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.
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
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.