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Range check for 5.8ghz vtx?

PostPosted: Thu Oct 04, 2012 12:04 am
by nevertoooldfortoys
My knowledge of electronics is confined to sub RF circuits, so what might work for audio (one of my other interests) might not work at these frequencies. Basically I want to be able to reduce the power output of my vtx so that I can do a range check (nice to have a way of checking growing collection of cp antennas). However since the power output is built into the 5.8ghz Chinese board I'm using I was thinking of just reducing the input signal. Ie turning the "volume" down on the input signal so the output signal is not as strong. However since I have no idea about the type of signal that the signal wire carries from the camera to the vtx, I'm not sure how to reduce it's power effectively? Would a simple voltage divider circuit work? (20k pot that i have lying around) Or would i need some sort of filter circuit? Any ideas before I just start soldering sh*t to my vtx in a trial and error type of experiment would be greatly helpful. My current method of hiding my fpv plane behind various bushes to reduce the signal is not very precise or repeatable ;)


Re: Range check for 5.8ghz vtx?

PostPosted: Thu Oct 04, 2012 11:21 am
by nevertoooldfortoys
Impatient so got soldering. Put a 2K 20 turn pot b/w ground and video signal wire (black and yellow from camera) and soldered the center pot wire to the video transmitter input. Set R=0 to check video still worked. Yep- haven't broken anything yet.
Started progressively sending more signal to ground by winding the pot up. Signal started to go snowy until it went black (not a blue screen as there is still plenty of carrier signal ?). This might actually work! My rx is about 2m away.
Wound the pot back until the picture came back. Now to see if I can use this for effective range testing.

Will re-post with results, otherwise: surely there must be someone (who actually knows what they are doing, as opposed to myself) that can design a proper range test mod for video transmitters?

It seems odd to be able to range test my radio but not my video.


Re: Range check for 5.8ghz vtx?

PostPosted: Sat Oct 06, 2012 11:17 pm
by nevertoooldfortoys
Tried and failed. This does not work (hardly surprised). Signal started bad and stayed bad but did not dropout. Hopefully somebody in the fpv community will produce a vtx with built in range check button.


Re: Range check for 5.8ghz vtx?

PostPosted: Wed Oct 10, 2012 9:47 am
by AltechMYOB
Hi Simon... yes, that would be nice, however I believe what you want to adjust is inside the RF module. It's the RF module that outputs the 200mW... it produces the carrier with the inserted A/V. Take away the A/V inputs & (IMHO) it will output whatever it's designer(s) designed it to do (if anything) when this happens, but probably just 200mW of static... that static looks to us as no signal is not really the case, it's a 200mW carrier full of video & audio or static. It's in an RF shield as even the environment affects this sort of electronics at these frequencies. It's for the same reason that test equipment to work at these frequencies are SO VERY expensive.

You could try putting 1 or more attenuators in line between the A/V Tx and it's antenna. Here's one location that sells reliable units... ... f+explorer

I purchased a 30dBm (and a dummy load) and it works fine. These are rated to 1 watt, so ample for this application. They also have an adapter if you need one. Scroll the list down to the bottom to see them.
No doubt there are a lot on ePray... I purchased one (20dBm) from there and it doesn't work, so always a risk.