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Author Topic: Antenna multicoupler (dist amp) CU-168 70 Ohm and 50 Ohm  (Read 4455 times)
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Patrick J. / KD5OEI
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« on: September 04, 2019, 04:24:57 PM »

The CU-168 Antenna Multicoupler takes a 70 Ohm input from the antenna is is made to drive up to 5 receivers which are supposed to be 70 Ohm loads. It's broadband, no tuning, and covers 2-30MHz.

Here's the short description:

This equipment permits operation of five receivers over the frequency range of 2 to 32 megacycles per second. No adjustment or tuning is required. For accommodation of additional receivers, two or more antenna couplers may be connected in tandem by removing a terminating resistance, and substituting another antenna coupler.

Input signals from an unbalanced 70-ohm transmission line are introduced through an impedance matching transformer to a balanced, 1,000-ohm, artificial transmission line.
The artificial transmission line is terminated through an impedance matching transformer to a 70-ohm load.
Five identical isolating r-f amplifiers are connected to this transmission line at points along its length and independently supply signal voltages to the respective receivers. Outputs to receivers are also nominal 70 ohms.

The equipment passes all antenna signals in between 2 and 32 mc with minimum frequency discrimination, distortion, noise generation, cross-talk, or response to spurious signals. The equipment has enough gain to overcome most of the losses of the coupling circuits while isolating the receivers. The maximum loss through the unit from input to any output is 4dB. (Vin <1.6 * Vout)

-->  My question is, how will this thing likely work when getting fed by a 50 Ohm antenna, most likely the 50-ohm-matched side of the shack antenna tuner during receive.

My concern is that the unit has this 'transformer-coupled broadband transmission line' that is supposed to see a certain impedance, and it's not what I am using.

-->   How do I fix the 50-70 Ohm mismatch at the input? A 20-Ohm series resistor? A small RF transformer?

-->   Do I ignore it and just terminate the unit with 50 Ohms instead of 68?

-->   I am not too concerned about what the outputs of the individual amps will do when seeing 50 Ohms because they are broadband transformer coupled class A push pull stage, but should I also add 20 Ohms in series with the outputs to the receivers?

The schematic is attached.

* CU-168 schematic tabloid.pdf (389.56 KB - downloaded 147 times.)
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Radio Candelstein - Flagship Station of the NRK Radio Network.
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« Reply #1 on: September 04, 2019, 05:43:54 PM »

My thought is to just ignore it and carry on.  I have set up a few broadband receive-only antennas for various customers.  Some were specified as 2 to one SWR over the specified band, and some were specified as 3 to one.   In each case, they were destined to feed multicouplers.  You are looking at roughly a 1.5 to one mismatch, assuming that all your receive antennas are tightly specified.  And your receiver front ends may be nominally 50 ohms but who knows what they actually are. 
If you do decide to worry about it, one option is to insert a 50 to 75 ohm matching pad.  These are resistive attenuators that have a different impedance at each end.   The beauty of attenuation is that it makes the match look better, even if it isn't.   The loss in the attenuator is not generally a problem below 15 or 20 mhz because band noise is relatively high.  Matching pads aren't easy to find.  I have one in the junk box ... somewhere.  It's possible that Pasternack has them.
But I'd just cable it all up and go with it as is.
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« Reply #2 on: September 04, 2019, 10:55:23 PM »

"Outputs to receivers are also nominal 70 ohms."
Nominal means: In name or thought, but not in reality. Thus, nominal is whatever you say it is. So, get out your Dymo labeler and print up some "50 ohms" labels to stick on that thing and...
...no worries.


Don
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« Reply #3 on: September 05, 2019, 11:52:08 AM »

Yeah, just run it... it's buffered by the amps, so it really won't matter except in terms of "gain"
if you compared a 50ohm load to a 75 or 100 ohm load on the output side... oddly the schematic shows
75 ohms at what I thought is the input (left side)??

What the purpose of the series, small chokes between each preamp is, I have no real clue.

Pix of the unit?

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« Reply #4 on: September 05, 2019, 08:32:37 PM »

Okay, I'll proffer a practical answer. Terminate it in a 50 ohm load. Not that it will make very much difference, but that's what I'd do. The H-pad calculates as a 3dB attenuator at 1000 ohms. So you'd get at least 6dB return loss no matter how you terminate it.

Does this thing really have twenty 12AU7's in it? Wow.
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« Reply #5 on: September 05, 2019, 09:07:12 PM »

I have several of these.  The wording of 70 Ohm input-output should have been prefaced with the word "nominal."  You do not need to match these to-the-Ohm. 70 and 50 are close enough to forget about it.  On the downstream end, receivers do not represent a pure static impedance at the antenna input across their range either. 

Yes, they do have that many 12AU7's.  Fortunately those are not as expensive as 12AX7's and I haven't needed to replace any yet. 

The Navy used these to feed R-390A's in another century.
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Geoff Fors
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« Reply #6 on: September 05, 2019, 10:15:34 PM »


12AU7s? Mine's got 6922/6DJ8s!

Not sure which version mine is, there are two very similar.
This one has been on the shelf for about 35 years or so...
Came in from a hamfest.

Missing the meter... Need to find a suitable meter.
Notwithstanding the face of the meter, wonder if anyone who has one
can check the actual meter to see what it really is?

Thing is that some 6922s today are highly coveted by the audio folks.
I never could stand the way they sound...

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« Reply #7 on: October 10, 2019, 04:24:11 PM »


Pulled mine out of the warehouse today.

It's a CU-656 masquerading as a KW-2A Coupler, Antenna mfg by Westinghouse.

A clean shot of the meter, and it's actual FS= rating would be nice, the pix on the
NAVY data website are low res. Need to replace the meter and repair all sorts of
cut wires... Sad
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MikeKE0ZUinkcmo
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« Reply #8 on: October 11, 2019, 09:26:24 AM »

If one were to carry this a bit further, you could feed your distribution amp with something like an old Johnson 250-39 T/R switch, and end ALL your concerns about antenna switching.



Edit; Pic is a link to more info.

 
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Mike KE0ZU

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« Reply #9 on: October 11, 2019, 11:01:22 AM »

That thing is for Break-in CW.

BAD JuJu if you use that thing with anything solid state!  P-N junctions will go off to join their ancestors in a twinkling of an eye.

BTW, 6BL7 runs nice and hot.

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« Reply #10 on: October 11, 2019, 01:25:27 PM »


Pulled mine out of the warehouse today.

It's a CU-656 masquerading as a KW-2A Coupler, Antenna mfg by Westinghouse.

A clean shot of the meter, and it's actual FS= rating would be nice, the pix on the
NAVY data website are low res. Need to replace the meter and repair all sorts of
cut wires... Sad

I have scanned and posted the CU-656 manual is at
http://www.navy-radio.com/manuals/cu656-93804b-6305.pdf

I have a CU-168 in my rack that has an original plaque reading "Multicoupler No. 25" - I just like thinking about some place with 125 receivers......

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MikeKE0ZUinkcmo
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« Reply #11 on: October 11, 2019, 11:04:12 PM »

Quote
...BAD JuJu if you use that thing with anything solid state!  P-N junctions will go off to join their ancestors in a twinkling of an eye...
I've no idea what "Bad JuJu" means, and it seems you didn't follow the link.   However, a pair of silicon diodes back to back at the "Receiver" output connector, takes care of any possibility of killing SS front ends.

Further, the subject here was tube type equipment and no harm will come of the distribution amplifier being discussed.
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« Reply #12 on: October 13, 2019, 04:52:03 PM »

Are the diodes in the attenuator in the link biased on always? Just curious about that.
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« Reply #13 on: October 13, 2019, 09:54:36 PM »

Meter? What meter?  The CU-168 don't need no stinkin' meter.  Or was that just about the other models?

Bad juju = inviting disaster by doing something risky, tempting Satan to blow up your boat anchor gear.  Examples: Letting your brother-in-law borrow your SX-88 for Field Day, or calling CQ on the lower CW band edge of 20 Meters using the fake callsign of 5Y3GT.  
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Geoff Fors
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« Reply #14 on: October 14, 2019, 07:11:29 AM »

Pat, If you're referring to my link, the diodes are simply back to back.   We're just looking at the plate of the T/R sw's receive signal output tube.  



--------------

Geoff, nice chuckle, I LIKE that call sign Smiley Smiley

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« Reply #15 on: October 14, 2019, 02:12:03 PM »

Or you could use this one... (which is what I intend to do)

Dismayed that most of the 6922s died the white cap death due to not being
in an environmentally stable situation, and being knocked off the shelf by varmits
that penetrated the perimeter defenses. Sad

Nick, already had downloaded the manual.

Turns out that someone had scavenged the HP/LP input filters before I got it...
The HP would have been nice to have.

Thinking about how to use Mosfets to replace the function of the 6922s... Hmmm...
20 tubes is a lot of filaments to burn...

It's said that the back to back diodes at the input of a receiver increases IMD. I don't know
the truth of the matter, or if it really matters enough to be important or not.

Trying to figure out if I need to mess with this unit or not. BUT IT's COOL! "Smiley

(Clearly, I have some "personal issues" if this is how I think about these things!)
 
I have a TMC solid state that I can expand to 16 out by cloning or substituting some amp
boards. I'd need to "fake" some small old school edge connectors to be able to slide them
in... Have 4 outs running now.


* DOW KEY DKC TR Switch.JPG (1440.57 KB, 2592x1944 - viewed 215 times.)
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