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Author Topic: Elevation control for HF beams?  (Read 1821 times)
ZL2HAM
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Posts: 52




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« Reply #15 on: September 28, 2008, 09:13:59 PM »

If you need ZL on 80, a bunch of us (and some VKs) have been on most nights lately on 3790, 3795 and/or 3799, from around 1000Z to 1200Z or so.  There's a whole crop of regulars from the US.  We've had good propagation, usually for a half-hour before and after your local sunrise or so -- although I've also seen great prop to the west coast well before then.
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G3TXQ
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Posts: 1510




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« Reply #16 on: September 29, 2008, 01:08:58 AM »

W8JI said <<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. >>

Tom, the limited modelling I've done suggests that a single, high, antenna always produces the lowest wave angle. I'm surprised that combining one at 180ft with one at 90ft does better than one at 180ft on its own.

Perhaps I've misunderstood what you are saying?

73,
Steve
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W8JI
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« Reply #17 on: September 29, 2008, 03:41:50 AM »

No, you didn't misunderstand anything I said.

What you are doing is the same thing I keep warning everyone about, you are looking at the useless term called "take off angle". TOA is meaningless.

Single Yagi at 180 feet:
TOA 11 degrees at 14.35 dBi

Stack of Yagis at 90 and 180 feet:
TOA 13 degrees and 16.26 dBi

But.....

The gain of the stack at 11 degrees, which is the "TOA" of the single high antenna, is 16.02 dBi. The slightly higher TOA stack has nearly 2 dB more gain than the single high antenna at lower wave angles.

As a matter of fact the stack has more gain even at a very low 3 degree wave angle, despite having a higher TOA.

We have to stop using TOA. It is a useless number and it just confuses people. What you always want is the absolute field strength or gain at a desired angle or range of wave angles you are talking about, not the TOA.

TOA is totally useless when comparing performance at various wave angles.

73 Tom

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G3TXQ
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Posts: 1510




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« Reply #18 on: September 29, 2008, 03:54:05 AM »

<<What you are doing is the same thing I keep warning everyone about, you are looking at the useless term called "take off angle". TOA is meaningless. >>

Even though I expressed it in "wave angle" terms I was actually overlaying the two EZNEC elevation traces and comparing them across all low elevation angles from 1 degree up. If we are seeing different results I need to re-check my modelling Smiley

73,
Steve
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WA3SKN
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Posts: 5458




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« Reply #19 on: September 29, 2008, 04:58:08 AM »

Lowering your horizontal radiation angle is easy... all you do is lower your ground!
At HF, this is usually done by adding sections of tower.  No elevation rotor needed!
73s.

-Mike.
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G3TXQ
Member

Posts: 1510




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« Reply #20 on: September 29, 2008, 05:15:38 AM »

>>Even though I expressed it in "wave angle" terms I was actually overlaying the two EZNEC elevation traces and comparing them across all low elevation angles from 1 degree up. If we are seeing different results I need to re-check my modelling Smiley <<

I just ran some models using simple 40m dipoles to keep things simple and looked at wave angles from 1 to 5 degrees.

One at 180ft plus one at 90ft was a significant improvement over one at 180ft.

I then dropped them to 50ft and 25ft. The single at 50ft was significantly better than the stacked pair.

Around about 120ft + 60ft was where the stacking began to show advantage.

So the message seems to be that it's height dependent.

Steve
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W8JI
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« Reply #21 on: September 29, 2008, 06:41:45 AM »

That's correct Steve.

As I said, the height sets the minimum wave angle. Stacking antennas can move things around in that height range but it can't make gold from straw.

If you try to force radiation into an area where the ground is creating a pattern null, it reduces gain.

That's what happens when you stack a low dipole that has very little low angle radiation with a higher dipole.

It's all pattern multiplication of the patterns of the two antennas at whatever height they are at, and we have to remember TOA is a mostly meaningless number that distracts people from what they should be looking at.

73 Tom
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N6AJR
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Posts: 9915




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« Reply #22 on: September 29, 2008, 11:53:15 AM »

The main reason the folks who do uhf and vhf cw and ssb etc work have the antennas able to change azimuth is not for take off angle.


they point it "up" to do moon bounce, satalite and other modes like meterior scatter, so they need to be able to find and follow a celistial body.

has nuttin to do with getting a better "bounce " on low angle take off.  
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