Help with frsky 2.4ghz antenna length.

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Help with frsky 2.4ghz antenna length.

Postby nevertoooldfortoys » Wed Apr 25, 2012 1:32 pm

I have just made a Biquad bow-tie antenna (IBcrazy), for a 2.4Ghz frysky v8 tx to extend it's range for FPV flight. However on testing, I am getting less range than with my ducky (tested only by reducing the power of the tx and getting my son to walk away until the servos stopped working). Thinking that I might have got the length of the sections of the bow-tie incorrect (30mm), I pulled apart the ducky to check the length of it. Each antenna should be based on 1/4 wavelength of 2.4ghz, right?. Anyway these are the measurements.

Frysky ducky tx dipole is 25mm!

Frsky rx has 33mm of core exposed, 8mm longer than the tx!

Biquad I made has 30mm sections.

The bow-tie is attached to the tx by about 20cm of rg58- don't know if this is relevant.

Have the frysky tx and rx been made way off spec or am I missing something?
Very confused - any words of wisdom would be greatly appreciated.
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Re: Help with frsky 2.4ghz antenna length.

Postby Shotglass » Wed Apr 25, 2012 6:41 pm

your lengths are correct
did you keep in mind that it is a directional antenna? meaning that if you dont aim it at the plane it can give you considerably shorter ranges than the standard antennas?
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Re: Help with frsky 2.4ghz antenna length.

Postby nevertoooldfortoys » Wed Apr 25, 2012 10:23 pm

Do you know why there is 8mm difference between the rx and tx frysky units?
If the tx is 25mm long doesn't this imply a frequency of about 2.88 ghz?
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Re: Help with frsky 2.4ghz antenna length.

Postby Shotglass » Wed Apr 25, 2012 10:28 pm

most because antennas dont really matter all that much with short range low data rate digital links secondly because even if frsky does make really good products they are made in china after all and lastly because calling a desleved coax cable an antenna is almost offensive to an rf engineer anyway
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Re: Help with frsky 2.4ghz antenna length.

Postby nevertoooldfortoys » Wed Apr 25, 2012 11:01 pm

Thanks for that. So i guess I should consider contsructing an antenna for the rx unit to increase my range further. Will also rebuild the biquad. I use an adapter so that I can convert sma to rp-sma, which I think might be a weak link as well.
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Re: Help with frsky 2.4ghz antenna length.

Postby Shotglass » Thu Apr 26, 2012 1:12 am

nevertoooldfortoys wrote:I use an adapter so that I can convert sma to rp-sma, which I think might be a weak link as well.


i doubt that causes much more than mabe .1 of a db loss; at such low frequencies its not really worth worrying about connector losses (keep in mind that genuine sma connectors are speced for operation up to 26ghz)
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Re: Help with frsky 2.4ghz antenna length.

Postby kaptain_zero » Thu Apr 26, 2012 3:25 am

Don't forget polarization..... horizontal bowtie requires rx antenna parallel to the ground, vertical bowtie requires rx antenna vertical.

I honestly think going circular polarization is the way to go for FPV.

Regards

Christian
"I'd like to live as a poor man with lots of money."
-- Pablo Picasso
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Re: Help with frsky 2.4ghz antenna length.

Postby nevertoooldfortoys » Thu Apr 26, 2012 12:06 pm

I hang my head in shame - dry solder joint to sma pin :oops: I thought I was past that sort of thing, clearly not.
Biquad is working perfectly now and I have changed the rx antennas to inverted V's polarized at right angles. Looking forward to testing.
Thanks for all the helpful advice.
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Re: Help with frsky 2.4ghz antenna length.

Postby nevertoooldfortoys » Sat Apr 28, 2012 2:42 am

Completed range check.

With range check on, the stock antenna gives 40m the biquad gives 150m (I ran out of room and into trees!) Same conditions-noise free environment. The frysky is quoted at 1.5 to 2.5 km range. If we scale up, 40:150 is the same as 1.5km:5.65km. A 5.65km range on a 2.4ghz $30 system got to be happy with that. Need to get in the car and find a place where I can do a real test.
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Re: Help with frsky 2.4ghz antenna length.

Postby RCModelReviews » Sat Apr 28, 2012 4:19 am

It's also worth remembering that those 2.4GHz transmitter antennas aren't made by FrSky -- they're made in huge volumes by some other manufacturer and I wouldn't be surprised if they'd decided to cut a few mm off the length to save money. A few mm multiplied by 500,000 units becomes a significant amount of cable that they'd save -- and that means more money for them.

It's been my experience that the sleeved dipoles used in RC and FPV video transmitters are very poor in design and construction. They vary immensely -- depending on the batch and the manufacturer.
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