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    geofrancis

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    Post  geofrancis Mon Sep 27, 2021 6:34 pm

    I have been reading your forum and you have very obscure Radio requirements that I found interesting. I have some ideas that might help some of you out.

    the main issue you have is signal penetration, the lower frequency the better so i was looking at modern 27mhz modules and came across these, I think these could be fed with a ppm signal from a handset and the output could be split into multiple channels using a basic ppm > pwm decoder giving up to 8 channels, I done something similar with a cheap 315mhz transmitter to make a head tracking system. there is a chance there might not be enough bandwidth but I suspect 8ch will be possible.
    even low power levels at 27mhz should have far more range than any uhf system.
    radiometrix.com /our-products/Transmitters/Single-channel


    Another idea I had was something that was used for long range drone work was using a relay. Basically, get a rc boat and set it up as a radio relay to follow the submarine or sit in the centre of the pond. that way the rf from the submarine only has to penetrate to the surface and not through the whole pond to get to you. so your using regular 2.4ghx to the boat relay and then uhf to the sub. its simple to set up since your just taking ppm out from the 2.4ghz receiver and plugging it into the ppm in of a uhf TX modue. Technically it wouldn't have to be a boat, even a small mast near the pond would work to minimize the distance the signal has to go through water.

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    Post  david f Tue Sep 28, 2021 8:15 am

    That is a very interesting idea!

    You are right about model submarine requirements being obscure! I think I would also say unique and niche.
    And you are right, signal penetration is THE issue.
    Could I refer you to an article by John Elin about antenna. (Best article I've ever seen about underwater antennae. Conclusions are TX height important, water conductivity important, shortened aerials may be better in water.)"

    Go to the AMS Facebook site "files" section here:
    https://www.facebook.com/groups/AMSmembers/files
    and download this magazine copy:
    AMS 1986 6OCR

    Your idea of using 27Mhz in modern equipment could be a real winner not least because it would be a global market which could get commercial manufacturers interested.

    I wonder if the modules could cope with the bandwidth? They would need to work for about 6 simultaneous users. Maybe frequency hopping would work with this and also reduce the problem of interference from CB radio and toys? The latency or servo response need only be very slow compared with aircraft use. Maybe their would be enough bandwidth for some telemetry? Only 2 or 3 channels needed.

    Lots of questions! Any comments Simon, Tim, Jonathan or Cheapsub?

    I will have a look for some Radiometrix data sheets.

    (Later edit: Could this also be a contender the AX5243 ?: https://www.onsemi.com/products/wireless-connectivity/wireless-rf-transceivers/ax5243)

    David

    The relay idea is also a nice one. A bank side mast may be best to get the height requirement.
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    Post  geofrancis Tue Sep 28, 2021 9:37 pm

    these were the modules i used for my head tracker, they are about as simple as it gets, all i done was connect a 8 channel ppm signal directly to the transmitter and it gave me a good enough signal out of the receiver so i think those 27mhz Radiometrix modules should work in a very similar manner.
    ebay.co.uk/itm/221297956541

    the main problem is the lack of bandwidth on 27mhz for any kind of encoding or packet headers, those modules run at a fixed frequency so you would have to replace the module to use another 27mhz channel but i dont see more than 10 being on at one time, they should still work fine with the classic coloured peg system. CB radios are pretty rare these days, even childrens 27mhz toys have went away so 27 is a lot cleaner than it used to be so i dont think it should be too much of an issue.


    the issue I had with your openlrs receivers was the lack of a ground on the antenna, monopole antennas always gave me awful performance on openlrs especially with telemetry activated.
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    Post  Cheapsub Wed Sep 29, 2021 8:04 pm

    In US, is very hard to find any 27MHz receiver, that is more then 3 channels.
    Most receiver is in AM and the RF module is in FM. ( I have 3 RF modules but can't find any matching receiver; there are parkzone 27MHz RX at Ebay but it only works with parkzone transmitter.) 20 yrs too late.

    I gave up, after I builded the Frankenstein transmitter [ER9X](cheap) with a used Novak 75MHz synthesized module and be done with. ( mix and match almost any RF module 27, 35, 40, 72, 433, 2.4 )


    May be in EU, is easier to find a old used 27MHz or 40MHz set.

    There is still some as its 40MHz synthesized receiver from China, but CQ is so so.( I got 3 out of 4 are good)

    Off the shelf from China 433MHz stuff, is replaced with the 900MHz stuff. So that's a dead end. Look for wolfbox.
    All 433MHz system has Some learning curve to use.
    Like hobbyking module, out of the box needs reprogram.

    If you want all the bells and whistles that flyboy's has in 2.4GHz. Run a long receiver antenna extension to the top. In snorkel mode.

    Toy's RF. No, the channels are on, off only.

    DIY, not everyone's cup of tea.

    Last thing good luck looking for crystal.


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    Post  SimonH Wed Oct 06, 2021 10:06 pm

    Thinking about the 27MHz & 40MHz bands, to be compatible with existing models we would probably have to frequency hop within the 10kHz channels or at least be able to not use channels already being used by other models. that would be quite challenging as the frequency accuracy required becommes so tight. That in turn would reduce the data rate, the old data rate vs bandwith problem.
    The thing with the GHz bands is that all the RF components (coils, caps etc) can be made as part of the chip, for the lower frequencies the have to be real springs and things, so more expensive to buy, fit and crucially tune. Ever tried tuning a superhet radio!
    Again, given the niche product and small numbers (compared with toys and white goods) thats probably why they are so difficult to get commercially.

    The alternative would be some open source design avaiable to all, and proven, with all compnents easily availble or at least someone prepared to hold a stock of tricky items (e.g. PCB). Even then I suspect that it would need the ability to buy ready built/tested units for those that don't want to assemble their own. Would that then required the design to be licensed or at least verified to support the contentions that it does comply with the regulations, that of course are different in different contries?

    The various forums (is that correct latin?) do show that systems at 458MHz (UK) work, but being able to produce consistently in larger numbers by less experienced constructors is different.  It may be that DIY electronic sklls becomme as important as machining skills when modelling subs.

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    Post  tsenecal Thu Oct 07, 2021 8:55 pm

    SimonH wrote:Thinking about the 27MHz & 40MHz bands, to be compatible with existing models we would probably have to frequency hop within the 10kHz channels or at least be able to not use channels already being used by other models. that would be quite challenging as the frequency accuracy required becommes so tight. That in turn would reduce the data rate, the old data rate vs bandwith problem.

    all available frequencies are covered (there are 30 discrete crystals in the 75mhz range), so any frequency hopping functionality would hit a used frequency if even one person at an event were using an "older" non hopping device.

    SimonH wrote:
    The alternative would be some open source design available to all, and proven, with all components easily available or at least someone prepared to hold a stock of tricky items (e.g. PCB). Even then I suspect that it would need the ability to buy ready built/tested units for those that don't want to assemble their own. Would that then required the design to be licensed or at least verified to support the contentions that it does comply with the regulations, that of course are different in different countries?

    The various forums (is that correct latin?) do show that systems at 458MHz (UK) work, but being able to produce consistently in larger numbers by less experienced constructors is different.  It may be that DIY electronic skills become as important as machining skills when modelling subs.

    that is exactly how OpenLRSng started.  a set of documented PCBs and parts that anyone with basic soldering skills should be able to make for themselves.  only after it became moderately popular did third party manufacturing show up.  after it died back down, the third party manufacturing dried up and left.  same thing is currently happening in the US with 915mhz equipment.  Jumper and FrSky are making commercial devices in that frequency at this time, but i am absolutely certain that  once the hype dies down, one or both will leave.

    our biggest problem is that there simply aren't enough people in the world interested in these niche areas of the hobby to warrant a big company making anything for an extended period of time. a single production run for them would saturate the market, and they would never come back.

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    Post  geofrancis Mon Oct 11, 2021 6:19 pm

    I think your over thinking everything. forget digital systems, to build an analogue 27mhz RC system you need is a transmitter and receiver pair that can send and receive an analogue PPM signal that's about as sophisticated as morse code. I suspect it would probably work over a walkie talkie if the signal was clear enough. that's why I linked the 27mhz modules in the first post, they should have everything there all they should need is a ppm signal from a handset and a ppm decoder connected to the receiver its its now an RC system. as far as i can tell they work on regular 27mhz channels and can be ordered on any frequency.



    If you want all the bells and whistles that flyboy's has in 2.4GHz. Run a long receiver antenna extension to the top. In snorkel mode.

    thats how i can live stream video my cheap Gopro camera clone over wifi, i extended the wifi antenna out of the water proof case to the surface so it has line of sight to my phone.

    I first tested my idea on these cheap modules, you can get them in 433 and 315mhz but they have no options for channels so they wouldnt play well with more than one person but its still enough to make a basic RC system using the method i describe. they are normally used for controlling things like home automation, garages etc

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    Post  david f Thu Oct 14, 2021 10:28 am

    Thanks for very useful discussions on here.
    It is clear from these that the overriding problem for us is lack of equipment. Commercial equpt.  Is probably never going to happen, as Tim indicates.
    Even RF modules seem to be heading ever upwards in frequency  terms.
    The ideas on use of 27 meg from Geo are very interesting.  This is a frequency which we know is very good for underwater use and is not much used now.
    The trouble is that I had a brief look for suitable RF modules at 27 mHz and there seems to be limited availability and at a high price $50 or so.
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    Post  geofrancis Thu Oct 14, 2021 12:43 pm

    david f wrote:Thanks for very useful discussions on here.
    It is clear from these that the overriding problem for us is lack of equipment. Commercial equpt.  Is probably never going to happen, as Tim indicates.
    Even RF modules seem to be heading ever upwards in frequency  terms.
    The ideas on use of 27 meg from Geo are very interesting.  This is a frequency which we know is very good for underwater use and is not much used now.
    The trouble is that I had a brief look for suitable RF modules at 27 mHz and there seems to be limited availability and at a high price $50 or so.

    yes those are pretty expensive but there really aren't many companies still making 27mhz equipment anymore, its almost a vestigial frequency at this point and has been surpassed in every way by newer systems apart from signal penetration.

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    Post  geofrancis Fri Oct 15, 2021 10:22 pm

    you should message the manufacturer of those 27mhz modules and ask them if they would support a ppm signal.

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    Post  david f Yesterday at 4:22 pm

    Good idea
    I  have just sent Radiometrix an enquiry on behalf of AMS members.
    Will let you know what they say and some idea of price. It could be a fairly simple retrofit for existing TXs. Their datasheet says they offer different 27mHz  bands, presumably without the nuisance of crystals.
    It is a longshot but worth asking.

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    Post  SimonH Yesterday at 8:12 pm

    Hi all, I had a quick look a at the Radiometrix 27MHz RF module datasheet and whilst not entierly clear (I would have to go through in detail with example frequencies) it looks as if it has 10kHz RF channels, so setting a specific RF channel within the model band shouldn't be a problem, but it looks as if they might only support a limited number for each s/ware version programmed in.
    Similarly, since the modulation input is  DC to 4kHz CMOS so a straight digital signal, PPM would be no problem as PPM is pulse position (time) not pulse amplitude. You might have to work on the antenna matching since no details of the RF port impedance are given as far as I can see.

    Assuming a standard 2ms servo at 50% gives a 500Hz signal (ignoring sync pulses etc.) so with an assumed channel bandwidth of 4kHz there might be some jitter since if the channel bandwith is too low it effectively slows the switching edges making them sloping, so any noise gets translatted into timming jitter.
    A 4kHz bandwith would convert a 4kHz square wave to a 4kHz triangle wave (sort of) with a 'rise time' of 125usec, so the max timing jitter could be +/-62.5usec due to noise
    Probably no worse than the old 'analogue' systems though, since they probably had a similar signal bandwidth.

    If noise or interference is an issue then going digital allows the use of digital message coding techniques that basically use multiple message bits to define each servo command bit, so you can use a majority decision system to allow for corrupted bits. If you had a max channel bandwidth of 4kHz, then that gives a max bit rate of 8kbits/sec, so spreading 8 servos of 10 bits each accross 4 bits (320bits per frame, say 500bits per frame) then that would allow 16 frames/sec (500 x 16 = 8,000).

    The 27MHz probably has the advantage of less volume, as the antenna is just a wire, as opposed to the rigid 'rubber duck' antennas that are probably not waterprooof, so have to the inside the WTC.

    Digressing slightly, I was interested in the commesnt about the origins of the openLRS, I did look at it but it seemd as if over time it has grownw to be more flexible and complex, but it seemed quite complex to configure and using an arduino mini-pro I had to go back to basics to fit the s/ware in the memory provided, hence my version of s/ware.
    Going to any commercial system, or even a new open source I think the trink is to be clear up front what is required, and to prevent the 'wouldn't it be nice if....' (professionaly known as WIBNI or creeping elegance!) for even more niche ideas.

    Even more of a digression, has anbody tried the new RaspberryPi pico instead of the Arduinos? This looks as if it has way more memory, 3 x anlogue inputs and 16 PWM channels .. and < £4
    That processing power might allow the use of a software radio module, I think been mentioned before here, such as
    https://thepihut.com/products/software-defined-radio-receiver-usb-stick-rtl2832-w-r820t?variant=27739418385&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=EAIaIQobChMIjLrg583P8wIVogV7Ch1BTwsMEAQYAiABEgLi1_D_BwE
    That claims to cover 24MHz to 1.8GHz all for £22 or so, but it still needs a transmitter design. Once I've finished my current project .......!

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    Post  Cheapsub Yesterday at 9:28 pm

    Very interesting they still make that new!

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    Post  geofrancis Today at 1:34 am

    SimonH wrote:Hi all, I had a quick look a at the Radiometrix 27MHz RF module datasheet and whilst not entierly clear (I would have to go through in detail with example frequencies) it looks as if it has 10kHz RF channels, so setting a specific RF channel within the model band shouldn't be a problem, but it looks as if they might only support a limited number for each s/ware version programmed in.
    Similarly, since the modulation input is  DC to 4kHz CMOS so a straight digital signal, PPM would be no problem as PPM is pulse position (time) not pulse amplitude. You might have to work on the antenna matching since no details of the RF port impedance are given as far as I can see.

    Assuming a standard 2ms servo at 50% gives a 500Hz signal (ignoring sync pulses etc.) so with an assumed channel bandwidth of 4kHz there might be some jitter since if the channel bandwith is too low it effectively slows the switching edges making them sloping, so any noise gets translatted into timming jitter.
    A 4kHz bandwith would convert a 4kHz square wave to a 4kHz triangle wave (sort of) with a 'rise time' of 125usec, so the max timing jitter could be +/-62.5usec due to noise
    Probably no worse than the old 'analogue' systems though, since they probably had a similar signal bandwidth.

    If noise or interference is an issue then going digital allows the use of digital message coding techniques that basically use multiple message bits to define each servo command bit, so you can use a majority decision system to allow for corrupted bits. If you had a max channel bandwidth of 4kHz, then that gives a max bit rate of 8kbits/sec, so spreading 8 servos of 10 bits each accross 4 bits (320bits per frame, say 500bits per frame) then that would allow 16 frames/sec (500 x 16 = 8,000).

    The 27MHz probably has the advantage of less volume, as the antenna is just a wire, as opposed to the rigid 'rubber duck' antennas that are probably not waterprooof, so have to the inside the WTC.

    Digressing slightly, I was interested in the commesnt about the origins of the openLRS, I did look at it but it seemd as if over time it has grownw to be more flexible and complex, but it seemed quite complex to configure and using an arduino mini-pro I had to go back to basics to fit the s/ware in the memory provided, hence my version of s/ware.
    Going to any commercial system, or even a new open source I think the trink is to be clear up front what is required, and to prevent the 'wouldn't it be nice if....' (professionaly known as WIBNI or creeping elegance!) for even more niche ideas.

    Even more of a digression, has anbody tried the new RaspberryPi pico instead of the Arduinos? This looks as if it has way more memory, 3 x anlogue inputs and 16 PWM channels .. and < £4
    That processing power might allow the use of a software radio module, I think been mentioned before here, such as
    https://thepihut.com/products/software-defined-radio-receiver-usb-stick-rtl2832-w-r820t?variant=27739418385&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=EAIaIQobChMIjLrg583P8wIVogV7Ch1BTwsMEAQYAiABEgLi1_D_BwE
    That claims to cover 24MHz to 1.8GHz all for £22 or so, but it still needs a transmitter design. Once I've finished my current project .......!



    that is very cool, 8 channels at 16hz would be more than any 27mhz system I have ever seen. A pure digital system would be a far superior solution, it would open up options like channel bandwidth allocation, since you dont need high-resolution or high speed on anything other than the first 4 channels the rest could be sent much slower like 1234+5, 1234+6 etc or at a lower resolution like 1 or 2 bit opening up 16 channels on the same bandwidth.

    openlrs wasnt always an open source project, it was originally started by flytron who then released the source code version 1, then "theundeadmod" firmware was made to upgrade the capabilities of the original flytron firmware by adding binding and tweaking the radio settings for maximum range lowering servo resolution and dropping the radio rate down to 9600k. that firmware eventually became the basis of openlrs-ng that was wrote by Kha, there are also forks of openlrsng like gitsly and some Russian versions, someone then started making them with modules using arduino boards connected to hope RF rfm22b radio modules modules, there is also some other firmwares out there like ULRS, Chiplrs, oplink, that are independent of openlrs but mostly hardware compatible.

    I designed the guide for converting 9xr radios to mavlink telemetry over openlrsng for ardupilot flight control systems
    http://marc.merlins.org/mirrors/flyingforfun.weebly.com/open-lrs.html

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