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Archer R4 and R6, enhanced antenna

Stuart Munro
(@boatrc)
Active Member

Hi,

I'm new to FrSky and also, not a flyer but a yacht sailer. No where near as complex as drones but getting more so with a 4 sail ketch that needs 3 sails operating independently but co-ordinated. The FrSky with Open Tx looks the part.

But I find that receiver range over water is much more limited than in the air. I'm not sure why this is but its true of other RC systems. 

I notice that FrSky produce an enhanced receiver antenna. The description is limited (actually missing!) from both UK and US FrSky sites - can anybody advise what this does, would it enhance range and can it be used with the Archer R4 as well as the R6 (R6 is specified but R4 not)

Stuart

Quote
Topic starter Posted : 08/07/2021 5:43 pm
Stuart Munro
(@boatrc)
Active Member

ps. I may also be confusing Telemetry range with receiver range. 

I understand that the receiver is moving out of range because the RSSI figure falls to around 35% half way across the lake. (ie 75 meters away) but is this just reporting the telemetry feed FROM the receiver or the signal strength RECEIVED by the receiver?

Stuart

ReplyQuote
Topic starter Posted : 08/07/2021 6:00 pm
Rich bago
(@richbago)
Admin Admin

the r4 has an Ipex1 fitting and the enhanced antenna are Ipex4 so your unable to use them.

Try the extended ipex1 antenna if you can get the antenna further up away from the surface that will greatly improve reception

ReplyQuote
Posted : 08/07/2021 6:19 pm
Stuart Munro
(@boatrc)
Active Member

Rich,

Thanks - its difficult to place the 2 antenna at 90 degrees to each other above deck in a 'scale' model that is supposed to look real. I've considered threading one up the wooden mast and running the other just below the (also wooden) deck.

But would the enhanced antenna work, with the R6 or another appropriate receiver? Does it amplify the signal?

Stuart

ReplyQuote
Topic starter Posted : 09/07/2021 6:50 am
Barry Cole
(@barrycole)
Member Admin

Stuart,

There are a number of things that can be done.

e-mail me:-  modelgliding@aol.com

I am sure we can improve things

ReplyQuote
Posted : 09/07/2021 9:16 am
Barry Cole
(@barrycole)
Member Admin

It seems the main problem was the RSSI warnings were set to 45/42 rather than 35/32 for the Archer receivers.

ReplyQuote
Posted : 11/07/2021 11:34 am
Stuart Munro
(@boatrc)
Active Member

Barry,

 

I was going to feed back when I had done a new range check - next Thursday. 

I don't think the RSSI warnings are the whole problem - they just woke me up to the low RSSI which I was then watching 'live'. I've done a more accurate measurement of the lake on Google maps so have a clearer view of where signal loss became a problem and will compare this to the signal next Thursday after a few fixes.

I get low RSSI warnings around 50 meters and Critical around 60. Around 70/75 meters the signal drops to 38%. Still not good. At this point the lake is 100 meters across (with dragons beyond the horizon - or at least reeds to snare the boat)

Now my 'fixes'. 

1. Antenna - for an unassociated reason I changed the receiver in a hurry (thought it was not working - turned out to be the on/off switch in the boat). I taped one antenna onto the deck longitudinally. The  position was also directly over the battery. The tape and/or the closeness to the battery may have an impact.

I've bought two 'enhanced' antenna and will change to these, with better positioning/fixing.

2. Being used to an internal areal on my old Futaba, I was unsure of the position of the areal on the QX7S. I no see it should be parallel to the boat.

If these both fail I will move onto more drastic solutions. I am building a large yacht which I want to range out further up the lake which could diagonally be 400 meters! So I'm starting to think of a XJT LBT module and L9R receiver. 

A question for you experts if I may: Does the XJT LBT have LR12 mode? The QX7S lost this capability when it was upgraded to ACCESS but I cant see if the XJT lost it when it became compliant with EU regs. LR12 mode is no longer mentioned in the specs.

An alternative is the 868 Mhz module receiver but this requires unsightly external receiver antenna. ImmersonRC make a JR module for 459Mhz......

I'd rather fix this without an external module but if I have to have one, with the L9R, but how can I make this work with the QX7S if the XJT LBT does not cover LR12?

Many thanks in anticipation.

 

Stuart

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Topic starter Posted : 11/07/2021 3:48 pm
Stuart Munro
(@boatrc)
Active Member

I promised to feed back once I had completed a detailed range check.

The problem

2.4ghz is not suited to use over water. After much reading on the subject I have been led to believe that the cause is not absorption of the TX signal by the water - for this to happen we would need to bend 'line of sight' signals  travelling from the TX to the RX and without the gravity of  massive star that is not possible! What is happening is that those signals that hit the water near to the RX are being partly absorbed and partly reflected. The reflected signals have their frequency changed and create considerable noise close the the water surface. Signal is lost because a coherent message is not being received. 

The Range Check

Being over water I decided not to use the range check function instead the range check was accomplished by tethering the boat at one side of the lake then walking round the lake reading the RSSI values and stopping at set points to 'wiggle' the rudder with a friend observing movement.

Checks were carried out at points approximately 50m, 100m, 150m, 200m and 250m. Reception remained good out to 200m (RSSI 38). However, the signal was lost before I reached the 250m point. 

More importantly, the signal strength was more impacted by blockages; people between the TX/RX would typically reduce RSSI by 10. Reeds that grow around parts of the lake would also reduce the signal but by 6 or 7.

As expected, orientation of the boat is important; the antenna are placed as high in the hull as possible and at 90 degrees to each other so with the boat stable it was never greater than 45 degrees from parallel to the TX antenna but at this point the RSSI fell by 12 - 15.

Similarly orientation of the TX antenna impacted RSSI, this time by a greater amount (15 plus) but I was able to turn it to 90 degreed from the boat.

After this range check I sailed the boat in rough conditions, as the yacht keeled over the RSSI was strangely little impacted.

Conclusions

Given operation over water the range of 200m is acceptable. The antenna are carefully positioned within the hull. One running lengthwise along the middle of the hull. This placement is critical, to place it at one side could result in it being below the water line as the yacht keels to that side. To be under the water line when its mounted in the centre the boat would need to sink!

The second antenna is mounted as high as possible in the hull across the hull. Both antenna spaced as far apart as possible. In this case about 20cm apart after fitting one 250mm extended antenna.

Thought on further enhancing the signal

3 options are possible:

1. Change the frequency to a lower one. Lower frequencies are less vulnerable to noise and less likely to have frequency alteration when being reflected. This option could be achieved by using R9 868 mhz TX/RX which is available from FrSky as a JR module. Given a range of 200m is achieved with 2.4ghz I do not (at least for now) consider this necessary.

2. Increase power of the TX. Not possible at 2.4ghz as the QX7S already provides 100mw output - I believe this is the legal max in the UK. But the R9 can go higher. (I am having difficulty establishing the legal power limit but I believe it is 125mw or 250mw.)

3. Using a redundancy receiver at 2.4ghz to increase antenna placement and orientation. This is what I will try next.

 

Stuart

ReplyQuote
Topic starter Posted : 15/07/2021 12:19 pm
Rich bago
(@richbago)
Admin Admin

Hi Stuart

in regard to the 868mhz legalities the max power of 25mw is a requirement on airbourne  transmission. im not sure that

there is the same restriction on power for ground use

ReplyQuote
Posted : 20/07/2021 10:56 am
Stuart Munro
(@boatrc)
Active Member

Thanks Rich, do you actually get longer range than 2.4Ghz in the ait with 25mw. 2.4ghz is allowed at 100mw.

I have searched govt sites on this and they hardly mention boats so my working assumption is that they follow the same rules as everything else. You can't legislate for everything I guess. Would it not be simpler if someone (from govt or a national RC club) actually produced a schedule of permitted frequencies and power.

Stuart

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Topic starter Posted : 20/07/2021 12:05 pm
Stuart Munro
(@boatrc)
Active Member

Argh - found OFCOM doc dated 2021 - doc IR 2030 referring to Licence Exempt Short range devices. Interestingly, Model Control does not schedule 2.4Ghz but 'Wideband Data Transmission (which may be used airborne)  does permit 100mw if it has 'frequency hopping modulation' 

To a non radio engineer this makes as much sense as Egyptian Hieroglyphics. Why not specify the main UK model control frequency under Model Control?

But any 868mhz I can find with airborne capability is, as Rich says, limited to 25mw. Yet I read online that the EU (I doubt we've changed this yet) permits 250 or 500. 

It appears that vendors have an obligation to sell appropriately but all I see is 'check regional legal requirements' which they can get away with because they sell multinationally.

Rage over - the conclusion is that 2.4Ghz appears to be permitted at much higher power than 868mhz so surely any range advantage to airborne users must be negligible.

ReplyQuote
Topic starter Posted : 20/07/2021 12:50 pm
Rich bago
(@richbago)
Admin Admin

868mhz was adopted more for the penetration benefits like trees and people mainly by the FPV community and with better spec antennae can see RF link up to distances of 10miles even on 25mw power, but that is the downside of 868mhz, the antenna needed are not always practical being large dipole T shaped or spiral domed and directional reliant

 

https://drones.stackexchange.com/questions/1226/why-is-900-mhz-better-for-long-range-than-2-4-ghz

This post was modified 3 weeks ago 2 times by Rich bago
ReplyQuote
Posted : 20/07/2021 1:02 pm
Stuart Munro
(@boatrc)
Active Member

Rich,

 

Thanks - found an interesting article on internet - rc life.co 12 nov 2018. It mentions 25mw for use with telemetry and implies higher for non telemetry. It is referring to EU rules (2018) but I believe is UK based.

Nevertheless, he was impressed with the range using 25mw with telemetry.

Worth a quick read. R9M Long Range module review (rclife.co.uk)

Stuart

ReplyQuote
Topic starter Posted : 20/07/2021 1:19 pm
Rich bago
(@richbago)
Admin Admin

Thanks stuart

Here is some important notes from Mike Ridley of Radiomodel Workshop, he is a specialist in this field:

2.4G Radio use and installation help sheet

With regards to problems with 2.4G radio, as 2.4G radiation is quite directional, aerial position at the transmitter and
receiver is quite important.

In the case of the models with carbon or metal in the way or close by the aerials this will have an effect on the aerials
and will reduce the overall range.

Even the wires to the servos will also affect the range, so having any of these items close to the aerials will narrow
down the active part of the aerial receiving the signals from the transmitter.
With all carbon models the only thing you can do is to get the aerials outside of the model, it has been found that having
the aerials some distance behind the wings is the best. Having the aerials in front of the leading edge of wings will tend
to give lock out and failsafe operation as the fatter part of wings will hide the aerials with the model in given positions.
I suggest that once a model has its radio fully installed that you take the model to the edge of its range by using the
power down setting on the transmitter and then at the edge of range position the model in different angles and attitudes
and see if you can lose the radio link. If this is the case then look at the aerials positions within the model to see if they
are being compromised in anyway.

UBec use
When using UBec power supplies avoid mounting the UBec close to the receiver as this can cause direct interference. I
suggest when you first install a UBec carry out a short range check with and without the UBec if you find a significant
change in the range then have the UBec check out or replaced. This problem can also apply to some speed controllers
that are now also using UBec BEC circuit.

NimH batteries
One of the other problem with today's modern radios is the batteries, as we've been forced to use NimH battery this
makes the possibility of battery voltage collapse much more likely, as NimH have a much higher internal impedance
sudden high loading of the battery can make the voltage collapse causing the receiver micro processors to reset. If you
have a lot of servos in your model you should consider using EneLoop batteries as they have much lower internal
impedance and can cope with high loads.

Transmitter aerial position
I recommend that the aerial be put into the vertical position as the signal that is radiated from the aerial is polarized in
vertical plane and is more likely to reach the receiver than in the horizontal plan. This also avoids the tip of the aerial
pointing directly at the model which is where the radiation is at its weakest.

Third party add ons
If you are using third party add ons with your 2.4G radio be aware that some units may radiate interference that can get
into you receiver, items like height limiters etc. Mounting such items on top of your receiver can lead to lock outs and
failsafe operation, try to avoid mounting electronic items close to the receiver as even CE marked thing may still upset
the receiver.

2.4G with boats
Because boats are at ground level and in water the strength of the 2.4G signal is very weak across the water, so the
aerial/aerials should always be mounted in the vertical where possible in a tube, fitting the aerials under the decking
can lead to lost of control when the model banks especially as the aerial get closer to the water level.
Once again metal or carbon parts in the model will deflect the 2.4G radiation so avoid mounting the aerials close to
these types of materials.

Range checking
Using the power down button on your transmitter to test the range does not tell you the whole story, if we say that the
power down output is 10 milliwatts, if there is a problem with the R.F module/board inside your transmitter then you
could have just 10 milliwatts when in full power mode.
This effect has caught many people out, one of my customers after crashing one model then carried out a power down
short range check on a second model and everything seems fine, and ended up crashing another model.
I suggest if you get any signs of loss of control with a model, carry out a full range check or at least a test that is great
than the distance where you lost control of the model.

Common faults with 2.4G R.F in transmitters are fracture aerial wire, damaged aerial, loose aerial, aerial connector not
fully home, faulty R.F I.C in the R.F module, all of the above is quite common in most make

ReplyQuote
Posted : 22/07/2021 9:29 am
Stuart Munro
(@boatrc)
Active Member

Barry,

 

Great source, thanks.

The only comment I'd make is that for boats, a vertical antenna is only considered for RC racing. In these situations realism is not the primary aim, speed and control are.

 

The other format is 'near scale' where the boat is as close to realistic as possible when on the water. In myu experience near scale modellers never place 2.4ghz antenna outside of the hull. But equally they never use carbon fibre - mostly wood some fibreglass. 

Oscar Liang also has a great site for info. With 868mhz he favours receiver and transmitter antenna being parallel to each other. May be different configuration depending on air/ground use I suppose.

 

Stuart

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Topic starter Posted : 22/07/2021 4:22 pm
Rich bago liked
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