Directive coupler with microstrip

[eddygo]

Hi all,

I am not a microstrip specialist and hope to find here some advices. I want to build for me a directive coupler on printed circuit board FR4, with 30 dB attenuation for measurements with my Spectrum Analiser, Scope, etc on my power amplifiers.

I calculated main line with AppCAD from Agillent to be 1.9mm width for 50 Ohm @ 145 MHz.

Hou do I calculate coupling line for exact 30 dB attenuation ?

I post a link with a picture with what I want to build

http://yo3.hcv.ro/directive_coupler

Thank you in advance,
Eddy

 

[Higgler]

Most couplers in that frequency range are lumped circuit. Standard printed circuit couplers use quarter wavelength in the dielectric, a bit long typically.
Check out

http://www.minicircuits.com

for low cost couplers at 20 dB coupling, then add a 10 dB pad.
Depending on design requirements, and if you don't need any directivity in your coupler, and you just want some power coupled off, you can simply make a 50 ohm line in microstrip, then have a small printed line touch that 50 ohm line, forming a T shaped metal junction. The thickness of the tapped line can set the coupling. That's acceptable in certain circumstances.

kchiggins

http://www.toyon.com

 

[eddygo]

Ok, I know Minicircuits Couplers but they support small power only. These days I setup one 50W 145MHz PA and I want to see / reduce his spur better than 40dB.

I don't have a flat high power resistive att to couple my Spectrum Analyser and I made a small line of Cu wire at some distance between PI filter and SO-239 connector... very rude... but worked well....

I need to measure the Power in the same time with SA connected, just to adjust IMD better I can so, I think directivity is not a problem, just like you said, couple some power.

Can you please describe more ? I don't understand quite sure...


Regards,
Eddy

 

[Higgler]

eddygo,
Build a test circuit with three connectors on any dielectric material, any connectors that handle your power, sma-f's would be ok at this low frequency.

Arrange the connectors in a T shape, two on opposite sides (In>Out) and one on the other side (coupled)
Print a 50 ohm line the In>Out and connect a thin line from this 50 ohm line to the coupled output connector. Or you could solder a wire to the third connector and onto the 50 ohm line, that way you can change the wire size to change the coupling. Measure the coupling at your frequency first. It will be fairly accurate if your VSWR's on the surrounding equipment are good, but VSWR interaction can make this measurement erroneous if you first measure it on a network analyzer, having nearly perfect loads, and then switch to your circuit which may have poor loads. It's not a great solution, but it'll get you some results.
A similar solution might be to take a cable, cut the outer conductor, along the direction of the cable. i.e. use a sharp knife or razor blade and cut a thin slot along the cable, it shouldn't hurt it's VSWR much since the current inside travels that same direction.
Then wrap some wire around the slot on the cable and connect the two ends of the wire to a connector, one end to the center, one end to the cable ground. This would be an antenna coupler. Not certain what coupling you'd get out, but with 50 watts thru the cable, and measuring with a sensitive spectrum analyzer, it might be ok if coupling was 40 dB. Place a box around the wire to increase the coupling from the cable to the wire. When the cable leaks, the box contains the energy and the wire can pick it up easier. I've had two loads at the end of a cable couple -100 dB when in air and 50 dB when in a box next to each other.
The coax antenna coupler might be easier to make since you don't have to etch circuits.

kchiggins

http://www.toyon.com

 

[Comcokid]

Just a suggestion: If you can use something other than microstrip, and since you already indicate you're trying to do moderate wattage in a amateur band, there are already amateur how-to articles you can adapt. Look for ham articles on SWR bridges or do-it-yourself (Bird) wattmeter articles. These are basically couplers with a forward/reverse pickoffs into a diode to rectify to drive a meter. Just leave off the diode. There were some good ones using common plumbing copper pipe fittings.

 

[eddygo]

Thanks Comcokid,

I will buy a Bird element and jump over diode
In fact, that's what I already have, a Bird with DummyLoad and I need to see the spectrum in the same time with RF Power. Great ideea.

Thank you all for reply,
Eddy