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Name of coax ground isolation device?

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SherpaDoug

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I need a transformer or maybe a waveguide device that can couple 2.4GHz WiFi RF but block DC and 60Hz traveling on the shield. My radio circuitry ground is connected to seawater but my antenna is at the top of a steel mast and the mast is at local power utility AC neutral which can be several volts from seawater potential. Any connection from the mast to my coax shield or antenna ground plane causes big galvanic corrosion issues.

I looked for a "coax isolator" but that is something else entirely. A "DC block" only blocks the center conductor. If I knew what it is called I am sure I could buy one somewhere.

Thanks
 

SherpaDoug said:
I need a transformer or maybe a waveguide device that can couple 2.4GHz WiFi RF but block DC and 60Hz traveling on the shield. My radio circuitry ground is connected to seawater but my antenna is at the top of a steel mast and the mast is at local power utility AC neutral which can be several volts from seawater potential. Any connection from the mast to my coax shield or antenna ground plane causes big galvanic corrosion issues.

I looked for a "coax isolator" but that is something else entirely. A "DC block" only blocks the center conductor. If I knew what it is called I am sure I could buy one somewhere.

Thanks
Click to expand...

The best way to block low-frequency and DC on the coax sheath (including center conductor) s to use a pair of waveguide-to-coax transitions, and use a thin Teflon foil to separate the waveguides. Foil thickness to be determined by expected DC voltage- a 10 mil foil can hold >100 V. I doubt anyone makes a commercial device, but the transitions are available.
If your system is WiFi, you can send both up and downlink by antennas- at 2.4 GHz WiFi range is typically up to one kilometer, and this way the solution will be much cheaper. Patch antennas may suffice for a short distance like meters...
 

I would simply use two coax connectors, with a capacitor (100pF or so) in both signal and ground connection.
 

Are you looking for a "Bias Tee" ?? That blocks DC and passes RF signals only??
 

I was hoping this was a common enough problem I could just buy a weatherproof widget to screw onto the SMA connectors, like they do with video isolation transformers to block ground loop hum bars.

The waveguide solution is elegant but much too expensive. I will probably make something with capacitors.

- - - Updated - - -

Not just a Bias Tee as I need to block the shield as well.
 

BigBoss said:
Are you looking for a "Bias Tee" ?? That blocks DC and passes RF signals only??
Click to expand...

He doesn't need to feed any DC, so his "DC block" was fine ... except that it isolates only the center conductor and not both signal and ground.

So we are looking for a DC-block that has capacitors in both signal and ground path. Normal coax devices all have a continuous shield (for good reasons), so it seems we need a homebrew solution. Two coax connectors with series cap in both signal and ground path will work fine.
 

SherpaDoug said:
Some sort of ferrite bead transformer would work at lower frequencies, but I am not so sure at 2.4GHz.
Click to expand...

There are ferrite transformers for 2.4GHz, but they usually have a different configuration that doesn't block DC path from input to output **broken link removed**
And the packaged ones will again have common ground for the coax connectors. It is very unusual in RF to isolate/float the return path.
 

??? ferrite beads? come on. outside dc block!

https://www.pasternack.com/outer-dc-blocks-category.aspx

you could do it with waveguide where the flange had a choke joint, the guide faces were insulated using kapton tape, and held tight with plastic screws, but there WILL be some emi leakage
 
biff44 said:
??? ferrite beads? come on.
Click to expand...

Yeah, we call this "transformer" and some guys use this to couple AC whilst isolating DC.

biff44 said:
outside dc block!
Click to expand...

Bingo, that seems to be the magic keyword.

biff44 said:
you could do it with waveguide where the flange had a choke joint, the guide faces were insulated using kapton tape, and held tight with plastic screws, but there WILL be some emi leakage
Click to expand...

Yeah, why use a simple solution (two capacitors) if we can create a complicated one.
 

Yes, I think using just two capacitors would be the best solution.
Have to be careful how to choose them, to get best DC blocking performances at the working frequency.
 

Outer-DC-Block seems to be the term I need! Thank you.

Two capacitors would work but I was hoping for a screw-on adapter that was weather tight. I was also hoping it would be under $100.

At UHF frequencies transformers are made from ferrite beads all the time.
 


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