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FrSky DJT to internal antennae options?

PostPosted: Tue Apr 03, 2012 1:50 am
by pldb64

I saw the attached pictures on RCG last week and liked the aesthetics of using the internal antennae placement within the 9x but using the plug in modules to allow the use of FrSky Tx Module or some other variant (not that i have cause to use anything other than FrSky at the moment).

However my question to the RF knowledgable folks on here is whether or not there would be any change in RF performance by using this approach?

The poster indicated they'd used an RP-TNC connector inside the 9x over the RP-SMA connector.

So the questions are:
1. RF performance impact of using this approach?
2. Is the connection between the module and the antennae simply an electrical connection with the RF magic (ie antennae length) only occuring in the attached antennae or are there RF considerations in what to use to make the connection and/or the length.
3. What material should be used for the flexible link between teh module and the base of the antennae
4. Does the different connector type limit what i can use for antennae further down the track? ... The stock antenna is the 2dB loss antenna, however it might be that i have use of a 5db or 7db patch antenna later on in life ... thoughts/suggestions as to the best options with that in mind?
5. Bruce - no doubt you'll have a video available on how to do this already :-) .... although i haven't seen it as yet i'm sure (since i've seen everything you've ever posted under RCMR and most of xjet!!)

external module to internal antennae
IMG_1866.JPG (84.48 KiB) Viewed 5041 times

external module to internal antennae #2
IMG_1869.JPG (215.31 KiB) Viewed 5041 times

Re: FrSky DJT to internal antennae options?

PostPosted: Tue Apr 03, 2012 4:26 am
by RCModelReviews
Any length of cable will cause an attenuation of the signal and every connector also represents another point of loss.

However, the FrSky system has an abundance of range in stock form so you could afford to drop a little signal without any noticeable effect.

I've gone away from using modules now though -- the interface between the pins and the module socket is a weak-link and that's why my SuperSports 40 bit the dust (albeit after 3 years of constant swapping modules for review purposes).

I hardware a DIY kit into my radios now because it eliminates that potential point of failure.

Using high-gain antennas on the transmitter is not really a good idea (IMHO) because it then makes it increasingly important that you have the radio correctly positioned relative to the model. It's very easy to forget this and suddenly find that "oops -- nothing's working" -- until you (hopefully) quickly reorient the radio to restore the signal.

If I could, I'd have an antenna with 0dB of gain on both the transmitter and receiver -- because that would be a perfectly spherical radiation/reception pattern. Unfortunately, that's not possible so the 2dB antennas are about as good as it gets (without making something that's cumbersome and easily damaged).

So in short -- yes, you can use the pictured method and it will likely cause absolutely no problems at all.

Re: FrSky DJT to internal antennae options?

PostPosted: Tue Apr 03, 2012 5:04 am
by pldb64
Thanks for your thoughts Bruce! ... so one follow up question?

Presumably the thickness of the cable will also cause a variation in the amount of attenuation (as well as the length)? Would it lessen the amount of additioanl loss to use something thinner than shown in the pics?

(Ok i lied - i thought of another question) :-)

Is it possible to ball park the affect on the cumulative losses through the above approach?

I think we're aware from your videos that a 9x with the FrSky DHT DIY kit can achieve approximately a 2km range (although admittedly that FPV video you did out over the forest beyond the motor bike track was using the one-way RF module rather than the two way kit).

So with the cumulative losses of the connector + cable, would that cause a 50% reduction in range? or are we talking about a 10-15% reduction? I know you can't be exact in your answer but a ball park estimate would be most helpful.


Re: FrSky DJT to internal antennae options?

PostPosted: Tue Apr 03, 2012 5:11 am
by RCModelReviews
It's pretty hard to exactly quantify the losses because it depends on the quality of the connectors and the terminations made to them. Also, there are a range of different coax cable types -- some have more loss than others but thickness isn't the major factor.

If I were you I'd just bite the bullet and see what difference you get when range-testing. I'd pick you might get around a 15%-20% difference in range but, if you use a low-loss cable and good connectors/terminations -- it might be as little as 5%.