Author Topic: Homemade line coupler  (Read 3252 times)

Offline mojoe

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Homemade line coupler
« on: June 29, 2013, 08:02:40 PM »
After missing out on a line coupler on ebay, then having another one that I had arranged a trade for sold to someone else, I decided that I would try to build one. I have a mostly complete schematic from the Radio Systems manual and a closeup picture that I got from somewhere on the net.

Three important pieces of information are missing from the schematic. First, the schematic shows the rotary switches as boxes. The switches for the taps on the toroids are straight forward. Not so with the switches for the mica caps (using four caps in combination to produce ten values). Second, there is no informaton about the number of windings on the toroids, or the type/size of ferrite used. Third, the schematic labels the meter as "microamps". The manual says "milliamps". After I build the circuit, I can easily determine which it is.

I haven't figured out what the switching arrangement is on the caps, as they are shown on the schematic (and backed up by the picture). However, I have figured out a way to accomplish the same thing with a different wiring scheme.

Looking at the picture that I found, I think I have also figured out the toroids. I counted the turns as shown on the picture. I believe that I have the correct numbers, as the primary/secondary ratios come out as whole integers that have a logical progression. For the size of the ferrite cores, I estimated that from the size relative to other components in the picture. As to the type of ferrite, I picked some common types that could be used at these frequencies and ran some calculations for impedance, based on the number of windings that I counted. To be on the safe side, I ordered two different ferrite types.

The manual states that the coupler matches 15 values of impedance, ranging from 1-50 Ohms. If I am understanding the circuit, I calculate the range as 1.47-25 Ohms. It is possible that I am missing something.

I just received most of the parts from Mouser and Amidon. I am still awaiting the ceramic rotary switches (NOS Russian surplus). I have some perf board and a suitable metal box to house things in. It won't be layed out like the original, but should do the job just fine.

Hopefully, the switches will arrive soon. The big question is, did I pick the correct size and type of ferrites? I think that even if I got close, things should work well enough to get a reasonable match. Keeping my fingers crossed.

Joe
« Last Edit: June 29, 2013, 08:04:13 PM by mojoe »

Offline RFBurns

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Re: Homemade line coupler
« Reply #1 on: June 30, 2013, 10:55:59 AM »

The manual states that the coupler matches 15 values of impedance, ranging from 1-50 Ohms. If I am understanding the circuit, I calculate the range as 1.47-25 Ohms. It is possible that I am missing something.

The bank of caps and combination with the toroid cores gives the impedance range, which actually can go above 50 ohms. LPB's TCU-30 can match up to an impedance of 75 ohms.

Let us know how it goes! As far as I am aware, you would be the first to replicate a coupler based on partial schematics of one. This is great because you can add to the design in case the base values don't quite help in matching up. I myself have built several couplers in the past but never taken a design and replicated it.

RFB

Offline mojoe

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Re: Homemade line coupler
« Reply #2 on: June 30, 2013, 12:35:18 PM »
RFB,

Perhaps you would share the details on the couplers you have built? What impedance ranges did they match? How well did they perform?

Joe

Offline RFBurns

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Re: Homemade line coupler
« Reply #3 on: July 07, 2013, 01:36:57 PM »
Most of them were built around tuned circuits. IE..air inductor coils and capacitor banks. I did not make any that had a wide range of matching capabilities, they all were designed to match up into 3 or 4 ohm loads which is typical for utility transformers. (hot side).

The most simplest coupler was with a 20 watt 50 ohm resistor across the TX output and tapping the + side with about a 50 foot long wire wrapped around a yard extension cord plugged into an outlet. Coupling tuning was obtained by moving the wire up or down the length of the extension cord. Worked darn well for what it was!

RFB

Offline mojoe

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Re: Homemade line coupler
« Reply #4 on: July 20, 2013, 04:31:06 PM »
In addition to here, I posted about my efforts to build a coupler over at Hobby Broadcaster, in the CC section. My last post concerned the number of taps on the first of the two cascaded toroid transformers. It is obvious from the picture that there are 15 turns each. I did a spreadsheet to calculate different tap positions. If you have time, would you comment on the two choices that I came up with? I'm favoring the one with less than 1 Ohm impedances. Is this necessary? I think either choice would probably work OK.

I'm still waiting on those darn rotary switches.

Joe

Offline RFBurns

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Re: Homemade line coupler
« Reply #5 on: July 21, 2013, 07:47:30 AM »
Most line (hot) coupling will be something around 3 ohms. Neutral coupling can vary depending on effectiveness of ground system between 75 and 25.

Hope those switches arrive soon. Looking forward to hearing how the replicated coupler pans out!  8)

RFB