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[Articles Home]  [Add Article]  

The Myth of Takeoff Angle

from Tom Rauch - W8JI on April 26, 2010
Website: www.w8ji.com
View comments about this article!

The Myth of Takeoff Angle

Antenna discussions and articles often emphasize take off angle. We commonly read or hear that a low takeoff angle is good for DX, and that a high takeoff angle is good for local or short-range work. This is exactly what a good friend of mine, the author of one of the most popular antenna modeling programs, often showed angst over. He often expressed great reservation in including TOA in his software, but felt he had to include take off angle because people expected it. In fact, if we go to the help menu in most versions of his software and search for instructions on using TOA (or takeoff angle), help is not there!

So what is takeoff angle? How can we use it? Is it useful for anything? My answer to that, surprisingly to the great believers in TOA, is TOA is not worth anything by itself. Let us look at some examples, and see how little value TOA really has.

Vertical vs. Dipole

We all know a vertical antenna is great for DX because it has a "low takeoff angle", and a modest height dipole is better for modest distances because it has a "high takeoff angle". Let us see how true this is.

Figure 1: Dipole at 1/3 wavelength

This dipole is 1/3 wavelength high (figure 1). The peak gain is 6.66 dBi, and it has a takeoff angle of 54 degrees. At 15 degrees, dipole field intensity is .42 dBi. Few who focus on takeoff angle would consider this antenna "DX worthy", or even think it would make DX contacts.

Figure 2: Vertical with 16 radials

We see now that a vertical with 16 radials (figure 2) has peak gain at 19 degrees, where gain is 2.21 dBi. Gain at 15 degrees, an angle considered very useful for long haul DX on lower HF bands, is 2.1 dBi. This vertical has only 1.7 dB more signal level than the useless cloud warmer dipole at the very useful low band DX angle of 15 degrees! At 19 degrees, the peak radiation angle of the vertical, the dipole and vertical are essentially equal in field strength even though the vertical has a TOA of 19 degrees and the dipole has a TOA of 54 degrees.

This does not mean the vertical is useless for closer contacts, but it certainly is at a huge disadvantage compared to the dipole. It also does not mean the dipole will not work DX, or will not at times be stronger than the vertical over very long distances. Small differences in location, propagation, and installation could easily make the dipole the better antenna for long distances. The only sure bet is the vertical will not be good for very high angle propagation.

Obviously, TOA by itself tells us little about an antenna's ability to be a good DX antenna. What then is important? With what should we really be concerned?

The answer is almost painfully logical; we want maximum available field strength over the entire useful range of angles and directions. Peaks and nulls outside of the desired range of angles and directions are of no concern when our goal is developing the maximum most consistent signal into a specific desired location. We should not care about or even bother discussing TOA; we only want maximum field strength in our target area!

Nulls, a Major Problem We Ignore

We know we want maximum signal over the useful and most desired angles and directions of radiation, but we should never confuse that with maximum peak gain in that area. Peak gain, by itself, is as useless and meaningless as TOA. Let's look at a few examples of high useless gain.

Let's assume we are in the center of the USA, and we want to work most of the USA with our multiband antenna. We all know a low band loop, operated up near the high end of HF, has considerable gain. That is a good thing, isn't it? After all, everyone wants a high gain antenna.

Figure 3: 80-meter loop on 15-meter band

Our eighty-meter full wave loop antenna (figure 3) has 14.37 dBi gain, about 6 dB more gain than a dipole, when used on fifteen meters. While this may sound attractive, this 6 dBd of additional field strength divides between four major lobes and four minor lobes. The average gain in the main lobes is 12.46 dBi or about 4 dB over a dipole. Worse yet, that gain is in an azimuth beamwidth of only 21 degrees for each of the lobes, and next to those peaks are holes or nulls up to 30 dB deep!

While we may have bragging rights of high isotropic referenced gain, our loop is very narrow area gain. Gain, at best, averages 4 dB over a dipole in four very specific peak directions of 44, 133, 224, and 315 degrees. The -4db points of the main lobes, at which point gain comes down to the peak gain of a dipole, is about 11 degrees.

It is statistically difficult to find contacts inside the main lobes, where the signal level would exceed a dipole!

Figure 4: Loop and dipole superimposed

A comparison to simple dipole antenna (figure 4) shows the loop is better by as much as eight db in four very specific narrow directions. The total azimuth area where the loop is equal to or better than the dipole is 125 degrees. On the other hand, the dipole is better than the loop over 235 degrees of azimuth! When located almost anywhere in the mainland USA, we would have far better average signal levels in the USA with a small half-wave dipole antenna.

This leads to a rule we often ignore or forget. If we cannot move the nulls, it is usually better to have a smooth pattern with a bit less gain. The last things we want are multiple nulls in the useful azimuth.

Elevation patterns

What is good for azimuth is also good for elevation. Once again, the last things we want are a plurality of deep nulls scattered throughout useful wave angles. A broader elevation pattern (with no change in efficiency or azimuth coverage) results in slightly less gain, with twice the elevation beamwidth typically reducing gain by 3 dB. This will never be the difference between someone hearing us S9 with one antenna and not hearing us at all with the other. Similarly, a difference in TOA from 30 degrees to 5 degrees will not make that difference either.

The sole exception to this is if we have a pattern with a deep null that happens to fall right at the target angle or direction. The nulls are what cause the problems of significant loss of signal, not gain or TOA!

Figure 5: Six-meter stack at W8JI with traditional phasing of high antennas

With this pattern (figure 5), propagation would appear very spotty. A change in wave angle from just 2 degrees to 4 degrees could result in a 20 dB signal reduction. This is not a TOA problem. The problem is rooted in narrow main lobes with deep nulls between sharp peaks. If the signal arrival varied just 1-2 degrees, signal level could go from excellent readability to unreadable.

Figure 6:Stack at W8JI, same mean height, with progressive phase lag

One method of correcting the deep nulls is to add antennas and use a progressive phase shift between antennas. Since antennas normally fire in the direction of lagging phase, and since we want to fill nulls above the main lobe, the logical phase shift would be a progressive lagging phase shift with increasingly higher antennas.

In Figure 6, the addition of two more antennas with a lagging 30-degree progressive shift results in an array much less sensitive to wave angle. It has virtually the same field strength, within 6 dB, from 0.9 degrees up to 11 degrees. It still has nearly the same peak gain at very low angles to open the band, or to use when MUF just crosses the operating frequency, yet does not have the problem with 20-30 dB deep nulls if the wave angle shifts as little as 2 to 4 degrees from a lobe peak. Such a pattern would provide much more consistent signals over time.

Other Stacking Mistakes

We make many other stacking mistakes. I almost made one here at my house when I was considering stacking four 20-meter antennas. Fortunately, I put the myths about peak gain and takeoff angle aside and looked carefully at the nulls and beamwidth.

Figure 7: Proposed 20-meter four-antenna stack

My proposed four-antenna stack (figure 7) would have a 23.5 dB null at 9 degrees, which would be right in the hot spot of 20-meter signal a large portion of the time. It also would have less than dipole gain, sometimes significantly less, at any wave angle above 8 degrees. While it is mechanically easier and cheaper to stack multiple antennas one above the other, it is a huge waste of hardware because the pattern is just not that good.

Figure 8: 2X2 H-stack for 20 meters

The antenna in figure 8 is an H arrangement of four antennas. This system uses half of the four-stack tower height, but requires either two towers side-by-side or one tower with two cross supports a full wavelength long. Since the antennas are small (four elements on a 26-foot long boom), I have decided that either arrangement would work.

Notice the peak gain is 20.05 dBi, which is about 11.5 dB over a dipole at optimum height. This system gives under 0.5 dB of peak gain, and gain is useful from well under 2 degrees up to 8 degrees. I have almost doubled the vertical beamwidth, making the antenna far less sensitive to wave angle variations.

Figure 9: H-stack with upper lagging phase

Lagging phase on upper antennas (figure 9) fills the deep pattern nulls while shifting the main lobe higher. It costs very little gain to do this, peak gain only decreasing 0.3 dB. Field strength is greater than a dipole at optimum height at any angle up to 20 degrees. By switching between these two patterns, this simple system covers most conditions of long distance propagation.

Summary

I hope the article shows why constructive conversation and debate, and looking at things from a different perspective, can lead to better understanding of systems and better antennas. If readers follow my thought processes as I progressed through these various antennas, they should notice I never considered take off angle at all. I looked at gain over the desired angles, focusing especially on nulls and null depth. My goal was maximum signal at all angles within the useful range of angles and directions. Nulls are especially poisonous to our signals. Few will notice a ten or more degree change in peak radiation angle, unless the lobe is very narrow and accompanied by deep nulls a few degrees off peak. If we focus on only one aspect, we can hurt ourselves in the end. We can obtain gain through a very narrow pattern, but if that pattern is full of nulls, we might lose a majority of contacts over time.

Sure, it is nice to brag and say we have 10 dBd gain. It is nice to say we have a low take off angle antenna, or a high angle for local work. Factually, take off angle by itself means nothing. Neither does peak gain. We need to focus on how well a planned antenna works over the most useful angles and directions. We need to pay particular attention to nulls, because the deep nulls, in particular multiple deep nulls, are really what prevents us from hearing and working what would otherwise be spectacular signals.

Member Comments:
This article has expired. No more comments may be added.
 
The Myth of Takeoff Angle  
by AD4J on April 26, 2010 Mail this to a friend!
Thanks for posting this excellent article, Tom. I found the stacked yagi array section particularly interesting because of the disappointing performance of a friend's stack of three 4 element yagis.

I recall that W4AN decided to put one of his 10 meter yagis at a lower height than he had planned. This was to avoid nulls at desirable elevation angles.
 
The Myth of Takeoff Angle  
by KB2DHG on April 26, 2010 Mail this to a friend!
Very nice article, There is much to be learned and an almost infinate source of information when it comes to antennas. My problem with most of my antenna instalations is conditions of space and restrictions. In a perfict world it would be easy to accomplish an instalation of an antenna sutible for whatever use we may desire. BUT not all of us have this luxury.
I live in a condo with restrictions. MY goal was to install an antenna of stealth. TOA had to take a back seat. During my research for this ideal multi band DX antenna, I came up with one which worked out ALL my concerns and problems. I was able to home brew a full size G5RV in an inverted Vee instalation. I don't know exactly what the take off angle is but I have worked good DX when conditions are favorable.
BUT this article is a good read and someting more to add to my understanding of antennas.
Thank you
 
RE: The Myth of Takeoff Angle  
by K1BQT on April 26, 2010 Mail this to a friend!
Brings back a recollection of one local manufacturer who swore his compact 2-element antenna had "super gain" because it miraculously outperformed a much larger Yagi in a side-by-side DX comparison. In reality, the smaller antenna had significantly less gain, but the elevation pattern had fewer deep nulls, creating the allusion of magical performance. Great article, and sage consumer information that is sure to raise sweat on the brow of antenna-ad copy writers everywhere. Nice job.
 
The Myth of Takeoff Angle  
by N5YPJ on April 26, 2010 Mail this to a friend!
Nice article Tom, thank you for the info. It's nice to have some one with an educated view cut through the advertising fluff and urban legends and give us the meat.

 
The Myth of Takeoff Angle  
by WB0KSL on April 26, 2010 Mail this to a friend!
This is a terrific article. Science and common sense makes for a formidable tool. I think exposure to expertise like this, via the internet, has become one of my favorite aspects of this hobby.

John, WB0KSL
 
RE: The Myth of Takeoff Angle  
by KU5Q on April 26, 2010 Mail this to a friend!
Thanks for the work Tom. Sure hope to see more from you. Between your work here, your website, and Walts books, you guys have more than enough information available to enable one to teach themselves a lot. And, most of it's free! Pretty good deal. Also thanks for you work on the Elecraft K3 reflectors.
 
RE: The Myth of Takeoff Angle  
by AC5UP on April 26, 2010 Mail this to a friend!
Regarding the cloverleaf pattern of an 80 Meter loop operating on 15 Meters...

While it's true that someone on Wichita, KS (very near the exact center of the CONUS) won't be wall-to-wall into North Carolina or Winnipeg, a cloverleaf arranged to favor the NE, SE, SW and NW does have DX potential on all four lobes.

A great circle map centered on Wichita, KS will show EU land to the northeast, JA to the northwest, OZ to the southwest and Sud Afrika on the far end of the southeast lobe. Considering that 15 is the better DX band while 75/80 is popular among regional rag chewers I'd say the combo of the two patterns is not a bad compromise.......
 
RE: The Myth of Takeoff Angle  
by KY6R on April 26, 2010 Mail this to a friend!
Excellent article - thanks!
 
RE: The Myth of Takeoff Angle  
by VO1FZ on April 26, 2010 Mail this to a friend!
This all makes sense to me, but I will raise an advantage to the vertical in some situations that is directly related to the lack of nulls.
I run an 80 meter horizontal loop (balanced feed) and a 102 foot inv vee doublet - each of which offers some gain in different directions on the higher bands. Switching between them will usually offer gain of some sort.
But the fly in my antenna farm's ointment is that I don't know where the common nulls are for each band. If choose to put up a dipole, I'm still getting a significant null off the ends.
A vertical should give me a nicely balanced omni-directional antenna - albeit at the cost of some gain in some cases as you point out. But overall, it fills a very nice complementary function to my overall 'farm'.
Great article. Thanks again.
 
RE: The Myth of Takeoff Angle  
by K6AER on April 26, 2010 Mail this to a friend!
Tom,

Excellent article. You did a great job tying the concepts together for those who dont work with antennas that much. Many articles tend to target other engineers and forget most hams are not in the business professionally.

With a large stack, 4 elements on 20 meters with a 180 foot tower how much does the phase angle change in relation to the deep nulls with ground reflection? We have a few large arrays out here in Denver and when I played with the antenna selection I wondered about how much ground reflection affected the gain on the top antennas versa the bottom antennas and is their any way to effective correct for that?

I often wonder with using SteppIR beams (two at 100 and 140 feet) if you could adjust the element length of each antenna there by changing the phasing and steering the beam up and down.
 
RE: The Myth of Takeoff Angle  
by WA7NCL on April 26, 2010 Mail this to a friend!
Great article. Now can anyone produce an article debunking the common eham lament about "living in a hole" and confusion of optical horizon, versus radio horizon at HF. For the typical ham on HF with a dipole at modest height of 30 or 40 feet, or a trap vertical ground mounted with 16 radials, what is the "real" situation for radiation pattern below 20 or 30 degrees elevation? Do we really need that location on a hill with the ground sloping away from us?

I think maybe not, but an article concerning the subject would be very useful to the HF ham community. Then maybe we could obsess less on location and spend more time minimizing the antenna losses.

Or maybe I am full of beans.........
 
RE: The Myth of Takeoff Angle  
by N3OX on April 26, 2010 Mail this to a friend!
"A great circle map centered on Wichita, KS will show EU land to the northeast, JA to the northwest, OZ to the southwest and Sud Afrika on the far end of the southeast lobe. Considering that 15 is the better DX band while 75/80 is popular among regional rag chewers I'd say the combo of the two patterns is not a bad compromise....... "

I don't agree.

Here's that map:

http://n3ox.net/files/80_loop_15_az.jpg

Southern Europe and N. Africa are good. Central EU has a deep null on it, as does (worse) Central Africa. You're covering Japan and some of Oceania well but New Zealand's in a null, a lot of the Pacific islands have a weak secondary lobe.

You can probably work 4X OK but you'll have a really hard time with A7.

You'll be OK with Indonesia but disadvantaged to Myanmar.

One whole lobe is useless for anything besides long path Central Asia which you'll never hear over your loud South American lobe :-)

If you rotate the whole thing a bit you get different peaks and nulls but I agree with Tom that it's just got excessively narrow lobes for a fixed antenna. One of those lobes as rotatable single beam antenna ? Sure, sweet. But not being able to move them? Bad news.

73
Dan
 
RE: The Myth of Takeoff Angle  
by K5TR on April 26, 2010 Mail this to a friend!
WA7NCL asks:
"Do we really need that location on a hill with the ground sloping away from us? "

I have long term operating experiences from stations with stacked yagi arrays on tall towers over flat terrain. I also have experience with antennas on a hill top (my current station) and I can without a doubt tell you that you really do want a station on a hill with sloping foreground.

Before I moved to my current location I thought I that big stacks on flat ground would be just as good or better than the hill - and they can be very good - but I will take the hill every time now.

 
Fan Dipole Applicability?  
by KA4KOE on April 26, 2010 Mail this to a friend!
First to say it.....

How does the much-vaunted fan dipole figure into all this learned discussion???


Philip
KA4KOE
 
RE: The Myth of Takeoff Angle  
by W8JI on April 26, 2010 Mail this to a friend!
Thanks all. I also encourage any critiques of things I might have missed, because that is how we really learn things. No one learns much when we all agree, except we agree. :-)

As for the pattern of the 80 meter antenna on 15 meters, it easy to overlay it on a map and see the disadvantage of narrow azimuth lobes. Narrow azimuth lobes with deep nulls adjacent to the lobes are just as big a problem as having narrow elevation lobes with deep nulls.

For example from my QTH, the populated European countries range from 20 degrees to 62 degrees azimuth. It is impossible to not have a deep null on some significant European country with the 80 meter loop on 15 meters. A small Yagi antenna, on the other hand, would easily cover all areas of Europe with more signal level and not have to be rotated.

Australia is another example (even thought there are many more). From here, southeastern Australia can be nearly southwest, and western Australia can be nearly northwest. The range is 248-300 degrees, or 52 degrees of azimuth, just to cover populated areas of mainland Australia.

I think the problem is we have not really looked at the large change in path for different areas of "the same general place", and overlayed our antenna patterns on a great circle bearing map.

Gain in too narrow of an area, especially when deep nulls are right next to the main lobe, is almost always a major hardship.

We might not want it to be so, we might really want the free lunch, but it won't often happen where one antenna is optimum for everything. It might be good salesmanship to sell an antenna as one that "covers all major areas with gain", but it is almost never true.

73 Tom
 
RE: The Myth of Takeoff Angle  
by WB4JZY on April 26, 2010 Mail this to a friend!
Looking at fig. 1 and fig. 2, it seems intuative to me that the "field energy" in total is much more concentrated at lower angles in fig. 2. So, would it not be accurate to say that the fig. 2 antenna is better for long distance work?
 
RE: The Myth of Takeoff Angle  
by K3AN on April 26, 2010 Mail this to a friend!
I believe in the advantage of a hilltop location. I'm near the top of a hill and the ground slopes down as much as a hundred feet in all directions except W through NW to N. That's the direction of the top, which is about 30 feet higher than my house. This is the best DXing QTH I've ever had.

At the top of the hill resides a top-of-the-honor-roll DXer, with just a log periodic on a modest 40 foot tower.
 
RE: The Myth of Takeoff Angle  
by K0BG on April 26, 2010 Mail this to a friend!
It's a great article Tom, and as busy as your are, I'm surprised you found time to write it. I thank you.

One would hope this article would lay waste to all of the "Gain Game" and TOA claims. Unfortunately, it won't as evidenced in the responses.


Alan, KBG
www.k0bg.com
 
The Myth of Takeoff Angle  
by N4DBC on April 26, 2010 Mail this to a friend!
DITTO
This is a really informative article.
73,
Dave N4DBC
 
RE: The Myth of Takeoff Angle  
by N3OX on April 26, 2010 Mail this to a friend!
"Looking at fig. 1 and fig. 2, it seems intuative to me that the "field energy" in total is much more concentrated at lower angles in fig. 2. So, would it not be accurate to say that the fig. 2 antenna is better for long distance work?"

No. That's what everyone's gut reaction is but it's because they can't easily see the total energy radiated.

The vertical actually radiates LESS energy overall, the rest getting eaten in ground reflection processes.

Ground mounted verticals over ordinary dirt only radiate something like 30% to 40% of the applied power to the far field, so comparing their pattern to that of a horizontal dipole (which is much more efficient... 80% ? 90%+ if it's high enough) is not like comparing a dipole to a beam. Switching from a horizontal dipole to a vertical is not a lossless "squish" of the energy into a better direction.

73
Dan
 
RE: The Myth of Takeoff Angle  
by N6RK on April 26, 2010 Mail this to a friend!
I did some quick modeling of a stack of two 2 element arrays. As the article suggests, retarding the phase of the upper one filled in the null above the first lobe. However, I got identical results by ADVANCING the phase of the upper array. It appears that the phases merely need to be different, to break up the cancellation effect that creates the null. I found that I lost 0.5 dB forward gain for a 32 degree phase offset of either sign.
 
The Myth of Takeoff Angle  
by AB2NM on April 26, 2010 Mail this to a friend!
Thank you Tom; excellent article! Mr. Cebik (SK) would be proud.

You've taught me to:
(1) Take a holistic view of antenna characteristics. All measurements (TOA, gain, etc.) must be viewed in context and in totality.
(2) Consider every antenna as a compromise. As Covey said, "Begin with the end in mind" and design your antenna to meet your target area(s).
(3) Pay attention to what isn't there: "Nulls kill" (signal, that is ...)

I look forward to reading more of your work. 73
 
The Myth of Takeoff Angle  
by N3KE on April 26, 2010 Mail this to a friend!
Great article!

I think it is important highlight another important negative impact of the deep nulls found in many high gain azimuthal patterns - namely extreme fading problems. Many folks seem to be under the misguided impression that a tight high gain lobe surrounded by nulls just means they'll hear this part of Europe real well at the expense of this other part of Europe.

In reality DX propagation paths can change their azimuthal arrival angle over short time scales by many degrees. That is, the great circle path to the source is not necessarily the propagation path and the propagation path changes with time. If you have a narrow azimuthal beam with nulls on either side as the arrival angle changes you'll experience deep fades in the channel. Of course the same holds true for the pattern in altitude (the focus of this excellent article).

The old classic "Radio Antenna Engineering" hammers this point home repeatedly and recommends broader and smooth azimuthal and altitude pattern lobes for reliable communications channels.
 
RE: The Myth of Takeoff Angle  
by KE7FD on April 26, 2010 Mail this to a friend!
Very good article; lots of details and the kind of science that we need more of.

I use an 80m loop on 80 - 10 (some 6 too), and found there is quite a contrast between computer models and my "real" antenna. The models do their best to approximate real conditions but they cannot account for everything. According to just about everything I ever read on using a loop, aka, "cloud-warmer", I avoided putting one up for many years; I wanted to work stations farther than a state or two away, and DX was reported not the loops strong point. However, some years ago, I worked a station on the other side of the country where the antenna was... (insert drum roll here) a cloud-warmer. So for several years now my loop has been hanging in the air not more that 25 feet up, zig-zagging between trees and branches, counterweighted against the effects of wind, Turkeys and flying pigs, working just fine. I attribute the fact that it works so well (affording me to work stations in North, Central, South America as well as Europe) to it being NOT very much like a computer modeled loop. It is nested inside trees and on the side of a hill, not square, round or any geodesic shape per se, just spread out as support points allow.

In short, I think modeling is like anything else, it's a tool that helps us plan and hopefully avoid wasting our time erecting air-borne dummy loads. But if your installation is far from ideal, there's a chance and perhaps a very good one that the modeled pattern will be very much off, which in my case is exactly what I needed. Model first, then experiment.

Glen - KE7FD
 
RE: The Myth of Takeoff Angle  
by KA4KOE on April 26, 2010 Mail this to a friend!
"Ground mounted verticals over ordinary dirt only radiate something like 30% to 40% of the applied power to the far field, so comparing their pattern to that of a horizontal dipole (which is much more efficient... 80% ? 90%+ if it's high enough) is not like comparing a dipole to a beam. Switching from a horizontal dipole to a vertical is not a lossless "squish" of the energy into a better direction."

Are you speaking of a vertical with a "decent" radial field? Lots of variables are involved, so please clarify your statement.

I term making judgements based on insufficient information "nailing jello to a tree," something I've learned to do in my daily job as an elektickle injuneer.

Thanks

Philip
KA4KOE
 
RE: The Myth of Takeoff Angle  
by N3OX on April 26, 2010 Mail this to a friend!
"Are you speaking of a vertical with a "decent" radial field? Lots of variables are involved, so please clarify your statement.
"

Yep.

I'm talking about a vertical with a perfect radial field maybe 0.5wl-1wl in radius, excluding only huge impossible copper plating of the entire region where you may have ground reflections. This is probably tens of wavelengths out for really low takeoffs.

The radiation leaving both horizontal and vertical antennas is a combination of direct radiation and reflected radiation off the earth, but at low angle incidence, vertically polarized radiation is partially transmitted into the earth.

In dirt that gets absorbed. But even if the earth were a low loss dielectric, like sand or glass, some of the energy would enter the dielectric medium at the reflection point and head off in a refracted direction inside the medium.

So a lot of the energy that leaves the antenna heading toward the dirt from a vertically polarized antenna actually enters the dirt and is gone. You can improve this for HIGH angles with a ground screen near the antenna, but to improve it at low angles, you basically need to move to the oceanfront.

Horizontally polarized antennas don't suffer the same problem because the reflection and transmission coefficients are different.

This is handled properly in modeling software. If you have some that can calculate the total power leaving the antenna, it's easy to see this. EZNEC has an "Average Gain Test" where it calculates the ratio of the far field radiated power to the source power.

If you use MININEC type ground, the one you can attach wires to, the near field region of the earth is treated as perfectly conducting like you have a really great radial system. The ground reflections are treated realistically and you can see the loss of power as you change ground conditions, even with an essentially perfect radial field.

73
Dan
 
The Myth of Takeoff Angle  
by KB5ZXM on April 26, 2010 Mail this to a friend!
I have be wondering if 2 or more vertical HF antennas spaced apart could be fed in such a way that the out going< and conversely in coming> signal pattern can be made "tune-able". I know that AM broad cast stations have used this technique to shape <restrict> their "pre and post Grey line" and night time radiation pattern, since way before I was thought of. I also recall the equipment used took up a cabinet 7ft tall 5 ft wide and 5ft deep, to tune a <10Kw/5Kw/2.5Kw/ 1/2> kw array of six antennas at 1520 Kc. To be fair Hermetically sealed antenna contractors were in there also .It was set/tuned and Locked UP Tight, By an Elmer Named Merl Saxon from Houston <Circa 1968>. I was 18 then and was employed to take field strength readings late at night.
One of the Monitoring points was in a Grave Yard.>>>>>
Try and Explain that to the Sheriff?
With things we have now like the LDG auto tuners, There must be away, to do this NOW DAYS with less hardware?
This is just a thought.
 
RE: The Myth of Takeoff Angle  
by W7ETA on April 26, 2010 Mail this to a friend!
Great prose, easy to follow.
Nice graphs to augment excellent prose.

73
Bob
 
Nailing Jello to a tree with an AT1011 Vertical  
by KA4KOE on April 27, 2010 Mail this to a friend!
N3OX

Thanks for the clarification. I "play" with a truck mounted AT1011/8-50' radials combined with a 20W manpack. The whole setup is "non-mobile", as the whip is 32' long fully deployed---even at half of it, 16', it must be bent over the front of the truck and tied down.

The most common comments I hear are "I can't believe you're mobile" and "I can't believe you're running 20W".

I use it primarily for quick setup at scouting and public service events. At a full 100W, its amazing.
When one doesn't have trees available, this setup can be handy.

Here's a link-

http://tinyurl.com/yjxasmp

Philip
KA4KOE
 
RE: The Myth of Takeoff Angle  
by N3OX on April 27, 2010 Mail this to a friend!
"With things we have now like the LDG auto tuners, There must be away, to do this NOW DAYS with less hardware? "

You'd have to re-write the code in the autotuner and get two of them to talk to each other or get a computer to talk to them...

Which, actually, W6RMK is working on:

http://home.earthlink.net/~w6rmk/antenna/phased/implement.htm


Sweet!

Still a long way from plug and play or even simple homebrewing, but this sort of thing is possible.

73
Dan
 
RE: The Myth of Takeoff Angle  
by N9VO on April 27, 2010 Mail this to a friend!
Interestingly, just had a friend recently proselytizing the "TOA" value for a VHF vertical for repeater work......
 
The Myth of Takeoff Angle  
by N3ND on April 27, 2010 Mail this to a friend!
The moral of the story:

You can never have too many antennas.

Great article, Tom
 
RE: The Myth of Takeoff Angle  
by AB9NZ on April 27, 2010 Mail this to a friend!
Dan said-
"Horizontally polarized antennas don't suffer the same problem because the reflection and transmission coefficients are different." Wow, new stuff for me. I've heard verticals can be noisier, but didn't know they could be inherently less efficient. Could you guys please point me toward some web or book resources so I can study up on this.
Tnx vy much, de Tom, AB9NZ, Mount Prospect Il.
 
RE: The Myth of Takeoff Angle  
by N3OX on April 27, 2010 Mail this to a friend!
"Could you guys please point me toward some web or book resources so I can study up on this. "

I don't know of any off the top of my head that are any good...

What you need to look at is the power losses associated with incidence near the pseudo-Brewster angle.

A lot of ham discussion focus on the pseudo-Brewster angle when talking about verticals, but they aren't very explicit about losses.

I think probably looking up "pseudo-Brewster angle" in non ham sources might be the most productive. If I find anything comprehensive and clear I'll post it here.

73
Dan
 
RE: The Myth of Takeoff Angle  
by AB7E on April 27, 2010 Mail this to a friend!
WA7NCL,

There is a huge advantage for DX work to be able to beam energy at low takeoff angles, and hillside locations achieve that better than almost any practical antenna installation over flat ground. The work by N6BV in the ARRL Antenna Book shows the statistical probability of the optimum arrival/takeoff angle for various propagation paths, and his program HFTA will show you the calculated antenna pattern for almost any combination of antenna (horizontal polarization only), antenna height, and terrain profile. The absolute accuracy of the data may not be perfect, but the general conclusions are pretty much accepted as being valid. Try it out and you will see ... all of it comes free with the Antenna Book.

I can also tell you from personal experience that living on a hillside lot provides a huge benefit in the direction of the downward slope. I have a high (2000' above my QTH) mountain ridge immediately to the west of me, and the ground slopes downward at a 16% grade for about a half mile to the east (continuing downward at about a 5% slope for another few miles beyond that). On 20m I have a single 4-element yagi at 70 feet, and based upon numerous on-the-air comparisons over several months I am typically one S-unit stronger into the Middle East than a fellow ham with three Force-12 4-element yagis stacked on a 130 foot tower who lives on flat ground about 25 miles away. On the other hand, he kills me in almost every other direction and to the west he is able to give honest S7 reports to stations I can't even hear.

Slope matters ... a lot.

73,
Dave AB7E
 
The Myth of Takeoff Angle  
by K6OK on April 27, 2010 Mail this to a friend!
Excellent article. There's another component to the takeoff angle myth: low wires with high TOAs sometimes outperform the models because of ionospheric refraction and ducting. The ionosphere is not a uniform mirror-like surface. When conditions are right and you work someone 6,000 miles away with a 599 signal using a cloudburner wire, you know those aren't four 1,500 mile NVIS hops. Clearly, your 30 degree TOA got bent on entry into an ion cloud into a 5 degree ray.
 
RE: The Myth of Takeoff Angle  
by WB2NVY on April 27, 2010 Mail this to a friend!
Tom you say
"Our eighty-meter full wave loop antenna (figure 3) has 14.37 dBi gain, about 6 dB more gain than a dipole, when used on fifteen meters. "
A dipole has a theoretical gain of 2.15dBi, so 14.37dBi is 12.22dB more gain than a dipole, not 6dB.
 
RE: The Myth of Takeoff Angle  
by KL7AJ on April 27, 2010 Mail this to a friend!
The real issue isn't really about whether takeoff angle is important or not. Some very simple geometry will show the effect of takeoff angle on ionospheric reflections to a pretty high degree of accuracy.

The real issue is that takeoff angle is dependent on ground conditions FAR beyond the near field of the antenna installation, sometimes on the order of hundreds of wavelengths. You just don't have any control of ground conductivity more than a wavelength or so from your antenna.


Eric
 
RE: The Myth of Takeoff Angle  
by N3OX on April 27, 2010 Mail this to a friend!
"Tom you say
"Our eighty-meter full wave loop antenna (figure 3) has 14.37 dBi gain, about 6 dB more gain than a dipole, when used on fifteen meters. "
A dipole has a theoretical gain of 2.15dBi, so 14.37dBi is 12.22dB more gain than a dipole, not 6dB.
"

A dipole in free space has a gain of 2.15dBi. A dipole mounted at the same height as this loop over real earth will pick up 6dB of ground reflection gain, so it will have a bit over 8dBi absolute gain.

14-8 = 6.

Tom is correct.

73
Dan
 
The Myth of Takeoff Angle  
by KL7AJ on April 27, 2010 Mail this to a friend!
Might also add that another frequently ignored factor is ANGLE OF ARRIVAL. Very few hams give any consideration to which vertical angle incoming signals are arriving from. This is, with a few caveats, reciprocal with the transmit launch angle, and can have major effects on signal to noise at the receiver's location.

At Hipas Observatory we put a great deal of effort into accurately determining angle of arrival from ionospheric signals. (as well as polarization, which a whole 'nother subject). We used an large baseline interferometer which covered a good portion of the Interior of Alaska. Our end goal was to obtain a "topographic map" of the ionosphere over Alaska, and this was quite revealing. The ionosphere is neither even nor horizontal...not even CLOSE....two assumptions that are generally taken for granted by most hams...and unfortunately most commercial models.

When optimizing takeoff angle, you need to know what angle the ionosphere is, as well! We're just STARTING to get reliable information on this!

Eric
 
RE: The Myth of Takeoff Angle  
by KE7FD on April 27, 2010 Mail this to a friend!
Aw Eric, who cares about how the signal is received? All we care about is how loud we are, that's why we turn our mic and compressor gain all the way up when using a full gallon. If the guy on the other end can't hear us, that's his problem; just as long as we can dim the lights in the neighborhood so we can hear the report on the other end say, "...20 over, old man".

Joking aside though, I never thought I had a whole lot of control over that other than using verticals for 2 meter FM...

Glen
 
RE: Dipole Gain  
by W8JI on April 27, 2010 Mail this to a friend!
by WB2NVY on April 27, 2010 Mail this to a friend!
Tom you say
"Our eighty-meter full wave loop antenna (figure 3) has 14.37 dBi gain, about 6 dB more gain than a dipole, when used on fifteen meters. "
A dipole has a theoretical gain of 2.15dBi, so 14.37dBi is 12.22dB more gain than a dipole, not 6dB.>>>>

That is true for a dipole in freespace, but the loop is not in freespace!!!

If we put a dipole over earth at reasonable heights gain of the dipole typically ranges from 8 to 8.5 dBi.

We should always compare to the same reasonable reference system. It is not reasonable or logical to compare a dipole in freespace to another antenna, like the loop, over earth to get the gain comparison.

When we model a 1/2 wave dipole over the same soil and at about the same mean height as the loop, we find the dipole has about 8.3 dBi gain. With the loop at 14.37, we have a dBd gain of 14.37-8.3 = 6.07 dBd gain.

It is a very popular idea, but totally wrong procedure, to simply subtract 2.15 dB from an antenna's gain over earth to obtain dBd gain. Subtracting 2.15 dB only works when ALL antennas being compared are in freespace.

http://www.w8ji.com/antennas.htm

73 Tom





 
RE: Dipole Gain  
by K0BG on April 27, 2010 Mail this to a friend!
It is interesting to me, that no one has mentioned the effects of ground loss with respect to low angle radiation field strength from verticals.

There was mention, however, by Dan, N3OX, I believe, about living close to the ocean as the only means to achieve the effects of far field conductivity advantages. Unless my memory is cloudy, the proposed effect of living close to water is more of an uncluttered horizon, than far field conductivity.

Am I amiss?

Alan, KBG
www.k0bg.com
 
RE: Dipole Gain  
by N3OX on April 27, 2010 Mail this to a friend!
"Unless my memory is cloudy, the proposed effect of living close to water is more of an uncluttered horizon, than far field conductivity. "

No... you can see that in EZNEC too ... The conductivity and dielectric constant of saltwater both contribute to good low angle gain.

Now, this does depend on your definition of "close to saltwater" I'm talking about island DXPedition close where you actually set up the vertical on the shore.

The vertical on a cliff a mile from the sea doesn't have the same effect: the ground reflections mostly or entirely happen from the shore, which isn't saltwater. Then the "clear horizon" and "sloping terrain" improvements would be the only ones.

But if you set up so that your vertical is within a wavelength or two of the saltwater, you'll get a huge advantage out in the direction of the ocean.

Here's a plot I made in EZNEC for 5 degree takeoff from a ground mounted vertical using a linear boundary between two ground media. If this is a compass rose with North at the top, firing "West" is over ordinary dirt and firing "East" is out toward the "ocean"

http://n3ox.net/files/20m_ocean.jpg

The different plots are different distances in wavelengths from the "land/sea" boundary. "minushalf" is actually like installing the antenna out on a dock or something... it's actually a half wavelength out from shore. Everything here is perfect flatland ... this is only the effect of the ground reflections for this elevation angle occurring on the saltwater instead of dirt.

I think this was actually a vertical dipole with the tip a few feet above land... now I don't remember. The antenna height matters to these distances, because higher antennas can "see" the shore further out for a given takeoff angle.

If I had done this for an elevation angle lower than 5 degrees, you would have to back away more than eight wavelengths to have the ocean stop mattering.

The shore-side vertical is a killer antenna. There's some good info including some experiments here:

http://www.k2kw.com/

73
Dan
 
RE: The Myth of Takeoff Angle  
by NZ5L on April 27, 2010 Mail this to a friend!
Results here have been very good with antennas at .3. .4, and .5 wavelength, so I "theoretically" don't have a very low TOA. I have often wondered why the results on these extended-double-zepp dipoles were consistently better than with the highly rated GAP vertical, which presumably "doesn't need radials" (I added six anyway).
Thanks to this article, I now see things were not as simple as I had first imagined. There is definitely a place for simple wire antennas at heights of 25 to 35'.
Very informative article.
 
RE: The Myth of Takeoff Angle  
by KB5ZXM on April 27, 2010 Mail this to a friend!
About phased verticals, I was just wondering if the average Hay seed , could do something with 2 or more vertical antennas , coils and capacitors, to achieve directive gain.As well as received gain.
Tomorrow Wednesday night and Thursday night <from like 5pm to 8pm> some friends and I were gonna play with some <omni> NVIS antennas on 75 and 40 meters. Possibly check in to the nets on 3.873Mc and 7.285 In Texas.
If my health permits. 73's de KB5ZXM
 
RE: The Myth of Takeoff Angle  
by WG8Z on April 28, 2010 Mail this to a friend!
Ropes and Pulleys vs Smoke and Mirrors ?
Once again Tom a great article.
73
Greg
 
RE: The Myth of Takeoff Angle  
by KL7AJ on April 28, 2010 Mail this to a friend!
ZXM:

Of course! Phased verticals are very effective on low bands. I highly recommend ON4UN's book "Low Band DXing" for everything you'll ever need to know about phased verticals...and a few things you probably won't. :)

Eric
 
RE: The Myth of Takeoff Angle  
by KC8VWM on April 28, 2010 Mail this to a friend!
We typically assume the big mirror in the sky is always in a fixed position with respect to our transmitted signal. But it isn't because the ionosphere consists of charged ionized particles and gas that are in a constant moving and evolving state.

The reality is signals arriving at the ionosphere experience dispersion. Therefore, taking into account an antenna's "fixed" TOA may not even be the right approach.

Attempting to calculating TOA is the technical equivalant of trying to accurately reflect a flashlight beam from a constantly moving mirror.
 
RE: The Myth of Takeoff Angle  
by ZENKI on April 29, 2010 Mail this to a friend!
Another good explanation was published in QST 2009 October, by K4IA.

His concept of "constructive and destructive" between the ionosphere and ground makes a lot of sense as well.
Its a good article for those wanting to know the "best height" for an antenna
 
RE: The Myth of Takeoff Angle  
by W4ZV on April 29, 2010 Mail this to a friend!
Here are a couple of related webpages regarding Takeoff Angles:

http://users.vnet.net/btippett/terrain_&_toas.htm

This takeoff angle stuff really does work. The above system holds all 6 current USA records on 10 meters (CQ WW, ARRL DX and CQ WPX...both modes). It also regularly places (QSO totals) with the top Multi-Multi stations who have several other advantages (e.g. packet, multiple operating positions, larger antennas, etc).

http://users.vnet.net/btippett/w3cra.htm

The above story about W3CRA solved the mystery of Frank's legendary DX location from over 50 years ago. Many old timers will recall hearing Frank's 20m backscatter signal working Asian after Asian that you couldn't hear a trace of!

73, Bill W4ZV
 
RE: The Myth of Takeoff Angle  
by OE6ZH on April 29, 2010 Mail this to a friend!
Tom, you asked:

"Could you guys please point me toward some web or book resources so I can study up on this. "

To the best of my knowledge, the answer is Les Moxons (G6XN) classic "HF Antennas for all Locations" Chapters 6 and 10.

73, Peter
 
RE: The Myth of Takeoff Angle  
by W4ZV on April 29, 2010 Mail this to a friend!
Tom AB9NZ: Could you guys please point me toward some web or book resources so I can study up on this.

Recent editions of the ARRL Antenna Book include information and software (CD-ROM) for N6BV's HF Terrain Assessment program (HFTA). This is the tool used to model take-off angle effects of stacking, height above ground and terrain effects customized to your antennas and QTH.

Unfortunately ARRL's new website isn't working well or you could read more about it there. Meanwhile you can Google "N6BV HFTA" which should give you some good info (remembering that all ARRL links are currently broken). This is the best resource IMHO and was used to generate the plots on the links I previously posted.

73, Bill W4ZV
 
RE: The Myth of Takeoff Angle  
by KL7AJ on April 29, 2010 Mail this to a friend!
In Part IV of my series "Plasma Physics for the Radio Amateur" I will also address the "Myth of the Great Circle Path" The ONLY time an H.F. signal follows the Great Circle path is when the Earth's magnetic field is neutral....which is almost nowhere. The presence of a magnetic field all but guarantees that the signal will NOT follow the great circle path. This is VERY obvious in Alaska, but not so noticeable in the lower 48.

Eric
 
RE: The Myth of Takeoff Angle  
by KC8VWM on April 29, 2010 Mail this to a friend!
Where can I read about that?

Is there a link?
 
RE: The Myth of Takeoff Angle  
by KL7AJ on April 30, 2010 Mail this to a friend!
VWM:

Here's a great place to start:


http://ulcar.uml.edu/digisonde_dps.html
 
RE: The Myth of Takeoff Angle  
by KC8VWM on April 30, 2010 Mail this to a friend!
Thank you!
 
RE: The Myth of Takeoff Angle  
by W9AC on May 1, 2010 Mail this to a friend!
> "The real issue isn't really about whether takeoff angle is important or not. Some very simple geometry will show the effect of takeoff angle on ionospheric reflections to a pretty high degree of accuracy."

The real issue is knowing the entire vertical elevation profile and not just the TOA. The TOA only describes the one elevation point of maximum field strength.

Paul, W9AC
 
RE: The Myth of Takeoff Angle  
by W8JI on May 2, 2010 Mail this to a friend!
That's right Paul, and that was my primary point. TOA is meaningless. It is a single angle in space where the peak radiation occurs.

We have to look at the entire pattern and the absolute gain over the desired range of angles, not just match the TOA to what we think is the most likely angle.

We can have an antenna with a TOA of 10 degrees that has more radiation at 2 degrees than an antenna that has a TOA of 2 degrees. Worse yet, if the useful angle ranges from 2-8 degrees and we have a TOA of 2 degrees with a big old null at 8 degrees, all we assure ourselves is the system will be much less reliable than one that has a little less gain over that range but no deep nulls.

73 Tom
 
RE: The Myth of Takeoff Angle  
by W5WSS on May 4, 2010 Mail this to a friend!
Having more major lobes increases the coverage but we seem to trade a more major and minor lobe condition with singularity in our thinking as we consider toa. We must ask ourselves where does the bulk of the power manifest? Any attempt to thicken the major lobes while thinning the minor lobes is worth further investigation. Tom has very eloquently taught some, reminded others to remember that antenna proximity to Earth surface is underestimated as an integral part of the global coverage we are striving for.
 
RE: The Myth of Takeoff Angle  
by K7DAA on May 4, 2010 Mail this to a friend!
Tom:

Excellent article, and very informative.

Ahhh...now one of these days I hope to live somewhere that would allow me to actually stack Yagis. At my present QTH, I can't even put up one, let alone four.

In the meantime, I can at least live somewhat vicariously though your activities!

Thanks for all the great food for thought (and dreams)!

73,

Dave - K7DAA
 
RE: The Myth of Takeoff Angle  
by N1LO on May 6, 2010 Mail this to a friend!
Hi Tom,

So it's having a range of TOA's (pleural) in useful directions that are important, not "TOA" (singular).

I think it's important to hit this point on the head and say that the "TOA" (singular) is just one angle calculated by the analysis software for *the single strongest lobe of gain* only!

Broader lobes of gain are more useful, as you point out.

I have found one simple solution that helps fill in the nulls for the everyday workhorse 80m horizontal doublet antenna: put it up as an inverted U. You put a corner in each leg and let the remainder hang down as vertical tails. Patterns in EZNEC are encouraging, with far fewer nulls as compared to the traditional flattop rigging.

The first configuration I have been playing with has a 32' tail on one side and a 22' tail on the other.

Every antenna is a compromise, of course, so as someone already pointed out, it's good to have a variety.

Thanks for taking the time to write the article, Tom.

Regards,

--...MARK_N1LO...--
 
The Myth of Takeoff Angle  
by NZ4O on May 8, 2010 Mail this to a friend!
I use a horizontal loop made up of 300 feet of #14 stranded wire. It's in a square configuration with each leg 75 feet in length. Each corner is up at 35 feet and it's fed with 40 feet of 450 ohm window line to a balanced link coupled antenna tuner.

It works very well on 80-10 meters and I regularly work all continents with it. I have dipoles and vertical up also and the loop always wins.

When I modeled the loop in EZNEC v5.0 I see the deep nulls that Tom is talking about but in practice the nulls don't present a problem at this particular QTH.

I have a web page about the antenna at http://www.wcflunatall.com/nz4o15.htm.

Just my .05 cents due to inflation.

BTW I've known Tom for 21 years and no one knows more about antennas then he does.
 
RE: The Myth of Takeoff Angle  
by W4EF on May 9, 2010 Mail this to a friend!
This thread reminds me of that old joke about the British and the Americans being two peoples separated by a common language :-)

When someone refers to "TOA", they could be talking about the elevation angle of the incoming signal (and by reciprocity the elevation angle of the outgoing energy that reaches the distant station we are communicating with) or they could be talking about the elevation angle corresponding to the peak gain in the elevation pattern of a particular antenna. I believe Tom's article was referring to the latter usage of the term TOA whereas W4ZV in his post was addressing the former usage of the term TOA.

What it really comes down to is determining which range of elevation angles are relevant to the communication path of interest and then matching that range up with an antennna system which will put down maximum possible field strength across that range of elevation angles without significant nulls inside the desired range.



Hilltops are great (wish I owned one) because they allow one to put a very high field strength into very low elevation angles with modest tower heights compared with flatlanders who have to spend a small fortune on tall towers to achieve a high field strength into that same range of very low angles (unless they are lucky enough to live on a saltwater beach). It has been my experience that these really low angles can be especially relevant when operating close to the MUF which unfortunately these days isn't all that high in frequency most of the time :-)

This was a great article and great thread. Thanks, Tom.

73, Mike W4EF.........
 
The Myth of Azimuth Angle  
by K6GC on May 9, 2010 Mail this to a friend!
Tom, Great Article !!

Let me point out to all that missed it in the Subject line, I substituted Azimuth for Take-Off.

They are quite similar.

It always amazes me how many hams are eager to put up a yagi at great expense and effort and lose all that azimuth their old vertical had.

Especially if that vertical was a ground plane (GP) with an excellent view of a low horizon !

A GP like this is an antenna to be feared !

It has very little problem with take off angle, only eliminating high angle signals that tend to have very great mV/M vs DX with low mV/M signals, a decided advantage if you're after DX. Not only that, you don't need rotors and you can hear 360 degrees. When you scan the bands for a DX signal, they stand out over the high angle signals, in all directions. The band is often open in multiple azimuths, and the yagi boys miss this advantage.

Let me mention cost.

This is a major consideration to many of us.

Putting up a yagi on a nice tall tower is an expensive endeavor. Not so, a GP

Get a 45' tower, a multiband vertical of 30' or so and you have 60' of actual height. String some radials on that and remember instead of a yagi, you have much less wind load. This equals a much cheaper tower. Use some compression insulators and it is cheaply guyed.

A big savings in money, and a big savings in Azimuth

73, TR

K6GC, The Great Circle Station
in the State of Jefferson
http://www.ijpr.org/Page.asp?NavID=1033
 
RE: The Myth of Azimuth Angle  
by WA7PRC on May 12, 2010 Mail this to a friend!
Tom,

Thanks for a well-written article. I learned something new today. Unfortunately, being an OT, I forgot two things! ;-)

73,
Bryan WA7PRC
 
The Myth of Takeoff Angle  
by WA1UFO on May 17, 2010 Mail this to a friend!
I like this! Separates the Crap from the Candy!
 
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