EMRFD Message Archive 14655

Message Date From Subject
14655 2018-04-28 04:10:04 Ravi Miranda Directional coupler - Transformer Saturation question

In a directional coupler I understand that there are two transformers,
say T1 and T2 while I can understand that T1 has 1 turn primary and
say 20T secondary and that makes sense. In T2 which is one end of
20Turns to the hot end and the other end to ground won't the 20 T be a
direct short? Maybe I'm missing something here? or my theory isn't up
to spec.

The question here is: which of the cores will saturate? would that be
T1 or T2, I'm thinking of using FT50-43 for both T1 and T2, but for
the T2, the one that takes the 20T to ground I am thinking of using a
stacked FT50-43. I hope to run 100W through this.

Also, another question, some couplers show a low value capacitor
(https://www.w6pql.com/hf/hfddd-assembly2.htm) from hot end to ground
and some others show a faraday shield?
(http://www.angelfire.com/electronic2/qrp/Zmatch.html or the stockton
version here
( http://www.eham.net/ehamforum/smf/index.php?topic=110739.0)

There are more versions here:

Do each of these have a difference of readings, quality?

Sorry about the long list, but any pointer to further reading will be

Many thanks for your time.

Wish you all a great weekend.


I'm here to add more value to the world than I'm using up.
14656 2018-04-28 09:32:38 kb1gmx Re: Directional coupler - Transformer Saturation question
The referenced articles cover two different bridge circuits and the basic ideas of a bridge circuit apply.

The w6PQL and the KC8AON are both examples of the Tadem Match or otherwise known as the Stockton bridge.

bridge used on some HeathKit directional power meters.

in both cases it was desired by the designers to get several properties.  One being the same or near the same result regardless of frequency.  The other was to obtain sufficient directionality to measure the reflected power.

As to the winding across the output, T2. What is the inductance at xxx frequency?   It is certainly greater 
than a RF short.  10 or more turns on a ferrite like 43 or 61 would have substantial inductance.

Core saturate in different for the two and in some cases you need to worry about the external components 
 in the circuit before the cores are a problem.  However w6PQL coupler as shown at www.w6pql.com/1_8_to_54_mhz_dual_coupler.htm is good for 1KW according to him.  I'd believe it
on inspection.

For that case and the other be aware the core sees the field generated by a single turn of wire so the ampere-turns are where you start for the T! core,  The T2 core is sampling a voltages and has very low currents.  By calculating the field and comparing it to what the core can tolerate you get an answer.  However most ferrites do not saturate until large values of applied field.  But the KC8AON tuner is limited by the elements used in the tuner section and the rather tiny ft37-61 cores are not stressed at 50W.

In both styles of bridges the sampled power the eare internal load resistors 50 ohms for the two stockton forms
and the borne uses a 200 ohm those have to stand the sampled power in the range of 20 to 30db less than
the applied power.  The borne also has a capacitive divider (compensation) that has to stand the applied RF voltages.

As to the quality question that is harder to answer as there are various ways one can evalate quality.
The usual is directionality, the ability to distinguish small reflected power from the applied forward power.
All do well for their application.   The other is power sensitivity and that does vary by design as well, the number of turns on both transformers determines that and interacts with directionality.



14679 2018-04-30 18:20:18 Russell Shaw Re: Directional coupler - Transformer Saturation question