Sacrifice antenna height for shorter cable run?
- Thread starter Jlinds
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If I follow you, in order to get the antenna 5 feet higher, you need to change how you route the cable and that change will add 60 feet to the total length.
Generally speaking, five feet isn't much in the big picture. But, how much difference it will make depends on the total height of the antenna. If you're 200 feet up, going to 205 feet won't make much difference at all. If you're 10 feet up then going to 15 feet will be more noticeable..
You can calculate the distance to the horizon. (How far the antenna will 'see'). This site assumes the earth is flat as a billiard ball, but it will give you a rough idea of how much difference 5 feet will make in your situation. Line of Sight Calculator - everythingRF
Generally speaking, five feet isn't much in the big picture. But, how much difference it will make depends on the total height of the antenna. If you're 200 feet up, going to 205 feet won't make much difference at all. If you're 10 feet up then going to 15 feet will be more noticeable..
You can calculate the distance to the horizon. (How far the antenna will 'see'). This site assumes the earth is flat as a billiard ball, but it will give you a rough idea of how much difference 5 feet will make in your situation. Line of Sight Calculator - everythingRF
Right. I can mount the antenna on my chimney which is on the opposite side of the house from my shack or I can place the mount on the siding just below the roof peak (with the antenna masted above the peak) on the side of the house where the radio room is located. The second option would be lower unless I make the mast taller to compensate (which I don't really want to do). Another way to ask the question is will the loss in signal power caused by a long cable run cancel out any height advantage of only a few feet? I know I can mitigate signal loss by using better cabling, i.e., LMR400 but I am also trying to keep costs down. BTW, I am mounting a j pole for 2m/70cm at 50w max power.
According to the calculator, the difference between 40 and 45 feet is about half a mile of radio sight distance. Does not seem that significant to me. I am leaning toward the easier install with shorter cable run. Why shouldn't I do it that way?
Cheers.
According to the calculator, the difference between 40 and 45 feet is about half a mile of radio sight distance. Does not seem that significant to me. I am leaning toward the easier install with shorter cable run. Why shouldn't I do it that way?
Cheers.
BF8609
Member
you would lose more signal thru cable attenuation than you would gain in five feet of height unless five feet gets you past an obstacle. Do some math using cable loss per foot at the frequencies your trying to monitor
mmckenna
I ♥ Ø
^^^ what he said ^^^
Only thing I'd add is that if that 5 feet gets you above/around an obstruction that would block reception.
When it comes to cable length, you need to not only consider how long the cable is, but what frequency you are using and the properties of the cable.
Higher frequencies have more loss in a given cable. Higher grade cable will get more signal to your radio than lower grade cable. If you were running something like large Heliax, then the extra length probably wouldn't make much noticeable difference. But if you're a cheap guy like most of us, another 60 feet of cable might really impact performance.
Also consider that coaxial cable losses can be over come by increasing antenna gain.
Only thing I'd add is that if that 5 feet gets you above/around an obstruction that would block reception.
When it comes to cable length, you need to not only consider how long the cable is, but what frequency you are using and the properties of the cable.
Higher frequencies have more loss in a given cable. Higher grade cable will get more signal to your radio than lower grade cable. If you were running something like large Heliax, then the extra length probably wouldn't make much noticeable difference. But if you're a cheap guy like most of us, another 60 feet of cable might really impact performance.
Also consider that coaxial cable losses can be over come by increasing antenna gain.
prcguy
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I've told this story many times but it looks applicable to this thread so here we go again.
A good friend of mine was a senior staff scientist for Raytheon, really smart guy. He did a study on coax length vs antenna height for VHF/UHF in an urban area like we live in with rolling hills, buildings, etc. It was a very complicated formula. He found raising the antenna every foot will overcome coax loss up to the point of about 8dB of coax loss. They type of cable doesn't matter either, this applies to a long run of RG-58 or 1 5/8" Heliax, it all works out the same.
More antenna height will allow you to hear more things at a distance that will overcome cable loss up to the point of 8dB of cable loss, then things reverse and reception starts going down the toilet.
A good friend of mine was a senior staff scientist for Raytheon, really smart guy. He did a study on coax length vs antenna height for VHF/UHF in an urban area like we live in with rolling hills, buildings, etc. It was a very complicated formula. He found raising the antenna every foot will overcome coax loss up to the point of about 8dB of coax loss. They type of cable doesn't matter either, this applies to a long run of RG-58 or 1 5/8" Heliax, it all works out the same.
More antenna height will allow you to hear more things at a distance that will overcome cable loss up to the point of 8dB of cable loss, then things reverse and reception starts going down the toilet.
I assume that this calculation is making a direct correlation between height and coax cable, 10ft more height=10ft more coax. The OP has a unique situation.
vagrant
ker-muhj-uhn
1. The short run using LMR400. Your antenna will be above the roofline.
2. Use the money saved on 60’ or 50’ less coaxial cable toward an improved VHF/UHF antenna later on. The gain it offers will really help.
2. Use the money saved on 60’ or 50’ less coaxial cable toward an improved VHF/UHF antenna later on. The gain it offers will really help.
Yes, he does have an odd situation, but; if he only raises it 1 foot he is breaking even (6.6db) with RG58 and is improved (1.8db) with LMR400 at 448MHz. With a 5' increase in elevation, the improvement will be significantly noticed.
Get a much better antenna.
prcguy
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Ooh, didn't see that. Yea get a better antenna, anything would be better than a J pole and you will notice an improvement.
If the OP already has the J-pole, go ahead and use it. While you probably won't find a J-pole at a commercial or repeater site,
it will be just fine as an economical antenna for hobby use. It will perform as well as a 1/4 wave ground plane or discone.
If you want to spend more money, then go for it.
it will be just fine as an economical antenna for hobby use. It will perform as well as a 1/4 wave ground plane or discone.
If you want to spend more money, then go for it.
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