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Aerial positioning

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Jeffrey Cottrell 213/04/2020 14:18:56
215 forum posts
33 photos

Hi Guys, couple of quick questions.

On a normal twin aerial 2.4 receiver, I understand it's onlt the bare end of the aerial that does the work.

However I am using on of these

They have a sausage bit half way down. Does the same still apply?

Also I understand the aerials need to be at 90 deg to each other for best reception. Normally I have one lengthways in the fuz and the orther spanwise, but on my current project this is simply not possible.

What's the 'second best' layout?

Cheers

Jeff

Bob Cotsford13/04/2020 15:21:16
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8799 forum posts
494 photos

I think the sausage is a part of the dipole aerial so I suppose both parts count as active but you really need someone with RF knowledge to chip in. As for mounting the aerial, can you put them both in at 45-ish degrees to the fuselage?

Peter Christy13/04/2020 15:42:55
1910 forum posts

Bob is right. That looks like a centre-fed dipole to me, with one of the two elements "sleeving" the co-ax feeder.

Ideally the two should be at 90 degrees, but its not super-critical. Anything better than 60 degrees will usually do in practice, but the nearer 90, the better.

I usually try and get one of them vertical in the fuselage somewhere, and the other either length-wise down the fuselage or across it. This is usually possible in even quite small models.

Hope this helps!

--

Pete

Dad_flyer13/04/2020 16:23:46
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319 forum posts
317 photos

Peter is correct. I use the FS-iA6b and I have one that I killed on the bench.

Taking off the heat shrink, it is like this

img_20200413_161045716~2.jpg

The brass can is soldered to the central wire.

img_20200413_161755861~2.jpg

It is loose over the coaxial feeder cable.

img_20200413_161513724~2.jpg

The FS-iA6 does not have the brass can part on its aerials, and is rated as 'Park Fly'.

Dad_flyer13/04/2020 16:25:49
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319 forum posts
317 photos

I had wondered about this before. Now I know what to do with these receivers in my models. smiley

Andy Stephenson13/04/2020 16:36:32
250 forum posts
40 photos

Jeffrey,

It's properly known as a sleeve dipole and as Peter says it's all part of the aerial. The sleeve and the wire extending away from it should remain straight. If there isn't room to put the two at right angles as you describe then try putting one at 45° to the axis of the fuz and the other at the opposite 45°. this will give you 90° between them. Also try to keep anything conductive away from the active part of the aerial by at least 30mm.

A.

Denis Watkins13/04/2020 16:52:30
4650 forum posts
131 photos

Chap on the internet explains it this way, maximising signal strength along the straight part of the antenna

As Andy says, aligned apart

As most manufacturers recommend.

img_4863n_rc-antenna.jpg

Phil Green13/04/2020 16:53:44
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1661 forum posts
344 photos
Posted by Dad_flyer on 13/04/2020 16:23:46:

Peter is correct. I use the FS-iA6b and I have one that I killed on the bench.

The brass can is soldered to the central wire.

 

I certainly hope not!  its soldered to the braid, the inner passes straight through.

Edited By Phil Green on 13/04/2020 16:55:21

Bob Cotsford13/04/2020 17:02:59
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8799 forum posts
494 photos

While we're on the subject of dipoles, I noticed aftermarket dipoles on Banggood listed as replacements for some FrSky receivers which are supplied OEM with the usual stripped coax type. Would fitting dipoles in place of the more common stripped coax affect the tuning, susceptibility to swamping, direction sensitivity or any other factor either improving or worsening the performance?

Dad_flyer13/04/2020 17:05:02
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319 forum posts
317 photos
Posted by Phil Green on 13/04/2020 16:53:44:
Posted by Dad_flyer on 13/04/2020 16:23:46:

Peter is correct. I use the FS-iA6b and I have one that I killed on the bench.

The brass can is soldered to the central wire.

I certainly hope not! its soldered to the braid, the inner passes straight through.

Edited By Phil Green on 13/04/2020 16:55:21

yes blush

Mike T13/04/2020 18:48:15
577 forum posts
40 photos
Posted by Phil Green on 13/04/2020 16:53:44:
Posted by Dad_flyer on 13/04/2020 16:23:46:

Peter is correct. I use the FS-iA6b and I have one that I killed on the bench.

The brass can is soldered to the central wire.

I certainly hope not! its soldered to the braid, the inner passes straight through.

Edited By Phil Green on 13/04/2020 16:55:21

Phil - In simple terms, what is the benefit of such a set up?

Phil Green13/04/2020 19:21:29
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1661 forum posts
344 photos

In simple terms there are two elements to each aerial rather than one, and its a better impedance match to the front-end yes

Mike T13/04/2020 19:55:00
577 forum posts
40 photos

OK - so the braid becomes an additional element, rather than just a grounded shield?

Peter Christy13/04/2020 22:27:54
1910 forum posts

Ideally, a monopole aerial (usually 1/4 wavelength) should be mounted vertically over a ground plane (a large area of metal, relative to a wavelength). The metal ground plane acts as a reflector, so that the monopole looks like a dipole, in the same way that standing a pencil on a mirror looks like two pencils end to end.

The more usual "bare-wire" aerials use the braid as the reflector, which is not terribly efficient. The sleeve dipole is a neat way of providing a genuine dipole aerial with a minimum of problems feeding it.

However, bear in mind that even the basic, wire ended aerial is still much more efficient than the dangly bit of wire which we used on 27 / 35 MHz! And on those frequencies we were also limited to 100mW ERP transmitted output. So any of the current crop of 2.4 GHz aerials will provide much superior performance to that achieved in days of yore, when we rarely had any range issues!

Sometimes it doesn't pay to worry too much over these things! wink

--

Pete

Jeffrey Cottrell 213/04/2020 22:44:52
215 forum posts
33 photos

Hi Guys, thanks for all the info.

Getting the impression the iA6b might be a better bet for range, yes? Have to say I have an iA6 in both my Riot and Foamie Wots Wot, neither of which is flown as a park flier, and never had an issue.

Internet reports also suggest although the iA6 is labelled as park flier, range is rarely a problem.

Back to the plot.

This is the layout I have.

Daresay I can arrange for one of the Dipoles to be vertical by cutting (yet another) hole in the cockpit floor.

The lengthwise one, not so sure.

Going forwards puts it next to the battery, back next to the servo leads.

I'm guessing the battery would be classed as conductive, but still looks the better option.

Your thoughts

Jeff

dscn0001#1.jpg

leccyflyer14/04/2020 07:42:19
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1632 forum posts
339 photos

Perhaps drill a small hole through the floor of the fuselage and put one dipole pointing vertically downward. Route the other backwards along the side of the fuselage, tidying those servo leads so that they are on the other side instead. The two aerials will then be at 90 degrees in a vertical sense, not in the same horizontal plane, not potentially masked by the mass of the battery and not next to the servo leads.

Peter Christy14/04/2020 08:41:13
1910 forum posts

I agree with leccyflyer about drilling a small hole and putting one pointing vertically downwards.

Its difficult to tell from the picture, but it looks as if the other aerial ought to be short enough to go transverse across the fuselage, between the receiver and servos.

Alternatively, again, poke one down vertically and move the one alongside the battery back so its alongside the receiver.

The whole point of having two aerials is so that if one is either screened or poorly orientated wrt the transmitter, the other will take over. In any model, this is bound to happen at some stage of the flight, so don't worry too much about it.

These systems typically have an in-the-air range of a few kilometers, so even with a relatively poor installation, you should have range beyond the limits of your ability to see it adequately!

Its always worth removing the prop and doing a ground range check with the motor running if you have any worries, but I'll be surprised if you have any problems.

--

Pete

Mike T14/04/2020 12:28:45
577 forum posts
40 photos

@Peter C - thanks for that - I get it now! (Perhaps I should have asked Phil for the slightly more complicated version )

In Jeffrey's installation pictured above, I'd be inclined to turn the rx around and have both aerials projecting at the extreme rear of the hatch, each bisecting the angle between TE and fus., giving them a 90 deg. angle relative to each other. The advantage of the bog-standard 'whisker' is that you can drill a small hole and pretty much poke them anywhere...

 

PS - I've just had a thought (staggering, innit?  Two in the space of an hour...).  Given Peter's comments about the braid being a reflector, does that mean that the entire 'whisker' should be out in the breeze for best reception?  I usually arrange mine so that just the 'active' bit is poking outside...

Edited By Mike T on 14/04/2020 12:42:00

Peter Christy14/04/2020 14:47:13
1910 forum posts

I don't normally have mine poking outside the airframe. Balsa and fibreglass are pretty transparent to RF! I usually have a couple of plastic tubes, or even drinking straws, glued somewhere convenient inside the fuselage, and poke 'em down those.

Having them "out in the breeze" makes them more prone to unnecessary bending and damage. Keeping them inside the fuselage protects them, and won't have any significant impact on range.

The one caveat to the above statement is where carbon fibre, or any other kind of electrically conductive material (sheet aluminium, even silver foil) is present. These can make a very effective screen against RF, and aerials should not be placed nearby.

--

Pete

Dickw14/04/2020 16:29:33
avatar
766 forum posts
103 photos

As an example of "how not to do it" have a look at the install for my 2m electric glider. The model is all carbon fibre (wing, fuselage, tailplane) apart from an approximately 6" section of the fuselage where the receiver sits which is glass or kevlar.

You can see the active portion of one aerial (yellow sleeved) almost taped to the side of the Rx. The other aerial is on the other side of the Rx with the slope reversed in the nearest I can get to a 90 degree separation!

I would never suggest anyone tries an install like this, but I have flown up to the safe limits of my elderly vision with a slim 2m glider - approx 500m LOS - without any problems.

It probably shouldn't work, but I have 3 models like this and they are all OK. If you are unsure of your receiver installation just make sure you do a really good range check with the model turned to all angles. Telemetry helps give confidence of course.

Dick

tight install.jpg

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