Replacing VHF Coax--Mix and Match?

[dlochner]

At the end of the season the SWR on the VHF antenna was a little high according to the built in SWR meter in a Vesper AIS. The SWR was at 3. If I recall correctly an SWR of around 2 is much better. Looking at the connectors the masthead connector is showing some discoloration on the center pin. This might be contributing to the higher SWR.

The current coax is RG 58 (skinny stuff) that is probably original the boat, or about 27 years old. It appears that I have several options,

1. Live with the higher SWR
2. Replace the coax
3. Replace the suspect connector

if I should chose option 2 I would like to use a low loss coax like the LMR 400 or RG 213 if it will fit in the mast conduit. If I go this route can the 2 cables be mixed, i.e., RG 213 down the mast and RG 58 through the cabin to the VHF?

 

[Captain Larry-DH]

Don’t kill yourself. Just use RG 58 or live with what you have and replace the connector. AIS is short range anyway

 

[Don S/V ILLusion]

If it was just the AIS, I would agree with the above but I infer you use the same antenna for both AIS and marine VHF as you say the coax is decades old.

High SWR is exacerbated by having high loss coax such as the RG58. Given its age, it would be prudent to replace the entire coax run. You can mix new RG58 with lower loss coax with no problem. It’s all the same impedance.

 

[AaronD]

... You can mix new RG58 with lower loss coax with no problem. It’s all the same impedance.

You might look at the 'ultraflex' variants - they have really tight turning radii, which could make your installation process easier and cleaner.
Specs and links for LMR-400 UF and LMR-240 UF at H26 Coaxial VHF cable.

 

[Brian D]

If you think the cable might be bad, then replace it. Personally I would get rid of the RG58 and go with LMR240 or RG8X. The loss is really insignificant. The size of either the 240 or 8x is the same as the 58 so there should be no issue pulling the old out while pulling the new in. Use the old cable as a snake.

When putting on the connectors, or if you plan on just replacing the connectors, make sure you use good silver UHF connectors. Then, wrap the connector with self-amalgamating tape or self-vulcanizing tape. This will keep moisture out of the connector.

If you want to mix that is fine. Just make sure what ever cables you use they are all 50Ω as mentioned by Don.

 

[Mark Maulden]

If you are going to replace coax in the mast, then replace the whole run and forget connectors in the middle. Every connector will have some loss. For a splice, that’s two connections. dBs can add up. 3dB is half power.

 

[Scott T-Bird]

If you are going to replace coax in the mast, then replace the whole run and forget connectors in the middle.

How is that feasible?

 

[dlochner]

If you are going to replace coax in the mast, then replace the whole run and forget connectors in the middle. Every connector will have some loss. For a splice, that’s two connections. dBs can add up. 3dB is half power.

How is that feasible?

While it is not essential to have a break in the cable, it would make the annual mast unstepping considerably more complicated as it would require removing the cable from the VHF and then pulling it through all the cabinets and nooks and crannies to the mast and in the spring re-running the cable. That would add several hours of work at the beginning and end of each short season.

 

[Don S/V ILLusion]

How is that feasible?

It isn’t really necessary. Some people think coax connectors have an inherent loss which might be true if someone does a really poor job of soldering, however, there are lots of test data showing good quality connectors have no loss.

I’ve seen one test where over 12 connectors were used as an experiment with no loss so breaking up the coax, for example at the mast step, is not a problem.

While on the subject of loss, there are all kinds of coax. Not all sizes such as RG 58 are equivalent in terms of loss. Picking good quality coax is the biggest consideration.

 

[Scott T-Bird]

While it is not essential to have a break in the cable, it would make the annual mast unstepping considerably more complicated as it would require removing the cable from the VHF and then pulling it through all the cabinets and nooks and crannies to the mast and in the spring re-running the cable. That would add several hours of work at the beginning and end of each short season.

Yes, I can see where it might be more feasible with a keel stepped mast. With deck stepped, you would also have to consider the seal at the deck for the cable runs. Mine are more or less permanently sealed. Any disturbance of the pass through for any of the wires and cables would require removing the sealant and re-applying in the spring. That's a project I don't really want to do. I sealed everything about 10 yeas ago and so far has been good. I don't want to disturb it until I absolutely have to.

 

[jnorten]

I have a very similar situation and have learned much from this thread.

I have to replace a cable run from the VHF to the mast base.

My entire run--radio to masthead antenna--is in two chunks. The part that needs replaced is about 25' through the cabin. Then a female-female connector attaches this first chunk to the the last ~43 feet or so to the masthead antenna.

The entire run is old school, fat RG-8U.

So, the question is should I pull new, fat, low-loss (probably RG-213 these days), or go with easier to pull RG-8X?

I've read a bunch of info--use quality marine cable (I'm probably going with Ancor), make high quality solder connections, etc.

Reading about cable specs, recommended maximum run lengths etc., I think 8X would be OK. One online calculator said 25' of 8U (or 213) would attenuate signal about 17% versus 26% for 8X.

Although probably OK, I can't help but feel this is a step backward for my system.

Aside from, "Your boat, your choice" (which I know is true), any insight would be appreciated.

 

[Don S/V ILLusion]

I have a very similar situation and have learned much from this thread.

I have to replace a cable run from the VHF to the mast base.

My entire run--radio to masthead antenna--is in two chunks. The part that needs replaced is about 25' through the cabin. Then a female-female connector attaches this first chunk to the the last ~43 feet or so to the masthead antenna.

The entire run is old school, fat RG-8U.

So, the question is should I pull new, fat, low-loss (probably RG-213 these days), or go with easier to pull RG-8X?

I've read a bunch of info--use quality marine cable (I'm probably going with Ancor), make high quality solder connections, etc.

Reading about cable specs, recommended maximum run lengths etc., I think 8X would be OK. One online calculator said 25' of 8U (or 213) would attenuate signal about 17% versus 26% for 8X.

Although probably OK, I can't help but feel this is a step backward for my system.

Aside from, "Your boat, your choice" (which I know is true), any insight would be appreciated.

Depends on how onerous running thicker cable is - obviously, there is some advantage to RG213 type but if it's impractical, go with what you can...

To reiterate, there are all types and quality of RG8X. They are not all equal in terms of loss, braid type, braid shielding, dielectric material, etc... Regardless of what coax you chose, go with quality cable from a reputable company, not marine store variety. Same with the connectors.

 

[dlochner]

Thanks for all the replies.

When it warms up, I'll take the masthead off to see how much room there is in the conduit and run new cable. I'll be looking for a reliable source then.

There is an article on Coax Connectors on MaineSail's site, as always it is informative and helpful.

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