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VHF/UHF antenna height vs feed line length

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jeepinjeepin

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#1
Right now I am using ~50' of RG-213 to an Ed Fong DBJ-1 at 20' AGL. I am going to replace that run with LMR-400, but am considering dropping back to 25' of coax and running a higher gain antenna at about 12' instead of 20'. Is that a good trade off, half the feed line loss vs less antenna height? Assume HAAT at 0.
 
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#3
A good friend of mine and one of my antenna elmers who was a senior staff scientist worked out a calculation for this exact question. His answer for an urban environment over relatively flat ground was you will benefit from more VHF/UHF antenna height up to the point of incurring 8dB of feedline loss.

This means straight up and not including coax running sideways to get to a tower, etc. It also doesn't matter what kind of coax or its loss. So consider when going straight up, your VHF/UHF distance will improve as you increase height until your coax loss reaches 8dB and then it starts to reverse and coax loss will then counter any additional height advantage.

Its good that your upgrading to to LMR400 with lower loss and considering a higher gain antenna but put it higher to get even more range.
prcguy
 
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#4
A good friend of mine and one of my antenna elmers who was a senior staff scientist worked out a calculation for this exact question. His answer for an urban environment over relatively flat ground was you will benefit from more VHF/UHF antenna height up to the point of incurring 8dB of feedline loss.


prcguy

tnx for this info... good to know, confirms my suspicions but good to see some science to validate it

Are your friend's calculations published anywhere? It would make interesting reading.
 
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#6
I wish they were published because he passed away a few years ago. He had all this on a spread sheet that took lots of things into account from free space loss to ground bounce reflection and degradation, foliage absorption, building reflections, radiation angles at different heights, you name it.
prcguy


tnx for this info... good to know, confirms my suspicions but good to see some science to validate it

Are your friend's calculations published anywhere? It would make interesting reading.
 
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#7
Generally speaking, use the best coax you can afford. The stuff lasts for years, and your antenna system has the biggest effect on your station's performance.
 

jeepinjeepin

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#8
A good friend of mine and one of my antenna elmers who was a senior staff scientist worked out a calculation for this exact question. His answer for an urban environment over relatively flat ground was you will benefit from more VHF/UHF antenna height up to the point of incurring 8dB of feedline loss.

This means straight up and not including coax running sideways to get to a tower, etc. It also doesn't matter what kind of coax or its loss. So consider when going straight up, your VHF/UHF distance will improve as you increase height until your coax loss reaches 8dB and then it starts to reverse and coax loss will then counter any additional height advantage.

Its good that your upgrading to to LMR400 with lower loss and considering a higher gain antenna but put it higher to get even more range.
prcguy
I wish I could go higher. The 20' section of chain link top rail with 5' of PVC on top sticks out like a sore thumb here on this small lot. I don't want to mount it on the roof and it's probably a few feet higher than the peak already. The other option would be a push up mast with guy lines, but that would have to be moved away from the house into the back yard to keep the guy lines on my property and I already have an inverted L for HF back there, plus it would need 100-150' feed line depending on location and height.
 

N4GIX

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#9
It's a proven fact that height is might and low loss is cost. IOW, begin with a high-quality antenna, as high as affordable, and use the best feed line you can afford.

Power is much less important than the antenna system. I was having a conversation with a friend yesterday and mentioned that too many people focus on power, power, and more power. As an example I asked him "If I can reach the W9CTO repeater from here (about 12 air miles) using my 4 watt HT with full quieting, how much power would the repeater need for me to hear it at full quieting?"

"Uh," he replied, "about four watts?"

Of course, this particular UHF repeater has the receive antenna at 520', the transmit antenna at 480' running at 40 watts continuous duty. The feed line is 1.5" Andrews hardline driving an 8-bay folded dipole.
 

jeepinjeepin

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#10
It's a proven fact that height is might and low loss is cost. IOW, begin with a high-quality antenna, as high as affordable, and use the best feed line you can afford.

Power is much less important than the antenna system. I was having a conversation with a friend yesterday and mentioned that too many people focus on power, power, and more power. As an example I asked him "If I can reach the W9CTO repeater from here (about 12 air miles) using my 4 watt HT with full quieting, how much power would the repeater need for me to hear it at full quieting?"

"Uh," he replied, "about four watts?"

Of course, this particular UHF repeater has the receive antenna at 520', the transmit antenna at 480' running at 40 watts continuous duty. The feed line is 1.5" Andrews hardline driving an 8-bay folded dipole.
I'm with you. With my current setup there is no difference between 2 and 20 watts into a 440mhz repeater at about 15 miles away. I know that my power supply and radio fans cycle less often when running minimum power!
 
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#11
50 Feet of RG-213 is going to have about 2.5dB of loss at 445MHz.
50 Feet of LMR-400 is going to have about 1.3dB of loss at 445MHz.
Losses will be less at VHF, but you get the idea.

So, looking at about 1dB of reduced feed line loss if you keep the length the same. Probably unlikely you'd hear that 1dB.
LMR-400 is a "better" cable if you want to look at it that way. 1dB might be a good thing, but it depends on what your expectations are. Installing new coax with properly installed connectors and weather proofing everything that's outside won't be a pointless exercise. You can do the calculations yourself here:
Coax Loss Calculator

I'd agree with the others, keeping the antenna up high is going to help more. Also, LMR-400 isn't the Holy Grail of coax, it's good stuff, but don't limit yourself to just looking at it as the only option. 50 feet isn't a lot of cable, so a little bit of extra cost won't add up to much.

On the flip side, chasing 1dB of loss in your cable is often something you won't be able to hear. In other words, spending money to improve system losses by 1dB can be kind of pointless, unless you are really on the fringes of coverage.
 

jeepinjeepin

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#12
50 Feet of RG-213 is going to have about 2.5dB of loss at 445MHz.
50 Feet of LMR-400 is going to have about 1.3dB of loss at 445MHz.
Losses will be less at VHF, but you get the idea.

So, looking at about 1dB of reduced feed line loss if you keep the length the same. Probably unlikely you'd hear that 1dB.
LMR-400 is a "better" cable if you want to look at it that way. 1dB might be a good thing, but it depends on what your expectations are. Installing new coax with properly installed connectors and weather proofing everything that's outside won't be a pointless exercise. You can do the calculations yourself here:
Coax Loss Calculator

I'd agree with the others, keeping the antenna up high is going to help more. Also, LMR-400 isn't the Holy Grail of coax, it's good stuff, but don't limit yourself to just looking at it as the only option. 50 feet isn't a lot of cable, so a little bit of extra cost won't add up to much.

On the flip side, chasing 1dB of loss in your cable is often something you won't be able to hear. In other words, spending money to improve system losses by 1dB can be kind of pointless, unless you are really on the fringes of coverage.
That's mostly why I was looking to scale back to 25 feet of coax. It would be like going from ~2.5dB loss to ~0.7dB loss and then picking up 5+ dBi on a new antenna, but at a lower height.
 
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#13
That's mostly why I was looking to scale back to 25 feet of coax. It would be like going from ~2.5dB loss to ~0.7dB loss and then picking up 5+ dBi on a new antenna, but at a lower height.
Look at LMR-600 or LDF4-50a 1/2" Heliax. Both of those will have enough lower loss to probably make a hearable difference. Even at 50 feet. The big gain will be at the UHF frequencies as the higher you go in frequency, the higher the feed line loss. At VHF, it won't make quite so much a difference.

LMR-600 is a bit more stiff and doesn't like to make sharp bends (compared to LMR400), but it's only slightly more lossy than the Heliax. Installing connectors is a bit more costly and takes some additional skill, but you can always purchase them pre-terminated.

The big cost with coax installation is in the labor, even if you are the laborer. Your time is worth something. Considering a properly installed cable should last you 15-20 years, the additional cable cost is pretty small. Not having to replace the cable in a few years because your needs change would be worth it.

But, we do what we can with the budgets we are given. In reality even cheap Radio Shack RG-8 will work for many hobby related uses. Chasing 1 or 2dB usually doesn't make a huge difference for amateur radio use. We just do the best we can with what we have to work with.
And, if you do everything perfect at the start, it sort of takes the fun out of the hobby!
 

chief21

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#14
That's mostly why I was looking to scale back to 25 feet of coax. It would be like going from ~2.5dB loss to ~0.7dB loss and then picking up 5+ dBi on a new antenna, but at a lower height.
Remember that VHF and UHF are essentially Line-of-Sight frequencies. By lowering your antenna, it can't "see" as far. Even if you replace the original antenna with a higher-gain antenna at a lower height, that higher-gain signal can't reach as far (or receive as far) as the higher antenna likely could. Although the pattern of the new antenna is more compressed (gain), it will have to overcome more near and far obstructions due to it being lower.

Think of yourself standing on a high deck. How much farther can you see just by climbing higher on on a small ladder?

Another note... Most coax loss ratings are in dBd versus antenna gain ratings which are often in dBi - you can't compare the two directly. In addition, antenna gain ratings are very often overstated for marketing purposes, so don't take them as gospel.

John AC4JK
 

N4GIX

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#15
On the subject of antenna gain. If expressed in dBi, subtract 2.15 to convert to dBd.

On the subject of line-of-sight to horizon:
20' height = 6 air miles
12' height = 4 air miles

Line-of-sight to a mobile or HT
20' height to 5' height = 6 + 3 = 9 air miles
12' height to 5' height = 4 + 3 = 7 air miles

* http://www.hamuniverse.com/lineofsightcalculator.html

What the above indicate is that when speaking strictly about a base station, unless you have a lot of height you aren't going to achieve much range base to mobile or base to HT.

Such relatively short heights however will work fine into repeaters! :D
 
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#16
Coax loss is measured simply in dB without any suffix. The second d in dBd would refer to gain or loss compared to a dipole antenna and would have nothing to do with a coax measurement.
prcguy

Another note... Most coax loss ratings are in dBd versus antenna gain ratings which are often in dBi - you can't compare the two directly. In addition, antenna gain ratings are very often overstated for marketing purposes, so don't take them as gospel.
John AC4JK
 
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#19
Go for the height. In almost all cases when you're talking about VHF and higher height will be of benefit. You do get to a point where line loss makes the height counter productive. You're only talking about 20 feet versus 12, not 200. Line of site will be a few miles better but at these heights you have to deal with ground clutter so the performance at 20 versus 12 will likely be more dramatic than a formula would suggest. Get signal into the air, not your attic. Line loss for the additional distance is not significant versus the better height. Keep in mind loss is notably less at 2m than at 70cm.
 
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