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

[S/V Illusion]

That 1.5 dB loss figure may not sound like much but it equates to approximately 20% power loss at the antenna. Usually indiscernible but you only need one time when it’s essential to be heard to make it worth the effort. Your choice.

[Jim Cate]

Originally Posted by transmitterdan

I would not rerun a coax for 1.5dB. It’s not worth it in my opinion. RG8U is not the high loss RG8X floating around. Don’t worry about tinned copper either. Even tinned will corrode if it gets salt water inside. Concentrated on sealing the heck out of the outdoor connector.

Agreed! IMO, your biggest concern here is doing a good job of attaching the connectors and then sealing them up... a job often poorly done by DIYers and pros alike.

Jim

[transmitterdan]

Originally Posted by S/V Illusion

That 1.5 dB loss figure may not sound like much but it equates to approximately 20% power loss at the antenna. Usually indiscernible but you only need one time when it’s essential to be heard to make it worth the effort. Your choice.

It will never be an issue. The 25W radio has 5 times more power than needed at that antenna height. This myth of “more power matters” does not survive the math of the physics.

As a counterpoint, a lossy cable provides some protection to the radio. If the antenna has higher VSWR such as might happen if it gets water inside or corrodes then the higher loss in the cable can allow the radio to continue to function. So there is an argument that a little cable loss is a good thing sometimes.

[S/V Illusion]

Originally Posted by transmitterdan

It will never be an issue. The 25W radio has 5 times more power than needed at that antenna height. This myth of “more power matters” does not survive the math of the physics.

s.

Id like to see that physics

[Paul Elliott]

Originally Posted by S/V Illusion

Id like to see that physics

Handwaving physics: Calculate the path loss over the radio line-of-sight distance, add in antenna gain, subtract cable loss and receiver noise figure. If the transmit power is adequate to yield full-quieting FM reception then more power is not necessary.

But remember, the calculations need to include the other guy's antenna, coax, etc., and any antenna pattern directionality. So a bit of extra power will provide margin.

I haven't done the math, but it's not difficult.

[S/V Illusion]

Originally Posted by Paul Elliott

Handwaving physics: Calculate the path loss over the radio line-of-sight distance, add in antenna gain, subtract cable loss and receiver noise figure. If the transmit power is adequate to yield full-quieting FM reception then more power is not necessary.

But remember, the calculations need to include the other guy's antenna, coax, etc., and any antenna pattern directionality. So a bit of extra power will provide margin.

I haven't done the math, but it's not difficult.

Implying that greater power to an antenna doesn’t result in greater signal strength to a receiver is ludicrous, even for theoretically straight line of sight path.


I see it virtually every day on ham radio when my signal is stronger with the amp on. This isn’t debatable.

[transmitterdan]

Originally Posted by S/V Illusion

Implying that greater power to an antenna doesn’t result in greater signal strength to a receiver is ludicrous, even for theoretically straight line of sight path.





I see it virtually every day on ham radio when my signal is stronger with the amp on. This isn’t debatable.

And yet that is how VHF FM radio works. Mr. Armstrong was a genius years ahead of his time. FM is a unique modulation type where power above a certain level does nothing to improve reception.

[S/V Illusion]

Originally Posted by transmitterdan

And yet that is how VHF FM radio works. Mr. Armstrong was a genius years ahead of his time. FM is a unique modulation type where power above a certain level does nothing to improve reception.

I was referring specifically to VHF FM in my prior description of observable results which cannot be discerned with simple marine VHF stuff commonly used by boaters. Signal strength can exceed what boaters hear as “full quieting” on marine radios clearly quantifiable on a radio with an S meter.

Far more important than this academic debate is what happens with weak signals or intermittent signals being greatly enhanced to readable signals with an increase in radiated power. That makes the difference between hearing a distress call and hearing nothing.

A20% power reduction from inferior coax can mean everything in an emergency.

Parenthetically, I think you meant to say Tesla, not Armstrong.

[transmitterdan]

Armstrong is credited with having first practiced and promoted FM modulation.

Tesla invented the AC induction motor. But due to his OCD issues with the number 3 insisted that AC motors should have 3 phases. Which is why power throughout the world is distributed in 3 phases.

Are you seriously arguing that a 1.5dB gain ham amplifier can make a huge difference in ability to decipher a radio message? I don’t think you can buy a 1.5dB gain amplifier. Probably because they would be considered pretty much useless by most hams.

[Paul Elliott]

Originally Posted by S/V Illusion

Implying that greater power to an antenna doesn’t result in greater signal strength to a receiver is ludicrous, even for theoretically straight line of sight path.

I probably should just have let this thread die peacefully, but nobody claimed that more power or less loss wouldn't result in more signal strength at the receiver. Of course it does, all other things equal, and that's also physics.

What *was* said was that this extra signal strength, if more than required for full-quieting FM reception, is not necessary. And given lowest-common-denominator setups at each end of a line of sight path (radio LOS, just a bit beyond visual LOS) then the reduced path loss obtained by knocking a dB off the cable attenuation isn't going to significantly improve the communications quality.

I haven't done the math, and one could certainly find corner-cases where that 1dB *did* make a difference, but I'm guessing that transmitterdan is for all practical purposes correct.

[Paul Elliott]

Originally Posted by transmitterdan

Tesla invented the AC induction motor. But due to his OCD issues with the number 3 insisted that AC motors should have 3 phases. Which is why power throughout the world is distributed in 3 phases.

I'm going to have to research this, but doesn't 3-phase make it easier to get the motors up and running smoothly without the need for phase-shift starting capacitors? And provide for more uniform motor torque over the full 360-degree cycle?

[KP44]

Originally Posted by ohgary

The Carol C1154-RG8U has about 2.5db of loss at VHF freq.

LMR400 has about 1db of loss at vhf freq.

Decibels are logarithmic.
A 1 db loss is about 1/5. (20.6%) - acceptable
a 1.5 db loss is nearly a third. (29.2%) acceptable?
A 3 db loss is half. (49.9%) - not acceptable

2.5 db loss per 100 feet means a loss of 44% per 100 feet.

It might not be important enough to rip out a line that is working.
But if you ever do replace the antenna lead line, replace with high quality, low loss line that matches impedance with your radio and antenna.

[Pete7]

Originally Posted by transmitterdan

I would not rerun a coax for 1.5dB. It’s not worth it in my opinion. RG8U is not the high loss RG8X floating around. Don’t worry about tinned copper either. Even tinned will corrode if it gets salt water inside. Concentrated on sealing the heck out of the outdoor connector.

Slight thread drift, but are you saying I wasted my time fitting this RG8X:

https://www.saltyjohn.com/product/cable-kit-20m/

[Dockhead]

Originally Posted by KP44

Decibels are logarithmic.
A 1 db loss is about 1/5. (20.6%) - acceptable
a 1.5 db loss is nearly a third. (29.2%) acceptable?
A 3 db loss is half. (49.9%) - not acceptable

2.5 db loss per 100 feet means a loss of 44% per 100 feet.

It might not be important enough to rip out a line that is working.
But if you ever do replace the antenna lead line, replace with high quality, low loss line that matches impedance with your radio and antenna.

Why is 3db per 100 feet a priori unacceptable? I think it depends very much on the use case and the installation, doesn't it? 3db vs 1db is probably irrelevant for a 50 foot cable run for marine VHF, with a 25 watt set. For a 100 foot cable run perhaps less so, but even that is not likely to ever matter.



My fixed VHF set is installed with a cable run of about 90 feet of RG214, which means more than 2.5dB loss through the cable. But I have installed it without any connectors anywhere except at the radio, with a continuous run of cable down the mast and to the nav table. And I've installed it very carefully. How many boats have cheap PL259/SO239 connections at the base of the mast, some of them even with rainwater dripping on them from down the mast? And another PL259/SO239 at the top of the mast? Such connections, even in perfect condition, can have an insertion loss at VHF frequencies of up to 1dB each (see: https://www.hamradio.me/connectors/u...t-results.html).



The marine VHF specification calls for enough power, 25 watts, to work satisfactorily with "average" installations and "average" antennae, plus average feedlines. With a good installation and a good antenna, you can afford to lose some power in the feedline. I keep my radio on 1 watt, actually, and rarely turn it up to 25 watts. I've had "weak but readable" signal reports from 60 miles away, speaking with Solent Coast Guard -- on 1 watt, and otherwise get nothing but "loud and clear" at normal ranges -- on 1 watt. Despite the 2.5dB attenuation in my feedline, 25 watts is obviously overkill, in my installation.



If your installation delivers 10 watts to a good antenna on the top of your mast, I believe that is more than enough for almost any possible use case for marine VHF. If you're not being heard when you want to be, it's very doubtful that losses in the feedline, in a feedline in good condition, are the problem. It's much more likely that the problem is due to a bad connection, a flaw in the cable, a poor antenna not properly grounded (if it needs a ground), or one of the other common problems with marine VHF installations.

[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.