FPV transmitter system modifications

What’s wrong with this picture?

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Stupidly, a cloverleaf antenna with a reverse SMA connector (SMA-RP) is supplied with a transmitter than has a “normal” SMA (f) jack, thus requiring an adapter.

These adapters can be relatively lossy at 5.8 GHz, and the semi-rigid coax feedline can be bent and maintain a 90° angle anyway.

At this point, a problem was noticed with the Carbonbird branded 200mW 5.8 GHz video transmitter, and the extremely poor quality SMA (f) jack that was fitted.

I have never seen an SMA jack like this, where the male pin is such a loose sloppy fit, to the point where the pin itself droops down, as seen in the following picture (you can even see the gap at the top of the pin).

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The loose and sloppy female contact can be seen here (left) alongside a better quality SMA (f) jack, on the right.

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Not wanting to risk an open circuit with the antenna and smoke the transmitter’s final stage, the jack on the Carbonbird TX must be replaced.

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Removing PCB edge launch style SMA connectors can be a challenge, when you want to avoid damaging the PCB at all.

From previous experience, I have found the following procedure works well.

• Using solderwick and a suitable tip, wick up as much solder as possible on the “other” side.
• Using small pliers, bend and fatigue each earth pin off
• Be careful – do not pull copper traces

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• Flip the board over
• De-solder the centre pin, and cut off

You can fatigue and bend the top earth pins, but simply heating both pins, and the connector should fall off by itself.

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Some PCB edge mount SMA jacks have earth pins with a wider pitch, so in this case, PCB solder mask was scraped away so that the earth pins could be soldered to the copper. Use a Stanley knife blade.

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New SMA (f) jack fitted:

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While we’re at it, we might as well change the connector to a normal SMA (m) on the antenna feedline.

This feedline is actually UT141 size, so we can use a normal 141 SMA plug.

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SMA-RP connector is simply cut off, tossed in the bin, and feedline prepared for an SMA (m) connector.

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Done!

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Aside from avoiding ~ 1dB loss with an adapter that’s not needed, we also save 2 grams!
(Weight with adapter = 19g)

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200mW transmitter is now mounted to the underside of an F450 lower board.
(A 50 Ω SMA dummy load was connected to the antenna port during testing)

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Don’t ever be fooled into thinking you need a 90° adapter – you don’t.

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