[Premium-Rx] FS: HF RECEIVER MULTICOUPLER (FROMDRAKE7-LINESTATION)

[Michael O'Beirne]

Thanks Clemens

Many thanks for the email.

I don't have the knowledge or software to emulate your grapgs, but I have 
noticed with my old HP8640B and th Racal 9301A RF mVmeter, that the output 
port-to-port isolation seems to be critically dependent on the input port 
impedance.

I have also measured quite a few nominal dummy loads and the DC resistance 
varies quite widely from about 48 to 53 ohms (with a Fluke 25).  My thought 
to improve matters is to stick the DL on the end of a very good 10dB fixed 
attenuator from R&S.

I have a CA612 in as new condition (via Pat) and also the same circuit.  You 
will probably know that he had to modify his one to achieve that superb 
isolation.

I suspect for higher frequencies that you could make a combiner for a fixed 
frequency from coax at little cost but quite a lot of effort.

73s
Michael
G8MOB

----- Original Message ----- 
From: "Clemens Paul" <cpaul at gmx.net>
To: "Michael O'Beirne" <michaelob666 at ntlworld.com>
Sent: Sunday, April 29, 2012 6:25 PM
Subject: Re: [Premium-Rx] FS: HF RECEIVER MULTICOUPLER 
(FROMDRAKE7-LINESTATION)


> Hello Michael,
>
> thanks for your interesting email,too.
>
> Star Splitter
>
> I was also surprised at the measured 6dB isolation.
> So I've drawn a circuit diagram and done a simulation
> with spice,see attachment.
> If you have a voltage of 1V at port 1 you get 0.667V after R1 (blue line 
> in
> the simulation,see att.) and 0.5V over R5 (red line).
> You can also calculate the voltage division by hand,paralleling and adding
> the different resistors,  which is a bit more cumbersome and one easily
> can make calculation errors (at least I did at the first try),but you get 
> the same result.
> I've also done a simulation with RFSim which also calculates 6dB isolation
> using power.
> Anyway, I found it a good exercise for me. :-)
>
> Hybrids
>
> I've measured my two HP 8721A directional bridges which are 6dB
> combiners/splitters,and my MSC ZFS-2-6,a 3dB design,see att.
> You can see different curves.
> The best curve (bottom) is from one of the 8721As with a very good HP 50
> Ohm termination from the 8407A VNA system.
> Next best is the other HP bridge,also with the good resistor.
> The next two curves are with the better HP bridge and two of the the 
> common
> HP 50 Ohm resistors  which were widely used to terminate unused
> spectrum analyzer ports.
> The top (i.e.worst) curve is from the 3dB MSC combiner with the good 
> resistor
> as termination.
>
> Racal CA612
>
> I have got a circuit diagram of this amazing combiner from Pat,G3YFGK,
> who you know well.
> Pat has measured a port to port isolation of 88dB.
> I wish I had also the combiner itself not only the diagram!
>
> 73
> Clemens
> DL4RAJ
>