FRSKY DIY Module

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FRSKY DIY Module

Postby mpjf01 » 24 Jan 2013, 14:29

I am trying to fit a DIY module into the module bay. Is the DIY aerial functionally the same as the stock 9XR handle aerial? Assuming I can find a way to connect the DIY aerial connector to the handle aerial connector will everything still work the same?
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Re: FRSKY DIY Module

Postby ShowMaster » 24 Jan 2013, 15:45

Yes they are basically the same. Being a 50 ohm system, be careful about trying to splice into the coax. Also every connection or connector added can cause an impedance mismatch which translates in to power lost and not radiated. You're only starting with 50-60 milliwatts.
Not discussed much, but for real antenna work a microwave field strength meter is a great tool. It's put at a distance from the tx and can measure radiated power as a number on a meter. If its tied down and the tx is tied down you can see if your antenna system is transferring more, less, or the same number after your antenna work.
Basically the same way your RX will act as you fly.
Coax if properly matched to the source and load becomes a transmission line transferring all power to the load(antenna).
Mismatch the coax impedance and it becomes a lossy piece of wire.

SM
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Re: FRSKY DIY Module

Postby mpjf01 » 24 Jan 2013, 16:35

Thanks for that advice. What is the difference between the Tx and the RX aerials, they both seem to me to be a bit of coax with a bare wire at the end plus there's a silver thing on the Tx wire. If it's the length that's important, the Tx bare bit is 26.4mm and Rx 33.4mm, could I cut 7mm off a Rx aerial and use it on a Tx, or is the lack of the silver thing the problem?
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Re: FRSKY DIY Module

Postby ShowMaster » 24 Jan 2013, 23:12

I've not modified these antennas myself because I haven't had to yet so I'll just give you some theory and you can find other forums and posts with more info on actually doing it.

The theory is that the little bare center part of the coax is actually a quarter wavelength antenna on 2.4 ghz.
Being that exact length should make that a 50 ohm (AC impedance, not DC ohms) load to match the power out circuit.
It's actual length from a formula also has to factor in the velocity factor of the coax used. That's why many antennas for this band are different lengths, because of the brand and type of coax used. The exposed tip will be a different length and still be correct because of the coax used.
The little silver sleeve is a decoupling sleeve. It's purpose is to basically decouple the feed point where the coax ends, from the rest of the coax length. Its connected at one end to the coax shield, just where the coax ends, and is slipped over the coax shield.This creates a dipole antenna in effect and increases the antennas efficiently quite a lot over just having that bare tip alone. Again the tip length is cut to a length with the addition of the sleeve length to once again be 50 ohms.
So the point is that the lengths can be different and still be correct but it depends on the coax used, and if used, the sleeves length interaction.
Usually this is done using test equipment and tuned for the middle of the frequency used.
So if done right your power out could increase and still be 360 deg pattern. Do it wrong and somewhere out there gravity will win out.
RF is magic and is a science and field of its own.
SM
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Re: FRSKY DIY Module

Postby mpjf01 » 25 Jan 2013, 05:58

Both the antenna connector on the DIY RF module board and the one on the Rx board seem to be the same so I was hoping that I could use a FRSKY receiver antenna extension as a Tx aerial and avoid having to splice the coax or add connectors. The extensions have something at their end that seems to be like the silver sleeve (it's under heatshrink). The FRSKY telemetry receivers are also transmitters with (presumably) the same effective range as the normal Tx RF module, although the Rx has two obviously. I hope the Tx and Rx aerials might be interchangeable. I'll connect one up later and do a range test and see if there's any difference.
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Re: FRSKY DIY Module

Postby ShowMaster » 25 Jan 2013, 12:53

Again, the silver sleeve actually add gain to the antenna and that's why the tx end has it. Almost all tx antennas internally have this sleeve. Many RX antennas have these sleeves when only using one antenna. The Frsky uses two non sleeved antennas but uses two in a diversity system with a rx preamplifier.
The link back to the tx is not at critical as the uplink for control.

The good news is there are other RC forums dedicated to making 2.4g rc antennas or adding the sleeve to existing ones.
If you're flying 500 ft out or less Italy not be critical.
Don't ever splice coax except with a 50 ohm connector, never just solder coax together and pit tape or shrink tuning over it. Again it may work but the SWR created will rob power getting to the antenna!
One more thought, Murphy's law only kicks in when it's your favorite plane and costs more than a foamy.
SM
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Re: FRSKY DIY Module

Postby mpjf01 » 25 Jan 2013, 16:01

The normal receiver antennas don't have the sleeve but the extended (longer) antennas available for these receivers I suspect do, at least there's something at the end of the coax under heatshrink that feels a lot like the sleeve on the original 9XR internal aerial, just slightly smaller diameter. See this http://www.hobbyking.com/hobbyking/store/__16667__FrSky_Receiver_antenna_40_cm.html
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Re: FRSKY DIY Module

Postby mpjf01 » 27 Jan 2013, 12:52

I have installed a FRSKY DIY module into the module bay of my 9XR.

The PCM/power connections were soldered to the back of the Tx pins and a 2s balance plug and socket put into the cable to allow the back of the case to be detachable.

The bind button/light was installed in the space behind the grille in the front of the Tx above the Tx on/off switch. Bits of plastic had to be ground away from the housing to make it fit. The 3 way switch was left out as I always plan to use telemetry Rx and there wasn't anywhere easy to put the switch except the case back, which I think is too vulnerable a place.

A FRSKY Rx aerial extension (see above post) was used as a Tx aerial, installed in the handle after removing all the stock aerial bits. This involved removing the daughter board and its support structure to get at the back of the stock aerial connector (had to be removed anyway to install the bind button). Watch out that the two small pins that the trim switches pivot on don't get lost as they can fall out easily.

In a range check test still had full control at 50m. Flight test tomorrow.
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Re: FRSKY DIY Module

Postby mpjf01 » 27 Jan 2013, 13:42

Interestingly I just checked the link above to the Rx aerial extension that I posted a couple of days ago. When I checked then the image looked like this

Image

This is a photo of the item that I received just before XMAS.

Now the product description on the HK website shows essentially the same thing without the heatshrink on the bare ends. I can't say whether or not this replacement item will work.
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Re: FRSKY DIY Module

Postby mpjf01 » 03 Feb 2013, 11:47

mpjf01 wrote:I have installed a FRSKY DIY module into the module bay of my 9XR.

In a range check test still had full control at 50m. Flight test tomorrow.


Forgot to post the result. Flew a 4m glider to the limit of my vision and still had full control. Using telemetry receiver, FRS_logger showed receiver signal strength at about 50% of scale at that range.
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