[Amps] hit a wall on bi-directional couplers

[Karl-Arne Markström]

Transformer coupled broad-band directional couplers are notoriously difficult to construct.

The reasons for this are that you have conflicting goals in the transformers,
one is that a high secondary reactance is necessary to have the proper phase relationships
at low frequencies, but this makes the self-resonance frequency of the windings lower.

This changes the phase behaviour at the high end of the frequency range, which manifests itself
as rapidly decreasing directivity as the frequency increases.

Only by very careful selection of core materials and equally careful attention to winding layouts 
to minimize stray capacitances, a larger usable frequency range than 4 octaves can be realized.

G4FGQ has made a program http://www.btinternet.com/~g4fgq.regp/swrmeter.exe whose program notes
discusses the question in some detail, also Sabin's "Single Sideband Systems and Circuits" has a
discussion in chapter 12.

73/

Karl-Arne
SM0AOM







----- Original Message ----- 
From: "Dan Sawyer" <dansawyer at earthlink.net>
To: "m.ford" <k1ern at direcway.com>; "amps" <amps at contesting.com>
Sent: Sunday, November 20, 2005 6:29 AM
Subject: Re: [Amps] hit a wall on bi-directional couplers


> Mike,
> 
> Thanks for the reply. The year of the handbook is 1995. There are 2 or 3 
> examples on bi-directional couplers in the back. The full schematic is 
> on page 35 of the link, however the coupler portion is the same.  The 
> basic framework is the same between the link and the arrl handbook. The 
> difference is the handbook uses a Faraday shield on the transformer 
> feeds and another between the transformers. I am not certain enough of 
> how these actually work to judge how that effects operation.  Do those 
> shields materially effect the operation or do they simply add to 
> sensitivity and isolation? You are correct, this behaves more like a 
> power splitter then a directional coupler.
> 
> Do you know of a schematic for a directional coupler and a 
> bi-directional coupler? This one claimed to be and was straight forward 
> to bread board.
> 
> The 8405a V.V. is coming along. I finally had time to figure out how it 
> works and bring it on line. It is all working except for the input on 
> channel B. Channel A locks on from 1 to 500 MHz with a -50 DBm input. I 
> was pleased to see that it was more sensitive then 0 db. The alignment 
> section of the phase system is in spec, however without the B channel it 
> is difficult to test the phase system.
> 
> Thanks for the help - Dan kb0qil
> 
> m.ford wrote:
> 
> >
> > ----- Original Message ----- From: "Dan Sawyer" <dansawyer at earthlink.net>
> > To: "amps" <amps at contesting.com>
> > Sent: Saturday, November 19, 2005 8:58 PM
> > Subject: [Amps] hit a wall on bi-directional couplers
> >
> >
> >> All,
> >>
> >> I have hit a wall on bidirectional couplers for HF. I have 
> >> breadboarded the bi-directional portion of the following attached 
> >> bi-directional coupler circuit. It appears in principal to be the 
> >> same as the circuit in the arrl antenna handbook.
> >
> >
> > Hello again Dan,
> >    I have not seen the circuit you refer to in the handbook. My latest
> > copy is 1977.
> >
> >>
> >> http://www.ramseyelectronics.com/downloads/manuals/PM10DC.pdf
> >>
> > The circuit shown in fig. 1, page 5 looks like a 180 degree hybrid.
> >
> >> I did not terminate either coupler output, I simply sampled them for 
> >> voltage with a high impedance scope probe.
> >
> >
> > Hybrids are difficult to construct. Symmetry is a must.
> > It is wise to use attenuators on the output ports when making
> > measurements with them.
> >
> >   The net net is there is no
> >
> >> difference between the outputs and they are both high, about 3 db 
> >> down from the input.
> >
> >
> > If you terminate port A and feed port B you have a 2 way power divider
> > that will give you equal outputs at D and C  and they will be 3db down.
> > Plus insertion loss.
> >
> >   I did not use a 'binocular' transformer but used two
> >
> >> tube types in the same configuration.  I was very careful to wind the 
> >> transformers exactly as described in the text.  The feed through is a 
> >> piece of copper wire centered in the transformer by an insulator. The 
> >> RF output is terminated in 50 Ohms.
> >>
> >> I would appreciate pointers, advice, links to primers, or links to 
> >> alternatives.  Thanks - Dan kb0qil
> >>
> >
> > If what you are really looking for is a bi directional coupler for your
> > vector voltmeter I would suggest a Model CH-130 made by ANZAC.
> > The coupling is 30db but it will handle 500 watts and covers the 
> > entire hf band.
> > Must be one out there somewhere.
> > Mike k1ern
> >
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