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Mr.RC-Cam

Banggood 5.8GHz 600mW Mini vTx review

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FPV'ers like bargains. Fortunately there's an endless variety of stuff from the offshore vendors at prices that are hard to resist. But the low cost Chinese FPV parts are a lot like a box of chocolates; You never know what you're gonna get.

Recently I saw that Banggood had a 5.8GHz 600mW vTx for only $27 USD. That's a crazy low price so I had to order one. Part of the fun is discovering what kind of "chocolate" I'll get. Spoiler alert: This one was not a tasty treat.

Here's the link to the item (Banggood part number SKU248588):

http://www.banggood.com/FPV-5_8G-600mw-32CH-Wireless-Transmitter-AV-Transmission-TX-for-DJI-Multicopter-p-984348.html

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So here's what showed up in a flimsy unpadded plastic bag / envelope from China:

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The hard-wired cabling provides the basic connections to power and camera. The 2-pin JST is for 6V-28V battery power. The 3-wire Futaba type connector has pass-through battery voltage (red), video in (white), and common ground (black). Mono audio is available on a solder pad near the white wire.

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Other remarks: The low weight and size is ideal for small models such as 250 class multi-rotors. The RF output is a RP-SMA connector. This is typical of the China based RF designs. I prefer SMA, but I can deal with RP-SMA too.

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RF power is advertised as +27dBm (600mW). Using an HP spectrum analyzer I measured +27dBm to +28dBm on randomly chosen channels. It was refreshing to find that this vTx was meeting its stated RF power spec since I often find the advertised power as wishful thinking.

Next I checked the channels for frequency accuracy. The online published channel list did not match the channel positions on the vTx. The supplier's messed up list will no doubt confuse some users.

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Here's a revised channel list that matches the vTx I received (as measured on my RF test gear):

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Next it was time to check out the displayed image and measure the video signal levels. The testbench's video source is a calibrated colorbar generator. Test were done with two different vRx's, a Boscam type FR632 and ImmersionRC NexWaveRF UNO5800. Both are 32 channel models.

The main reason I used these two competing vRx's is because they were on my workbench at the time. Plus, their modulation methods are not the same because the two companies do not follow the same de-emphasis and bandwidth specs. I expected that one would work "better" than the other. By comparing the results I can usually determine which competing modulation spec the vTx is attempting to follow.

I randomly saw a video image on the UNO5800 vRx. The test monitor did the dreaded blue screen dance a lot. That was a clue that there's a compatibility issue with the vTx. I switched over to the FR632 vRx and it displayed an image. But there were two faint horizontal noise bands crawling through the image. Color and contrast was slightly disappointing too.

Given the poor video performance it seemed like a good idea to get out the o-scope. It was time for some measurements to see what was going on. Here is what I observed:

1. Video Input impedance was measured to have an non-standard 47 ohms termination. It should be 75 ohms. The termination mistake is amateurish and can contribute to video problems.

2. The full-white video level on the output of the UNO5800 was only 0.45Vpp instead of the expected industry standard of 1.0Vpp. No wonder the video image was unreliable. There was also some video tilt/sag at the vertical sync interval. These issues often occur when the modulation specs used on the vTx and vRx are incompatible.

3. The video level on the output of the FR632 was only 0.66Vpp. That's still much too low. Some displays might not like the severely attenuated video signal. As a possible workaround I modified the input impedance of the vTx to 75 ohms by increasing the video terminating resistor's value on the vTx. This increased the video level to about 0.9Vpp at the FR632, which was promising. But the syncs became distorted and that is a worse problem to have.

Final remarks: My new Banggood 5.8GHz 600mW vTx is now in the waste bin. Paying more for a better design is fine with me. I'm not saying it won't work good enough for someone on a very tight budget. But it's not something I want to install in my FPV models.

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