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Old Sep 03, 2014, 05:34 AM
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Perplexing 5.8Ghz problem.

Hi there... I'm having a problem with a 5.8Ghz transmitter that has me scratching my head.

Before I start I'd like to make it clear that I have no intentions of using any of this gear as a primary flight control, spatial awareness or navigation system. I would like to home brew an FPV system in the future but at the moment I'm just cutting my teeth with some experimentation.

Anyway, I have an old 100mW 5.8Ghz AV transmitter that I bought from Jaycar. It was a kit for "enthusiasts". To satisfy local laws you had to build part of it yourself, roughly proving that you were a bona fide geek and not just a householder looking to get a better TV reception to the cost of the neighbors.

I bought it to put in my plane, mostly to keep the kids at our flying site entertained with a TV screen and a live video down link from one of the planes flying around. It never really worked all that well, I think I only used it once..

It also has a rather nifty UHF data link that is used to relay the the IR remote control, which would be really easy convert to a serial data interface! Another reason why I was so interested in this kit.

Anyway, I pulled it out of the cupboard yesterday, dusted it off and got myself a couple of Fatshark Circular Polarized antennas from the hobby shop.

It works just fine with the FS Ant plugged in to the TX, but the minute I plug the FS Ant in to the RX I lose signal completely.

Could someone with a bit more knowledge please enlighten me?

Cheers
Stuart
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Old Sep 03, 2014, 08:39 AM
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Are the transmitter and Fatshark receiver on the same channel?
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Old Sep 03, 2014, 12:35 PM
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You said it works fine until you plug the FS ant. into the RX, what are you using as an ant. before you plug in the FS ant.?
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Old Sep 03, 2014, 02:42 PM
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I suspect the problem is caused by a DC bias voltage on the vRx's RF port (SMA connector). When you install the Spironet it effectively shorts the power supply and some (or all) of the vRx circuitry loses power. To check for the presence of a DC bias you can use a common voltmeter to measure the empty RF port's SMA. If you find it has voltage then you have your smoking gun.

The DC bias is only needed for active antennas (those with amplifiers in them). It is not needed by the Spironet, so with some luck you may be able to hack the RF port to disable the voltage. If you need help doing this then post **clear** close up photos of the input circuitry of the vRx module and someone may be able to assist.

- Thomas
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Old Sep 03, 2014, 08:39 PM
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Thanks Thomas!

I recall that the kit had different ants for TX and RX and they weren't innterchangable, so I'd say you're right on the money.

I will check it out as soon as I get home.
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Old Sep 04, 2014, 04:09 AM
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Thomas. It definitely has antenna power. I can't see a way of turning it off. I suspect that the second earth on the antenna output might do it. It isn't mentioned in the data sheet.

The other idea I had was to use the amp in the existing antenna. Would the two solder points be for connecting a second antenna? Photos attached.

Thanks
Stuart
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Old Sep 04, 2014, 10:42 AM
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Quote:
It definitely has antenna power.
As seen in your photos, your vRx has an active antenna. That kind of antenna requires a DC bias on the RF port because it needs power to operate the RF Amp.

Quote:
I can't see a way of turning it off.
Sadly nothing obvious in the circuitry can be seen. But there's the outline of a component (obscured in your photo) on the upper left of the module. If it is a resistor, and has roughly the same voltage on it as the RF port, then temporarily remove it to see if the DC bias on the RF port goes away.

Quote:
The other idea I had was to use the amp in the existing antenna. Would the two solder points be for connecting a second antenna?
See photo.

- Thomas
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Old Sep 08, 2014, 05:52 AM
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Sorry about the late reply. I think that component is a diode. It's labeled BX and is noted on the data sheet but they don't say what it does. There is also one on the TX so I've pretty much ruled that one out.

I did see a rather high end home AV sender the other day. It had an on screen menu and among the settings there was a setting to turn off the antenna bios.

Now I'm assuming that these RF modules are all very similar. Could one of the pins be for sending settings data to the radio?

I'm not sure if you can see it on that photo, but there are three markings that have me quite excited. Next to CH1/CH2/CH3 (which I think is for the DIP switches for channel selection) there is CLK/DATA/SPI

There is an unused pin... I think this might be RSSI. Pretty cool.. I'll be using it if it is.

Cheers,
Stuart
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