Jump to content

Transmitter Power


Recommended Posts

Just looking at the Radiomaster Tx 16 and see you can now get it with the 4 in 1 module which transmits at 100 mW or ELRS which transmits up to 250 mW. I've not kept up to speed with the regulations around this, but is 250 mW within the regulations, or is it up to the user to dial down the power?

Link to comment
Share on other sites

Hi Frank,

Last year I spent a LOAD of time ensuring that a range of mini drones my company sells were fully compliant. At that point in time 250mW would not have been permitted within our frequency allocation. However, I haven't looked at the frequency the ELRS version uses.

 

One thing though, the directive states that the equipment cannot transmit on frequencies not permitted, and at power levels above those permitted. This means you can't rely on a user to dial it down. It is permitted to dial the output down within firmware though.

 

I may have a quick look later and see what the ELRS frequency range is and check the directive.

 

One thing; I got my new TX16mk2 this week and there is an option to select your country. This concerns me a bit as it implies that the hardware can therefore operate to US levels. The UK spectrum is a lot more restrictive. If this is the case, the compliance to the EU/ UK directives may possibly be a little 'fast and loose'!

 

I will have a check later though

 

Graham

Link to comment
Share on other sites

In legal terms, the short answer is no:

 

https://www.ofcom.org.uk/spectrum/radio-spectrum-and-the-law/licence-exempt-radio-use/licence-exempt-devices/Radio-controlled-models

 

I don't know if the ELRS system uses frequency hopping, and that may provide a higher ceiling.

 

The responsibility for this lies with the manufacturer and importer; to a lesser extent to the user.

 

From my company's point of view, we took a black and white view. It either complies with the ofcom advice, or we won't sell it. However, we all conceded that this is not clear, and many other sellers take a different view.

 

As John says, actual spec may well differ. Don't forget that radiated power (e.i.r.p) is also dependant on the antenna, so whilst 100mW may well be the specification of the output stage, the radiated power you will have seen when testing may be considerably different.

 

Clear as mud!

 

Graham

Link to comment
Share on other sites

There can be a big difference between that actual power output of the transmitter module, and the "Effective Radiated Power" (ERP).

 

In the UK, all our bands (27, 35, 459 MHz and 2.4 GHz) are limited to 100mW ERP. (There are other provisos on 2.4 GHz, due to the peculiar nature of spread spectrum, but lets ignore those for the moment!)

 

On 27 or 35 MHz, the telescopic antenna on a hand-held transmitter is incredibly lossy. To get 100mW ERP, you need to measure something like 1 Watt at the terminals - ie: you only radiate about 1/10th of the input power.

 

At UHF frequencies (300 - 3000 MHZ), antennas get much more efficient, and can actually have gain, at the expense of becoming more directional. Most of the standard antennas at 2.4 GHz have a gain of 2dB, which means that to get 100mW ERP, you need an input power of 60mW.

 

If you have a suitable power meter and connect it to the antenna connector of the Tx, it should read 60mW.

 

ERP is very difficult to measure accurately. You can do comparative tests against a known "good" transmitter with a Field Strength Meter, but these only give a rough guide. (Hint: A microwave leakage detector for microwave ovens makes a useful FSM at 2.4 GHz!)

 

It is much easier to calculate the ERP at 2.4 GHz, as the antenna will have a published gain, and therefore you will know how much power to apply to it to meet the legal requirements. It is not so easy at 27/35 MHz...

 

BTW, 100mW ERP will provide more than adequate range on 2.4 GHz. Some of the wildlife documentaries seen on TV have used standard 2.4 GHz RC gear for their "animatronic" cameras and recorded several kilometers of ground range. This range will be even greater in the air, where there are fewer obstructions.

 

But don't expect your RC buggy to have that kind of range in a crowded city environment...! ?

 

--

Pete

 

  • Like 1
Link to comment
Share on other sites

Reading up on ELRS, it looks like this is an opensource system where you can set the power and data rate, so you can have a shorter range system with a very fast data rate or longer range with a lower data rate. ELRS will work with both 2.4GHz and 900MHz systems, with 2.4 you can have a higher data rate than 900. On 2.4 at 100 mW power with a 250 Hz data rate a range of 10km has been quoted, so it looks like the system is aimed at long range use.......................................

 

The specifications for 250 mW power output was quoted by several UK sellers.

 

I came across it as I'm looking for a transmitter to drive older DSM2 receivers I have as my old Dx6i is getting a bit rickety and Spektrum no longer support DSM2 in the UK and the new Radiomaster Txs come with the option of the 4 in 1 module or ELRS system.

Link to comment
Share on other sites

Lets look at this another way. The amount of power transmitted is set by government body to a limit that seems arbitrarily set on a finger in the air "this errs heavily on the conservative so must be OK" basis, and so seems in reality not much more than based on guesswork.

 

In recent years manufacturers of both 2.4GHz control radio and 5.8GHz video radio have pushed the available power upwards. There are now tiny transmitter devices capable of outputting FAR above the set level.  Factors of eight times above or more are it seems near routine.

 

Control power up to over 1W is now readily available against a UK level of 100mW, as is video power of 1.4W against a UK level of 25mW.

 

One must assume that there ARE localities where this higher power is legal and therefore must NOT be causing issues, or it would be banned.

 

Equally there has been a proliferation of additional uses added in bands that are not exclusive to model flying. Video Internet connected Door Bells as one example.

 

The two factors are in theory not making it any easier, but is this so?

 

Inevitably some will use what is available regardless of local rules.

 

The question that seems largely unanswered/unproven is, what effect ACTUALLY is there to these rising levels.

 

ARE the set levels sensible/practical, are they really needed to be that low.

 

Be good to see some unbiased factually accurate data on real world UK interference levels.  Anything out there that can be trusted?

Link to comment
Share on other sites

The reason Spektrum don't 'support' DSM2 in the UK and EU now is because it is illegal. All 2.4 transmitters now have to 'listen before talk' to meet our regulations. DSM2 and early FrSky protocols don't do this so Spektrum and FrSky had to discontinue them and replace them. Spektrum went to DSMX which is legal. Spektrum had to go to considerable expense and inconvenience to do this and I imagine they feel somewhat aggrieved that the likes of Radiomaster and Jumper and their local retailers happily sell DSM2 radios willy nilly and with no regard for UK/EU regulations at all.

Link to comment
Share on other sites

@Dave Bran: The present UK (and EU) power levels were not set by "holding a wet finger in the air". They were set on the basis of "what is necessary to do the job, without causing issues to other users". As such, they have proven remarkably effective.

 

As I stated, the 100mW ERP goes right back to the days of 27/35 MHz. Even at those frequencies it proved more than adequate. In the early days of experimenting with 459MHz, we played with ready made Tx modules that only output 10 mW, and never experienced any range issues.

 

Unlike 35 MHz, which is pretty much for the exclusive use of RC aircraft, 2.4 GHz is a "shared" band. We have to live with other users. Spread spectrum goes a long way to mitigating the effects of this, along with the fact that most of those other users are low powered and urban - unlikely to cause issues at a flying site.

 

The problem with using higher power levels is the risk of "deafening" other nearby users on lower power levels. Even spread spectrum can ultimately fall foul of this. OK, an increase to 200 mW may not make much difference, but it won't make much difference to range, either. And since range is already "out of sight" at 100mW, why bother? All it will do is increase the battery drain.

 

Yes, there may be locations where higher power levels may be desirable. I'm thinking of areas where there are already high levels of RF "pollution", but such areas are probably not ones you would be flying in anyway. And of course, there are quite a few totally unregulated areas world-wide!

 

BTW, there is a misconception that you *have* to use Listen-Before-Transmitting (LBT) on 2.4 GHz. You don't. But if you don't, you are limited to a Media Utilisation Factor of 10% - in over simplistic terms, an output of 10mW. Based on my experiences on 459 MHz, that would still provide adequate range for our purposes, though personally, I am far happier with the higher margin afforded by 100mW.

 

I'm reminded of the old joke: "To a pessimist, the glass is half empty. To an optimist, its half full. To an engineer, the glass is twice as big as it needs to be!"

 

100mW is fine. We don't need any more, as demonstrated by the thousands of people flying with it without incident.

 

--

Pete

 

  • Like 2
Link to comment
Share on other sites

Hear hear Pete.  The thing I am at a loss over is the desire for more power. Everyone is limited to visual line of sight, which is easily obtainable using current power levels. Unless there is some new mode of operations that find the current legislation limiting then I think it should be left well alone.

 

As soon as you start down the slippery slope you will run into problems like beamed high power 2.4G wifi signals turning your Rx deaf because some bloke wants wifi service in his allotment down the road and has installed a 100 watt burner in his shed.....remember CB?

Link to comment
Share on other sites

Just a thought, but very little is said about the sensitivity of our receivers. I wonder how their quality and performance differs between makes?

I saw a very interesting programme about how spacecraft communicate with Earth from Mars and from much further away in deep space. The inference was that reliable communications over huge distances can and are reliably maintained with only a few watts of transmitter power, providing the aerial and receiver is good enough to detect it.

 

 

Edited by Cuban8
Link to comment
Share on other sites

7 minutes ago, Andy Stephenson said:

Is R/C operation on 868MHz legal, as there are systems set up to use this frequency as a backup against 2.4GHz failing and in some cases as the primary R/C frequency.


Both Jeti and Frsky have out of the box solutions for resilient RF across 2.4GHz and 800/900MHz; the exact frequency of the latter link varies by geography due to differences in legislation.

Link to comment
Share on other sites

3 hours ago, Cuban8 said:

Just a thought, but very little is said about the sensitivity of our receivers. I wonder how their quality and performance differs between makes?

I saw a very interesting programme about how spacecraft communicate with Earth from Mars and from much further away in deep space. The inference was that reliable communications over huge distances can and are reliably maintained with only a few watts of transmitter power, providing the aerial and receiver is good enough to detect it.

 

 

Jeti quote the sensitivity of all their receivers. I have no idea what it means though, other than they work!

e.g.:

 

image.thumb.png.b20a82407363c2e3f8b13626c16bea10.png

Link to comment
Share on other sites

Decibel (dB) is a logarithmic measure often used for sensitivity measurements - not only of RF signals, but noise measurements too. A dB on its own doesn't mean much, as it is a ratio - like a percentage - and has no real meaning until you say " a percentage of what?". IE: 50% on its own is pretty meaningless, but if you say 50% of two, then you immediately know it means "1".

 

a dBm means that 1mW (1/1000th of a watt) is what it is measuring against. So, a figure of -10dBm means 1/10th of a mW, -20dBm means 100th of a mW, -30dBm a thousandth, and so on.

 

Essentially those figures for the Jeti (rounding them to -100dBm) are saying that its limits of detection are 1/1(followed by 100 zeros) of a thousandth of a Watt.

 

Pretty adequate, I would say!

 

Remember that radio waves weaken in accordance with a square law. If you double the distance, you quarter the received signal strength. So if the ERP of the transmitter is 100mW - well, I leave it to the mathematicians amongst you to work out the theoretical range based on those figures! (Don't see why I should do all the work! ? )

 

Let's just say its more than adequate!

 

--

Pete

 

  • Thanks 1
Link to comment
Share on other sites

Not understanding of reading all of the above,? I was glad to see that Cuban8 wrote, "sensitivity of our receivers" and what about aerials,,,

 I use 3 different receivers

the Futaba 617, around 500 metres of range 

the 6014Hs gives me over 2.500 metres

the Frsky Tfr 6 gives around 800 metres, all have the same aerials and fitted to the same drone, in the same position,

 Not forgetting the orientation of your Tx aerial,,,,,

Also with my FPV gear on 5.8, different aerials give different results,

 

Link to comment
Share on other sites

7 hours ago, Peter Christy said:

...Essentially those figures for the Jeti (rounding them to -100dBm) are saying that its limits of detection are 1/1(followed by 100 zeros) of a thousandth of a Watt.

 

Pete are you sure, I make that 1/1(followed by 10 zeros), Just asking?

 

 

 

 

 

 

 

 

 

Link to comment
Share on other sites

8 hours ago, Andy Stephenson said:

 

Pete are you sure, I make that 1/1(followed by 10 zeros), Just asking?

 

Ooops! Yes, quite right! Typo on my part - too many zeros! ?

 

Still a big enough number, though!

 

Note to self: Must get some new glasses....

 

--

Pete

 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...