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RobanyBigjobz

UHF/LRS Radios at CAMFC

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While I haven't had the chance to fly it at Fickleshole, I have a quad that I fly FPV. I've had problems with losing the 2.4GHz RC link while the video is still fine even while using an omni Rx antenna, a 25mW Vtx and well within the accepted limits of LOS. After much fiddling around with no success, I gave up on 2.4 with this quad and picked up a TBS Crossfire to provide the RC link. The Crossfire is a frequency hopping digital radio, similar in principle to the current crop of digital radios, that operates at 868MHz instead of 2.4GHz. TBS claim a telemetry range of 30km but all I wanted was for my RC link to be solid at a larger range than my video link so I could turn back if video degraded (or, obviously, if I was reaching the limit of LOS to stay legal). I know that plenty of people have no trouble with range while using a 2.4GHz RC link, it may be something particular to my quad but I'm not looking to diagnose that issue anymore.

 

The only problems with other radio equipment I am aware of from my research is with 1.3GHz FPV gear used in the US. This is not due to the Crossfire emitting RF outside its allocated band but the Vrx's not being selective enough. 2.4GHz digital radios are resilient in the presence of lots of other 2.4GHz RF and the third harmonic of 868MHz should be out of that band anyway. 5.8GHz UK-legal FPV gear doesn't seem to be affected by the Crossfire.

 

I don't know of any reason I shouldn't be able to use the Crossfire (FPV or otherwise) at Fickleshole but I wanted to put the info out there and ask if anyone else, particularly the committee, had any objections or wanted to discuss it.

 

ttfn

 

James

 

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Interesting that OFCOM, the UKRCC, the LMA, the BMFA and a variety of other organisations currently either fail to list or fail to link to any info proving that 868MHz is a legal UK frequency for airborne model use.

 

VERY interestingly the BMFA fails to mention in its "The Law" section ANY MENTION AT ALL with regards ANY legal Radio Control frequencies, being locked solely into CAA/CAP matters.

 

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Crossfire is an SRD (Short Range Device), OFCOM's license-exempt SRD page is here. Quote from section 3: "SRDs have little potential to cause interference to other radio users, provided they operate under the correct technical conditions. In keeping with Ofcom's general policy of deregulation and reducing unnecessary burdens, we have removed the need for most SRDs to be licensed."

 

From the Interface Requirements (IR2030) page 20:

 

  • IR2030/1/14

  • 2010/0168/UK Oct 2010

Non-specific short-range devices

Equipment may be used airborne

Analogue audio applications other than voice are excluded.

Analogue video applications are excluded

865 – 868 MHz

25 mW e.r.p.

Techniques to access spectrum and mitigate interference that provide at least equivalent performance to the techniques described in harmonised standards adopted under Directive 1999/5/EC must be used. Alternatively a duty cycle limit of 1% may be used

EN 300 220

2013/752/EU Band No.47

 

 

This section of the Interface Requirements, especially in the context of the quote from section 3 of the SRD page, seems to adequately cover airborne use of the Crossfire.

 

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My thinking was rather the other way around, with the possibility of SRDs, especially airborne SRDs in densely populated areas, receiving interference FROM other radio users. If not now, soon!

 

Too many sources are springing up well above normal/legal power levels, fuelled by other countries much more lax control, as with 5.8Ghz where sellers in the UK are perfectly legally selling units more than 20 times more powerful than UK law allows to be used.

 

Crossfire's "advantage" may well be extremely transient.

 

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The proliferation of non-conforming RF devices is definitely a problem though I'm not sure that it would be limited to just the Crossfire's working band. 2.4GHz is used for a huge range of things from WiFi to baby monitors and TV repeaters so there's scope for 2.4GHz radios to be negatively affected near densely populated areas too.

 

TBS do make some pretty strong claims about the noise resistance of the Crossfire in busy RF environments. How true these are I can't answer personally. What it does provide is RSSI and link quality metrics on the OSD so the pilot has real time info on the RC link. This should allow monitoring of the control link and enable the pilot to turn back in the short term and notice the increase in the noise floor over the longer term.

 

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Since data is king and we don't trust anybody, here's a data point. N=1 isn't brilliant but it's a start.

 

Quad with Crossfire Rx placed on the ground, antenna in 90 degree V orientation. I walked away from the quad back down the track watching the RSSI, SNR and LQ, my body was between the Tx and the Rx. By the time I reached the gate the RSSI was -110dB (TBS claim sensitivity down to -130), SNR was 3-4dB (TBS docs say max is 8dB and Rx can maintain link at -12dB, SNR is how far above the noise floor you are) and LQ hovered between 90-95% (LQ is fraction of packets received successfully).

 

Given the shape of the hill between Tx and Rx, completely blocking direct line of sight and definitely violating the Fresnel zone, this doesn't seem bad to me. I have no idea what a 2.4 system would do in the same test so it's hard to compare.

 

RangeTest.jpg

 

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Could have checked the RSSI SWR reception figures against Taranis and Horus, maybe do this next time we meet?

 

(I'm not expecting anything like as good results BTW!!)

 

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Who has a Horus?

 

I thought we settled this? Do you want me to get you a table for you to put it on when you fly, because you need a metal re-enforced neck to hold that thing up?

 

The Horus is for looking at and walking past, not buying! The results from the test would be interesting though... :wink: I am sure it is a great tx,

but it is M-ha-sive: I could watch tv on that screen!

 

B :wink:

 

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I have a Horus, Development Model doing Beta Testing, currently on FrSky's own OS.

 

I saved up for a set of scaffold poles, clamps, and some extra large castors, and I'm good to go. Don't mess with me lad, three weeks using it and I have muscles that provide the punch of Superman.................................... :mrgreen:

 

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Sounds formidable!!!

 

Seriously, joking aside, how is it really? I know its heavy but just how heavy? Does that big screen aid flying?

B :mrgreen:

 

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I'll try and get up sometime when you are there so you can see. I'm signed into a Non Disclosure Agreement so I cannot comment to questions on how wonderful it is, but you can look over my shoulder and draw up your own opinion. It weighs currently 1.3kg and after a day or so you do not notice.

 

Weight Comparison:-

 

Futaba 10CG - 988g

Spektrum DX9 - 856g

Taranis (with FASST module) - 882g

Horus - 1297g

 

This Sunday end of day might be good for that (and for signal tests James?)

 

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Second test, same Rx position as last time but using the Crossfire micro Rx with single dipole instead of the diversity Rx with its coil loaded dipoles. Another difference is the carbon fibre frame instead of fibreglass so not an ideal comparison with two variables changed.

 

-117dBm RSSI, -3dB SNR and Link Quality down to 50% by the time I got to the gate. Unlikely to be easy to fly with that amount of packet loss but it didn't lose link completely.

 

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