Graham Graham wrote: > > 1. the requirement was half duplex not full > > 2. a directional coupler has nothing to do with what is needed. > > 3. the suggestion of a 1/4 impedance switcher was a good one, except at vhf > it will be probably be prohibitively large for a small project and hard to > set up without test equipment/knowledge. At, say 144MHz (amateur band?), 0.25lambda ~ 0.5 metres less a few %? Hmmm ... borderline. > > 4. if you really have to do this then a small RF relay is the easiest > solution for a 'hobby'project, switching time will probably not be an issue > because most low cost modules need a 'stabilisation' time that will exceed > antenna switching time. Isolation is not an issue , use the pic to power > down the tx at rx times. That's a good point. You'd have to allow stabilising times if switching modules on/off so a relay is not won't look so bad in that case. > > PIN diodes is the second easiest solution, they are cheap and easy to find > but a bit lossy, prohibitively so if you have not laid up an RF based board > before, but, given you have a PC and an internet connection then you can > surely find all you need to know on that....make that search/layup challenge > part of the project ? Ha, all I know about RF is you need things widely spaced on the board, often mounted at right angles wrt one another, and don't forget the ground plane! > > HOWEVER, and in any case, there is no 'free to use' band at vhf for this > application... Well, the freq depnds on what's avialble and how much ($$). Might even finish up in the CB (27MHz) band. Things seem pretty free-for-all down there, is the impression I get!! Seriously though, isn't there an amateur band at around 140 - 144MHz? Maybe the school could help out with an experimental licence? The legal aspect is a another good point though - better look into it. > > the idea of using 70 mhz is not good from a) a licensing viewpoint b) a > noise viewpoint c) obtainable modules d) inefficiency of small (in terms of > wavelength) antennas. True. Wavelngths of around 1m, 25 cm 1/4lambda, are at the top end of the VHF. you can get SAW resonators at around 303MHz, so that might be the way to go? There's a slot for garage door openers up there, I think(?) and they're all jumping-key encoded so interference shouldn't be such an issue? Hmm ... maybe? As I said, just kicking round ideas at this stage. I'm thinking that there are plenty of UHF short range data Tx/Rx systems around but not too many able to be used out of doors, round corners etc. So one tends to think VHF in that case. It's always a good idea to try and do something innovative in a project IMHO - who wants to retread the retread? However as you say, finding a legal slot might be a problem. > > Using other vhf spectrum is not a good idea, most of it is occupied, and not > free to use or licensable for simple one off projects, UHF or SHF will offer > better indoor performance and free to use ('ish) bits of spectrum. > > if you want to escape most of the issues you face, ie two modules at each > end, antenna switching, no free frequency to use, no experience in rf > design/layup Yep .. suffering all those disadvantages is _slightly_ discouraging, I have to admit. > > then look at using the *transceiver* from NORDIC vlsi, which does it all for > you on 433 mhz (or others) at less cost and complexity than most else > available....the features available are adequate for what appears to be a > school or 'hobby' project. > > their web site (http://www.nvlsi.no) has all info available in .PDF format, > they have a board layout that you can just 'use' so no need to worry about > tricky rf pcb layouts ....they also have 'oven ready' artworks for a range > of loop antennas with a fully documented explanation that discusses antenna > efficiency (or rather lack of it !) and design....that in itself is a good > educational project. > > The PCB is a bit tight for hand building and also contains a lot of 'VIAS' > which MUST be there...the components are all smt and very small series parts. Cool. Thanks - will check it out. > > If you look first, plan second and then are still serious....write them a > note explaining your project and see if you can 'beg' a couple of > pcb's......if your project/school/college (?) has a 'budget' then they may > have an evaluation kit and may take a kindly view of an educational > application for one ....OR you can try their distributor(s), the list is on > the website. OR you can ask them for a local customer and write some begging > letters to them to try and scrounge a couple of gash/test boards they may > have lying around. Begging letters ? Now _there's_ a topic I'm gaining experience in! More than RF design anyway. > > if not, and you think you can make a double sided pcb by hand to the > necessary tolerances (and you probably can with care) then contact me off > list and I will send you a revised artwork (not yet tested, but will be next > week) I have done which will accomodate hand inserted copper 'via' rivets > (0.8mm) and 1206 size smt parts...*but that will be it*....after you get it > you are on your own, so if you are spending limited resources and not sure > of success without further hand holding, then take that into account. Hey thanks very much. And don't worry, I'll strive mightily to avoid holding hands .... :) > > Graham > > -Need *high IP3* AND good N/F ?? try this- http://www.rfham.com > Thnax again. you're a resource! I'll have a look at the nvlsiv site and see what I can come up with. Regards & New Year's greetz - Ian. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body