Best frequency for medium range data transmission - Page 1
Forum section: electronics / projects

author: mackrc1
author_rank: Contributor
timestamp: July 22, 2015, 01:18:05 pm
content: Hi,I'm in the process of designing device which needs to transmit telemetry over a frequency using FSK (probably), I am having trouble deciding on the frequency and having difficulty understanding UK law for a start!Here are my criteria:- Transmission at at least 100mW (looking for around 1KM range so probably more but around that figure.)- Readily obtainable parts for chosen frequency.- Legal! (Looking for it to be legal in as many countries as possible)I am not entirely sure what to make ofthisdocument which seems to say I can use any frequency within the VHF and UHF bands as long as power stays below 500mW. My interpretation doesn't sound correct.Thanks

author: LA7SJA
author_rank: Regular Contributor
timestamp: July 22, 2015, 02:22:15 pm
content: This documenthttp://www.ofcom.org.uk/static/archive/ra/topics/research/rrac/members/rrac01_19.docor?Is this distinct point to point or multipoint communication? 100mW and 7dB gain = 500mW in isotropic antenna. The more info you provide the more people can and want to answer.Johan-Fredrik

author: mackrc1
author_rank: Contributor
timestamp: July 22, 2015, 03:12:40 pm
content: Quote from: LA7SJA on July 22, 2015, 02:22:15 pmSorry me being a tit as usual, forgot to attach the webpage! Corrected it but here it is as well:http://stakeholders.ofcom.org.uk/spectrum/information/licence-exempt-radio-use/licence-exempt-devices/short-range-devices-informationThis is simply point to point communication, one way. One device transmits the other receives. At the actual physical end I'm looking for 100mW ish. Antenna gain etc is another thing for me to worry about later. I was thinking about the 433mhz as off the top of my head is used for many devices which do similar tasks to mine. The biggest problem which I should have mentioned earlier is that the point it will be transmitting from will potentially be unknown which means it will have to be effective on ground level. Sorry for the lack of detail earlier, internet is playing up here (0.25mbps download) and had to rewrite the last post several times!Thanks.

author: LA7SJA
author_rank: Regular Contributor
timestamp: July 22, 2015, 03:33:02 pm
content: My condolences regarding your internet connection, I am currently 270 Km north east of Bangkok so I know all about the lack of internet speed. I'll read up a bit more for that and get back to you later. But first I need to get an idea of the amount of data and frequency of broadcasts, as this will influence the choice of modulation, protocol and the importance of short response time and so on.Johan-Fredrik

author: mackrc1
author_rank: Contributor
timestamp: July 22, 2015, 04:31:02 pm
content: Total data transferred per transmission will be no more than 32 bytes. The time between transmissions will be 1 to 2 minutes so not very often. Speed is not a priority in this situation, it is much more important that it gets decoded successfully.Really appreciate your help on this.Ewan

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: July 22, 2015, 05:28:58 pm
content: doesn't the physical environment make a huge difference - line of sight, open land, forest, mountains, dense city, from indoors?relative size and expense of receiver, transmitter - vehicle mounted, man portable, pocket sizedor:https://www.google.com/search?q=MENWITH+HILL+nsa+photo&biw=1920&bih=1052&tbm=isch&tbo=u&source=univ&sa=X&ved=0CB0QsARqFQoTCOm_1rKj78YCFcPUgAod-EQCbwI can't see how any "answer" could be given without lots more constraint info

author: mackrc1
author_rank: Contributor
timestamp: July 22, 2015, 07:14:14 pm
content: Quote from: f5r5e5d on July 22, 2015, 05:28:58 pmIt does!Again the position that it will transmitting from will potentially be unknown, it could (if your lucky) be line of sight, or it could be in dense woodland, or it could be a mix. I'm not looking for a definitive answer I am just looking for a good all round one.As for the receiver RTL2832 based USB dongles are readily available and cheap. A transmitter of 100mW power is likely to be size of a thumb, so without a doubt pocket portable!

author: Marco
author_rank: Super Contributor
timestamp: July 22, 2015, 08:08:36 pm
content: In the US you can get away with a lot if you use frequency hopping or low duty cycle or directionality, but I think in the EU the only ISM band with that kind of power (also with low duty cycle) is 869.4 – 869.65 MHz.

author: nali
author_rank: Frequent Contributor
timestamp: July 22, 2015, 08:26:52 pm
content: Quote from: mackrc1 on July 22, 2015, 01:18:05 pmTry this document for a full list of devices:http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-policy-area/spectrum-management/research-guidelines-tech-info/interface-requirements/IR_2030.pdf

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: July 22, 2015, 09:18:37 pm
content: if both stations can be finely synced (GPS?) then you can do extreme spreading, have big processing gainif duplex channels you can add protocol options for improving error ratetoo many options for "best", no other info - a big part of engineering is defining the problem

author: AG6QR
author_rank: Frequent Contributor
timestamp: July 22, 2015, 10:03:06 pm
content: Quote from: mackrc1 on July 22, 2015, 01:18:05 pmNali already pointed you to IR2030, which lists devices explicitly. I'll also point out that the document you linked to lists many device types and their associated band restrictions in 7a, 7b, 7c, ... 7ab, 7ac, 7ad.Which of those paragraphs best describes your device?Also, note that the power limits are given in terms of ERP or EIRP. The ERP/EIRP is essentially the transmitter power multiplied by the antenna system's gain (relative to a dipole or isotropic antenna, depending on whether it's ERP or EIRP). So if you use an extremely efficient, highly directional antenna on your transmitter, then you will be restricted to using a transmitter with much lower power output to get the same ERP value. In other words, assuming you stay right at the legal ERP/EIRP power limits, a high gain antenna on your transmitter might save some battery life, but it doesn't increase your legal range. A high gain antenna on the receiver, however, can increase range, provided you can keep the receiver antenna pointed properly at the transmitter.How reliable does this thing have to be? If an occasional 32-byte transmission gets garbled or dropped, how bad is that?A higher band means that efficient antennas are smaller and cheaper, and the waves will penetrate through smaller apertures (like windows of vehicles and buildings) better. A lower band means that waves diffract around larger obstacles better. Which is best? Depends on the situation. Going from my ham radio experience, comparing the 2m band (147 MHz) to the 70cm band (443MHz), 2m diffracts over hills better, while 70cm penetrates buildings better. Neither is superior in all situations. Neither will reliably allow FM voice communications over a km of arbitrary obstacle-filled terrain with a Watt, but either will do fine if there's a clear line of sight.