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Author Topic: Elevation control for HF beams?  (Read 2317 times)
ZL2HAM
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« on: September 28, 2008, 03:13:14 PM »

One of the oft-quoted issues with respect to working DX using horizontal antennas, including elevated beams, is that the gain they deliver at low angles is poor compared to verticals.

Elevation controls for beams are commonly used in satellite work on VHF/UHF.  Why aren't they also used on HF?  Is it because the nearby ground prevents any reduction in radiation angles when the elevation angle of the beam is changed?  Or is it more of a cost/complexity issue?

If it's the former, is there a height at which elevation control would become effective enough to make it worthwhile?
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K0OD
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« Reply #1 on: September 28, 2008, 03:41:30 PM »

"the gain they deliver at low angles is poor compared to verticals."

?
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N3OX
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« Reply #2 on: September 28, 2008, 03:48:57 PM »

"Why aren't they also used on HF? Is it because the nearby ground prevents any reduction in radiation angles when the elevation angle of the beam is changed? Or is it more of a cost/complexity issue? "

Both.

There is a guy who built a ridiculous SteppIR stack with Az-El control... maybe it's good for contesting but there are cheaper and easier ways to do the same thing.

It's a much different world in VHF/UHF satellite where you might be pointing a 15dBi beam at a spot 45 degrees or 80 degrees in elevation.  The only bands where the ionosphere really supports such high angles are the lowest ones, and it's impossible for anyone to put a rotatable beam up there.

On the higher HF bands, there's not much point in going to *extremely* high elevation angles.

For moderate elevation angles, I'm going to tentatively say that you can get more gain at a given high-ish angle by using a lower antenna in a stacked array with its ground gain included than by tilting up a higher antenna, but that's based on about 5 minutes with EZNEC, so you'd want to do a more thorough analysis of that ;-)

When you point too high skyward, you lose good ground gain... ask a moonbounce guy.

There's probably a situation in which mechanical HF elevation control can be shown to have a performance advantage over not having the same, but I bet once you factor costs and difficulty in, it swings far the other way.

73,
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #3 on: September 28, 2008, 03:54:37 PM »

I agree with K0OD's "?" as well.

Verticals shine at low angles over good conductivity soil or even better, saltwater, but it doesn't take a whole ton of height to get more gain at extremely low angles with a horizontal over not-great dirt.

Oh, and by the way, it sounds almost like you were thinking of tilting your beam down... a quick check with EZNEC of tilting a 5 20m element yagi at 70 feet down 30 degrees does indeed show that you get a slightly lower take-off angle that way, but you end up losing a few dB of gain to the development of a high-angle bulge.

The net effect is that the gain at extremely low takeoff angles doesn't change at all, and the gain at the takeoff angle of the down-tilted beam is 2dB less than the gain at that same angle of the horizontal beam.

Certainly no advantage there ;-)

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
ZL2HAM
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Posts: 52




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« Reply #4 on: September 28, 2008, 04:08:38 PM »

To the "?" response above: the gain of a vertical over salt water (I live at the coast) at 5 deg takeoff angle is about 4.5 dBi at 80 meters, vs. about 0.5 dBi for a 2 element Yagi at 45 m .

Yes, I was thinking of pointing the beam down, not up.  The goal would be to improve low-angle gain, particularly on 40 and 80 meters.  From the EZNEC results, it sounds like it wouldn't buy anything though.

N3OX:  what are the "cheaper and easier ways to do the same thing"?
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N3OX
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« Reply #5 on: September 28, 2008, 04:14:55 PM »

By "same thing" I meant increase gain at *higher* elevation angles/fill in nulls for contest purposes.  There, I'd recommend stacking.

You can't really increase gain at low angles except by going higher..

If you live on the coast and want good 40m/80m low angle gain, why not just do a vertical array?  Guess it's not so rotatable, but how many directions do you want to cover?

73
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K0OD
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« Reply #6 on: September 28, 2008, 04:21:15 PM »

"To the "?" response above: the gain of a vertical over salt water (I live at the coast) at 5 deg takeoff angle is about 4.5 dBi at 80 meters, vs. about 0.5 dBi for a 2 element Yagi at 45 m ."

So, you're talking about an 80 meter yagi. And you're talking about verticals near salt water. (I thought maybe you were quoting a ZeroFive ad Smiley )

On the high bands, verticals don't shine compared with Yagi's of even moderate height. 10/15 meter verticals are actually pretty high angle radiators.

Don't know for sure, but I've alway heard that arrival angles on 80 and 160 are quite high compared with 10-15-20. Lower angle may not always be better on the lowest bands.
 

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ZL2HAM
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« Reply #7 on: September 28, 2008, 05:31:04 PM »

A vertical array is certainly one option.  From my QTH, I only need to cover about eight 30-degree wide slices in azimuth (planning eight Beverages for receiving) -- Antarctica and open ocean occupy most of the other 120 deg.  

I'm trying to weigh a 4-square array vs. an optimized beam of some sort...  I suppose adding more vertical elements in the array is another option, although the complexity of such a setup seems to increase pretty quickly.

I saw some charts in ON4UN's book that say that arrival angles on 40 and 80 are between about 2 and 20 deg, depending on distance, with peaks at around 5 deg for US to Europe on 40, and around 11 deg on 80.
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W8JI
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« Reply #8 on: September 28, 2008, 05:44:10 PM »

First, take off angle is meaningless.

The thing that means something is the absolute gain at a desired angle, not the angle of maximum radiation.


Second, unless the antenna is far above earth (nearly in freespace) the primary influence on the lowest angle is the earth for some distance from the antenna.


It is possible, to a limited extent, to control wave angle so long as that wave angle is ABOVE the lowest angle determined by the height above earth. People often do that with stacks of antennas. For example my 40 meter antenna has two Yagi antennas, one at 90 feet and the other at about 180 feet. I can pick a low wave angle by feeding them in phase, a medium wave angle by feeding just the lower one, and a high wave angle by feeding them out-of-phase. We are doing the same on 80 meters with antennas at 150 and 300 feet high, and on other bands.

Simply tilting the antenna, unless it has a **really** long boom, doesn't do much. You actually have to stack antennas to control gain at various elevation angles. Even so, the height above earth limits the low angle gain.

Vertically polarization, because the phase of reflection does not invert 180 degrees like a horizontal, does not have a zero degree wave angle null. Instead the vertically polarized signal has earth losses that dissipate the low angle energy.

While you get a lower angle from a vertical, a considerable amount of power is consumed by earth losses. The stuff at zero degrees isn't useful anyway, unless you work groundwave.

73 Tom

 
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N3OX
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« Reply #9 on: September 28, 2008, 05:57:25 PM »

"I suppose adding more vertical elements in the array is another option, although the complexity of such a setup seems to increase pretty quickly.
"

That's true of any low band directional setup though.

I dunno, if you put in 40m and 80m  four-squares, I expect you're going to be very loud in all the saltwater directions.

What's that going to be?  9.5dBi at 5 degrees elevation?

Might be all you need...

Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K6AER
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« Reply #10 on: September 28, 2008, 07:21:02 PM »

My four element SteppIR beam at 100 plus feet is consistently 10-20 dB stronger than my ¼ wave vertical mounted on the 40 by 60 metal roof of the horse barn. I have never heard a signal that is stronger on the vertical than the beam when the beam is pointed to the same station.

Now the vertical will perform in all directions and that is why I have it ready to switch to when needed.
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N3OX
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« Reply #11 on: September 28, 2008, 07:40:21 PM »

"My four element SteppIR beam at 100 plus feet is consistently 10-20 dB stronger than my ¼ wave vertical mounted on the 40 by 60 metal roof of the horse barn"

There are so many differences between that situation and trying to get on 40m/80m looking over saltwater ;-)

You've got a good ground return but the pattern of the vertical is formed out much further by ground reflections off of your Colorado dirt.. uh... rocks?

And ZL2HAM would have to put an 80m beam at, what, 400 feet to be the horizontal equivalent of a 20m beam at 100 feet.

If you put a 4 element 80m beam 400 feet above ordinary dirt, it would probably beat a ground mounted vertical over the same dirt by 20dB no problem.  I'm not sure that bears much on ZL2HAM's choices when his 1/4 wave vertical is maybe 8dB-10dB stronger at good DX angles than yours, relatively speaking, and 4 elements at 400 feet is... well... ambitious at least ;-)

Think about VP6DX's signals on the high bands with those little vertical two element yagis.  This is the difference ;-)  

It's pretty clear to me that the rules of what's loud when you're actually looking out on saltwater are completely different than when you're not.  I find time and time again that a modest height horizontal beats a ground mounted or groundplane vertical.  You're not going to find that if you set up on the ocean beach, and if you set up a couple vertical elements on the beach, then you're really cooking.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K0OD
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« Reply #12 on: September 28, 2008, 07:43:54 PM »

"My four element SteppIR beam at 100 plus feet is consistently 10-20 dB stronger than my ¼ wave vertical"

--
And I bet the difference on receive is even more impressive since yagis provide noise and QRM rejection up to 30db. If yagis have "issues," the guys winning the big contests don't seem to suffering from those issues.  
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K6AER
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« Reply #13 on: September 28, 2008, 08:46:41 PM »

The original posting said nothing about 80 meters. It was a general question on the original post.  Yes the beam on 80 meters would have to be 400 feet to be 20 dB over the vertical but then again who cares on eighty meters about QRM, gain and noise floor issues. Crank up the amp and make noise.

Most VFO’s on eighty have been frozen in place for years.
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N3OX
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« Reply #14 on: September 28, 2008, 08:52:22 PM »

"The original posting said nothing about 80 meters. It was a general question on the original post."

The info came out later in the thread.

"who cares on eighty meters about QRM, gain and noise floor issues"

I do.  Heck, I care about ZL2HAM's gain ... I need ZL on 80 ;-)

73,
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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