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Sep 17, 2019, 03:00 PM
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Thread OP
Question

Soldering on new or different antenna on RX


Hi,

from time to time i destroy/cut an antenna on an RX. Then i allways simply solder a new on on like the old was put in. Yesterday i repaird one RX for a friend, he had torn of hiw UFL and i soldered the new antenna on directly. I checked the connection and the "antenna in"-pad and shield/ground pad were not connected (Flysky FS-A8S), that was what i expected.
After that he wanted me to put on a longer antenna to an FS82, this one normaly has a single wire as an antenna. I thought i could change it to an coaxial cabel from my R-XSR spare antennas. I soldered the core to the antenna pad and the shield to the ground (scratched of the mask). Checked the connection and was surprised! Ground an antenna pad were connected, not like the FS-A8S. I removed my mod and still the same, so it is normal!

Checked with other Rx that i have:
FRsky XSR - Ground and antenna pad connected
FRsky R-XSR - (has an U.FL/IPEX) core to shield, connected

Why are the ground and antenna pad not connect on the FS-A8S ? But on all others?

PS: When i tried the FS82 with the longer shielded antenna (same antenna length only + shield length) it got realy hot, so i turned it of and soldered the old one on again. Now it stays cool, why is that so?
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Sep 17, 2019, 10:43 PM
Registered User
RF works in mysteriuos ways.
What is low ohm for DC is high ohm for 2.4GHz, and vice versa.

Do not think the cable did not add some load to the transmitter, and load is current either way.
Even changing the length of the shielded part may debalance the circuit and shorten effective range while increasing the rf levels above legal limits.
Sep 18, 2019, 12:00 AM
Registered User
vollrathd's Avatar
Quote:
Originally Posted by kleineMax
Hi,

from time to time i destroy/cut an antenna on an RX. Then i allways simply solder a new on on like the old was put in. Yesterday i repaird one RX for a friend, he had torn of hiw UFL and i soldered the new antenna on directly. I checked the connection and the "antenna in"-pad and shield/ground pad were not connected (Flysky FS-A8S), that was what i expected.
After that he wanted me to put on a longer antenna to an FS82, this one normaly has a single wire as an antenna. I thought i could change it to an coaxial cabel from my R-XSR spare antennas. I soldered the core to the antenna pad and the shield to the ground (scratched of the mask). Checked the connection and was surprised! Ground an antenna pad were connected, not like the FS-A8S. I removed my mod and still the same, so it is normal!

Checked with other Rx that i have:
FRsky XSR - Ground and antenna pad connected
FRsky R-XSR - (has an U.FL/IPEX) core to shield, connected

Why are the ground and antenna pad not connect on the FS-A8S ? But on all others?

PS: When i tried the FS82 with the longer shielded antenna (same antenna length only + shield length) it got realy hot, so i turned it of and soldered the old one on again. Now it stays cool, why is that so?
You're dealing with very high 2.4 Ghz frequencies. (That's 2,400,000,000 cycles per second) A piece of wire an inch and a half long is just a piece of wire on a battery circuit. But, connect that inch and a half piece of wire to 2.4 Ghz, and that wire starts to act like a tuned circuit. If the wire length is off just a little bit, that puts the tuned circuit off frequency, which can be very hard on the 2.4 Ghz radio frequency amplifier located inside the transmitter.

Add to that, many of those RC 2.4 Ghz antennas uses shielded wire, which is another can of worms if not properly wired and tuned up.
Sep 18, 2019, 03:11 PM
Registered User
Thread OP
So coming to this:
Quote:
PS: When i tried the FS82 with the longer shielded antenna (same antenna length only + shield length) it got realy hot, so i turned it of and soldered the old one on again. Now it stays cool, why is that so?
I can not change the original antenna (non shielded) to a shielded antenna with the same unshielded length by simply soldering the shield to the ground of the RX.
Because it simply is not simple

OK

But what about this:
Quote:
Why are the ground and antenna pad not connect on the FS-A8S ? But on all others?
(the others are the R-XSR and the XSR from FrSky) Is there something broken?
Sep 18, 2019, 03:58 PM
65 years of RC flying
Daedalus66's Avatar
You simply canít test RF continuity with a DC meter. The results you are getting mean nothing.
Sep 23, 2019, 12:42 AM
Registered User
vollrathd's Avatar
Quote:
Originally Posted by Daedalus66
You simply canít test RF continuity with a DC meter. The results you are getting mean nothing.
Yeah
At those 2.5 Ghz frequencies, a very short piece of wire can be an open circuit when dealing with 2.4 Ghz.
Oct 10, 2019, 03:38 PM
Registered User
Hi,
Receivers using an LNA often appear slightly capacitive to the antenna so to restore the input impedance back to a resistive one a small inductor is often added across the input to ground. This effectively resonates with the LNA's input capacitance and the antenna now sees a resistive load. Not all input devices will need it so it's a lucky dip if the antenna has a DC path to ground or not. If the input device looks like 2pF then the inductor would be about 2.2nH so will appear as a dead short to a multimeter.

Cheers,
David


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