I found that an easier way to make a high power dual directional coupler is the
old 'monimatch' SWR bridge approach. Here you have a mainline (which I did with
a 50 track on PC board, and two adjacent p[ick up line running parallel to it.
In my case, I spaced it aby about 3/16 inch. Each pick up line has a socket
(BNC), obviously at opposite ends to each other. So if the main line runs left
to right, pick up line 1 hasa the BNC at the left end, pick up line 2 has the
socket at the right end. Each line is terminated in a resistor and if needed, a
small shunt capacitor adjusted for best directivity. One way round gives more
coupling than the other, and off hand, I can't remember which it is. Take the
way round that give min signal on a pick up line (turn the main line 'ends'
round) and adjust the termination for minimum. Use the highest frequency you're
going to use for this. A 6 inch line will work well up to 30MHz, maybe beyond.
You now have a reasonably high power directional coupler. With decent PC board
and low SWR, it should handle a kW or more. I've seen a simlar one handling a
kW at 432MHz.
The problem you have is that the coupling is frequency sensitive, and reduces
at 6dB/ocatve. If this is a worry, a suitable R-C on the BNC end will flatten
the response pretty well. Because you have two lines, you have a dual
directional coupler. I get about 40dB coupling on 3.5MHz, dropping, as
mentioned above, at 6dB/Octave.
73
Peter G3RZP
========================================
Message Received: Nov 20 2005, 05:30 AM
From: "Dan Sawyer"
To: "m.ford" , "amps"
Cc:
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"
> To: "amps"
> 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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