Choosing the the right VHF Coax

[Jammer]

Originally Posted by Timccarpenter

We sail on Lake Superior, Tartan 27-2 with a 40' mast and about 12-15' feet from the base of the mast to the radio. I'm having a hard time understanding the potential range difference between using RG-58CU or RG-8X (due to signal loss over that distance).

I can see the signal loss numbers for Rg8x compared to rg213, but what does it really mean with respect to transmit and receive distance. Are we talking about a range of 5 miles vs 13 miles 9 vs 13?

With approximately 50 feet of cable at 100 MHz the loss for RG58C/U is 2.5 dB. For RG8X it is 1.6 dB, a difference of 0.9 dB. At 156 MHz these losses will be about 25% higher, and there are also compounded losses related to SWR so in practice you're looking at an effective difference of 1.5 dB.


Here is a coverage projection for a sailboat like yours in the middle of the Apostle Islands communicating with another, similar sailboat. The bright green indicates areas with coverage using RG58C/U. The yellow indicates areas of additional coverage that could be achieved with one of the boats using RG8X. The modeling software includes coverage in land areas which you can disregard.


While the projections aren't perfect, and while they presume that both sailboats have proper installations, I find them to be fairly accurate over the water. The coverage in the Apostles isn't as good as projected because the modeling software does not consider the attenuative effect of vegetation. But I can hear other sailboats calling the Aerial Lift Bridge when I'm not far past the last of the Islands on the way to Duluth.

[Jammer]

Originally Posted by Timccarpenter

I could probably put RG213 in the mast (probably only 5" in diameter inside). I doubt I could run RG213 in the cabin from base of mast to the radio.

You don't have to use the same cable for the whole run. You can use 213 where it is feasible and RG58 where it is not.

Separately, I'm reading that the Shakespeare connectors are not great. DX engineering are much better quality. Any insight into this?

What you have heard matches my experience. I use the DX Engineering connectors that have a solder center and a crimp shield.

[Dockhead]

Originally Posted by Timccarpenter

I don’t mind having a few around, the whole winter project has gotten far more expensive than planned! The nature of anything with “marine” in the name.

I ordered a new Shakespeare 5215 (but can change this plan - seems like still a great antenna) . I have a 20 year old one that I’ll test but suspect this is something I should just replace after doing all of the rewire.

I ordered an SWR meter to test the setup (at least from the base of the mast) before we re step the mast. I’ll need to haul a battery and radio to where I’m working on the mast for this as I don’t have a dummy load. Of course, I could find issues with the radio! We shall see.

5215 is what's called a coil-shortened vertical, and I believe is cut for AIS frequencies. That is in fact what I'm using for my AIS. I think this is sub-optimal for VHF radio.



This is what I'm using for VHF radio: https://shakespeare-ce.com/marine/pr...-band-antenna/.


This was recommended to me years ago by Jedi of this forum, and I have been extremely pleased with it. It's an internal dipole so doesn't need a ground, which can be hard to provide.

[deblen]

https://marinehowto.com/easy-vhf-terminations/

[Timccarpenter]

Originally Posted by Jammer

RG8X .....Not sure where on Superior you are. I will be in Duluth on Sunday and if you're in the area you can stop by my boat and I'll give you one.\

I live in Duluth. Thanks for the offer. I was out of town this weekend and ended up ordering a 4-pack from DX engineering. Extras will become part of my boat supplies inventory!

[stanhoney]

http://honeynav.com/wp-content/uploa...ax-testing.pdf

some reference background

[Dockhead]

Originally Posted by stanhoney

http://honeynav.com/wp-content/uploa...ax-testing.pdf

some reference background

That's a fantastic document!

Lots of good advice there -- a veritable bible for VHF installation.

Note in particular the point about mini-UHF (PL259/SO238) connectors at the masthead -- just don't. The Galaxy Little Giant antenna which I like doesn't have a socket, it has a pigtail. I cut off the connector of mine, installed a Type N connector on it, filled the connector with dielectric grease, made the connection, then wrapped the connector with self-amalgamating tape, then wrapped that in electrical tape (that's a "telecomms wrap"). The Type N connector is theoretically weatherproof (unlike the Mini-UHF), but it's better to give it another barrier against water intrusion.


There is also a good explanation of why you do NOT want to use quarter wave whip antennae which need a ground.

[stormalong]

Originally Posted by Dockhead

That's a fantastic document!
...

There is also a good explanation of why you do NOT want to use quarter wave whip antennae which need a ground.

It is a good document, not fantastic. Stan makes the often repeated interchangeable use of the terms ground and counterpoise. They are two different things.

A counterpoise is needed on any antenna regardless of wavelength. It is the other half of the antenna. Think of it as the thing the radio waves push against to radiate effectively. A mast is a good counterpoise which is why the shield of all VHF antennas is electrically connected to the mast. Fortunately the counterpoise does not need to be resonant at the frequency of the transmission.

A ground it a safety connection to conduct static electricity and nearby lightning strikes away from the radio. A direct lightning strike is an unimaginably huge amount of power on any radio transmission line which is why direct or very close hits will do the damage that they do.

This is 40 years of ham radio experience writing this.

[Jammer]

Originally Posted by Dockhead

There is also a good explanation of why you do NOT want to use quarter wave whip antennae which need a ground.

I dunno. One of these days I would like to run some experiments.

1/4 wave antennas work extraordinarily well for VHF and can use various counterpoise arrangements instead of a large grounded surface. The designs with four wires bent to a 45 degree angle down from the horizon are particularly good in terms of pattern and characteristic impedance. For many boats the aluminum mast approximates this shape or can also be seen as a large sleeve dipole.

For land mobile antennas on cars etc, a 1/4 wave antenna with an NMO mount is standard, in part because they seal well and are easy to service.

1/2 wave end-fed antennas are largely seen as antennas of last resort outside the marine world.


On my last boat I experimented with a 9' long high gain antenna designed for base station use and never was able to get the results I expected, plus the windage was noticable (1" diameter fiberglass...). Perhaps it was defective. Perhaps I had feedline or radio problems that I was unable to figure out.

[Mike Burch]

The VHF range that can be relied on varies as the square root of the antenna height. It does not vary directly as the height.

VHF antennas with high gain achieve that gain by concentrating the signal into a plane at right angles to the antenna. They are intended for use on fizz-boats that a) do not have high masts, and b) stay more-or-less upright. If you fit one on top of a yacht's mast, as soon as the yacht heels, the signal goes up in the air (to windward) and down to the water (to leeward). The extra loss in the extra coax needed to feed it will reduce any extra gain you might get when the boat is upright, and the extra weight and windage aloft won't do you any favours either.

RG58 is cheap. Make sure you avoid the version with a solid centre conductor—only ever use the braided stuff. Fitting RG213 or LMR400 will give you perhaps a few percent more power at the masthead, but the effect in the real world will probably be negligible. The usual 50 feet of RG58 supplied fitted to most yacht antennas has 3dB of loss at 156.8MHz (Ch16). That means that half the transmitter power gets to the antenna and the other half heats up the coax. And the vast majority of yacht VHF installations have no problem with that.

The statement that any antenna requires a counterpoise should be read as meaning that any vertical antenna (other than a dipole) requires a counterpoise. And as the counterpoise is indeed the other half of the antenna, it should present a low impedance at the feed point. The simplest way to achieve that is to make it also a quarter of a wavelength long.

For maximum receive sensitivity, you need to keep the squelch set so it only just quiets the radio in the absence of a signal. This is often overlooked.

[Dockhead]

Originally Posted by stormalong

. . . A counterpoise is needed on any antenna regardless of wavelength. It is the other half of the antenna. Think of it as the thing the radio waves push against to radiate effectively. A mast is a good counterpoise which is why the shield of all VHF antennas is electrically connected to the mast. Fortunately the counterpoise does not need to be resonant at the frequency of the transmission.. . ..

It is not true that all antennae need counterpoises. Dipoles (and other types of balanced antennae) don't need a counterpoise, and work well without grounding the antenna base or connecting the coax shield to the mast.

[stormalong]

Originally Posted by Dockhead

It is not true that all antennae need counterpoises. Dipoles (and other types of balanced antennae) don't need a counterpoise, and work well without grounding the antenna base or connecting the coax shield to the mast.

Half of a dipole is THE counterpoise. A "balanced" antenna is half radiator half counterpoise.

[s/v Jedi]

The point is that there are antennas that do not ground the coax shield to the mounting bracket and thus not to the mast. There are very good reasons for that:

- the coax shield to ground is what introduces lots of noise to radio reception.

- the coax shield to ground at the antenna often creates ground loops.

- any connection to the mast provides a direct path for damage due to near lightning strikes that cause transient surges.

A dipole like the “Little Giant” antenna is very effective and has showed the best VHF radio reception ever in 50 years of using marine VHF and HAM licenses in two countries.

Yes to RG213 down the mast, where it should connect to a surge suppressor. From there you can use RG8x to the radio. If RG213 is too big for the mast then use RG8x. Don’t use RG58 because it’s cheaper.

Here is the surge suppressor to use: https://www.arraysolutions.com/surge...ection/as-302n

[Mike Burch]

Originally Posted by stormalong

Half of a dipole is THE counterpoise. A "balanced" antenna is half radiator half counterpoise.

Are you trying to tell us that only half a dipole radiates?

That's a novel theory, but I don't think you'll find a lot of support for it.

[stormalong]

Originally Posted by Mike Burch

Are you trying to tell us that only half a dipole radiates?

That's a novel theory, but I don't think you'll find a lot of support for it.

No, I am telling you that each half of a balanced antenna pushes off of the other half. That is why it is BALANCED.