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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics
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  #11  
Old 05-28-2012
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Re: RG-8X or RG-213 for a 50 foot run?

I strongly agree with Bill. No one has ever complained about low loss in coax. 8X is okay. RG213 is better. LMR 400 is better yet.

Try therfc.com for coax and tell 'em I sent you.
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  #12  
Old 06-17-2012
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Re: RG-8X or RG-213 for a 50 foot run?

I worked this out... I don't know... maybe a year or two ago. I want to get a masthead antenna on Abracadabra, but I don't want heavy, difficult, RG-8/U type coax running up the mast and I can't run it through the mast step.
Code:
Marine VHF frequencies: 156MHz - 174MHz (center: 165Mhz)

Assuming mast height of 50' (15m) and another 20' (6m) in addition to that,
that's a cable length of 70' (21m).

Assuming 25 watts at a frequency of 165MHz, with an SWR of 1.1:1

Belden #    Cable Type    Power at antenna
--------    ----------    ----------------
 9913         RG-8/U            19W
 8237         RG-8/U            17W
 9258         RG-8/X            12W
 8240         RG-58             11W

The above calculated from http://www.saarsham.net/coax.html and
http://allenk.home.infionline.net/coaxtest.html
Note that loss works both ways: On receive as well as transmit. It's all RF. The physics don't care which direction it's headed .

Note: 8237 has loss characteristics similar to the more commonly-seen 8214. 8214 has a foamed dialectric. I think I'd be partial to 8237 for a sailboat. (It's slightly lighter, too.)

In a marine VHF application, I don't feel the extra expense and hassle of Belden 9913 is worth the gain. I wouldn't use RG-58. If you're not racing (don't care about the weight aloft), and can deal with the connectorization: By all means: Go RG-8/U. Otherwise you'll probably want to use RG-8/X.

I recommend a known, reputable manufacturer, such as Belden (my personal favourite), Alpha, CommScope or Times Microwave. If the brand of cable isn't mentioned or prominently labeled on the cable: Shun it.

Here's another thing: If you're going to go with RG-8/X, you can improve connectorization loss by the use of either BNC or TNC connectors. They make crimp-on male and female in-line connectors for both types. BNC and TNC connectors have better insertion loss figures and using M/F connectorization at both the mast step and at the top of the mast will reduce your connection count by 40%.

Lastly: Get your masthead antenna up above the other metallic clutter on your masthead. Anything metallic w/in 1/4 wavelength (about 20 inches or 50 cm for marine VHF) and on the same plane is going to seriously affect antenna pattern.

Jim
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Old 06-17-2012
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Re: RG-8X or RG-213 for a 50 foot run?

I install quite a bit of Belden 9913 RG8U and it is very good cable. The measurements I get are damn close to LMR 400, which I can't grab locally. In terms "wire" I find it dirt cheap. Course I have over 3k in inventory just in battery cable so 9913 is cheap to me, in the whole scheme of "marine" wire...

I just installed some 9913 last week on a customers Sabre 34 spar. To test I bring my own 12V battery, a fixed mount VHF radio, SWR tester and Metz antenna so I can compare a known good antenna to the owners. The owner was shocked at the before and after test results but to put the icing on the cake we did an automated Sea Tow test. Before, with the old cable which was RG8X we could not discern our voice, just scratchy static. The "after" with the new 9913 it was clear as day and this spar was laying flat in his yard 9 miles as the crow flies to the Sea Tow automated antenna. I don't find the 9913 tough to work with in the least...

EDIT Sunday 6:00 PM (On the hook Quahog Bay Maine):
Interestingly enough, after posting this, I condiucted a Sea Tow automated test from where we are on the hook to the Sea Tow antenna in South Portland, ME. This is not a clear open water transmission and we have numerous peninsulas, with high land, between us and the antenna for the Sea Tow automated VHF test. We are currently 17.4 nautical miles away and the recording was perfectly understandable. While not "crystal clear" it was not half bad for a 17.4 nm distance. We have crappy Ancor RG-213 wire. I got it free so figured I'd use it. I KNOW it tests worse than Belden 9913 and Times LMR-400. Still not bad performance from what I would consider a medium to lower quality VHF cable.. Next time I re-wire the mast it will get Times LMR-400. It has tested better than any other wire I have installed but 9913 come as damn close and is less money (at least locally).
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Last edited by Maine Sail; 06-17-2012 at 06:08 PM.
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Old 06-17-2012
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Re: RG-8X or RG-213 for a 50 foot run?

Belden 9913F7 is a very good coax for VHF use. It's the same diameter and has many of the same characteristics of LMR-400, including double shielding and very low loss at VHF frequencies, but has a 7-strand AWG10 center conductor, and is thus much more flexible than the solid conductor LMR-400.

It costs just over $1.00 per foot.

As in all things coax, The RF Connection is your friend: RF Connection Product Release - 9913F7

Bill

Last edited by btrayfors; 06-17-2012 at 07:13 PM.
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Old 06-17-2012
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Re: RG-8X or RG-213 for a 50 foot run?

To each his own. 9913 is good coax, but is a PITA to work with, comparatively speaking, and overkill for the vast majority of marine installations, IMO.

As for your test Sea Tow automated test: Something awry there. There's no way, on VHF, with all else being equal, and at those distances, that a change from RG-8/X to 9913 should produce that much difference.

Jim

Last edited by SEMIJim; 06-17-2012 at 07:49 PM.
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Old 07-02-2012
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Re: RG-8X or RG-213 for a 50 foot run?

And don't forget good connectors! The best solder in and you need moisture protection. Corroded connectors can easily push the SWR (a measure of the efficiency of the cable and antenna) into the 4+ range which means over a 50% reduction in transmission power.
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