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FlySky 9x FM antenna and 2.4Ghz

Old 04-05-2013, 03:40 AM
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solentlife
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Default FlySky 9x FM antenna and 2.4Ghz

Here's an interesting question :

The OEM / factory 9x has the FM telescopic antenna still fitted. This allows you to fit FM module if you so desire. But most of us fit the 2.4Ghz and ignore the FM antenna.

I like to use the FM antenna to check wind direction - by having a ribbon attached and extending before launch. Once I have direction - I then retract antenna fully and launch. I do not extend this FM antenna at all while flying.

The question is : If anyone left the FM telescopic antenna extended during flight - would this create a problem for the 2.4Ghz signal ? It would be nice if the FM antenna could stay extended partly enough to provide a continuous wind indicator.

I ask because I know from my College days and radio theory that inductance etc. can occur between an antenna and any metal run nearby ... or especially another antenna too close. So for that reason I retract the antenna before flying.

Interested to know ...

Nigel
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Old 04-05-2013, 04:08 AM
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fhhuber
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In theory it shouldn't matter if you leave the antenna out.... but theory means it needs testing to confirm.

The issue that might come up is if the antenna length is a harmonic of the 2.4 ghz frequency(ies) the radio selects.
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Old 04-05-2013, 05:41 AM
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kyleservicetech
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Originally Posted by solentlife View Post
Here's an interesting question :

The OEM / factory 9x has the FM telescopic antenna still fitted. This allows you to fit FM module if you so desire. But most of us fit the 2.4Ghz and ignore the FM antenna.

I like to use the FM antenna to check wind direction - by having a ribbon attached and extending before launch. Once I have direction - I then retract antenna fully and launch. I do not extend this FM antenna at all while flying.

The question is : If anyone left the FM telescopic antenna extended during flight - would this create a problem for the 2.4Ghz signal ? It would be nice if the FM antenna could stay extended partly enough to provide a continuous wind indicator.

I ask because I know from my College days and radio theory that inductance etc. can occur between an antenna and any metal run nearby ... or especially another antenna too close. So for that reason I retract the antenna before flying.

Interested to know ...

Nigel
Hi
I copied my response from your thread in a different posting:
Ah ha, good to hear. I've seen more than a few very nice models lost when the pilot forgot to extend the transmitter antenna.

As far as the FM antenna, they run at 72 Mhz. These 2.4 Ghz radios run at 2400 Mhz, or 33 times higher in frequency. IMHO, the 72 Mhz antenna would have no effect on the microwave frequency.

In fact, by retracting the 72 Mhz antenna, you'd actually be increasing the "Tuned Frequency" of the 72 Mhz antenna somewhat closer to the 2400 Mhz frequency. Even so, that FM antenna tuned frequency would still be far below the 2400 Mhz (2.4 Ghz) of the microwave antenna.

Just about the only way you'd affect the 2.4 Ghz frequency would be to have another antenna tuned to 2.4 Ghz, (or a much higher 2.4 Ghz harmonic frequency) and it would have to be very close to the transmitter itself.

Some years back, I took a look at the transmitted signal of a 72 Mhz transmitter on my 100 Mhz Tektronix oscilloscope. Got a very nice sinewave. Then, another 72 Mhz transmitter on a different frequency was placed a few feet from the first unit. I was stunned by the amount of interaction between the two transmitters. To say there was a lot of interaction would be a major understatement. Moving the two transmitters 10 feet apart completely eliminated the interaction.
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