Help with small transmitter problem - Page 1
Forum section: electronics / projects

author: vk3pb
author_rank: Contributor
timestamp: December 26, 2015, 10:35:56 am
content: Hi Dave & allI have a small balloon transmitter project that I am working on that needs a little advice.Basically my plan is to build a lightweight battery powered transmitter to transmit geographic coordinates derived from a gps on an amateur radio frequency, and then launch this on a picoballoon (small balloon like you get in party shops).To do this without needing permission or a license my payload must be less than 50 grams total!The good news is that I have a working prototype which uses an Arduino mini, AD9850 direct digital synthesizer (dds) and Neo-6 gps. It runs well, transmitting on 10.140 Mhz however I have two problems.The first is that the output of the AD9850 is very low - probably less than 10 mw. I need to amplify that power a little - say 10x. I also need a little filtering because the output of the dds is not so clean. I am getting output on other frequencies but at lower levels.I have looked around the internet for circuits to achieve the above but the ones I have seen are way too complicated and more importantly - heavy. I need a simple circuit of no more than say 4 or 5 components that can be put on a small piece of breadboard and the total would weigh less than 10 grams. Can anyone help with a simple circuit that can give some amplification and a little filtering. The filtering does not need to be perfect as at these power levels no one is likely to receive the harmonics anyway.Thanks Peter

author: mikerj
author_rank: Super Contributor
timestamp: December 26, 2015, 12:10:00 pm
content: The frequency you are using is in the 30meter Amateur radio band, and there are hundreds if not thousands designs for low power transmitters and power amps on the internet. You should be able to get plenty of power output from a single transistor stage, and you won't need any kind of exotic/expensive transistor.Search for "30m QRP amplifier", e.g. the class C driver stagehereshould be more than adequate. It's also possible to use CMOS logic devices such asthis design based on a 74HC240, which also includes an output filter.There is a useful design guide for harmonic filtershere.

author: retrolefty
author_rank: Super Contributor
timestamp: December 26, 2015, 12:29:42 pm
content: Probably need a little more information. What form of modulation are you using? A small MMIC IC amp chip should could get you a good +10db gain with little weight but the added current flow may require a larger battery capacity for the same flight time duration?Your choice of 10MHz is rather unusual as well as probably technically illegal. Applications like this are usually line of sight VHF or UHF as antenna efficiency is much better with a higher operating frequency, but of course your DDS limits your choice to HF.

author: vk3pb
author_rank: Contributor
timestamp: December 26, 2015, 01:38:29 pm
content: Thanks mikerj and retroleftyI shall look at those links.As an amateur I'm permitted to use the 30m band. The plan is to use wspr mode which means that even with a small amount of power my transmitter can be received over long distances. If combined with some Arduino sleep routines I will hopefully be able to get the batteries to last a few days.As for modulation wspr is fsk with 6 hz bandwidth.Cheers Peter

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 26, 2015, 01:45:44 pm
content: But doesn't using lower frequencies mean you need a physically larger antenna to achieve any decent efficiency?

author: davebb
author_rank: Frequent Contributor
timestamp: December 26, 2015, 01:53:59 pm
content: Hi Peterif you use your aprs frq on 2m vhf,your signal will be digipeted on to google maps.fi and your antanna wll not need to be so long i dont know what power you are alowed to use unatended in vk land,uk is 500mw,i would go for 2m aprs and it has been done a lot ,so loods of info on the net and yahoo aprs groupsgood luck Dave 2E0DMB

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 26, 2015, 04:38:26 pm
content: Yes, if you are operating in a covered area, APRS will provide real-time tracking.Ref: APRS Australia:http://www.aprs.net.auExample: Sydney area:http://www.aprs.net.au/live/sydney-area-interactive-aprs

author: vk3pb
author_rank: Contributor
timestamp: December 26, 2015, 10:42:25 pm
content: ThanksYou are correct that one can use APRS on 2m to send positional data into the aprs network but one you go out to sea you can't track the balloon any more. Although hf requires a larger antennà it can be used with wspr to send data over very large distances.Cheers Peter

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 26, 2015, 10:57:39 pm
content: How far out to sea are you talking about? I see many maritime stations being tracked on the aprs.fi website.If you are so budget-constrained on weight, how do you expect to deploy an efficient HF antenna? Of course, since you have not revealed your location, we have no idea what the APRS coverage is in the area of interest.