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Directional Coupler Vers 0.1

My first attempt to make a directional coupler turned out OK but not brilliant. The goal

was to be able to sample forward and reflected power using a simple stand-alone box. I

think I can call this a success through 6M. M it starts to fall apart and fre!uencies

abo"e M are simply relati"e. #ere is a schematic of "ersion $.%&

images'(irectional)oupler'(irect)oupl*$%+chematic.,pg

The transformers are %$ turns of % on a T-/$ type 6% toroid. 0y making this a 1

port coupler2 I can re"erse either the input or output to measure reflected power.

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images'(irectional)oupler'#omebrew)oupler.,pg

It seemed like a good idea to put some isolation between the input and output sections.

The nifty #ammond box pro"ided a place for a di"ider so I put it into the circuit. Two

'34 holes were punched through some double sided board to allow the passage of the

wires. #ot glue was used to ensure stuff remained where it was supposed to be. 5fter

initial measurements2 the copper board was remo"ed because it added about %./d0 of 

insertion loss. The two parallel wires were separated as much as possible because they

were coupling and damaging the isolation between ports at higher fre!uencies. I

experimented with a di"ider between the two parallel wires but couldnt obser"e any

meaningful results so the di"iders were omitted completely.

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The design is completely re"ersable. The inputs can be outputs. The point is there is

supposed to be -$d0 of separation between in'out. The two hal"es are completely

symetrical.

Inside 20dBCoupler2

images'(irectional)oupler'Inside-$d0)oupler.,pg

Initial data was collected to measure the performance. I used a 7a"etek /%$

generator and a 0oonton 1$ power meter. 5fter calibrating the power meter2 the rest

of the fixture needed to be documented. 8irst I calibrated the 9:/; and the related

coupler'adapters I used to get 98 from type < connectors on my instruments to 0<).

#ere is some raw data. The =)ontrol> column is the measured loss of the

cables'connectors with only a 0<) barrel connector inserted instead of my new box.

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images'(irectional)oupler'(irect)oupl*$%9aw(ata.,pg

8irst I measured insertion loss. The secondary output ports 3 and 1 were terminated

with /$ ohm terminator 0<)s. The cable losses ?control@ were subtracted from the

measured responses. The table therefore isolates the true insertion loss of the (AT2

which I ha"e indicated as +%. #ere is a graph of the input port % to output port

insertion loss o"er fre!uency. 5s you can see2 measuring down to $.%d0 has some

funny anomalies. I belie"e you can safely attribute the weirdness to cable

transformations as the roughly 14 of coax can do funny things at these fre!uencies. If 

you smooth out the line from /$ to 3/$ you will see the likely true response of the

circuit. If you look at the raw data you will see that insertion loss really goes into the

dumpster abo"e 1$$M#B2 which is why I ,ust ended my graphing of the data at 1$$.

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DirectCouplV01S12Chart

images'(irectional)oupler'(irect)oupl*$%+%)hart.,pg

<ext I measured the forward power indicated at port 1 with port 3 terminated using a

/$ ohm 0<). The isolation between ports should be about %./d0. 5t low fre!uencies

it works well. It starts to fall apart at about 6M and is still usable as a forward power

tap point through M.

DirectCouplV01FWDChart

images'(irectional)oupler'(irect)oupl*$%87()hart.,pg

Then I re"ersed port 3 and 1 to measure reflected power. )onsidering the 0<)

terminator has a pretty flat response2 the re"erse power measured should remain

minimal. 5bo"e 6M2 the re"erse power is not much different than the forward power.

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This does not bode well for my new gismos ability to tell the difference between forward

and reflected.

DirectCouplV01REVChart

images'(irectional)oupler'(irect)oupl*$%9C*)hart.,pg

5nd finally2 using the data collected2 I prepared a chart of the 8orward minus 9e"erse

power. I belie"e this to be a measure of how effecti"e the de"ice is at measuring

 Ddirecti"ity of the power. 5s you can see2 when terminated into /$ ohms you would

expect there to be "ery little re"erse power. 5s fre!uency goes up2 the measured

reflected power increases dramatically2 which tells me that the de"ice is not able to

determine directi"ity "ery well. I was hopeful to use this circuit to measure impedances

at some point. Im not so sure this will work as hoped.

The transformers were wound on type 6% ferrite. 7ould 13 be betterE +ome other

fla"or perhapsE Fart of the high fre!uency anomaly likely relates to the transformer

being self-resonant at some fre!uency. I didnt attempt to measure what that was as it

would likely mo"e around o"er temperature !uite a bit. +omebody suggested adding

turns but this would lowere the +98 and likely create other problems. 5ttempting more

than $d0 of wideband isolation in a simple enclosure like this is wishful thinking.

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DirectCouplV01FWDREV

images'(irectional)oupler'(irect)oupl*$%87(9C*.,pg

5lthough the picture abo"e shows the circuit board used to compartmentaliBe the

#ammond box2 I pulled it out because it was actually increasing the insertion loss for

some reason. I also obser"ed that separating the two parallel wires helped reduce

insertion loss. Initially2 my losses were as high as 1./d0 at %:#B and I was able to pull

that down to about 3d0. I also noted that holding a screwdri"er across the #ammond

box with the lid remo"ed changed the insertion loss at certain fre!uencies dramatically.

5 ground loop needed attention. 5t 1/$2 $$2 the obser"ation was most noticable with

a change of o"er %d0 possible using the blade of a screwdri"er. C"entually I pulled the

weather proof insulation out of the box lid and pro"ided a decent electrical path through

the lid2 which helped a bit. These measurements were taken after all inter"entions were

completed.