30MHz long range trasmission questions - Page 1
Forum section: electronics / rf-microwave

author: simmetry.breaking
author_rank: Newbie
timestamp: July 12, 2018, 10:52:00 am
content: Good morning from Italy, I’m an electronic student and I’m planning to make a small project for school, so I decided to began thinking around a water analyzer placed near rivers that sends data remotely to a PC.The idea (for now everything is only on my mind) is based on a PIC MCU that collects and stores data (only temperature for now) and every hour sends the measured values to another PIC microcontroller connected to a PC through RS-232 or USB, last there is a LabView program to view the data collected.The problem is this: I came up with the decision to use the HF range (3-30 MHz) but I don’t now (since I have no experience) if it’s possible to make a long range transmission in this band(about 10-15km) and how much power do I need.It’s legal to use this frequency band?I’ve asked you this question because I’ve have no idea and I don’t know how complex this project is, so feel free to tell me “leave your idea, it’s too complex”, but if you want to help me you can also send me text or videos around this subject.Thank you for your time, and sorry for my bad english.P.S. I feel strange posting this stupid thing while the other topics in the forum are at higher level…

author: HB9EVI
author_rank: Frequent Contributor
timestamp: July 12, 2018, 11:19:34 am
content: That's not going to work out like that.There are the so called ISM-Bands on Shortwave - but their dedicated use is on short range (SRD) only.After all, self constructed transmitters wouldn't be allowed either.The only way to get that project running is obtaining a hamradio licences which permits the usage of HF-frequencies. Radioamateurs are allowed constructing their transmitters without the need for any certification.

author: simmetry.breaking
author_rank: Newbie
timestamp: July 12, 2018, 11:52:50 am
content: thank you for the fast reply, sorry to hear that.

author: Ian.M
author_rank: Super Contributor
timestamp: July 12, 2018, 12:18:28 pm
content: That sort of range for a point to point link is going to be difficult to achieve legally in *any* licence free band, and will be highly dependent on the intervening terrain unless there is a direct line of sight.Rather than jumping through the hoops to get an amateur or experimental licence, it may be preferable to use a GPRS/GSM modem module, with a PayG data sim to send the data over the mobile phone network to an internet server. As long as the device location has mobile phone coverage with reasonable signal strength, data collection and display can be done absolutely anywhere in the world with Internet access. Another option would be to use a similar module at the receiver and send the data as a SMS text message.Here is one example of implementing such a system:https://circuitdigest.com/microcontroller-projects/send-data-to-web-server-using-sim900a-arduino

author: CJay
author_rank: Super Contributor
timestamp: July 12, 2018, 12:25:54 pm
content: Might it be worth investigating LoRa?https://theamphour.com/376-an-interview-with-richard-ginus/

author: iMo
author_rank: Super Contributor
timestamp: July 12, 2018, 12:44:55 pm
content: QuoteThe HF range 3-30Mhz includes a lot of "bands", each band being suitable for a different transmission ranges and other specific operating conditions.Moreover, the HF propagation conditions change with day/night and solar activity, so it may happen the connectivity breaks anytime for longer periods of time (hours/days).For such short distances the 160/80m bands may work (1.8-1.9MHz and 3.5-3.8MHz in Europe) with few watts power and a good antennas (we talk ~40m++ long wires!!).You need a license for that (from your Telecom Office). Most probably you have to be licensed ham radio operator too. Also I doubt the "data modems" in that bands are allowed in your country.PS: the HF 10kHz-30MHz range is used for data transmissions by governmental bodies, military and other agencies and services..There are modern transmission modes allowing QSOs with milliwatts power, but slow data rates and these cannot be used as a "data modem".http://www.hfradio.org.uk/html/digital_modes.htmlAlso mind to get a ham radio operator license to be able to play withThus, for 10-15km distances you rather go with different technologies as mentioned above.

author: xaxaxa
author_rank: Regular Contributor
timestamp: July 12, 2018, 12:49:26 pm
content: If you have near-line-of-sight, the ax5043 at 915MHz/863MHz with a power amplifier (acpm-7868) can easily give you >10km range; sensitivity is exactly the same as lora at equivalent bitrates. I have some reference designs that I can post if you are interested.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 12, 2018, 01:10:32 pm
content: 30MHz can't be used with the sort of compact antennas you would use at UHF.A helically wound whip as used on cars is smaller than a full 1/4 wavelength ground plane antenna, but it isn't small.10-15 km isn't "long distance" at 30MHz,The characteristics of HF propagation are such that you may have relatively poor results over such a distance, but your signal may be audible on another continent.As others have said, the only band you can use unlicensed around 30MHz is the ISM bands around 27MHz,adjacent to the HF CB band."ISM" means Industrial, Scientific & Medical, so there will be a lot of signal competing with yours.You can't use wideband modulation,either.

author: Howardlong
author_rank: Super Contributor
timestamp: July 12, 2018, 02:15:08 pm
content: Quote from: simmetry.breaking on July 12, 2018, 11:52:50 amIf you're an electronics student, you should have little trouble passing the technical aspects of the amateur radio exams. The only real work you'd need to do is to get a handle on the regulatory aspects, a day's effort of rote learning and practice papers.Having said that, I'm not convinced that HF is the right place for your application, it usually means big antennas, and the noise floor on HF is very much higher than VHF and UHF.

author: simmetry.breaking
author_rank: Newbie
timestamp: July 12, 2018, 09:21:34 pm
content: I’m having a very interesting conversation with you and gave me some good ints.Well the GSM/GPRS module seems to be only way, it offers everything I need and doesn’t require big RF skills to work.The option that I like most is the one proposed by xaxaxa, it combines a chip with all the dirty stuff inside but doesn’t remove all the fun and magic behind playing with RF, I was searching a chip like the ax5043 for quite a long time, thank you very much!The LoRa module is also a good competitor, but like the other solutions (except the GSM/GPRS one) “isn’t legal” on long range.I know that I need big antennas at this low frequency but the bigger problem is what is the best frequency? For me higher is better because everything becomes smaller, but how higher frequency signals behave during the transmission and the enviroment attenuation doesn’t increase with frequency?I’m quite sad that the GSM module is the best competitor because I really like stuff like PLLs and similar and making a wireless system out of these less integrated components should be amazing, unfortunately my knowledge is to limitated (at least for now, I hope to learn a lot from school) and this type of things is too far from me; chips like the ax5043 seems to be a good trade off between fun and not too complex design (I’m really curious to see the schematics about this IC).Another interesting solution is the one offered by the rfPIC12F675 that combines the PIC MCU and UHF transmitter, the only problem is that includes only the transmission module but not the receiver oneThanks to everyone, I appreciate your accuracy while sharing with me your experiences!

author: Lord of nothing
author_rank: Super Contributor
timestamp: July 21, 2018, 08:22:20 pm
content: Why not a CB Radio an Sound Modem? You can transmitt on CB 24/7 who i legal.The problem with 433, 868 Stuff is there are limited to 0,5W and an Duty circle with 10% .And a CB Radio have 12W SSB Peak and 100% Duty circle.

author: Towger
author_rank: Super Contributor
timestamp: July 21, 2018, 09:32:18 pm
content: Quote from: Lord of nothing on July 21, 2018, 08:22:20 pmThe OP is in Italy. Back in the day it seamed the average Italian CBer was running at least a KW.

author: Lord of nothing
author_rank: Super Contributor
timestamp: July 22, 2018, 08:30:10 pm
content: the are still transmit with a kw transmitter.