New radio and coaxial install........

[Dockhead]

Originally Posted by transmitterdan

Something like 99% of the time 2.5 watts delivered to the antenna is more than enough. When that isn’t enough is maybe when trying to contact to a big tall shore station from 100 miles out to sea.

People get hung up on percentages like 20% as a big deal. But 20% only matters if you have several of them in cascade. For example, 3 20% attenuation sections in series is only about 50% net (0.8*0.8*0.8).

And a dB is a dB no matter how much people feel the urge to convert them to %. We are talking about total path losses between two radios of about 110dB. An extra 1.5dB gain in that path just isn’t going to make a big difference. Heck the path changes by about 10dB all by itself depending on environmental factors such as humidity, temperature etc.

If the signal is too low, increasing gain by 1.5dB will barely make a difference. That’s why no one sells 1.5dB amplifiers because they would get no sales. 1.5dB more power simply wouldn’t make enough of a difference to matter.

No one wastes their time or money installing good quality low loss cable unless the installation is not right. I am not saying good cable is “bad”. All I am saying is, there is no urgent need to do major retrofitting to only improve the signal by 1.5dB. If you have water in the coax then you could have 10dB or more attenuation. That’s worth fixing.

All that sounds exactly right to me.


I did NOT use LMR400 cable for my VHF antenna exactly because I was concerned about water getting into the foam dielectric, and I was concerned about the foam getting dented somewhere in the mast, whereupon all that performance goes away. I went with RG214/U for the solid plastic dielectric and robust cover, and the tinned conductors.



I did, however, use LMR400 for my UHF/VHF ham antenna on the first spreader.

[S/V Illusion]

Originally Posted by Dockhead

The marine VHF specification calls for enough power, 25 watts,....

Therein lies the fundamental flaw in logic. There is no such thing as “enough”.

You based your position on having “enough” power to overcome losses, path loss and receiver noise floor, none of which is either fixed or constant.

This is another of those pointless academic debates which serve no purpose other than to confuse folks looking for advice.

Anyone who suggests a boater not strive to create the most effective radiated power to overcome weak signals at the receive point which could be the only person hearing a distress call is giving bad advice. That includes low loss coax.

Good quality, low loss coax, properly soldered and weatherproof connectors and maximizing height are good. The converse is not.

[Dockhead]

Originally Posted by S/V Illusion

Therein lies the fundamental flaw in logic. There is no such thing as “enough”. . .

Well, sure there is. This is not logical at all.


If that were true, then we would all be carrying 10,000 watt transmitters.



The problem is that 10,000 watts will not get you much past line of sight. There is very little you can do with 10,000 watts (or a 0 loss feedline, or whatever) that I cannot do with my 1 watt (with 2.5dB attenuation in the feedline), including talking to the Coast Guard at 60 miles.



And if more is always better, and "there is no such thing as enough", then besides the 10,000 watt transmitter, isn't the 1.5dB per 100 feet of LMR400 unacceptable as well? Why not then LMR1200 with 0.481dB per 100 feet? Or why not operate your VHF from the top of the mast, and eliminate the feedline altogether, if "there is no such thing as enough"?


No, of course there is such thing as enough. And people make reasonable compromises with ERP of their setups, rationally trading some ERP for other attributes like more robustness from RG214 or RG213 coax vs. foam dielectric coax, and indeed using any feedline at all, in order to gain antenna height at the expense of some ERP lost in the feedline.

[S/V Illusion]

Originally Posted by Dockhead

Well, sure there is. This is not logical at all.


If that were true, then we would all be carrying 10,000 watt transmitters.



The problem is that 10,000 watts will not get you much past line of sight. There is very little you can do with 10,000 watts (or a 0 loss feedline, or whatever) that I cannot do with my 1 watt (with 2.5dB attenuation in the feedline), including talking to the Coast Guard at 60 miles.



And if more is always better, and "there is no such thing as enough", then besides the 10,000 watt transmitter, isn't the 1.5dB per 100 feet of LMR400 unacceptable as well? Why not then LMR1200 with 0.481dB per 100 feet? Or why not operate your VHF from the top of the mast, and eliminate the feedline altogether, if "there is no such thing as enough"?


No, of course there is such thing as enough. And people make reasonable compromises with ERP of their setups, rationally trading some ERP for other attributes like more robustness from RG214 or RG213 coax vs. foam dielectric coax, and indeed using any feedline at all, in order to gain antenna height at the expense of some ERP lost in the feedline.

You keep limiting your argument to what happens when the receiving station can hear you well. That scenario is irrelevant when discussing loss. I’m talking about the important consideration of when you may be barely heard if at all in which case considerations such as loss become critical.

[Jim Cate]

Originally Posted by S/V Illusion

You keep limiting your argument to what happens when the receiving station can hear you well. That scenario is irrelevant when discussing loss. I’m talking about the important consideration of when you may be barely heard if at all in which case considerations such as loss become critical.

In the end we all have to decide at which point we will stop worrying about very small probability situations. The chances of that 1.5 db being the difference between copy an d no copy in an emergency of life/death severity are small enough that they have vanished from my concern horizon.

Obviously, everyone's mileage will vary; I have better things to worry about, and I imagine the OP does too.

Jim

[Dockhead]

Originally Posted by S/V Illusion

You keep limiting your argument to what happens when the receiving station can hear you well. That scenario is irrelevant when discussing loss. I’m talking about the important consideration of when you may be barely heard if at all in which case considerations such as loss become critical.

I'm talking about what is enough power. With marine VHF, all the ERP in the world will not get you much past line of sight. So within the radius of even 60 miles (talking with a very high antenna like the CG), more then maybe 10 watts at the antenna just doesn't do much of anything useful. There might be an issue from the FM "capture effect" if you're being overspoken, but again, if you are 60 miles away and someone is talking to the same station from 2 miles away, you're not going to get through with 20 watts, 50 watts, or even 100 watts ERP -- in any case, you have to wait until the frequency is clear.



And here we're talking about many decibels of difference, not 1.5dB per 100 feet of cable. The chances of that ever making the difference between being heard and not being heard are roughly zero. So it is an error to sacrifice some other important value (antenna height, robustness and waterproofedness of the feedline, ability to snake the feedline into the mast) for the sake of this meaningless 1.5dB per 100 feet.


All other things being equal, of course -- choose the lower loss coax. But if you have to sacrifice some other value to get a couple of dB better performance -- then you need to weigh it up, and usually you can easily afford a couple of dB.

[transmitterdan]

A general rule of thumb i like to use is not have more loss in cable than gain in the antenna. So a 3dB gain antenna would set my cable loss budget at 3dB.

But an equally important factor besides cable loss is antenna VSWR. If you really want to improve coverage for your transmitted signal get the lowest VSWR antenna you can. Put it well away from other metal bits. Which on a sailing vessel means top of the mast with no other antennas within 1 meter. This is a non-trivial requirement given the wide frequency range of marine radio. So you should always compromise the antenna VSWR in favor of channels 9-16. These are going to be the important ones for emergencies in most cases. A good marine radio technician (or ham) should know how to tweak a 3dB gain VHF antenna to optimize VSWR.

It is simple not a valid engineering argument to say that minimizing cable loss is the driver of coverage. It takes a systems approach to maximize coverage.

[Rotten Ricky]

I am in a similar position of installing a new coax cable in my 52 ft mast while it is on the hard, the total run being about 75 ft long. I already possess both RG8 and RG8U new cable, so cost is irrelevant. They are obviously old stock and not made any more but have been stored in the warmth of my house most of this time. They both seem to have about the same loss (1.8 Db per 100 ft). My instinct says to go with the RG8 as the heavier conductor and thicker dialetric inside the 0.4" diameter case would be better with lower losses than the smaller 0.25" diameter RG8U. Yet I am told they have almost the same loss? And is it possible to support the cable in the mast so all the weight does not hang from the mast head? Maybe at the mid-mast lights? If someone smarter than me could advise which one to use I would be grateful. Or should I just bite the bullet and buy new RG213? Is it worth it?

Thanks,

Rotten Ricky.

[KP44]

Originally Posted by Rotten Ricky

I am in a similar position of installing a new coax cable in my 52 ft mast while it is on the hard, the total run being about 75 ft long.
…. Or should I just bite the bullet and buy new RG213? Is it worth it?

Thanks,

Rotten Ricky.

My only comment on cable is: This is a one-time expense and not the place to save a few dollars.
That said, RG8 cable is good stuff. I replaced RG8U with RG113 and have noticeable improvement for a nearly 80' run. (60' mast) But I think RG8 is lower loss than the thinner, lighter RG8U.

I grabbed the cable near the top of the mast and secured it to the race. I don't think I secured it any other place inside the 1.25 inch conduit I used for a race.

Someone talked about antenna gain (in dcb) and that is an often misunderstood concept. A 3 dcb gain antenna does not amplify your signal by 3 dcb. It sends 3 dcb more signal laterally and the vertical signal is less. So it sends and receives better to all points level to the antenna mast. A handheld directly below your antenna or a plane directly above will be in the weakest area of your signal.
The Standing Wave is like an ocean wave against a strong current or river. The wave coming in at 4 knots against a 4 knot current might create an enormous wave on a river bar or in the Gulf Stream. Likewise, a radio wave reflected back from the antenna at the right frequency can create a "standing wave" that becomes powerful enough to damage your radio. Modern radios sense a wave and limit their power output to keep the standing wave weak enough that it does no damage. But that means that instead of transmitting at the nominal wattage, your transmission leaves your radio at a much-reduced power. Maybe 7 watts instead of 25 watts. Then it loses strength through the lead-in cable and might have 60% of 7 watts getting radiated by the antenna.
We match impedance of the radio, the lead-in line (cable), and the antenna to have a standing wave ratio (SWR) as close to 1:1 as possible. Anything under 1:2 is acceptable. Different frequencies resonate differently. That's why we use an antenna designated for our VHF frequencies and we use connectors and cable designated for VHF.
Don't be overly concerned as long as you use 50 ohm cable and a 50 ohm antenna. Almost all VHF radios have 50 ohm impedance match at the connector. I am just trying to give information so people understand concepts like antenna gain and SWR.

[Rotten Ricky]

KP44, thanks for your response.

It sounds like RG8 was intended mainly for use on towers etc that required the least loss over greater cable distance, and little requirement for tight bends etc, whereas the RG8U will do better in tight places and shorter distances. The RG8 also has a heavier conductor with solid, less-crushable dialetric with greater ability to reject absorb moisture, compared to the lighter duty RG8U, right? It makes little sense to reject either cable, already in my possession, for the added expense of the ony slightly superior RG213, etc.

It seems that what will decide the choice is whether I can manipulate the heavier RG8 into place around all the bends it must take, especially at the ends where I will try to do a perfect install of the PL59 fittings and get it into position for the radio and especially the antenna, which could be really awkward. I note that one can get a 90 degree PL59 which might help. I hope to not have any join in between the ends.

Thanks also for your info on SWR, as it was always a bit of a mystery to me, but I think I get the idea. It is more a matter, with the same power output, of the signal propagation being squashed into a more powerful horizontal pattern compared to the near omni-directional output of a whip antenna. Being as my multihull, like many power boats, seldom heels more than about 15 degrees, I had wondered about installing a fibre-glass, 6Db antenna on top of the mast, but it seems that it will not make a marked improvement over the top-grade, whip-style, 3 Db antenna I already have, as at about 60 ft tall, it should already have more than enough signal to achieve good transmission and reception to the horizon. Incidentally I happen to possess an SWR meter so I hope it works well for testing the cable.

Thanks again, Rotten Ricky.

[S/V Illusion]

Originally Posted by Rotten Ricky

Or should I just bite the bullet and buy new RG213? Is it worth it?

Thanks,

Rotten Ricky.

Be aware there are various types and quality among RG8 cables. Some have poor quality dielectric, others have a semi-permeable outer insulation and some cheaper brand RG8 cables have very poor shield density, all of which affects loss. You didn't mention what brand you have so it's impossible to tell how good/bad it is.


Stay away from the marine store variety as well as any big box store. They all sell poor/poorer quality coax. even among the Rg213 labeled coax, quality varies and that affects attenuation and loss. Similarly, there are a myriad of PL259 connectors, some junk and some good silver plated connectors.


It's senseless to do this and impossible to advise on replacement without first knowing the quality of the cables.

[KP44]

Your RG8 will probably make it through all the areas. I used RG213 and it is same thickness. We both have long runs.

I replaced a piece of television cable that I think was rated 75 ohms on my first boat. That is where people unknowingly create an impedance mismatch and get a higher SWR Even 450 ohm line can be used with the right impedance-matching devices. That is what a marine SSB antenna tuner does automatically.

A HAM with an antenna analyzer for VHF frequencies can help you tweak your SWR if necessary. But with your good cable and antenna there should be no need

[KP44]

Originally Posted by S/V Illusion

It's senseless to do this and impossible to advise on replacement without first knowing the quality of the cables.

We do our best to share what works. No one can remotely control cable quality or connector quality. But we can share information, as you have. It all helps.

[Rotten Ricky]

KP44 and Illusion,
Thanks for your guidance.

The two cables are from Tandy Radio Shack. I don't know how they stack up in your experience, but I have had some great stuff from them in the past. I picked the two reels up for peanuts when Radio Shack went outta business in Canada about 10 - 20 years ago or so. The residual stores are now called "The Source" and have very little radio or hobby electronic stuff in them.

The 0.4 size cable is printed "Tandy Wire and Cable, RG8U, 13AWG, Type CL-2, 75 degrees C, (UL), E111378A"
The 0.25" cable is printed "Tandy Wire and Cable, RG-8/U, Mini-Foam"

There is no other stamped or other identification on either cable.

Maybe the first 0.4 size is better than I thought before, being RG8U instead of just RG8. The lighter cable might be less desirable than before, being as it has not so much to identify it.

It looks like I will try to get the heavier cable through the passages it needs to go, but will wait for your suggestions.

Thanks again, Rotten Ricky.

[S/V Illusion]

Originally Posted by Rotten Ricky

KP44 and Illusion,
Thanks for your guidance.

The two cables are from Tandy Radio Shack.

The Tandy coax cables suffered from the same lack of shielding as the braid wasn't as dense as good quality coax. As a practical matter, what that means is stray RF is more likely to affect other stuff onboard but it won't noticeably degrade performance.


Most of the other hams I know who, like me, have been building and using all types and sizes of antennas for decades would use that stuff only to tie up the garbage. However, if it's in good condition, hasn't wicked any moisture and the cable hasn't been exposed to sunlight for a prolonged period, you probably won't gain much effective radiated power by using better quality cable.


Make sure you don't skimp on the quality PL259 connectors.