Repair/Diagnosis Questions

Two questions for my repairs I will need to make after yesterdays maidens.

First one, probably quite easy. The Cine8 lost it’s VTX antenna, not entirely sure when or how long it was running without an antenna, oops… The design and layout meant it would easily happen tbh. I believe it’s an IPEX antenna and Unmanned tech do a direct replacement (IPEX 5.8Ghz FPV whip antenna for CineFlea / Cine8 – Unmanned Tech UK FPV Shop), is it worth just getting that or are there better options, things worth considering (the VTX I believe is this one): Happymodel OVX306 openvtx 5.8G 48ch 400mw VTX replacement of whoop_vtx – Happymodel

Second question. The VTX works fine as I put on a spare UFL/IPEX 2.4 Ghz aereal at home (a ERLS module one I had spare), probably not the best idea, hense asking above for what to replace with. The Betaflight overlay is appearing, but there is no camera image. This is weird as when were flying all was fine (although when the antenna fell off, anyones guess!) So my question is about diagnosing the camera. I can check it’s getting 5v. Can check the continuity of the cables, no problems. If I was to check for signal from the camera would I link up the 5v and gnd to a supply, and then make an RCA plug with the “yellow” video signal cable to the centre pin, and ground the outer circle bit of the RCA plug? I hope that makese sense. Can then check if it’s the cam or the VTX? Any other suggestions of why a cam might not be working be great.

Easiest way would be to swap the camera from one of your other quads. I dont know if a 2.4GHz antenna will work at all on a 5.8GHz vtx? It may work enough for short range testing possibly but certainly wont be effective. Im sure one of the smart guys will be along shortly to confirm :+1:t2:

They do make small TrueRC Singularity antennas with ufl connection that could be worth a try? Hard to find in stock at the moment though

@BeardandBraces had some for sale. I’ve had several from him. I don’t know if he has any left.

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It might work better :man_shrugging: It still may be a whole round number of the wavelength (or a fraction of it if that makes sense).

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:exploding_head: lol I thought it had to match wave length to be most effective? I thought a fraction like half might work but not to the same or better degree as full wave length. Learn something new everyday :grin:

2.4 is twice the wavelength, so the antenna might be 1/4 for 2.4, but will be 1/2 for 5.8, or 1/2 for 2.4 so full wave for 5.8. The only thing that might be out is its impedance.

Could just be talking crap

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I doubt that mate, you know alot more than me about these things :grin:

So it’s true that it’s all about getting the antennas to be fractions of the wavelength.
2.4GHz is 12.5cm
5.8GhZ is 5.17cm

So… half frequency does not correspond to a similar change in wavelength… It’s an inverse law…


So your VTX works :+1:
So drop its wire off the Video Out pad on FC and put the camera video out wire on the now floating VTX wire
If camera works you’ll see just it’s image in the goggles😀


A word to the wise.

The VTx expects to see a load close to 50 Ohm, which a resonant 1/4 wave length provides. Anything else will have a different impedance and subsequently require some form of matching. You can get away with a halfwave dipole (2x1/4wave back to back) as that has an impedance of around 75 Ohms. Anything else will be extremely lossy and may damage your VTx.

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OMG you all went off topic, lol. Love this place, learn so much! Talking about waving at people at length or something…

I don’t want to keep the 2.4Ghz antenna on there, put it there temp to check the VTX/Camera was OK still. Also 2.4 isn’t half of 5.8? Or am I going mad? 2.4 doubled is 4.8… I dunno…

I think this makes total sense…


95% of fixing quads (and INAV planes) isn’t actual fixing, it’s running a series of experiments to isolate which bit isn’t working and why :grinning:


LMAO… I knew summat was up… I’ve done this calculation before. Why did my mind completely miss that?

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Im no expert in wavelengths and antenna lengths by any means, and was doubting my maths (even opened calc!)… I was like… Come on… One of the guys will spot this… Had to say something

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I’ve always considered antennas to be the physical manifestations of complex mathematical calculations.

In simple terms they can be considered as three components, capacitor, inductor, and resistor, connected to each other. But to accurately calculate the dimensions of an antenna you need to ensure the feed impedance equals the output impedance (relative resistance at a particular frequency) of the device connected to it, and that the capacitive reactance and inductive reactance are as close to zero as possible to ensure efficiency. So if you input 4.8GHz to your 2.4GHz antenna the feed impedance changes from 50 Ohm (ideal) to anything between 1.8K Ohm to 5K Ohm depending on the material used and the diameter, which would be like not attaching an antenna to your VTX in the first place.

Sorry to go all Tefal Toasterhead but some days I just can’t help myself.



I vaguely remember this stuff from my degree 32 years ago… but I’ve never used it…


I have no friends, because I still remember it.

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Told you I was talking crap @SirGunner :laughing: