# Gates M-5693 Modulation Monitor

Discussion in 'Amplitude Modulation' started by W2WDX, Jul 11, 2018.

1. ### AC0OBPlatinum SubscriberPlatinum SubscriberQRZ Page

DTS: "I do not understand why an inductor is needed rather then a resistor, or a variable capacitor.
Its said the capacitor will induce deadly phase shift, but does not an inductor do the same?

Because it is the least intrusive and most economical method.

DTS: "I would worry an inductor across a feedline would interact at points (in bad ways) as you adjust it and move around bands and adjust the antenna tuna."

You misunderstand the Pickup Coil system. Nothing is across or touching the transmission line wire from the transmitter to the antenna tuner or to the antenna. There are no phase changes or impedance changes caused by the pickup coil (Least Intrusive).

By the theory of induction, an oscillating current in a single Primary wire will Induce a Voltage in a coil of wire (secondary) surrounding it due to the oscillating magnetic flux surrounding the Primary wire, from V = - L dI/dt.

https://www.electronics-tutorials.ws/transformer/transformer-basics.html

L = Inductance of coil in Henry's, V is the induced voltage in Volts, dI/dt is Change in current through coil wrt time.

The suggested pickup coil systems are essentially transformers (RF transformers) in which there is One primary turn and N number of secondary turns. The voltage induced in the N secondary turns multiplies the Primary voltage by N turns.

The resistive voltage divider concept can be expensive because you would need high power, non-inductive and expensive resistors (not economical) in order to not modify the impedance or phase of the signal in the transmission line..

SO, the simple RF transformer that has been suggested by at least three of us, is the least Intrusive and most Economical method.

See the Schematic:

Pheel

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• ###### RF Transformer.pdf
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2. ### W7TFOPlatinum SubscriberPlatinum SubscriberQRZ Page

Well, traditionally it DOES touch the trans line, usually inside the TX cabinet. A choke to ground, tappable to derive the necessary 'oomph' to push a vintage MM.

Hey, it keeps deadly HV off the line if a coupling cap fails...

73DG

3. ### AC0OBPlatinum SubscriberPlatinum SubscriberQRZ Page

But I am not talking about an old vintage BC transmitter's system.

I am discussing a simple coil of N turns surrounding the transmission line that makes no contact to the transmission line.

For this amateur application, why complicate things?

BTW, here is a simplified Mod. Monitor Pickup coil schematic for a BC-1T and a schematic of the BC-500GY

Pheel

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• ###### Gates 500GY schematic.jpg
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4. ### W7TFOPlatinum SubscriberPlatinum SubscriberQRZ Page

It's complicated...this relationship. :-}

73DG

5. ### N2DTSHam MemberQRZ Page

Oh I thought you were connecting a coil between both sides of the transmission line...

6. ### N1BCGHam MemberQRZ Page

Actually, restoring this mod monitor is complicated. Whatâ€™s with these wax caps?

7. ### AC0OBPlatinum SubscriberPlatinum SubscriberQRZ Page

I don't think the Roller Inductor across the RF output line is a good idea and here is why.

Your roller inductor is rated in microHenry's (uH).

In the vintage transmitters that BCG and others are referring to, this inductor is rated in milliHenry's (mH) AND also acts as the Lightning/Static choke as in the schematic below of an RCA BTA-1R1.

A 2.5 mH coil has a reactance of 59k at 3.75 mHz, whereas a 10 uH coil has a reactance of 236 ohms at 3.75 mHz.

A coil in uH across the RF output line will severely load the output, and only confuse your PI network and really mess things up.

Phil

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8. ### W2WDXSubscriberQRZ Page

OK now wait a minute ... I got the the bit about the inductor around the coax thing like Jeff NBC suggested, however when Dennis mentioned the roller inductor I was thinking in terms of this arrangement:

GRRR ... OK so Phil, you think the best way is the coil around the coax thing. Should the resistor across the coil be variable, or should it be fixed non-inductive, like a 2W carbon comp? Will this be broadbanded enough for most of the amateur bands?

However, wouldn't a coil of about 340uh be enough to be invisible to the tank at amateur frequencies, especially 75m and up, yet still serve with a movable tap for the MM.

Last edited: Jul 14, 2018
9. ### AC0OBPlatinum SubscriberPlatinum SubscriberQRZ Page

No, the schematic I gave clearly showed the RF line (broken line) going through the center of the Pickup Coil (solid line).

Phil

10. ### AC0OBPlatinum SubscriberPlatinum SubscriberQRZ Page

Yes, same type of system as I showed in the schematic in Post #47. The L43 you showed in your schematic has a value in the milliHenry's and acts as a Protective/Lightning/ Static choke.

What I am saying is that with your roller inductor, it is a bad idea, because unless your roller inductor has a value in the milliHenry's, it is going to cause problems.

OK so you think the best way is the coil around the coax thing. Should the resistor across the coil be variable, or should it be fixed non-inductive, like a 2W carbon comp?

The toroidal coil system will work as well. A simple coil in a metal box with SO-239's in and out and a BNC connector for the Mod. Mon. will work as well,.

The loading resistor can be a fixed 2W carbon film or a 2W carbon composition type.

Phil

#### Attached Files:

• ###### M_5693 PickUp Coil.pdf
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Last edited: Jul 14, 2018