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Author Topic: Coax Applications...  (Read 336 times)
KC0OHP
Member

Posts: 66




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« on: April 26, 2004, 10:15:28 AM »

Question...

  HF Amateur low -power application.
RG-8X.  Low power of 500 watt or less.
Below the 30mhz.  Is this type of rg-8x
acceptable for using on the HF bands.

  If the length of coax is under a
100' foot run without a great loss in
signal?  Or should I use RG-213 type
coax instead?

  Also can rg-8x be used on the VHF band's
also without great loss in signal too?  Or
should I use the Rg-9913 type of coax for
VHF bands?  Also how good of coax is the
RG-9914F. For VHF applications.  I see so
many views about the different's in coax.

  It gets to be confusing.  Any information
about this, greatly appreciated and help.
As to what type of coax should be used for
HF and VHF applications.  Thanks again for
all information on this subject..

  Dwight...KC0OHP...
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W7DJM
Member

Posts: 1




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« Reply #1 on: April 26, 2004, 10:50:21 AM »

I don't know why this would be so confusing.

Type something like "coax loss chart" (in quotes) into Google.

The very first link is this chart

http://kn5l.net/antenna/coaxloss.html

The left vertical scale is db loss per 100 ft of cable.  The bottome scale is frequency.  The chart ASSUMES that the cable is being used in a "matched" condition.  So, for  example, RG-58, when matched, at 144mhz, is about 4.5-5db

Your mileage may vary, depending on the brand, age, quality of the coax.

Just about ANY coax will work below 30 mhz, including believe it or not, TV coax.  If, for example, you are using a simple 20 meter dipole, fed with TV (75ohm) RG-6, I doubt that your friends on the other end would ever be able to measure the difference.

IF however, you are trying to feed ANY nonresonant antenna through a tuner on "all bands" then it pretty much does not matter what you use, because at some frequency(s) there is going to lots of losses.

For that, refer to the bottom chart on the page.

Notice the chart says "ADDITIONAL LOSS" This means, that for the example on two meters above, that if we had say a 3 swr, you would have to ADD more loss yet.


Here is an online calculator, you just plug in the coax type, the length, and the frequecy. Note that there is NO allowance for hi SWR here

http://www.timesmicrowave.com/cgi-bin/calculate.pl

Ain't "Google" wonnerful?
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W4TYU
Member

Posts: 518




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« Reply #2 on: April 26, 2004, 10:51:51 AM »

You are going to receive many opinions But the real answer is that it depends.

RG8X has a loss of 1.5 db/100feet at 30 MHz
and
a loss of 3.8 db at 150 MHz

For 2 meters and a gain type antenna with a run of 25 feet RG8X would be quite acceptable.

The loss below 10 meters can all most be neglected unless you are running QRP power.

RG58 has a loss of 4 to 6 db/100feet depending on exact type.

RG213 has a loss of 1.3 db/100 feet at 30 MHz
and 2.6db/100 feet at 150 MHz
I do not have figures on 9913 handy

Basically, you need to balance your type of operation,antennas and cost including connectors against the feedlines available.

Much information is available in the ARRL Antenna Book and in the Wireman's Wirebook IV

Ole man JEAN
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K0IZ
Member

Posts: 737




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« Reply #3 on: April 26, 2004, 11:18:04 AM »

As the man said, it depends.

Foam type 8X is rated at 30 mhz for 350 watts at 104 degrees F.  More power at lower frequencies and lower temperatures.  So if you run 500 watts on 10 meters, you are pushing 8X.  Also if you have SWR much above 1.5 to 1, that's even worse.  The voltage rating of the foam type 8X is not very high, and high SWR can cause puncture through from center conductor to shield.  The marine type 8X has solid poly insulation and would be a much better choice.

THe best attribute of 8X is that it is small and lightweight compared with the 8AU type coaxes.  So for some applications that might be important.

I don't recommend 9913 as it has a tendency to absorb water.  The 9913F7 gets generally very good marks.  Just about top of the line in 8AU sized coax.  And any good foam type 8AU coax should meet your needs except at the highest VHF/UHF frequencies.  The LMR series of coax is good.

Don't buy used coax.  Age, storage conditions, and original manufacturing processes make this a risky proposition.  Also don't buy real cheap coax.  Pay attention to % braid coverage (95%).  Among the 8AU type coaxes, those with heavier center conductors usually have the best specs.  
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WB2WIK
Member

Posts: 20601




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« Reply #4 on: April 26, 2004, 01:01:46 PM »

RG8X is an uncontrolled product, i.e., it's not a mil-spec cable type, so anyone can make anything they wish and call it "RG8X."  There would be absolutely no consequences for doing so.  I could label a piece of string "RG8X" and sell it.

And that's the problem.  Some RG8X is really nice stuff, easy to work with, very well shielded, good jacket material and overall construction, and well worth its price.  Some isn't.  Some of the "cheap" RG8X I've purchased via internet dealers wasn't worth the cost of shipping it, and was pure junk.

Even the best RG8X has two properties which make it unsuitable for use by me, in an outdoor environment where it's an actual antenna "feedline:"  One, it's mechanically fragile; and two, it has a low breakdown voltage, especially after being pulled, stepped on or taped to anything, because those operations compress the air-cell filled dielectric and further reduce the dielectric withstanding voltage of an already questionable design (for transmitting).

Some have said they've used RG8X at 1500W on HF, and had no problems at all.  I think that's either very lucky, or truly unbelievable.  I've had no problems making RG8X flash over internally, and fail altogether, at far lower power levels than that, especially during the hot summer months where the cable temperature is already 120F before applying power.

RG213/U, on the other hand, is somewhat heavier, thicker and less flexible than RG8X but usually has better properties for outdoor use.  It's tough, and unless there's really something wrong with it, nearly impossible to break it down at amateur power levels.  I stick with RG213/U as the "minimum" coaxial feedline for any outdoor (antenna) applications, and go to heavier cables for VHF-UHF runs of more than fifty feet.  "Heavier" = lower loss.  

If I need a small-diameter cable to wind a balun or something, and it's to be installed outdoors, rather than using RG8X I'll use PTFE cable like RG400/U.  It costs a bit more, but is very tough and will withstand lots of abuse.

WB2WIK/6
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W7DJM
Member

Posts: 1




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« Reply #5 on: April 26, 2004, 01:58:06 PM »

RG-8X is not the only "uncontrolled" coax.  Three is a LOT of differenct between coax of different makers
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W7KPQ
Member

Posts: 69




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« Reply #6 on: April 26, 2004, 04:45:04 PM »

Check out this great web site

http://www.ocarc.ca/coax.htm
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W8JI
Member

Posts: 9296


WWW

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« Reply #7 on: April 26, 2004, 07:35:40 PM »

In your situation, the ONLY thing you need to worry about is power loss and what you consider acceptable as a power loss in the cable.

I know people who love to quote specification tables will beat on me for saying this, but I use RG8X regularly at 1500 watts and never have a problem. As matter of fact we used RG8X for testing 1500 watt amplifiers all day in a testing environment up to 30MHz. We have been doing that since the early 1980's and never had a cable fail!

Part of the problem is although we can all read a specification chart, most of us don't rationalize if the data makes any sense or question how the ratings were determined. If we look at it factually (which some of us will probably refuse to do) we never should have a problem unless the cable is in a hot location or has other extenuating circumstances.

Factually, the 300 volt rating is just plain goofy. The cable would have to have an air gap of a few thousandths of an inch to break down at 300 volts! All you need to do is grab a hypot machine and test the cable, and you will find virtually any RG8X holds off more voltage than the coax connedctors ever will. We normally hypot RG8X cables to several thousand volts, at which point the properly installed UHF connectors flash over from the center pin to the shell!!

As for power capability, it's all a function of temperature rise and ability of the cable to get rid of heat. Just measure the power loss per foot and that is the energy dissipated per foot. In normal open 100F air a cable like RG8X can dissipate at least one watt per foot without damage.

All of this is easy to measure and confirm.

I suspect the 300 volts is some sort of bad guess that made it into a spec someplace. The power rating is probably based on an enclosed cable with an upper limit in a high ambient temperature plus a safety factor. This is an extreme of circumstances we are not likely to run into.

Often the bottom line with most cables is how much loss we can tolerate and how well the cable can transfer that heat to the outside world. 100 watts of loss spread over 100 feet of cable really isn't much heat.

I've run 1500 watts through RG59 and RG58 just to save weight on the center of dipoles, and never had a problem. RG8X is much better than those cables.

My long runs back to towers are 7/8th inch heliax, but the stuff hanging from dipoles is all RG8X or a smaller. My six meter beams from M2 have RG6 CATV cable in the matching systems. They handle 1500watts CW on six meters.      

73 Tom
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WB4QNG
Member

Posts: 362




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« Reply #8 on: April 26, 2004, 10:05:34 PM »

Find you a good supplier of Coax and go from there. Do to where I have to run my coax I use RG8X and have no problems. You will find that there is quite a bit of difference in the coax. I know I always ordered a RG8x from a certain company and they told me it was back ordered for three weeks They pointed out they had a RG58 that was just about as good for less money. I looked at their specs and they were right so I got it and have been happy with it. Are their specs. right or are they a bunch of bull. I don't know but else can you do. I agree rg8 would be better and hardline would be better yet but for normal applications if you want to use RG8X I think you will be fine. I know it is a whole lot easier to work with.
Terry
WB4QNG
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WB4QNG
Member

Posts: 362




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« Reply #9 on: April 26, 2004, 10:09:25 PM »

Oh yes I forgot to say I have 8 ants leads coming into my shack it is a big enough mess with the RG8x and the RG58. I hate to think would it be like with RG8 or bigger
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N6AJR
Member

Posts: 9908




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« Reply #10 on: April 26, 2004, 10:16:11 PM »

I use 8x for almost everything except 440 mhz. keep vhf runs short (under 50 feet) 6 meter runs fairly short (say 75 feet) and hf way out there.. I keep poser on hf undet 1100 watts in to a good swr or 500 watts with a bad swr, I don't like 58.  I buy rg8x on ebay from gssec for $85 per 500 feet, so I don't worry about a little loss.. its a hobby  after all
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