23, 2013, 06:33:47 am
content: How do you calculate what will the output power of the transmitter be?Lets say a transmitter has 12V@48MHz(at some arbitrary output impedance) of at the antenna connector, and the antenna is a monopole 6.25M long?And how do you calculate the output power for other bands?

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: June 23, 2013, 11:14:11 am
content: Quote from: Dajgoro on June 23, 2013, 06:33:47 amThe antenna (or more precisely its gain) can affect 'effective radiated power' but not output power, which is determined by the final stage of the transmitter.Output power can be measured with an RF power meter. This is effectively an RF voltmeter across a 50 ohm dummy load.DC Power input can be calculated by multiplying the current drawn by the power amplifier's collector with its voltage. This would be twice the RF output power if the stage is 50% efficient.The antenna needs to present a 50 ohm resistive load to the final stage for there to be maximum power transfer and minimum feedline loss. This can be achieved with a matching network at the transmitter and/or antenna end. The latter is favoured if using relatively lossy feedline (eg coaxial cable at HF/VHF/UHF).Generally an output transistor is more efficient at lower frequencies than higher frequencies. Especially at VHF/UHF it is hard to have the same stage amplify evenly over a wide range of frequencies unless there's some smart circuitry that alters drive and/or bias settings at various frequencies. Transistors designed for HF probably won't work that well on UHF while UHF transistors may have excessive gain on HF.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 23, 2013, 03:00:16 pm
content: QuoteHow do you match the antenna end, if the antenna is just a wire?Can you explain this matching business a bit more?

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 23, 2013, 06:42:27 pm
content: An antenna is not just a wire, it needs to have the correct properties for a certain wavelength. It is also not a monopole, there is always 2 poles but one of the poles could be the ground, the transmitter housing or something like that. The basic antenna is 2 quarter wavelengths of wire, a dipole. ___ ___ The characteristic impedance of such a dipole is about 75 ohms, but this can be varied by choosing the thickness of the wire. The radiation pattern is like an 8, with the maximum radiation at right angles to the dipole -8- . To get more gain out of an antenna you add extra passive elements in front (directors) and back (reflectors) of the dipole, and that is called a yagi antenna. The more you bundle the beam, the more gain you get in the direction of the directors and the less gain you get in the other directions. |--[-|-|-|-| ->Adding more elements also lowers the impedance a bit, and 50 ohms is what most modern antenna's are.Matching the impedance between the power amplifier and the antenna is done to because your source and load need to have the same impedance to get the optimum power transfer. This can be done in different ways like an HF transformer, an LC network and even with a piece of coax with a different impedance.