Posted: 8/30/2015 10:34:18 PM EDT
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I know RG58 has more loss compared to RG8x or LMR400, but what is the maximum acceptable length for both 5w and 50w transmitter output?
Reason I ask is I have 100' lying around asking me to do something with it and have yet to run a VHF/UHF line to my shack. The total run will be about 30', initially being used by my HT before getting a true shack radio. At some point I'd run LMR400 but this is a stop gap until probably next spring. |
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You can use the calculator at Times Microwave to work out a lot of things. You're asking for an 'acceptable length.' That is a flexible and subjective concept. You may get good performance from a 50w mobile with 75% line loss, but get nothing with a 5w handheld and 40% line loss. Also, loss is based on length and frequency. Power does not change the loss percentages, only the multiplier. Frequency has a big impact on loss. It may be better for you to use the RG-58 for an HF run where its pretty good, and use a better cable for the VHF/UHF ranges with much worse losses.
Here is a chart I worked up with different cable types, frequencies, and the lengths at which you will will have 50% and 75% of your starting power at the end of the run. ie: 100w over rg-58 on 2m will only have half (50w) of your power at 54 feet. On 70cm, it only takes 30 feet to loose half your power. Please note that power and range are not a linear concept. Going from 100w to 200w does not double the distance you can talk. The topic is more complex that I can do justice to right now. With luck someone will step in who can explain that aspect of it.  Please let me know if you see any errors on this chart. |
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I use this chart http://www.universal-radio.com/catalog/cable/coaxperf.html#atten
They list the numbers for 100', so figure half of that and you should be more than good. Note on this 6.1db at 100'. So half of that and your at -3db of loss at 2 meters. 10.4 or so at UHF per hundred, so -5.2db. Anyway you look at it, your going to need some real good height, and a good antenna on the end to make up for what you lost to the cable. I also heard somewhere that doubling your power only gains 3 db. (100 watts to 200 watts gets you 3 db.) So at 2 meter frequencies, your losing half your power to the cable. I'm not doing the math for UHF because its going to be ugly on how much power you lose to the cable. |
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Quoted: I use this chart http://www.universal-radio.com/catalog/cable/coaxperf.html#atten They list the numbers for 100', so figure half of that and you should be more than good. Note on this 6.1db at 100'. So half of that and your at -3db of loss at 2 meters. 10.4 or so at UHF per hundred, so -5.2db. Anyway you look at it, your going to need some real good height, and a good antenna on the end to make up for what you lost to the cable. I also heard somewhere that doubling your power only gains 3 db. (100 watts to 200 watts gets you 3 db.) So at 2 meter frequencies, your losing half your power to the cable. I'm not doing the math for UHF because its going to be ugly on how much power you lose to the cable. |
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Yes, at VHF/UHF FM with garden variety antennas the most important thing is having a path. If you do you can afford to lose quite a bit of power in the feed line and still function. Just keep in mind RG58 isn't particularly rugged so it won't last very long where it is being wind blown, etc. For me its always the same thing, the center conductor eventually breaks. If I didn't always seem to be finding chunks of it for free I wouldn't use it for anything.
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Power / db info: WeekEEpadeau and Something Completely Different. "A change in power ratio by a factor of two is approximately a change of 3 dB." "... an increase of 6 dB corresponds to approximately four times the power and twice the voltage, and so on" |
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Quoted:
I'm fairly certain that is a General test question. Quoted:
Quoted:
I use this chart http://www.universal-radio.com/catalog/cable/coaxperf.html#atten They list the numbers for 100', so figure half of that and you should be more than good. Note on this 6.1db at 100'. So half of that and your at -3db of loss at 2 meters. 10.4 or so at UHF per hundred, so -5.2db. Anyway you look at it, your going to need some real good height, and a good antenna on the end to make up for what you lost to the cable. I also heard somewhere that doubling your power only gains 3 db. (100 watts to 200 watts gets you 3 db.) So at 2 meter frequencies, your losing half your power to the cable. I'm not doing the math for UHF because its going to be ugly on how much power you lose to the cable. 
It is. |
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Quoted: It is. Quoted: Quoted: Quoted: I use this chart http://www.universal-radio.com/catalog/cable/coaxperf.html#atten They list the numbers for 100', so figure half of that and you should be more than good. Note on this 6.1db at 100'. So half of that and your at -3db of loss at 2 meters. 10.4 or so at UHF per hundred, so -5.2db. Anyway you look at it, your going to need some real good height, and a good antenna on the end to make up for what you lost to the cable. I also heard somewhere that doubling your power only gains 3 db. (100 watts to 200 watts gets you 3 db.) So at 2 meter frequencies, your losing half your power to the cable. I'm not doing the math for UHF because its going to be ugly on how much power you lose to the cable. It is. |
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Quoted: Well - good to know the flash cards are sinking in. my exam is tomorrow evening. Quoted: Quoted: Quoted: Quoted: ... It is. Good Luck! |
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For future reference…
To get Watts to decibels. dBW = 10*Log(P) To get back… P = 10^(dB/10) To get from dBW to dBm… dBm = dBW+30 Play around with some stuff to get ERP and EIRP levels. You'll see a 3 dB change isn't exactly double or half, but it is pretty close. |
