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Old 22-07-2013, 15:23   #1
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Looking for DC-Block for PL259

I need to block the DC ground path between my mast and engine through the VHF ground shield. I would like to install a DC block but have not been able to find one with a PL-259 / SO-239 (or is it PL-259 / SO-259?). Does anyone know where I can purchase this configuration?
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Old 22-07-2013, 18:23   #2
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Re: Looking for DC-Block for PL259

... why do you need to block DC from the VHF shield? What sort of problems are you having?

if there's a valid reason to do so, I would be more tempted to do something like insulate the antenna mount from the mast.
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Old 22-07-2013, 18:47   #3
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Re: Looking for DC-Block for PL259

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... why do you need to block DC from the VHF shield? What sort of problems are you having?

if there's a valid reason to do so, I would be more tempted to do something like insulate the antenna mount from the mast.
Yes, an insulated antenna mount would achieve what I am trying to do, but with the stick up, a DC block would be the easiest at this point.

I had the mast down two years ago (the winter after the fall that we purchased the boat) but didn't realize the problem until last year. We consumed three MaxProp zincs in about six months. We are on a mooring.
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Old 22-07-2013, 18:50   #4
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Re: Looking for DC-Block for PL259

Can't you install a plain old capacitor to block the DC?
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Old 23-07-2013, 01:18   #5
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I'm confused, why is a dc connection between your mast and motor causing you prop zincs to burn up so fast? Also don't forget that all the lights on your mast will be bonded to the dc negative. Blocking capacitors are normally used to prevent a dc path from a lighting strike damaging your electronics
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Old 23-07-2013, 05:09   #6
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Re: Looking for DC-Block for PL259

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I'm confused, why is a dc connection between your mast and motor causing you prop zincs to burn up so fast? Also don't forget that all the lights on your mast will be bonded to the dc negative. Blocking capacitors are normally used to prevent a dc path from a lighting strike damaging your electronics
Blocking a lightening strike? I may be wrong but I would think lightening would fry and jump any capacitor I know about except possibly some giant, commercial stuff. Can you point me to a capacitor for this application?
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Old 23-07-2013, 05:12   #7
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None of this is the problem of your zincs frying

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Old 23-07-2013, 05:14   #8
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Re: Looking for DC-Block for PL259

I insulated the vhf mount, but a temporary fix would be to disconnect the antenna from the back of the radio when you are not using it.

Yes, its possible that this could be the cause of your zinc's frying, especially if you have grounded the mast to your keel.
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Old 24-07-2013, 12:53   #9
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Re: Looking for DC-Block for PL259

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I insulated the vhf mount, but a temporary fix would be to disconnect the antenna from the back of the radio when you are not using it.

Yes, its possible that this could be the cause of your zinc's frying, especially if you have grounded the mast to your keel.
Yes, I've started disconnecting my radio, but I'd like to get to a more permanent solution that avoids me forgetting to do this, or forgetting to reconnect before the radio is used.

To an earlier comment, lights and instruments aren't an issue because their fixtures are plastic or otherwise not connecting ground to the mast.

Why do we have this problem? The mast is grounded to our iron keel, the antenna mount and base are not insulated from the mast. This results in our keel being electrically connected to the boat's ground, including our prop. Our keel is not fully encapsulated, though we try. This all creates a battery that results in about 300 mV. I measured this between the coax connectors when I had them apart.

I could also disconnect the bonding wire between the mast and keel, though I'm pretty sure that would go against ABYC standards.

As far as building my own DC block with a capacitor, I have thought about that too but in the interest of time, I'd just as soon buy one. I have seen several offered but not with the PL-259/SL-239 connectors that I could easily install.
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Old 24-07-2013, 13:30   #10
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Re: Looking for DC-Block for PL259

Try 'The RF Connection' they have all kinds of co-ax stuff, Bruce.
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Old 24-07-2013, 14:09   #11
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Re: Looking for DC-Block for PL259

My experience with DC blocks has been blocking power on the coax center conductor. I don't believe a standard DC block will accomplish your requirement to block DC on the coax shield. I suspect that you are going to be faced with designing your own.
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Old 24-07-2013, 20:20   #12
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To pass RF but not DC while not changing the impedance the radio sees will be quite tricky. I haven't heard of this being done in this way, but that may not mean much. If you simply try to DC block with a capacitor, you WILL affect the impedance seen by the radio, and might end up with a radio damaging SWR. if you want to experiment, I strongly suggest you borrow an antenna analyzer to see what effect your blocking appliance has on SWR at the VHF frequencies you will use. Or you could use an inline SWR meter but keep power on LOW when you test.

I think you will be better off isolating the antenna from the mast.
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Old 25-07-2013, 19:52   #13
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A 1:1 balun might be worth investigating?
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Old 25-07-2013, 20:10   #14
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Re: Looking for DC-Block for PL259

West Marine recommends this type of device but I could not find it on their web site.

A typical DC block only blocks DC on the inner conductor of the coax. The outer conductor is DC connected. They are typically used to eliminate 50/60Hz causing problems with a receiver or to allow DC to be fed to an antenna.

A 1:1 balun does not break the DC connection either.

Pasternack makes DC inner-outer blocks:

http://www.pasternack.com/images/ProductPDF/PE8215.pdf

Unfortunately they do not make them in PL-259/SO-239 style. But they make PL-259 adapters to convert type-N.
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Old 25-07-2013, 22:02   #15
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Re: Looking for DC-Block for PL259

What you are looking for is an inner/outer DC block. Unfortunately, all I have ever seen ( not that many) have been rated for low power, like 2 watts max. That wouldn't last long on a 25 watt VHF transceiver. All I have ever used in other applications had SMA connections, but I know they make them with N type connectors. Suppose you find one with N connectors, it's easy to find N connectors for any type coax cable.
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