Another RDT question

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GarydNB
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Joined: 15 Feb 2018, 23:12

Another RDT question

Post by GarydNB »

Me again.....I'm making a RDT transmitter for free flight using an 'orange' dsm2 module and was wondering if the aerial must be entirely outside the plastic project box or can I have only an inch or so protruding?
Thanks
Gary
jackdaw
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Location: Wet and Windy North Wales

Re: Another RDT question

Post by jackdaw »

Plastic should be no problem. But where will your hands be as a hand can be quite a good shield for 2.4gHz RF?
Martin
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Re: Another RDT question

Post by Martin »

If it's a normal plastic project box then it will work okay with most (or even all) of the antenna inside the box. Lots of transmitters now have internal antennas inside their cases and they work fine.

It's best to avoid having wires or other conductive items close and parallel to the antenna - but don't worry too much about it if that can't be achieved - it will reduce the range a little but it should still work.

A few times I've taken a set of mole grips to one of those plastic antennas and crushed away all the plastic to get to the 'working part' inside. All they consist of is the co-ax cable trimmed back to leave the inner 'whisker' protruding (like many of our 2.4 GHz receivers have) but they usually have a bit of tube, about the same length as the whisker, connected to the outer co-ax sleeve and running back outside (surrounding) the cable the opposite direction to the protruding whisker. It makes the 1/4 wave receiver style antenna into something more like a dipole (though unbalanced) That gives a bit more gain then you get from just the whisker without the tube. Phil made a post somewhere about how to make your own version using brass tubing. If you don't mind crushing your antenna like that, it makes it easier to mount the actual 'works' entirely inside your project case.
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PaulJ
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Re: Another RDT question

Post by PaulJ »

What Martin described so eloquently is this, which I posted in "another place" way back......
Antenna.jpg
Peel back the screening to expose the correct length of the centre conductor. Cut most of the conductor off but leave enogh to solder it round the top of the brass tube. From memory I think the exposed length was 29mm with the same length of brass tube. It seems to work well enough though of course you should range check it on the ground before flying it. I suspect your RDT won't be very different to this conversion of a Futaba FT-3A
IMG_0427.JPG
The 27MHz antenna is just for show and the 2.4GHz antenna is normally hidden within a pretty frequency pennant. There is an Orange DSM2 DIY transmitter module hidden behind the battery.
Martin
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Re: Another RDT question

Post by Martin »

Sorry Paul. I wrongly assumed it was Phil that posted that. :oops:

Thanks for reposting the information and the photo.
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Phil_G
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Re: Another RDT question

Post by Phil_G »

Evening chaps
The old "Internal 2.4 aerials" thread was intact on Wayback so I reposted it here:
viewtopic.php?f=62&t=110&p=260&hilit=aerials#p260
The old-faithful Futaba 6EX with the aerial in the handle is a very practical setup for slope soaring,
its so easy to break aerials when you're battling through 6' ferns to find your glider!

Looks like me & Paul separately covered the same ground, which at least proves that they work!

Cheers
Phil
Martin
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Re: Another RDT question

Post by Martin »

Thanks Phil - that must have been the post I was half-remembering. :)

I've seen comments that because the antenna isn't balanced the 'feed' part of the cable should ideally be some multiple of half a wavelength - but I suppose you have to figure in the velocity factor for the cable, and the cables I've measured that come with commercial 2.4GHz stuff (WiFi routers as well as R/C transmitters) seem to have cable lengths that don't fit any obvious pattern. I suppose it isn't that important, though obviously you should keep the cable fairly short because the small diameter cables typically used in these things are very lossy at 2.4GHz
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Phil_G
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Re: Another RDT question

Post by Phil_G »

I wouldnt think so Martin. VSWR is a ratio, moving the point of measurement or the load point only changes the levels, forward and reflected equally - the ratio remains the same. So for high power systems a wavelength specific feed might reduce flashover etc, but for our puny milliwatts I think it makes no difference but thats just my own take on it Image

Those particular measurements in the linked post have been optimised for minimum VSWR, of course its a compromise as it always is, taking account of velocity factors & the weird concept of a half-wave dipole coaxially-fed through one of its elements! Image

I've been using these for 5 years now, (since 2012) and they do work very well, in fact the 6ex gives fewer of the occasional rssi pips than it did with the standard aerial, maybe because of the 'ideal' 90-degree orientation.
Image
Cheers
Phil
GarydNB
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Re: Another RDT question

Post by GarydNB »

Right....here goes.....could this teensy weensy rx be used with a servo?

https://hobbyking.com/en_us/frsky-xm-mi ... rsion.html

Gary
Martin
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Re: Another RDT question

Post by Martin »

I think the output from that receiver is s.bus only, so you'd need an s.bus to servo decoder to use it to drive a standard servo or servos - and that decoder would likely be bigger and heavier than the receiver itself. :x

You can get s.bus servos and there are some small ones 6 grams or less. That way you don't need a decoder. But do you also need a speed controller? I guess there are s.bus speed controllers too. I've never used them myself. I think you also need some kind of tool that you connect to the s.bus servos so you can set their address - 1 for aileron, 2 for elevator or whatever.

I would look for the smallest standard receiver with normal servo outputs you can find - that's what I do. There are some receivers that use smaller than standard connectors for the servos, which is a good idea as the connectors tend to be the biggest and heaviest parts of these tiny receivers, but you either have to buy servos to match or mess about with a soldering iron attaching new connector leads to servos where you've chopped off the standard connectors. Depends how small and light your model is and whether it's worth going to that extra trouble.
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