Range with low-cost TX-Rx modules
The range claimed for such Tx-Rx pairs are usually of the order of 50-100m. I expect that such figures are rather optimistic ones made for ideal LoS conditions with low interference levels. To those of you who have used such products in practice, what kind of ranges have you managed to get reliably in clear LoS and with what kind of antenna?
Any low-power transmitter-receiver like those you are testing need a suitable antenna. You could start with a quarter-wavelength monopole on each end, and test the transmission over some distance. To expand the distance, I would try dipoles or yagis. Open space without obstacles like (other) cars or buildings is also good for such transmissions.
I am not testing them and I don't have a unit with me now. I am weighing the pros and cons of various means of sending short infrequent signal bursts over a distance. I am familiar with the basic issues involved and the factors I'm considering include cost, reliability, portability, ease of setup, adaptability, data rate, etc. I'm weighing low power + yagis against higher power + monopoles, as well as RF against IR (I've designed an IR system with a range of 500m in sunlight).
The system is intended to supplement custom-designed equipment that I made for a local body two years ago. The whole setup is used 2 or 3 times a year. It has to be set up on-site before each event, used for 2-4 consecutive days and then dismantled again. There are plans for furture upgrades and added functionality.
Easy availability of components and materials is an important factor because I'm in a remote location where procuring such things is not as simple as ordering them from Digikey. This is why I wanted to have at least some idea of how those Tx-Rx modules perform in real-world conditions. It will give me a chance to estimate the possibility of tweaking them to meet my needs.
If you want a reliable radio link, I would suggest to become a licensed radio amateur. You can then build or buy a transceiver for amateur radio band which, for example at 144, 220 or 432 MHz band, offer you a reliable temporary link over a reasonable distance.
To communicate at 315 or 420 MHz , the commercial devices (key for car locks, garage openers) intentionally have a limited range. You can extend it by coupling a directional antennas to transmitter and receiver. I tested the low-cost pair at 315 MHz, with quarter-wave monopoles the range was 10-20 meters. I did not need more. Many years ago I used a 432 oscillator and a superregenerative receiver, again with short whips, to signal over 30 meters through a concrete building. It worked well but interference from local airport caused false alarms.
A good antenna does make a difference even with a low RF power.
If you use a good antenna then you can generally get the specified range without too much trouble.
As an anecdote, I once played around with the 433MHz modules from Linx, with some good whip antennas, and me and a friend (who each had one end of the link) easily got over 100m line of sight. I walked behind a cement parking garage, still at around 100m, and was still getting the signal. Only after walking inside the garage did I lose the signal. This was in the middle of a college campus with WIFI and radio towers everywhere. Maybe not as much interference as you'd find in an industrial setting, but still pretty good.
It's not worth the hassle and expense to obtain a ham licence for this one application. I'd rather use a an approved commercial product that doesn't need a licence and tweak it if it looks doable. The site where it's to be used is a disused airfield. It's clear LoS and there's no source of man-made interference for miles - that is, except for vehicles brought to the event.
The distance between Tx and Rx will be about 400m and they will be in fixed positions. If users could get a reliable link at, say 50m with whip or wire antennas close to the ground, I thought a 10db (real-world) yagi each at transmitter and receiver, aligned some 12ft above ground, could be used.
mtwieg, your account of practical experience is just what I was looking for and is very encouraging. Even if it had been a negative report, it would still be valuable in helping me avoid wasting a lot of time and effort.
400 m is slightly above reliable reach for 10 mW 433 MHz modules, at least at standard data rates. A higher performance small bandwidth system can easily achieve it in open field. Higher gain directional antennas aren't permitted by the general SRD standards (at least in FCC and ETSI).
In ETSI domain, there's however a number of applications that allow for 100 mW transmitted power. FCC allows high output power (up to 500 mW in 915 MHz band) with wide bandwidth systems. If they apply spread spectrum with low effective data rates, higher reach is possible. But the technique is somewhat beyond low cost methods.
As for the ham radio idea, as far as I know to stay within the terms of license, transmissions need to be regularly identified with callsign, so I'm not sure how you
would apply that to dedicated standalone transmissions from multiple units as suggested here? Additionally you are not allowed commercial application.
However, maybe using citizens band radio, there is either HF or UHF band, and power is usually limited to 5W, and you don't have the identification requirement,
you will have to deal with interference, however the transmission coding could use some sort of error handling/retry.
You say the setup is only used a couple times a year, so I think its ideal. Use one channel, the raucous noise from your data transmission will probably
keep other users off that channel, just don't select a repeater channel.
The CB license is now free in my country, maybe yours too?
Further idea: cheapie handheld cb units are now easily obtainable, it shouldn't be too hard to dismantle and gain access to their speaker and mic for external
modulation.
Yes exactly right. so in the OP's situation, amateur radio could not be used
Definately the best way to go :)
the 433MHz LIPD devices are designed for low power and short distance, Tho you may often get up to 100m it isnt likely to be overly reliable
To the OP....
The moment you tweak it, it will no longer be an approved product.
Dave
Thank you all for your continued interest. I had to go out for some time and couldn't reply earlier.
I don't know if I'll be able to obtain CB gear without too much hassle. The region is such that I'm quite sure there are no CB radios to interfere with and the nearest ham operators must be at least 100km away in any direction, probably much farther. The police and government-run public utility services have their own allocated bands. The only private wireless communication devices are very short range walkie talkies and very few people use those.
There's no one to know or care if I violate some national regulations. Nevertheless, I'd prefer to stay within legal limits or at most commit only minor violations such as the tweaking I mentioned.