For anyone contemplating a VHF or HF radio project

[Don S/V ILLusion]

I came across this neat little plug-in which clearly shows the loss of various type coax cable at various frequencies.
It is worth plugging in a few of the listed types of coax to show how significant the loss can be for the type typically used by boaters vs. some of the better types.
Simply scroll to the bottom of the page, change the frequency to approx 150 MHz, estimate the length of coax and voila.

With a 100 ft run of marine RG8X and 25 watts input, the antenna will see less than 10 w. With a decent coax, e.g., 9913, the antenna will radiate twice that power.
That doesn't equate to twice the signal strength but it can make the difference between intelligibility and noise when you most need it.
Something worth considering the next time you have to call for help.

http://www.ocarc.ca/coax.htm

 

[sailormanbigd]

coax

Ok Don, nice little tool. It would be nice if they included a price per foot for each wire grade so one could decide if the better wire was worth the price. If I make any sense?

 

[Ken Palmer]

Go to http://www.universal-radio.com/CATALOG/cable/coax.html for specs and prices.

The 9913 specs include: OD=0.405"
Attenuation= 1.7dB per 100 ft
Solid center conductor
Price = $0.89 per ft.

The RG-8X specs include: OD=0.24"
Stranded 16 GA center conductor
Attenuation = 4.7dB per 100 ft.
Price=$0.29 per ft.

Communications distance at 150 MHz is line-of-sight (about 17 miles).

 

[wufibugs]

On the site Don linked to they refer to "SWR". Is that
short wave radio?

 

[Don S/V ILLusion]

On the site Don linked to they refer to "SWR". Is that
short wave radio?

SWR = standing wave ratio. It is an indicator of how much power is reflected back from the antenna on the coax. A not very well understood term and not easily measured without a specific meter (SWR meter). It can be thought of as how efficient the antenna/coax are at radiating power. For all intents and purposes, you might as well assume the SWR is 1:1 for this purpose (of comparing coax).

 

[kendall]

SWR means standing wave ratio, plain speak it's power reflected back to the radio from the antenna. High SWR means reduced radiation, and a fairly heavy load on the final RF circuits which can blow out a radio.

quick search shows this on wikopedia:

"When an antenna and feedline do not have matching impedances, some of the electrical energy cannot be transferred from the feedline to the antenna.[2] Energy not transferred to the antenna is reflected back towards the transmitter.[3] It is the interaction of these reflected waves with forward waves which causes standing wave patterns.[2] Reflected power has three main implications in radio transmitters: Radio Frequency (RF) energy losses increase, distortion on transmitter due to reflected power from load[2] and damage to the transmitter can occur.[4]"

Which illustrates why it's now a good idea to just buy hardware and consider it good when installed as-is.

Ken.

 

[kendall]

gotta type faster, Don's response wasn't there when I started!

Ken.

 

[kenn]

I've found that the weakest links in the path from VHF radio to masthead antenna are usually the outside connecters - at the masthead antenna, and the deck to mast connection. Dirt or corrosion in the mating surfaces, or corrosion in a poorly weatherproofed connector will cause more loss than the difference between average and superb coax cable.

If you are using the "wrench-on" type of PL connectors, be sure they're screwed on far enough.