VHF Coax Types

[MKirk51]

One of my winter projects is the replacement of my VHF due to age and no DSC capability. As I also pulled my mast this year, I figured I might as well replace coax and antenna. The mast is keel stepped so there's about a 50 ft run from antenna, down the mast, thru the bilge and sole, and ending at a PL259 barrel connector 'splice' under the quarter berth. From there, it's a short run thru bulkheads to the back of the radio.I've googled and checked archives and there doesn't seem to be a consensus on coax types. RG-8X seems popular as it's fairly light with a small diameter, but RG-213 seems like a better choice as it has a lower db loss. It's a little bigger (.405" vs .242") and heavier, and more expensive, although it's something I hope to never do again so the cost difference is minor. Is the lower db loss trivial when looking at the big picture, and not worth the extra size, weight and cost?

 

[Paul Akers]

Check the recommendations in the manual.

That would be your best shot at this time.And use the old cable as a messenger to pull the new one through to the bottom. You may need to add coax connectors to either end of the cable to accomplish the job. Just be carefull so that you won't have to re-drill any new holes.

 

[Rick D]

Go For Big

We have a YC member who has a good masthead antenna and very good connections on his Cal 34 who puts out at least half again the renge of the others. He has the larger coax. However, what is really important is to get the very best connectors and to seal them well and a good antenna for lasting performance. RD

 

[Don S/V ILLusion]

coax loss

MattThe difference between RG8X and RG 213 in terms of loss is approx 0.6 dB which, at 25 W input, is equivalent to a difference of about 3 watts. 0.6 dB or a 3 watt difference is not discernable.You can play with the numbers for different coax at the following link.

 

[Frenchie3958]

Coax Loss

Matt,Just take a look to the related web link:And select your cable length, frequency (160Mhz),power and you'll be surprised by the result. The loss is also affecting the signal you are receiving which is more important to my point of view.The only disadvantage with the RG 213 is the radii of curvature equal to five times diameter of the wire(because the cable is stronger and bigger).For more details visit Ham radio web sites.Regards

 

[Capt. Kermie]

How about RG8U ?

I originally bought a 50' pre-made cable so I did not have to worry about connectors. Turned out to be a waste of money even though I got it cheap in a boat show bin. It is one of those thin guage cheap ones with plenty of signal loss, I discovered how bad while out there. I have done a bit of research on them through various sites, WestMarine being one of them, and found a few calculation sites where one can enter the coax type and length and get the results. The RG8U appeared to be a better choice but it is also heavier and more expensive, not sure yet. The folks at WM informed me I could use another less expensive one with similar results as the RG8U and ultimately I would like them to put the connectors on or sell me a pre-fab of the same quality at 50'. I could install a thru-deck fitting then use a smaller length inside but I would still need 31' from the mast head to the deck and another 12' inside. It seems to be more simple to just get 50' and run it the same path as my existing coax which just fits the path with the mast down and bolted to the bow pulpit on my trailerable but has about 8' excess with the mast up in the step. Not sure where the most loss would occurr if it would be in the extra 8'-10' or in the extra connectors, kind of six of one, half dozen of the other. The upside to a prefab is that it is entirely weather proof. The RG8X showed too much loss in the online calculators over a 50' span.

 

[Brian D]

A lot of these cables being pointed out...

Are really for HF use, not VHF. If you really wanted to get good performance then the LMR series cables are a better choice. However, now we are getting into the area of big.I think standard RG58 will work for you. RG8X is about the size of RG58 and is very flexable. RG-223 is better shielded and that is what we use in the UHF range in our labs. We only use Belden 9913 or RG-213 when we have really long runs and have amplifiers attached.I really think you are looking too deep into this. VHF is VHF. Yes, we would like to have a good signal hitting the antenna with the least amount of attenuation, but you are going get attenuation regardless. That is in the connectors, barrels, and antenna coil. Go with the cable that you feel is best. Most marine grade coax is fine.Just my 2¢.[added] RG-58 is going to give you about 3dB of loss. With 25 watts, that's 13.98 dBW (decibel Watts). Subtract the attenuation in the cable, about 3 dB gives you about 11 dBW, or 12.5 Watts. So there is loss regardless.

 

[Peter18012]

Remember

that VHF signals go "line of sight", and that the power you put into the antenna thru less loss coax is not as important as the need to have a good "view" of the station at the other end. I have a 146 mhz ham radio that I have installed on my boat with an antenna switch to use it with the 3db 156mhz marine VHF whip antenna at my masthead, which is fed by comparatively lossy RG-58. Marine vhf radios put out 1 watt at low power and 25 watts at high. I usually use the marine VHF at 1 watt. I use the ham rig at its lowest setting - 5 watts. I never use it at any higher setting, because at only 5 watts I can easily work thru all the ham repeaters in my area (Northern Cal), even though many are over 40 miles away, because they are all on mountain tops and can "see" a great way. I once talked to a guy who was on top of a mountain in the sierra who was using a handheld 5-watt ham radio with a very inefficient "rubber duck" antenna. He was over 140 miles away. If you're looking at transmitting between boats, the distance the antenna can "see" to the horizon from your masthead is probably no more than 7-8 miles. If the other boat's mast is approx. the same height, the most distance you're going to be able to communicate between boats is probably about 15 miles. You'll be able to talk to the USCG farther, as their antennas are mostly on mountain tops. So a db of loss more or less isn't going to make any practical difference, all other things being equal. But believe me, working with RG-8 or 213 is A LOT more user unfriendly than the cables with a smaller cross section when you're dealing with a lot of bends - from the antenna into the mast, down the mast, thru the bilge, around bulkheads, etc.

 

[oreana123]

Another Cable Type

Check out LMR 400 cable. It is used by many high quality conector applications. It has one of the best signal loss qualities. It is available on ebay for a very reasonable cost. Impedence matches most VHF radios.I am in the process of running a VHF antenna cable in a 26 Laguna cabin and mast. It is a trailerable boat, so there is a deck connector. I am using the LMR 400 and soldering on pl259 connectors. The cable reducer threads on the inside of the pl259 connector thread onto the outside plastic shielding of the lmr 400 (after the rg 8 reducer is removed) for a great strain relief. The center electrode is a copper wire. The shielding is both a foil and a braided type. It has a minimum bend radius of 1 inch.The word on lmr 400 is that the developer started completely over in designing the high quality shielded cable, which is why it does not have a R or RG name.John S

 

[Bay Sailor]

Low loss coax does make a difference

Since almost all your contacts on marine radio will be with other boats the low loss coax can make a difference when the other station is coming in weak. Another advantage is that the coax is much stiffer than RG-8 so the connector at your antenna is less likely to go bad from movement. On the down side LMR 400 or 9913 coax are expensive, require careful soldering to get the connectors on right and may require that you enlarge the holes in your mast. If you can install a single run of cable from your antenna to the radio, with no extra connectors along the way, that would be best. Each pair of connectors you add degrades it and is a potential failure point. If a connector near the radio opens up, the next time you key up, especially on high power, your radio may get damaged.

 

[MKirk51]

Thanks everyone

As usual, you folks are the best. I especially like the link in Don and Frenchie's post. It makes for easy comparisons.I went up yesterday to look at the routing again. The segment from the connector under the q-berth to the radio is RG-58A/U. It's a short run but I'd still like to eliminate that splice. I might be able to use a different route and do that. I'll need to borrow a fishtape and see.I think I will go with the thicker stuff. It should be a pretty straight run, no sharp bends. I don't foresee pulling the mast anytime soon so it should be a 'do it once' and forget it.

 

[ruedi ross]

one more thing

use coax tape on all your connections. Keeps moisture out and signal strength in.

 

[Brian D]

I'd recommend heat shrink...

(but I am serious)

 

[oreana123]

LMR 400 Secifications

Even the best connectors will cause a loss. Many boats have at least a connector at the back of the xceiver, one at the deck and one at the masthead. LMR 400 will allow the installation of these connections with acceptable total power losses.Flexibility is relative. If you are talking about installation, the LMR 400 has a recommended minimum bend radius of 1 inch! No other cable type allows that kind of bending with acceptable power losses. In most applications on a sailboat, one would want to minimize any movement no matter what type cable.As far as the need for exotic fittings, the standard reducer on PL 259 fittings when removed, has threads that perfectly thread onto the outer black plastic jacket of the LMR 400 cable creating a very positive strain relief. I just installed LMR 400 on a 31 foot masted sailboat. It is a far superior cable, which becomes obvious as you cut through. So, I feel that the superior power transfer, incredibly small acceptable bend radius, and the adaptibility to standard PL 259 fittings makes LMR 400 cable a very good choice for sailboat installation. John S