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Author Topic: Bias-T power injectors to run screwdriver motor.  (Read 2069 times)
K7JQ
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Posts: 281




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« on: November 07, 2012, 02:12:15 PM »

Anyone have experience using bias-t power injectors (such as MFJ-4116) to run 13.8V from  a shack power supply through coax to power a screwdriver antenna motor, thereby eliminating a control line? This is actually for a fixed installation with a long coax run. Thanks for any input.

73,     Bob, K7JQ 
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KB4QAA
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Posts: 2235




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« Reply #1 on: November 07, 2012, 10:04:39 PM »

Not much to say.  They are basic devices taught in any electronics curriculum.  They are widely used in industry.  It is easy to build your own, plans are readily available on the net, and in textbooks and reference books.

It would be easier to help you if you listed specific questions.

73, bill
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WX7G
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« Reply #2 on: November 08, 2012, 03:06:52 AM »

It will work and is how the Yaesu ATAS-120 screwdriver antenna is powered.
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K7JQ
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Posts: 281




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« Reply #3 on: November 08, 2012, 03:58:23 AM »

Thanks for the info, guys. Gonna check into it.

73,   Bob, K7JQ
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AA4PB
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« Reply #4 on: November 08, 2012, 05:33:04 AM »

I'd say it depends on the antenna and how you want to control it. Many use a 4-wire control cable with two of the wires being used for sensors for automatic control. Obviously those can't be used with a bias-T. The manual 2-wire type might work with a bias-T provided it is rated to handle the motor current.

Because the bias-T only provides one conductor and a common shield ground you won't be able to use a controller that swaps the motor leads to reverse the motor. You'll have to have something that provides + and - voltages to the bias-T in order to reverse the motor direction. Be certain that your bias-T doesn't contain any polarized capacitors.

As you can see, there are a number of variables to consider - it's not a simple yes/no answer.
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KG4RUL
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« Reply #5 on: November 08, 2012, 05:52:28 AM »

It is possible to inject DC power into the coax in either polarity, thereby allowing up/down control of the screwdriver antenna motor and eliminating the separate control cable.  However, you lose the ability to count turns on the motor making it rather hard to return to a previous setting.  Also, depending on the antenna being used, running the motor to the stops repeatedly could cause damage.
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K0BG
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« Reply #6 on: November 08, 2012, 06:01:09 AM »

It is possible, but it isn't an easy ABC scenario. You have to RF isolate the shield of the coax at both ends, and that causes even more problems. I suspect you could use the ATAS way, and change voltages to effect a relay change to reverse the motor, but most screwdrivers won't operate very well with just 8 volts on the motor.

It seems to me that doing so one heck of a lot of work, just to save a few feet of wire. It doesn't answer the common mode issues either.
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K7JQ
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« Reply #7 on: November 08, 2012, 10:19:12 AM »

Thanks again, fellas. I was also thinking about common mode and RFI issues with such a set-up. Alan, you're right (as always :>)....gonna forget about that idea and just run  a separate control cable. The antenna will be in a fixed location....ground mounted on a hill behind my house and out of sight (HOA issues). I'm not really concerned with having a turns counter (I've had them before, and they're not accurate)...just want to know if it's actually moving, and when it stalls at upper or lower limits. I have a light on the up/down controller that lets me know when it stalls, and I can also watch the ammeter on my power supply to see that it's moving, as it draws current. If the current spikes, that'll also tell me when it stalls (and activates the light). I'm replacing a High Sierra HS-1800PRO (blown up by a direct lightning strike) with a Tarheel 200A-HP, and using the existing HS controller with stall indicator light. Also have to replace the coax and control line...both pretty well shredded.

73,

Bob, K7JQ
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WX7G
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« Reply #8 on: November 08, 2012, 02:27:23 PM »

The bias-T injector and controller will work with a floating 12 VDC power source. It can be as simple as a 12 volt lantern battery. The coax cable shield does not have to be galvanically isolated from ground.
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AD5TD
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Posts: 112




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« Reply #9 on: November 10, 2012, 08:57:31 PM »

It is possible, but it isn't an easy ABC scenario. You have to RF isolate the shield of the coax at both ends, and that causes even more problems. I suspect you could use the ATAS way, and change voltages to effect a relay change to reverse the motor, but most screwdrivers won't operate very well with just 8 volts on the motor.

It seems to me that doing so one heck of a lot of work, just to save a few feet of wire. It doesn't answer the common mode issues either.

Exactly!  The YAESU ATAS 100/120 uses 12vdc to go up and 8vdc to go down.  The electronics sort it out in the antenna.  That's how they do it without isolating the shield. 
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