VHF antenna coax DC Block, help

[Barnacle]

I need to block DC voltage between a VHF radio and its antenna. I understand capacitors can do this without affecting the RF signal performance of the radio. I'm not sure exactly what info you need to help me, but perhaps this helps:
DC voltage 12 volts / VHF radio / LMR 400 coax / Metz Manta 6 VHF antenna / PL-259 connectors

Thanks in advance for any help you can offer

 

[ccurtis]

You need a DC Block. They are made with various connector types but I could not find one with PL/SO-259 connectors. Yes, they have a series capacitor in-line between the center conductors (called an "inner" block).

Like these https://www.ebay.com/sch/i.html?_sacat=0&_nkw=rf+dc+block&_frs=1

You can make your own with chip capacitors and suitable connectors and box to mount them to.

Edit: Alternatively, you could open up the radio and disconnect the source of the DC power from the antenna connector.

 

[KeepItSimpleStupid]

These could work, I think: https://www.dxengineering.com/searc...f-female-so-239?autoview=SKU&keyword=dc block

The Pl-259 connectors are also known as UHF connectors.

It probably would not be a bad idea to have a lightning arrestor in the system. A good ground will be required too. Check the frequencies allowed.

 

[ccurtis]

Good find, KISS. One of those should do the job nicely.

 

[davenn]

I need to block DC voltage between a VHF radio and its antenna. I understand capacitors can do this without affecting the RF signal performance of the radio. I'm not sure exactly what info you need to help me, but perhaps this helps:
DC voltage 12 volts / VHF radio / LMR 400 coax / Metz Manta 6 VHF antenna / PL-259 connectors

Thanks in advance for any help you can offer

hi Barnacle
welcome to the forums

both those links take you to appropriate DC blockers but they are VERY expensive

Maybe it would be a good idea for you to tell us what DC and why you are blocking it ? .....
what are you actually trying to achieve ?
is this just a receiver or a transceiver ?
if transmit as well, what power level ?

cheers
Dave

 

[Barnacle]

Hi thanks for your help. I posted a reply but don't know what happened to it, so I'll reconstruct what I said.
I'm using a 25 watt VHF transceiver radio on a metal sailboat and need to block DC voltage that is grounding to the mast through the antenna (Metz Manta 6) . I could isolate the antenna bracket, but sea spray (electrolyte) may bridge this pad and recomplete the circuit.
Why do I need to block the DC voltage? I am on a metal sailboat and any 'stray voltage' will cause corrosion. Metal hull in sea water (electrolyte) and voltage = damage. Sacrificial zincs will be eaten very quickly.
Unlike a car chassis, a metal boat cannot utilize the metal hull for any electrical ground. Metal boats have to have an isolated DC wiring system from the hull.
As Ccurtis suggests crafting a box and some capacitors together should achieve what the suggested links sell, and ought to cost considerably less. The trick is in learning what size and type of capacitor I would need...
Thanks

 

[ccurtis]

A total value of 1000 pF (1 nF) or above will be fine for VHF. The actual value is not critical. I recommend connecting several smaller value (say five or more 200 pF) ceramic dielectric capacitors in parallel to achieve the total value (the individual values add up when connected in parallel) while maintaining a high self resonant frequency. If using leaded capacitors, cut the leads as short as possible. I'd use 500 volt capacitors, but for lightning protection only a gas tube, or transient voltage suppressor, across the inner conductor and outer shell can assure protection. Besides connecting the outer shells of the connectors together by virtue of mounting them in a metal box, I would solder thin sheet copper, or braided wire strap directly across the gap and over the capacitors to connect the connector shells together with the shortest length to reduce the inevitable impedance discontinuity. Dimension the box so that the connectors are as close together as possible. Instead of a metal box, maybe a copper pipe with a suitable diameter can be found and slipped over the whole affair and soldered to the shells. A purest would build a 50 ohm strip-line PCB (not a job everyone is equipped for) and mount chip ceramic capacitors and the connectors to the board to avoid any impedance discontinuity. But, hey, it should be good enough at VHF and below.

 

[Barnacle]

Thanks ccurtis! I'm game to give this a try but don't have a lot of experience in this field, so I'd appreciate any tips you may think are 'obvious'. I have soldered a PCB before and always tinker with projects, so am not a complete liability..... (perhaps).
Anyway, you say, "A purest would build a 50 ohm strip-line PCB and mount (five or more 200pF) chip ceramic capacitors and the connectors to the board to avoid any impedance discontinuity".
First off, what is a 50 ohm strip line? Do I use the inner copper core wire from a 50 ohm coax cable to link the capacitors in parallel?
I'm familiar with R..shack for electronics parts, could you be more specific with which capacitors I ought to use as there appears to be numerous options (tantalum, metal film etc etc) Also, I see MF and UF rating but not pF. Lastly, the voltage seems much less than 5oo volts you mentioned.
Is the high voltage rating for lightning? The DC voltage I hope to block is just the 12 volts. However, I don't know if RF voltage is different?
Thanks again for you time and patience.

 

[ccurtis]

Hi again. The rule of thumb is that a distance of less than 1/16th wavelength is needed to prevent significant impedance discontinuity. So at 200 MHz that is 3.7 inches. I think you should be able to easily build it within less than half of even that distance between the connectors, the less distance between the connectors, the better.

Stripline is a general term for various configurations of etched and layered printed copper traces and various dielectric materials, used to maintain a constant impedance along its entire length. I'm not aware of it being available for purchase in suitable form since it is used with components of almost infinite variety, and therefore is custom etched/fabricated from raw stock material. For that you will need etch resist and acid. If you want to proceed along that line, I don't have that experience, but would point you to this one of many sites on the subject: https://www.electro-tech-online.com/custompdfs/2012/10/RF-Microwave_PCB_Design_and_Layout.pdf

The blocking capacitors are connected between the center pins of the two connectors. I suggested 500 volts because they are readily available from most outlets (such as digi-key mail order electronics), without getting too large in size, and the higher the voltage the merrier. I'm thinking static charge, induced voltage from nearby lightning, and such. I don't know what your transmitter output power is (assuming your radio is a transceiver) but the voltage rating of the capacitors for operation of the transmitter, alone, should be at least V=1.41*sqrt(power*50), power in watts, which is likely below 500 volts (50 watts is 71 volts). Radio Shack in-store doesn't have the specific ceramic caps you need, but are available on the web as RS Catalog #: 55047596 (they're 1000 volts at that).

 

[Barnacle]

Thanks again ccurtis. I appreciate you finding the capacitors.
Having read the pdf link you provided, my confidence waivered a little as I recognize how out of depth I am, but regardless, I'm keen to give it a try.
I've reread your description how you would solder the capacitors a number of times and am not certain I follow. I drew a simple diagram in paint. Am I right in thinking, I cut the coax. Strip the insulation, say two inches, exposing the inner core and outer braided sheath of both ends. I bridge both of the outer sheaths with a jumper wire for continuity. On one end of the inner core I solder one leg of the capacitor, then solder the other capacitor leg to the other end of the inner core wire. I re-insulate the wires ensuring less than 1/16th gap, and insulate the inner core from the outer sheath....
View attachment 67810
If I've totally misunderstood, please let me know. Any chance you can direct me to a picture or drawing so I may visualize the project?
Thanks again.

 

[ccurtis]

Hello, barnacle. This is what I was describing.

 

[KeepItSimpleStupid]

Here are two more at $60. https://www.amateurradiosupplies.com/category-s/73.htm

I think you get a 10 year Warranty and a vibration specification. https://www.tessco.com/products/displayProducts.do?groupId=350&subgroupId=10

I don;t know about a protected equipment Warranty.

This thing is outside surrounded by water. If you do make a mistake and cause the cable to short, your looking at a repair cost for the VHF receiver.

I want to try to convince you real hard to that this is something that you should buy rather than build. IF there is a protected equipment Warranty all the better.

At work when I started specifying surge suppressors for the AC line, I settled on a Tripp lite ISOBAR. The protected equipment Warranty is like $50,000 or more. Yep, someone though that a surge suppressor is a surge suppressor. They are also an insurance policy. Only real issue is that you have to have the receipt. We did collect on the policy once. WHo knows how many times it did its job.

Other systems got more protection. 17 years and not even a hard drive crash for a system that was on 24/7. Just some dust and a Floppy drive replacement.