Antenna Height VS Coax Length

[Cajun Invader]

I have been pondering putting an antenna up in a tree in my back yard. The tree is a pine and is 55ft from ground to top where the antenna mast could be mounted. I estimate I could go another 20' up and have an antenna roughly 75' up.

The tree however is 80' from my house. After all is said and done I am looking at roughly 200' of coax.

I also purchased a 40' tower this week. I figure I can add another 15' of mast and have my antenna 55' from ground to base. The total amout of coax needed for this would only be 80' due to the fact that I can locate the tower close to my house.

So here is my question. Taking into consideration the amout of coax needed VS the difference in height between the two antenna placement scenarios would there be any performance difference at all between the antennas? Would the amount of loss from the distance of coax run cancel out any gain in height?

 

[C2]

I think that 1/2 coax means 1/2 the loss.

The difference between 55' high and 75 feet high is probably not enough to worry about.

Besides, 55' is rather good anyway.

 

[Robb]

The only way that is possible with current technology to minimize loss would take a fat wallet. It would mean buying the Andrew Heliax cable. It is really expensive.

There are also three different sizes available. The grades in order are: expensive, outrageous, and nuts. But they have the lowest cable loss available from anyone. A 80 ft run would be about $250-300. It is solid core type cable; so once it is in place you will want to have a switch box to keep it secure from movement inside the shack.

Your idea with the tower sounds like it will work out for you. Too bad you can't get the tower closer to your radio room, so you can keep the feed length as short as possible. I have a 40 ft push-up pole guyed to a 10 ft flat garage roof with 50 ft of 9913 Belden Flex. It's right above me and my radio. I dont use power yet; but when I do I'm going to put some corrugated tin between me and that antenna. And ground it.

 

[W5LZ]

Gee, I wish I had that problem!
Since you just got the tower, why not use it? There will be some difference in what you can hear with that difference in height. That difference will vary according to propagation mainly, some days the lower one would 'hear' more rather than the taller one.
Feed line losses. Depending on what feed line you use those losses could amount to something you just might be able to hear. So, I think I'd use something other than RG-58, maybe even not use RG-8x. After that, I very seriously doubt it that 200 feet of coax will negate any increases that the height would yield, at least at HF. At VHF/UHF, sure, get a little worried about those losses. At HF, as long as the losses are under something like 3 - 6 dB, forget it, just not worth the trouble.
- 'Doc

 

[Moleculo]

RG213 is around 1db loss / 100 ft at 30Mhz

LMR400 is about 0.7db loss / 100 ft at 30Mhz

That's not much.

I'd use the tower...easier to climb and you don't have to worry about it growing taller over your antenna.

 

[n8fgb]

So you adding 29 feet in height in exchange for the loss of 100 feet of feedline.With LMR 240 that is 1.3db, with LMR 400 it is .7db. I would go for the extra height.
Rich

 

[Beetle]

RG213 is around 1db loss / 100 ft at 30Mhz

LMR400 is about 0.7db loss / 100 ft at 30Mhz

That's not much.

I'd use the tower...easier to climb and you don't have to worry about it growing taller over your antenna.

These figures assume an SWR of 1:1. If it's greater than that, there will be more loss, and it can add up rather quickly.

 

[hotrod]

imho more coax equals more loss no matter who makes it.
with that being said id go with the tower being ya already have it.
if ya really worried bout it put a 15 ft or so pipe above the tower
then the antenna.id use low loss cable either way but thats me

 

[Marconi]

The chart below indicates the RF thru-put for several feed lines I use. Feed lines without length noted are 50' - 75' and those that are noted 100'+ are at least 150'.

I connect the feed line to the metered dummy load below and my KW TS570D.

Your numbers for your lines may be different. All numbers above are based on their having a perfect load.

 

[Moleculo]

These figures assume an SWR of 1:1. If it's greater than that, there will be more loss, and it can add up rather quickly.

Yep.

Cajun, do you have squirrels? They LOVE the taste of coax in their favorite tree. Of course one way to cure that is time it just right and key up with a bunch of power as they bite through. It might hurt your equipment, but you can at least have squirrel for dinner.

 

[Cajun Invader]

Man do I have squirrels! My dog and them play games in the back yard all day long. It's actually amusing.

I plan on using the tower whether or not if I use the tree. Chances are I'll even keep the push up pole and keep an antenna hooked up to it also. Every neighborhood needs some cooke with an antenna farm in his back yard.

The main reason I was interested in the tree aspect is that I was considering using it to put the omni directional on and use the tower for a set of beams. The tower came with a rotor so I figured I'd try to utilize it.

Marconi, great intel you posted up. I'm taking your advise by the way and purchasing a dummy load with SWR and power meter so I'll be able to check the loss myself. Also I'm investing in a MFJ 259B antenna analyzer. Now all I need is my safety bonus and I'm set.

Three new topics coming soon:

Tower Restoration

Building A Set Of Horizontal Beams

Antenna Rotor Reconstruction.

Oh yeah, if you notice the little shack by the tree, it has power run to it and it weatherproof. What exactly is a repeator station?

Too many projects brewing. I'm glad it summer time so I can get some done.

 

[Mr. Doug]

I would go for height, get good coax and you are good to go.. HEIGHT is everything.. makes up for coax loss for sure..
:drool:

 

[C2]

How do you quantify the performance difference of an 11-meter antenna at 55' vs. 75'?

That is approximately 1.5 wavelengths vs. 2 wavelengths over ground.

Could you even assume 33% improvement (1.5/2)?

0.5 dB is ~11%
1 dB is ~21%
2 dB is ~37%
3 dB is 50%

It seems to me that once you get over a wave up, you have to go up quite a bit more to realize significant gains.

Plus, the tower...it would not even be a question for me.

You might consider putting a 20' mast on the tower rather than 15'. If it's good for the tree it should be good for the tower too.

 

[Marconi]

I would go for height, get good coax and you are good to go.. HEIGHT is everything.. makes up for coax loss for sure..
:drool:

What Mr. Doug says is true if the added height can be added in the same location relative to the station. If you have to use over 200' of coax just to reach a higher support, then the added height has to provide a lot more than just 20' additional feet.

Plus the wind movement with trees and for sure pine trees can present problems for antennas and maybe visa-versa.

 

[C2]

Height is good to a point. What I recall most is that it changes your TO angle.

I don't think you can really quantify any sort of gains talking the differences here.

Here was a good article I found just doing a quick search:

http://www.arrl.org/FandES/field/regulations/local/antplnr.pdf

You will note that antenna heights up to 120 feet have the biggest gains in the lower HF spectrum. The plots for 10 meters comparing 70' to 120' don't show as significant.