Looking for DC-Block for PL259

[smac999]

the bounding wire on the keel / mast should be tied to DC neg at the main neg bus down below anyways. so this path should be there anyways.

though you can buy antennas that are isolated from their mount. you could also buy a plastic mount, but it'll be weaker.

[RandyBatchelor]

Any progress on this? I also want to ground my masts to a seperate grounding plates, and I'm concerned about this same issue, and can't find these DC blocks online...

[transmitterdan]

Polyphaser makes UHF DC blocks with gas discharge lightning protection. Not cheap...

https://www.dxengineering.com/parts/ppr-is-50ux-c0

[RandyBatchelor]

Thanks, I was actually looking at these yesterday.... It seems like they only block the center pin (inner) while I'm looking to block the outer, ground, to prevent corrosion.

I reached out to their tech support. I'll post back here if I find something useful. Cheer,

[smac999]

Originally Posted by hlev00

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.

most boats are like this...

[RandyBatchelor]

I'm mainly going off of what I read here. As an EE, this all makes good sense to me, and I like to be prudent. I've also cross checked this against a book on the topic and a couple other resources that seem to align. So really just trying to do all the best practices in hopes of preventing any corrosion issues.

Marine Grounding Systems | West Marine

[RandyBatchelor]

I got confirmation from the techs here... This will do it.https://www.dxengineering.com/parts/ppr-is-50ux-c0

[MBWhite]

My suggestion is if this is truly a valid concern then get a different antenna that has its own ground plane or matching stub.

As an example, the Shakespeare 5104 is 48" long and has 3db of gain in addition to being electrically isolated from whatever it is attached to, so no sort of isolation blocks are needed. Another benefit is the entire antenna will be cheaper than what a quality grounding block would cost.

[mlucitt]

Like this one:

[SoonerSailor]

Can you identify the capacitor?

[mlucitt]

Sooner,
I tried to take a clear picture of the components but my eyes are not as good as they used to be, either.


The high quality capacitor I used was 2nF, a value that I computed to be almost a short circuit at 156 MHZ but blocks spurious DC very effectively. See https://electronics.stackexchange.co...citor-block-dc


This is the second DC Block I have built and in testing showed no range or signal degradation of the transmit or receive signal at all. I do not have my measuring equipment on the boat so my results are qualitative and not quantitative.


Because the plastic box breaks the outer shield connection between the VHF radio and the antenna, there is no DC voltage passed from the radio ground to the antenna mount (the cable is still grounded on one end, the radio chassis). The bleeder resistor is 4.7 Ohm and designed to dissipate any blocked DC to ships ground, it keeps the antenna side of the cable shield 'floating' above the ground level of any other DC that may be present in the mast bonding system, even when the radio is off.


Mark Lucitt
Amateur K4MLL

[transmitterdan]

Originally Posted by mlucitt

Like this one:

That is a nice idea. But a true DC block requires 2 capacitors. Many antennas and radios have a DC connection between the inner and outer. So instead of the red wire between centers put another cap in its place. If possible get a smaller box so the connectors are not so far apart.

[mlucitt]

Dan,
There are 'inner' DC blocks, 'outer' DC blocks, and 'inner-outer' DC blocks. This is an 'outer' DC block. The OP was trying to solve a galvanic/bonding problem by preventing stray DC from the outer shield of the VHF cable or, the stainless steel antenna mounting base which is mechanically mounted to the aluminum mast.


I agree that a second capacitor installed between the center conductors would also block DC on that conductor. That technique is used when DC voltage is inserted onto the center conductor to power a down converter or low noise amplifier on the cable near the antenna; and then to remove the DC further down the cable. We are trying to maximize signal here and there should be no DC on the center conductor, so I chose not to place a DC blocking capacitor on the center conductor.


We could not find an 'outer' DC block with PL-259 connectors, so I built this one. We are operating at 25 Watts so a little leakage outside the component box is not a problem. The VHF center frequency of the marine band is close to channel 16 at 156.8 MHz, so the frequency is not as build-critical as it would be in the UHF or microwave bands.


I could wrap the box in aluminum foil and ground this shielding, but tests showed good signal output at the antenna and no loss of receiver sensitivity.



Thanks,
Mark