Thanks again Bob,
The explanation of the shield on the primary winding makes sense. I
thing that is referred to as a Faraday shield. Your explanation makes
sense, it shields the E field but not the H field.
I have all the materials to construct a bi-directional coupler. We are
leaving to visit family, I will have a chance to try to build one next
week.
I would like to thank everyone who provided an answer. Your replies were
all very helpful.
Dan kb0qil
Bob Alexander wrote:
>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@contesting.com [mailto:amps-bounces@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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