[Amps] hit a wall on bi-directional couplers

Tue Nov 22 11:42:15 EST 2005

The old ARRL SSB handbook had a schematic of the Collins power meter
directional coupler. I built one years ago and set it up with 2 meters. The
forward was calibrated 0 to 2000 watts and the reflected 0 to 200 watts.
It tracks the bird 43 quite well 160 through 10 meters. I think the greatest
error was on 160. I had 1 failure in 20 years when I left it connected to
the antenna during a lightning storm. The reflected diode 1n5711 got fried.
The forward one was fine. Dual meters are nice I give it a quick glance to
see the forward is higher than reflected and know I'm less than 2:1 SWR. gfz

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Bob Alexander
Sent: Tuesday, November 22, 2005 10:57 AM
To: Dan Sawyer; amps
Subject: Re: [Amps] hit a wall on bi-directional couplers

Hi Dan,
After re-reading my description of the coupler I realized I was a bit
careless.
So, to revise part of my description:

There are some problems with the schematic drawings found in the ARRL book.
If you look at the schematic in Figure 19 on page 27-17 the shields are
shown between the primary and secondary windings of both T1 and T2.  There
is also a shield between the coupler input and output sections.  The shield
should be drawn, from left to right, below the multi turn winding of T1 and
above the multi turn winding of T2. This can be seen in the pictoral
drqawing of Fig 20.   The turns ratio of T1 and T2 determine the amount of
coupling. The shield between the input and output sections assures that the
physical design of the device does not defeat the electrical design. It
prevents stray coupling.

The shield between the 1 turn primary and multi-turn secondary of T1 and T2
serve a slightly different purpose..... I hope I get this right.   Note that
the shield is shown grounded on one end only in both the schematic and
pictorial drawings.  Ground both ends and there is no transformer action at
all. With one end open only a current sample is induced in to the secondary
of the transformer.  In the coupled output side of the device this current
is applied across the 50 ohm resistor producing a voltage used by the
measuring part of the circuit.  As I said, I hope I got this right.  If not,
I am sure someone on the reflector will point it out.

If you want an RF output, remove the cap and diode.  They are there only to
provide a DC voltage output for the metering circuit .

73, Bob, W5AH

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]On
Behalf Of Dan Sawyer
Sent: Monday, November 21, 2005 8:42 PM
To: Bob Alexander; amps
Subject: Re: [Amps] hit a wall on bi-directional couplers

Bob,

Second quick follow up on the role of the Faraday shield. I reviewed a
second, 20th addition of the handbook today. The second diagram with the
horizontal shield is still, unchanged, in that version.

1. what is the role of the Faraday shield?

2. can the capacitor and diode be removed, and leave only the 50 Ohm
termination resistors?

Thanks - Dan

Bob Alexander wrote:

>Hi Dan,
>The newest ARRL Antenna Book I have is the 18th Edition (1997).  I have
>built several of the couplers as shown on page 27-17.  I followed the
>drawing in Fig 20 exactly and they work great.
>
>There are some problems with the schematic drawings found in the ARRL book.
>If you look at the schematic in Figure 19 on page 27-17 the shields are
>shown between the primary and secondary windings of both T1 and T2.  This
is
>incorrect.  The shield separates the coupler primary from the secondary so
>it should be drawn, from left to right, below the multi turn winding of T1
>and above the multi turn winding of T2. This can be seen in the pictoral
>drqawing of Fig 20.   The turns ratio of T1 and T2 determine the amount of
>coupling and the shield assures no stray coupling.
>
>The drawing in Fig 16 on page 27-14 is incorrect.  The shield should divide
>the box vertically.
>
>Look at Figure 12A on page 27-10.  Remove both resistors and install RF
>conectors at V1 and V2 and you have a bi-directional coupler.  A signal
>applied on the Tx/antenna side will be 30 dB down on the V1/V2 side.
>Likewise, a signal applied on the V1/V2 side will be 30 dB down on the
>Tx/antenna side.
>To clarify: assume all terminations are 50 ohm Z. Apply a 30 dBm signal to
>the Tx port and terminate the antenna and V1 port.  The signal level
>measured at the V2 port will be 0 dBm.  Move the termination from V1 to V2
>and try measure the signal level at V1. If the device is well built, there
>should be no measureable signal at V1 (no reflected power).  Now, move the
>termination from the antenna port to the Tx port and apply a 30 dBm signal
>at the Antenna port.  The signal level now measured at V1 should be 0 dBm
>(foward power).  If you apply the signal to V1 and V2 and measure at the Tx
>and antenna ports the same results should be obtained.  Measurements are
>meaningless without all ports terminated with either a load or source.
>
>Hope this helps explain this type of coupler.
>
>73, Bob, W5AH
>
>P.S. I am not impressed with the RF part of the Ramsey design. B
>
>
>
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