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Author Topic: coax question  (Read 1049 times)
FOXBAT426
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Posts: 274




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« on: April 05, 2010, 09:58:27 AM »

hi, i have to replace a 75ft run of coax. its rg-58 (the thin stuff) and has performed great with operating conditions upto 500 watts transmit. its wearing out now and i would like a new 75ft run. this is strictly for HF use. my questions is would i see a big difference going to rg-213 vs the thinner rg-58?? LMR-400 is out of the question - too stiff and a pain to work with. thx

john ki4ucw
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WA9UAA
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« Reply #1 on: April 05, 2010, 10:04:47 AM »

Hi John,
To answer your question, I'd like to know what kind of antenna is at the other end of the coax and what bands it is operated.
73,
Rob WA9UAA
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W5RB
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Posts: 565




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« Reply #2 on: April 05, 2010, 10:21:37 AM »

At relatively high power levels on HF , the main advantage of the fatter cable is an extra margin of protection against dielectric breakdown . You'd only reduce loss by ~1 dB at 30 MHz , but given the small cost , I'd definitely upgrade to the 213 , especially if you have any substantial mismatch on any band .
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WB2WIK
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« Reply #3 on: April 05, 2010, 10:25:08 AM »

How does coax "wear out?"

In any case, if RG58 "wore out" the first time, I'd have to assume new stuff will also wear out in the same application.  Maybe too much mechanical stress?  Abrasion to the jacket?

RG213/U is certainly more robust.
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WB6BYU
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« Reply #4 on: April 05, 2010, 10:42:36 AM »

Using VK1OD's transmission line loss calculator here: http://www.vk1od.net/calc/tl/tllc.php
there is about 1dB improvement on 10m using RG-213 on a matched line, or perhaps half a
dB on 40m.  The difference could be 2 to 3 dB if you are running a high SWR and matching
it with a tuner in the shack.

Beyond that, it comes down to a personal decision whether the reduced loss (and greater
power handling capability) is worth the added cost in your situation.  You could also
consider RG-8X, which is smaller and more flexible than RG-213.  The loss difference
would be roughly half, but, depending on the mechanism that is causing the RG-58
to wear out, the foam insulation might be a problem.
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FOXBAT426
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« Reply #5 on: April 05, 2010, 11:13:56 AM »

my antenna is a cushcraft MA5B beam at 40 ft. - actually i have three runs barrel connected together to give me a 75ft run. i would like to go with a steady run of 75ft. also one of the connectors on one of the coax sections has become rusted, so technically its not really "weathered", although the florida sun and the heat of the roof shingles does take its toll, the line could be salvaged, but at this point replacing the whole thing is easiest.
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VK1OD
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« Reply #6 on: April 05, 2010, 12:49:26 PM »

... one of the connectors on one of the coax sections has become rusted, so technically its not really "weathered"...

We speak a common language, English, but I have trouble understanding the above. These words have a common meaning, don't they?

Rust is corrosion of iron / steel, and can be caused by weathering (exposure to the weather).

What kind of connectors are you using that rust?

They have rusted, but not by weathering?

You need to use quality connectors, and ensure that they remain dry, effectively protected from the weather.

If you don't properly weatherproof the cable and connections, buy the cheapest stuff you can because it will need replacement after the next rain.

Owen
« Last Edit: April 05, 2010, 01:50:55 PM by Owen Duffy » Logged
N7WS
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« Reply #7 on: April 05, 2010, 04:45:26 PM »

Replace it with the best coax you can.  Personally I use 1/2" Heliax for my 200' runs, but have some 7/8" to replace it with when I get the ambition.

You will hear, "Well, it's *only* 1 dB" or words to that effect.  To me 1 dB is a loss of 20% of the transmitter power.

With some modern amplifiers costing in the neighborhood of $4.00 per watt, why would I want to waste 80 cents for every watt generated, just to save a few bucks on a piece of cable?
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KI4SDY
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« Reply #8 on: April 05, 2010, 05:52:43 PM »

I would run the best cable you can reasonably afford and try to eliminate the all barrel connectors you can, to prevent weather problems. In Florida's tropical storms, it rains sideways and upside down. The RG213 would be my first choice for 75'. The second choice would be RG8U (I am running 50' of that now with great results). My last choice would be RG8X, if you are on a budget. All of these cables should handle your transmitter power issues well.

If you are not experienced at installing coax PL259 connectors, buy the coax with them installed.
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WE1X
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« Reply #9 on: April 05, 2010, 07:11:41 PM »

It never ceases to amaze me that some operators will invest considerable dollars acquiring transceivers, shack accessories and antennas only to skimp on quality feedlines and connectors. I don't get it. If power has to get from "here" to "there" with a signal back again, why not invest in a decent feedline from the start?

Harry WE1X
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K0ZN
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Posts: 1531




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« Reply #10 on: April 05, 2010, 07:45:28 PM »

Hi,

75 ft. is not a long run at HF. Any good quality "large" coax will be just fine; you don't need LMR-400. The important thing is the percentage of shielding. Try to get 95% if possible. You don't need some super high dollar stuff for a 75 ft. run.

Regardless, DON"T use RG-58. The comments about spending money for a good radio and antenna and then cutting corners on coax are totally true. Would you buy a BMW, Corvette or Cadillac and then put the cheapest Wal-mart tires you could find on it??
Yeah, they are round, black and hold air....is that what you want?

73
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KC0FTC
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« Reply #11 on: April 06, 2010, 04:37:07 AM »

LMR-400 Flex is super flexable.  Not stiff at all
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N5XO
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« Reply #12 on: April 06, 2010, 04:41:33 AM »

Personally I would not skimp on quality coax. Here at my shack I consider LMR 400 the lowest quality coax I will allow in my shack, with the only exception being quality cable that is more flexible for my beam/rotor loops.

For my HF antennas all of my coax runs are LMR 400, yes it's a little stiffer but it is not hard to use.

My 6 meter antennas are all fed with 1/2 andrews hard line and my 2 meter, 220 and 440 are all fed with 3/4 andrews hard line. All of my runs to tower #1 {HF} are in the 100ft range and all of my runs to tower #2 are in the 150ft range.

Every DB loss impacts your receive and transmit signal, you have to decide what is acceptable. But why spend money on amps, etc only to save $50 on quality coax.
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W8JI
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« Reply #13 on: April 06, 2010, 06:41:51 AM »

Every DB loss impacts your receive and transmit signal, you have to decide what is acceptable. But why spend money on amps, etc only to save $50 on quality coax.

Loss or gain dirctly impacts transmitting, but on HF and lower VHF receive S/N is not impacted at all by cable losses up to 5 or 10 dB.

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VK1OD
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« Reply #14 on: April 06, 2010, 11:33:47 AM »

... Here at my shack I consider LMR 400 the lowest quality coax I will allow in my shack...

A more intelligent approach is to consider the needs for each application.

The OP had 75' of RG58CU on a multi-band beam.

Using TLLC, the loss of 75' of RG58CU with a worst case load end VSWR of 1.5 is 1.9dB, an feedline efficiency of 64%.

An option suggested my many was RG213, easy to obtain both cable and connectors, and not very expensive. With a worst case load end VSWR of 1.5 is 0.85dB, an feedline efficiency of 82%, ie 28% more power radiated than the baseline case, all other things equal.

Another option suggested was LMR400, not so easy to obtain both cable and connectors, and more expensive. With a worst case load end VSWR of 1.5 is 0.54dB, an feedline efficiency of 88%, ie 38% more power radiated than the baseline case, 7% better than RG213, all other things equal.

Another option suggested was LDF4-50A type, not so easy to obtain both cable and connectors, and much more expensive. With a worst case load end VSWR of 1.5 is 0.29dB, an feedline efficiency of 94%, ie 47% more power radiated than the baseline case, 15% better than RG213, all other things equal. (Of course, you could not use LDF4-50A alone, it isn't flexible enough for the rotator loop, so there would be a section of more flexible, lossier cable which erodes some of this benefit.)

The analysis ignored receive performance because given the external noise level, the small change in line loss will have an insignificant effect on receive S/N.

Somewhere you draw the line about value for money. You can make those decisions on a case by case basis.

If it were me, for my interests and values, I would think of a minimum of RG213 in this case... but probably no more. But, it is a personal decision, and one that can be an informed decision if you take the interest in exploring the options.

Owen
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