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Old 17-06-2008, 19:15   #1
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What co-ax for masthead VHF?

I am replacing the co-ax (and masthead aerial) for the VHF.

The co-ax I am removing is about 1/2" diameter. Being somewhat "anally retentive" about saving weight up the mast, I am hoping to find something smaller/lighter... somebody mentioned to me about mil-spec co-ax, possible abotu 5/16" diamter that might be good in this application, but I am pretty clueless when it comes to electrons and radio waves, so I'm hoping y'all might be able to help me out...


The total distance from masthead to radio is about 70'
Thanks
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Old 17-06-2008, 21:37   #2
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You can use RG-8x

Dan DiTucci
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Old 17-06-2008, 21:41   #3
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Smaller diameter (assuming it's still the right impedance of 50 ohms) will get you less weight but more signal loss. Take your pick.

Steve B.
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Old 17-06-2008, 21:42   #4
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Thanks for the input Dan. I like the look of the RG-8X, but it seems to be only recommended for runs of up to about 50-55'. The recommended for 70' is RG-8U, whihc is about 3/8", which is an improvement, butI was hoping for better... still, since I'm not planning on doing any ISAF Category 1 offshore races, maybe the RG8X is good enough.
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Old 18-06-2008, 00:04   #5
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I suspect your old cable was RG-8U and the insulation materials were either heavier or double shielded. I would stick with the RG-8U. Bigger core, less loss. I have installed that cable in runs a lot longer than 70 feet, with good results.
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Old 18-06-2008, 03:51   #6
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Weyalan,
I haven't got my reference books close at hand but will look it up tomorrow at work. There are some 50 ohm low loss coax cables available that are similar to RG 8 in loss but only about 6 to 8 mm diameter. I will also check out some east coast suppliers as they can be a bit hard to source except through specilaist RF suppliers. Hang tight for more info.
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Old 18-06-2008, 04:53   #7
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Attenuation calculator for various coax types.

Bob Stewart
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Old 18-06-2008, 07:30   #8
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If you are weight concious, the RG-8X will provide an acceptable signal with a 70 ft run, and is better than the RG-58 used on most boats. Since VHF is line of sight, you are not noticeably extending the range if you go with the heavier RG-8.
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Old 18-06-2008, 10:59   #9
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There is a 20% drop in effeciency between the RG-8/U and RG-8X. Kinda depends what trade off you want to lose maybe 10 lbs.
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Old 19-06-2008, 08:48   #10
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Looks RG 8X maybe the way to go given your stated considerations. I am not sure how available it is in Tassie (or the east coast) but could check it out if you want.

Other alternatives are LMR 195 ultraflex or RG 400. Both of these are 6mm diameter and able to terminated in the field with the LMR being slightly better loss and weight wise - available ex Sydney at less than $4 / metre.

If the loss is a consideration, I would suggest runing low loss (say RG 213) from the radio to the mast and then LMR 195 ultraflex up the mast. Use "N" type connectors to join, IMO.
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Old 19-06-2008, 16:47   #11
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RG213 is always the best choice for both its lower loss and its also being structurally very strong. I would not use or advise anything else myself - and on our current boat the builder put in an extra mast conduit as a matter of course purely so 213 could be used.

But if using RG8X or the likes of LRM-195 then make sure you get a version with a multistrand rather than single strand centre conductor (they both come with either) as will likely save you heartache later. Also be careful that bend radiuses are not too tight with these and with how it is supported so immune from the possibility of crushing as both have soft foam plastic dielectrics which the centre conductor can migrate through in a tight bend.
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Old 19-06-2008, 21:05   #12
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Iv'e used several different brands of rg8x over the years and have never seen one with a solid center conductor. Also, many brands of this coax have the foam dielectric but it is available in solid poly as well. I would recommend the Ancor brand from marine stores. It has a tinned center conductor, tinned braid, UV resistant jacket and solid poly dielectric.

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Old 19-06-2008, 21:44   #13
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Thanks for all the great advice, people.
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Old 19-06-2008, 22:33   #14
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Quote:
Originally Posted by fairbank56 View Post
Iv'e used several different brands of rg8x over the years and have never seen one with a solid center conductor.
Have a look at some Belden 7808A (which is one of several single solid conductor RG8X's made by them) and you will then have seen some .

Other cable manufacturers make it too.
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Old 20-06-2008, 08:34   #15
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Well, I wasn't saying that it doesn't exist. Iv'e been in the marine electronics field for 33 years and have physically seen many brands of rg8x but never one with a solid center conductor. It's not something you would use on a boat.

Eric
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