Protection of VHF radio antenna from lightning

[captainwireman]

My dear friend Mother Nature burned my sweet new Shakespeare antenna to a burnt matchstick. Thinking of putting a well grounded copper pipe for a lightning rod ( theory that high voltage mostly travels on the perimeter of a conductor) but what would be the proper placement? Right around the antenna Fariday cage or next to and higher than the antenna? I am thinking the antenna may get no reception if plan A was used. Opinions?

[Paul L]

You can't protect the antenna itself. You can protect the equipment past the antenna with an inline block B50 Series | Polyphaser | Lightning Management | RFI Wireless - RFI Wireless
You can also use a lightning rod at the mast head cabled to an exterior ground plate to help protect the boat.

[Captsteve53]

Also, If thinking about a lightning rod be sure your mast is preferably well grounded to the keel as many find out the hard way the charge exits at the water line with obvious results,

Cheers Steve

[masthugg]

Originally Posted by Paul L

You can protect the equipment past the antenna with an inline block B50 Series | Polyphaser | Lightning Management | RFI Wireless - RFI Wireless
.

I don't think a DC block would do anything. First of all, lightning isn't DC, second, a DC block has a delicate in-line capacitor (likely) that the lightning will burst right through.

[Paul L]

Originally Posted by masthugg

I don't think a DC block would do anything. First of all, lightning isn't DC, second, a DC block has a delicate in-line capacitor (likely) that the lightning will burst right through.

The Polyphaser units are used in many land applications all over the world. Here is their explanation on the DC block.

. POLYPHASER
Coaxial Protection

Lightning damages equipment at radio communications sites every day. Although lightning is a DC pulse, the time from zero current to peak current can be very fast. When lightning energy travels through a coaxial cable, there is a slight propagation delay that occurs due to the unbalanced inductances of the shield and centre conductor, and the centre conductor’s capacitive relationship through the dielectric to the shield. The higher-frequency shield energy will arrive at the equipment first, followed by the centre conductor energy. Since the pulse energy arrives at different times, a differential voltage occurs. A properly designed coaxial protector equalises this potential difference, which prevents current flow and therefore damage to the site’s equipment.
However, the choice of a standard gas tube type coaxial protector without DC blocking may not offer the user complete protection. The fast rise-time lightning pulse can produce over 1000 Volts across the gas tube before the gas can ionise and become conductive. Since there is no DC blocking mechanism, this high voltage is applied directly to the equipment input before the gas tube turns on.
A quarter wave stub coaxial protector creates a band-pass filter, at a frequency determined by the length of the quarter wave coaxial section from the horizontal centre conductor to the grounded base. However, if the equipment input is DC-shorted, the quarter wave stub can allow significant divided DC and low frequency energy to flow towards the equipment input.
A “DC blocking mechanism” inside the protector (no DC continuity through the protector) will prevent harmful levels of throughput energy from reaching the equipment. RFI stocks and distributes the patented PolyPhaser DC-blocked coaxial protector line, which has the lowest throughput specifications in the industry.
There is also a series of PolyPhaser coaxial protectors that block DC in the RF path to the equipment, and either inject, pass through, or pick off a specified DC voltage on the feeder’s coaxial cable centre conductor. This series of protectors is particularly suited to applications requiring DC to be passed up the coaxial feeder cable to power tower-top amplifier electronics.
Remember that no matter how good your lightning protector is, it’s not a fuse. It still needs to be correctly installed and connected to a suitable grounding system. RFI offers a complete range of products to protect your system, including the coaxial protector, grounding rods, copper strapping and grounding kits for the feeder cables.

[masthugg]

Originally Posted by Paul L

The Polyphaser units are used in many land applications all over the world. Here is their explanation on the DC block.

I'm still a bit sceptical to the explanation but if it is commonly used that is a strong argument.

[S/V Illusion]

Trying to protect a pointy metal object on top of a bigger metal object is the essence of futility. Buy a so-called spark gap lightning protector for the coax and electronic equipment, any of which costs a lot more than a vhf antenna.

[Paul L]

Originally Posted by masthugg

I'm still a bit sceptical to the explanation but if it is commonly used that is a strong argument.

Take a look at their website. Lots of info there.
Lightning Protection For Industrial Applications - PolyPhaser
They have been at this lightning and surge protection game for a long time. They have a really broad product line and deal with military, Telecom etc.

[Triumphant]

Might be worth looking at some Ham radio sites as they sometimes fit Lightning protection. I googled "lightning protection radio antenna" and quite a few things came up.

If anyone can confirm these things work I am sure a lot of us would be interested

[masthugg]

Originally Posted by Paul L

Take a look at their website. Lots of info there.

Lightning Protection For Industrial Applications - PolyPhaser

They have been at this lightning and surge protection game for a long time. They have a really broad product line and deal with military, Telecom etc.

I am convinced now, thanks.

[Frankly]

If all you lost was the antenna consider yourself a lucky guy. Using a gas tube surge protector is pretty standard for lightning protection (had them on both my ham antennas and sailboat for years). The DC blocking business is of questionable value but probably won't hurt. Might look at an Alpha Delta coax switch with built in gas tube for max protection of the radio ect. Got to remember to turn it off.

Adding a lightning rod/ air terminal to the top of the mast that extends above the VHF height can help divert some of the energy directly to the mast and should have minimal effect on the VHF function if you can get it a few inches away. 3/8" solid aluminum rod or tubing would be a better choice than copper. As others have mentioned have the but end of the mast well grounded to seawater via the metallic keel or a copper ground plate.