Feed line loss

[temchik]

Hi All,

I am reading a chart of cable loss for RG-8x and it says it has a loss of 8.6db per 100ft on 440MHz. So.. let's round it to 9db which is aproximately 8x. Does this mean it only lets 0.625W of power to the antenna (for a 100ft of cable of course) if my HT outputs 5W? I am a bit confused...

Thanks!

 

[prcguy]

That's exactly what it means but only when the source and load are well matched to the cable impedance. If you have a real bad VSWR at the antenna the losses go up even further.
prcguy

Hi All,

I am reading a chart of cable loss for RG-8x and it says it has a loss of 8.6db per 100ft on 440MHz. So.. let's round it to 9db which is aproximately 8x. Does this mean it only lets 0.625W of power to the antenna (for a 100ft of cable of course) if my HT outputs 5W? I am a bit confused...

Thanks!

 

[12dbsinad]

If you have a flat VSWR (antenna match) than yes that would be a correct calculation of RF power to the antenna. If you are considering 100ft lengths of RG8X for 440, an increase in coax size is a must. RG8X works good for short runs such as mobile installations which are typically 17ft or under. 100ft will represent serious loss at 440Mhz.

 

[temchik]

Ah, I really hoped I had it wrong, so... for permanent shack antenna requiring 50-100ft of coax I pretty much have to go with LMR-240 cable?

 

[temchik]

Or LMR-400? I don't want to get into 75Ohm+ cables and add matching networks

 

[n5ims]

8x is designed for HF use (it doesn't weigh down the center of a dipole as much due to the smaller size vs. regular RG-8) and performs quite poorly at VHF and UHF frequencies. LMR-240 is also not the best choice for long runs on the UHF band. Go with the thicker (half inch or so) RG-8 (belden 8237) as a minimum starting level for your coax run. Better will be LMR-400. Even better will be LDF4-50A. Note that these are STARTING types of coax for good performance. Larger diameter (read lower loss) coax (e.g. LMR-600 or LDF5-50A or even larger LMR-900 or LDF6-50A) will work even better.

LMR-400 generally is 50 ohm coax. Be aware that they do make a 75 ohm variant, but unless stated, the LMR-400 is most likely 50 ohm.

Since you indicate you'll be using a hand-held radio, you can (and probably should) use a short (6' or shorter) length of smaller, more flexible cable between the thick low loss coax of your main run and the radio. This should make it easier to move around and put less stress on the tiny coax connector on the handheld.

 

[mmckenna]

It depends on how much cable loss you are willing to accept. You can certainly run whatever you want, but performance will be dictated by that choice.

LMR-400 is going to lose about half your signal to cable loss over 100 feet. LMR-240 is going to lose a bit over 2/3's of your signal over 100 feet. Keep in mind that the loss works both ways, your received signal will be reduced due to the same cable losses that affect your transmitted signal.

You can play with different variations here:
Coax Calculator

There are many different options for cable, and prices can vary.
Personally, I'd probably use something better than LMR400. LMR400 is good stuff, but it is certainly limited. It's often suggested since it's a reasonable price and manageable for hobbyists.

Get the best cable you can afford. Good cable properly installed will last a long time. Getting cheap cable will result in disappointing performance.

 

[ShawnInPaso]

You might consider the larger issue at hand. Why have a 100' run of coax for a 5 watt HT? The perceived gains by the elevated antenna significantly reduce your effective radiated power (and to some extent your reception too).

Your signal would more than likely be improved by selecting another antenna location and using half the length of coax. While you didn't say it, I would imagine much of the 100' of coax is vertical while the remainder is for the run through an attic or other horizontal space?

 

[temchik]

Thank you for your replies.

My issue is that I get poor signal inside and around my home and wanted an elevated outside antenna. I probably will get a base rig as well at some point, but I don't want to loose all those watts on heating the feedline

Basically, the antenna would be about 30-35 feet off the ground and then the feed line needs to travel another 20-25 feet inside my home. I don't have much of an attic and almost everything is finished, so running coax inside is a big pain. I do have another path for the feedline where it will only be about 35ft run almost straight down into my basement shack, but I don't have a ground rod nearby, so I will probably have to install the lightning arrestor right at the antenna and run the ground wire down.

 

[zz0468]

Don't lose sight of the bigger picture here amidst the clutter of everyone's replies. Sure, a 100' cable run will have some loss. But look at it this way. If your 5 watt HT (with it's negative ERP) has an additional 20 or 30 db of loss being at ground level inside behind a bunch of trees, cars, fences, walls, etc. etc. ad nauseum, and 100' run of cable has 8 db loss, you're already ahead of the game by a considerable margin by running that cable and putting the antenna up 35 feet.

The only decision you need to make in this otherwise no-brainer, is what is the best quality cable that you can afford and work with. Once you've made that decision, buy it. You'll notice an improvement.

 

[KG4NEL]

Hundred-foot runs at 70cm are nothing to sneeze at. LMR-400, 600, or Heliax...

 

[KQ4BX]

A valid point was made, if you have a good signal on the ground, and it is 5 watts, but has to go through the tree's, and homes aground you, having that slight loss is all relative to the power you have, and pushing that signal out above all that is around you, plus having an antenna with gain, you will do much better even if you use RG8x. The gain can offset the lose in the cable. Put your money into the antenna. There is a nice duel band Workman fiberglass antenna that you will find on Ebay, or other places on the net. it will do a great job for you, and give decent gain.