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vhf antenna questions

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Lee Panza
Captain

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USA
468 Posts

Response Posted - 04/04/2012 : 23:11:43

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
 RG-58, RG-8X, RG-84 and RG-213 Of these, use the cable with the largest diameter that you can easily route for the least loss....
<hr height="1" noshade id="quote"></blockquote id="quote">

RG-8X and RG-213/U are really your two best choices. The length of run from masthead to a radio anywhere near the companionway would pretty much exceed the practical limit for RG-58/U. The mil-spec for RG-213/U is apparently the replacement for RG-84.

8X is about 1/4" diameter, with a solid core conductor, and 213 is about 1/2" with a woven core. Because of its woven core 213 isn't much more difficult to run than 8X, but it's more visually conspicuous wherever it's exposed. The signal loss for 213 is down around 40% of the 8X, which could make a significant difference if you need to call for help from a remote or obscured location.

Get a good marine grade coax, with tinned core and tightly-woven tinned braid. UV resistant PVC is important, even if most of the cable is within the mast and only small sections are exposed. Ancor and Berkshire are two manufacturers to trust (Belden, also, but their products tend to be expensive). I took a chance and ordered some 213 from GenuineDealz (they wouldn't state who they got it from, but they said it's American made, which narrows the options), and I was pleasantly surprised to find it was Berkshire (old, reliable company) at a very good price.

Just about as important as the cable are the connectors. Amphenol is widely considered the standard. To connect to the radio you'll need a PL-259 (ie. Amphenol 083-822), and you'll probably need the same at the antenna, but you'd have to check. To disconnect at the deck, go with a Type N male and female combo like Amphenol 082-202 and 082-63. Unless you have access to the right size crimping tool you're better off getting solder-solder connectors (solder for the core conductor and for the braid) than crimp-type (the radio geeks continue to argue about which type is technically superior). For mail-order purchases of components I've been quite satisfied with the service from Mouser Electronics.

Wrap the joints where the cable enters the back of each fitting with Coax Seal tape (and you might want to cover that with vinyl electrical tape for appearance). Coax Seal is good material for sealing pretty much any exterior electrical connection that doesn't have to be disconnected with any regularity. Another hint is to use a light coating of conductive grease (rather than dielectric grease) on the internal male and female pieces within the connectors. The black carbon-based stuff is more economical than the silver-based grease and is more than sufficient for this kind of application. If you're re-using an old existing connector, at least clean it up with CAIG DeOxIT; I was surprised at how well it restores lightly corroded fittings.

If you do go with the mast-head antenna (very strongly recommended for coastal cruising), the best is probably the Metz Manta-6. It was $39. (plus $9. shipping to California) from Northeast Marine Electronics.

Where the cable exits the mast I used Ronstan RF6031 exit plates, but that's not really necessary. Two holes with the metal between them jig-sawed out (and with the edges VERY thoroughly de-burred inside and out) would do. For RG-213 do 9/16" holes 2" - 3" apart.

This is a highly-condensed version of a complex topic. A good article to start with if you're going to do research on your own would be:

http://www.saltyjohn.co.uk/resources/Marine%20VHF%20Antennas%20%28aerials%29%20and%20their%20installation.pdf

A good radio installation is like insurance - if you ever get to the point where you really need it you'll regret not doing it right. For coastal cruising I wouldn't feel comfortable with anything less than what I've described here. I'm still even considering someday springing for the cost of single sideband (SSB) equipment like offshore cruisers have to use.

When I get caught up on all the work I'm doing I'll post pictures; probably early this summer.

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 04/04/2012 : 23:17:44

Nice write up Lee, thanks!

Edited by - on

Davy J
Master Marine Consultant

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USA
1511 Posts

Response Posted - 04/05/2012 : 05:24:57

It sounds like you have decided to mount an antenna, but if you want to try this, you will know if you will have a problem.

I use this ridiculous contraption in an attempt to keep the birds from sitting at the mast head.

The PVC pipe has a couple off holes cut in to hold sail slugs. They are clamped in place. The main halyard raises the device to the top of the mast. A second small line acts as a downhaul.

Instead of the CD rack, you could just fix a 3'-4' wire to the PVC and raise it up.

Edited by - on

JimGo
Admiral

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USA
962 Posts

Response Posted - 04/05/2012 : 06:06:23

Davy, with as often as your mast is down, I'm surprised you don't take advantage of that fact and put some spikes on the mast head.

Lee, thanks for the info!

Edited by - JimGo on 04/05/2012 06:06:45

awetmore
Master Marine Consultant

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USA
1144 Posts

Response Posted - 04/05/2012 : 06:36:34

Thanks for the write up Lee.

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 04/05/2012 : 07:37:11

Very thorough, Lee, gold standard stuff. I would caution that unless you are experienced or know some one who is, soldered connectors might not be the best approach. I have used solderless mechanicals and crimps for many years and am sure that I would butcher a fair amount of coax getting my skills back up.

Edited by - on

awetmore
Master Marine Consultant

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USA
1144 Posts

Response Posted - 04/05/2012 : 09:15:33

I'm good there, I have decades of experience with soldering.

I trust crimped connectors too, but know for RF that the best ones are compression style which use an expensive tool. I don't want to get a tool for this one off operation.

I do have a long section of RF cable with connectors on it already that came with my boat. I'll investigate what it is when I'm back at home. The radio install was never complete and I don't know what the PO intended to do there.

When you run the cable down the mast compression post in the head compartment where do you have it enter the bilge? I'm guessing that I'd route the cable through the bilge, up into the dumpster, then out and around the edge of the quarterberth to my radio. I'm getting to the point where all of my accessory electronics are mounted in that region. The open area and access panel above the quarterberth are very handy for wiring purposes.

I always found it odd that our boats don't have some sort of emergency drain in the head compartment anyway, the head floor is the lowest point in our boat and water can just pool there instead of getting into the bilge (and thus the bilge pump) if there is an issue.

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 04/05/2012 : 10:56:49

My 8X comes out of the mast near the base to a through-deck connector, across the overhead beside the aft bulkhead of the head compartment to the wiring channel on the starboard side and aft to the radio. That makes a shorter, nearly invisible run and friendlier environment for all but the short, exposed section at the mast. That section is wrapped in self-fusing tape, as is the section at the masthead.

edited for comma overload

Edited by - Dave5041 on 04/05/2012 11:02:42

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