One receiver or two?

[Steve W-O]

I notice that the new TH9X sets are being shipped with a receiver that only has one receiver and one antenna.

 

I'm a little bit hesitant to switch over to them, as I have had excellent results from the ones with the extra satellite receiver.

 

Theoretically if your receiver antenna is at 90 degrees to your transmitter antenna, you lose half the signal strength (3dB).

 

I have been installing all mine with the front antenna usually just in front the wing vertically, and then the other one under the fuselage horizontal as far back as the extension cable for the satellite receiver will allow.

 

I sometime fly in a park near town, with houses all around, and have never had even the slightest glitch.

 

I wonder what other receivers only have one receiver fitted, and if anyone has had any problems?

[Bill R]

Hello Steve,

I dont think you need to worry too much over losing signal strength. Theoretically there is a 3db drop if one antenna is vertical and the other is horizontal. What you may not realise is that 2.4GHz is the cooking frequency of your good old microwave. With an uninterruped horizon, 1 Mw of power  to matched antenna will send a signal in good conditions some 20 miles or so. This is using dishes of course so wire antenna are much less efficient than a dish. Theoretical distance with wire antenna [1/4 wave, equvalent to a hairpin] is around 2 miles line of sight. with 1Mw output. 

I have no idea what the design of the average 2.4GHz Tx antenna is but I suspect it may be helically wound to give circular polarised radiation. This emits both horizontal and vertical oriented waves plus most directions in between either clockwise or anticlockwise, depending on the winding of the helix. This will mean in effect that no matter what angle the model is at, a good signal will be received. This is not the case with 35Mhz where you could have, in theory, antenna end point to end point situations where the signal can be reduced by some 30db+ and lead to complete loss of signal.

Hope this helps a little.

Regards,

 

Bill R.

[Steve W-O]

Thanks for the comments.

 

Most of the ones I have looked at are just a 1/4 wave vertical, formed by not having the shield of the coax for the last 1/4 wave. Some have a 1/4 wave balun sleeve, but many don't.

 

There are many other factors like reflections that can affect the signal too, which on a LAN don't matter, but if you lose a few seconds on a model, it may be the few too many.

 

I would also point out that a helically wound antenna such as used on two way radios or radio control, does not produce a circular polarisation, it operates in "broadside" mode, which electrically is just about the same as a 1/4 vertical.

 

To produce circular polarisation, the diameter has to be much greater, and it operates in endfire mode, which would be quite unsuitable for RC as you would have to keep it pointed at the model. Also receiving a circular polarised signal on either a vertical or horizontal antenna, you again lose 3dB

 

Helical endfire antennas are often used for satellite communication and would have a reflector on them. The reason they are useful for satellites is that it prevents the signal level going up and down as the satellite rotates, or other causes of polarisation change.

 

I have decided to stick to the dual receivers for now, and am ordering spares, until I have heard more feedback about the single receivers in use.

Edited By Steve W-O on 03/10/2010 09:48:05

[Former Member]

[This posting has been removed]

[Steve W-O]

Posted by Eric Bray on 03/10/2010 23:03:18:

Although not quite the same, My old Reftec 459 Meg set only had a single antenna, formed by a bit of wire hanging out of the rx case, and I never lost contact between tx and rx, unless there was a solid (brick or stone) structure between them. The tx antenna was helically wound and about 4" long. I never bothered to work out the electrical (rf) length. The rx antenna was the same length, and didn't seem critical regarding location.

Early Reftec rx antennas were a dipole, with a stiff wire 'inner' and a flexy wire counterpoise, but experience showed that the counterpoise wasn't necessary, and the stiff wire was prone to getting snapped, so the flexy replaced the stiff, and the 'earthy' side was disregarded. There was no significant difference in the effective range of OOS!

 

 

While there are similarities, as you say not quite the same.

 

There are so many variables in a model, and the way people install the receiver, that make or could make a huge difference at 2.4. With the "working" bit of the antenna only just over an inch long, it is quite easy for it to get hidden behind a motor, servo, piece of carbon spar, undercarriage leg etc.  The receiver antenna being basically a 1/4 wave will also suffer from the doughnut effect most people associate with 35MHz transmitters.

 

It is also possible at the transmitter end to get reflections from the transmitter (if the antenna is on a module, and not in free air on top) or even from a nearby car, and if these reflections are out of phase with the signal, it would reduce the effective signal. If that happened at the same time as the RX antenna was hiding behind a servo, or masked by the undercarriage, it could result in loss of signal. Depending on the attitude, speed and altitude of the model, there may not be time to wait for it to recover.

 

When you play with wifi, you find that just by moving the antenna 10 degrees off vertical, you can get a hugely improved signal, or move the laptop 4" on the table, or somebody puts a metal trolley 10' from the laptop and kills the signal, then you start to see how unexpected things can effect the low powers at this frequency.

 

I feel that I would eliminate many of the variables and possible snags by having two receivers separated by a good couple of wavelengths, and the antennas at 90deg to each other, so for that reason for now I'm going to stick with this system.

 

Maybe later when I can see some proper test results, and more people have used and tried different systems, I may change my view.

[John Privett]

Posted by Bill R on 03/10/2010 09:30:19:

Theoretical distance with wire antenna [1/4 wave, equvalent to a hairpin] is around 2 miles line of sight. with 1Mw output. 

 

 

 

That can't be right, can it?  Only 2 miles range with 1MW - that's ten million times more power than our 100mW transmitters?

 

[David perry 1]

Posted by John Privett on 04/10/2010 19:58:45:

Posted by Bill R on 03/10/2010 09:30:19:

Theoretical distance with wire antenna [1/4 wave, equvalent to a hairpin] is around 2 miles line of sight. with 1Mw output. 

 

 

 

That can't be right, can it?  Only 2 miles range with 1MW - that's ten million times more power than our 100mW transmitters?

 

 Yeah I saw that.  I should have thought at two miles a Megawatt would melt the object in line of sight!!

 

All tongue in cheek Bill R, we know what you meant to type.. Damn those keyboard fairies!!!

 

Back to the serious point, How often and for how long will your aerial be in the wrong orientation?  Presumably "very" and "for very, very short periods" are the answers.  So the same answers as for 35MHz then?  Few modellers fitted 2 receivers then, and few modellers did much else than trail the aerial to the fin. 

 

Ive had a few glitches on 2.4 but not sure how many of these are attributable to aerial orientation.  I also fly in parks with houses, but I cannot imagine a park flier harming one.  

 

Re helically wound antennas: this does not in itself give circular polarisation, which requires quite specific aerial design!.  The helix simply helps load the antenna making a shorter length possible. Technically its still linear polarisation.  Quite why you'd want circular polarisation to aim a signal at a moving  "long wire" receiver aerial I would not know!!  The bottom line is that the 2.4GHZ Tx aerials (the ones Ive looked at anyway) are simple loaded long wires.  Recall also that circular polarised RF is "aimed" at the target whereas our aerials are designed to be bent down and not aimed at the model.  

 

I suspect two receivers, or one and a satellite, will make a considerable improvement in signal integrity, but (and this is just my take) its a bit like saying that building my house walls from six foot stones will make the wall integrity better to. It does, but do I need it and do I notice it?  For me it comes back to an earlier post: the chances of an outcome and the effects of same.   

 

It it makes you feel happier Steve, and if it improves your pleasure...you do it! 

 

 

 

D 

Edited By David perry 1 on 05/10/2010 10:29:11

[Chris Channon]

It is all so much easier on 35 megs !!!!!!

regards

Chris.

[David perry 1]

Posted by Chris Channon on 05/10/2010 17:44:23:

It is all so much easier on 35 megs !!!!!!

regards

Chris.

 

[Bill R]

Hi Guys,

Error the capitals!  Should have been mW [milliwatts] . Here is another one, if the antenna is a dipole then there is gain generated if the two legs are pointed at 45 degrees to the tx antenna. Hopefully with a RX satellite antenna it is always additive signal. You will always lose the signal when the frequency changes. [FASST etc] This is only milliseconds [to "L" with caps and things] or even fractions of a milliseconds [nans or even pico's] before communication is re-established. This is where the money comes in. Speed is expensive [relatively speaking] for the response time but do you really need it. Loss of, or the signal going into the noise floor would necessitate a frequeny shift to improve matters after a tx/rx exchange just in case. I am not a designer so could not say for certain exactly how it works. Reading the blurb it looks like there are genuine full performance receivers and short range receivers out there.

I dont think there is anything to worry about. Two receivers are belt and braces or the Manufacturer's way of overcoming shortfalls in the receiver design by designing for two. for the larger, more expensive models then two or even 3 Rcvrs would be the way to go as two belts and braces. Dont forget that small metal objects on the body could act as parasitic antennas for the TX!.

 

Nuff said. The whole idea of the hobby is what you want to do or see as the best in your eyes. What works for some may not work for others.

 

regards,

Bill R.

 

[Steve W-O]

Morning Bill, maybe the two receivers would seem to make more sense if I put briefly how they work.

 

The two receivers are basically identical, they have the same "range" (sensitivity) and the same overall performance.

 

The digital output from the two receivers is then fed to the circuit that takes that output and drives the servos. This part of the circuit monitors the quality of the signal from both receivers (there is no difference in strength, as it is monitoring two digital signals, not the radio signal) and automatically uses the signal that has less errors.

 

The only difference between the two receivers is the way the user mounts the antennas, which should be at 90 degrees to each other and well separated in the model. Being at 90 degrees to each other means that if one signal suffers from cancellation or reduction due to the path from the transmitter to the receiver, the other one will not usually be affected at the same time. Separating the antennas in the model helps to eliminate things in the model that might "shadow" the signal.

 

It doesn't really overcome any shortcomings in design as the system will work full range with the satellite receiver removed, because signals at 2.4GHz behave quite differently to signals at 35MHz, different things can help maintain a signal when the environment changes.

 

The same system is used on higher end wireless networks, and on longer distance communications links commercially.

[Bill R]

Afternoon Steve,

I understand the theory now. In effect with two receivers you would have 4 antenna separated by 90 degrees [2 horizontal, 2 vertical] so that all the angles are covered to some degree. Signals do behave quite differently at different frequencies as I have performed some amateur experiments from 1.8MHz to 432 MHz over the years with various powers and antennas, most home built.

I have read about the higher end of UHF and SHF communications using backscatter and point ionisation but this is outside the low powers we use in the hobby.

two receivers are better than one but the benifits would only be realised on the failure of one or the use of two where high levels of interference could be encountered.

Have fun!

regards,

 

Bill R.