Yagi Design - W/O Computer Modeling ?

[WA8FOZ]

Modeling can be useful for unorthodox designs, and for multiband antennas. My Force 12 C3S places 3 two-element yagis (for 10, 15, and 20m) on one boom. Works well for a compromise antenna. Modeling made the design of this antenna much easier.

[KO6WB]

There is no doubt that computer modeling can make it much easier to fabricate an antenna and have all variables defined. You can change things and then change them back again and see what difference it makes. But to consider the classic designs as inferior to computer aided designs is not always correct. There have been well defined antenna perimeters that will always dominate the design. It has become a problem that some operators have that they cannot put an antenna if the computer says it has .1 db difference form what it should have under perfect operating conditions. This is information overload. Instead of putting up an antenna and trying it the operator may spend a lot of time trying to find that perfect antenna. It is not a bad thing to have an antenna that is 1.5 to 2 db down from the next antenna. The effort to improve in that level is not a good application of resources. Even 3 db greater is not that great. So, quit over thinking and get something on the air.
73
Gary

[W8JI]

I have a friend literally pulling his hair out because he waned to make a 3 element 10 meter Yagi. Back in the Old Days, we simple made all the element spacing the same (from .15 to .2 wavelengths) and simply cut the reflector 5% longer, and the director/directors 5% shorter.

Here is something interesting.

The ideal, or near ideal, length of a reflector is almost perfectly at self-resonance for the lowest frequency of the Yagi.

Let's consider a wire element.

If we cut a dipole and tuned it for self-resonance at the lowest frequency, and shorted it, and then added a driven element, the reflector would be near perfect as a reflector at reasonable spacings. A reflector is typically within about 1% of self-resonance when at maximum gain, 5% extra makes it too long, gain is rolling off pretty fast at that length.

The reason the reflector element sometimes can wind up about 4-5% shorter is mostly because the driven element is mis-tuned by the reflector, and the driven element has to be shortened slightly from self-resonance to be made resonant. It isn't because the reflector has to be made 5% longer.

If I add a director and make it 5% shorter than true self-resonance, it is mis-tuned also. The director barely improves pattern. It actually needs to be around 2% shorter than self-resonance.

So the old 5% rule makes a pretty sloppy Yagi. Using the 5% rule, a three element Yagi has about the same gain as a properly tuned 2 element Yagi and a F/B of about 20 dB. The user would think it was working OK, because he would see F/B, but the gain would not be better than a two-element.

If we make the director about 2% shorter than resonance, and space it much closer than the reflector, the system has more gain. Not only that, we can get 30 db F/B, or more, at the optimum frequency.

The spacing also does not wind up the same. The director is closer, but the reflector is typically at .15 to .2 wavelength.

If we look at Yagi's now, we will see a definite improvement from old rules of thumb. We might feel like people were smarter in the 50's or 1960's, like we do with building cars, but it is mostly nostalgia at work. We can do things a lot better now. That's why I take my old designs and usually wind up throwing an element away, like my 5 element 20 meter 205BA Hygain beams, that I was always happy with in the 70's, are now all 4 elements and work better.

The real optimum tuning of a Yagi is about 1% or less low for the reflector than self resonance, and about 2% higher than self-resonance for the director at optimum spacings. The driven element needs to be slightly shorter than self-resonance.

73 Tom

[WB2WIK]

Is W1BX still with us Steve ?

Nope. Les died in the early 1990s.

[KA7NIQ]

Here is something interesting.

The ideal, or near ideal, length of a reflector is almost perfectly at self-resonance for the lowest frequency of the Yagi.

Let's consider a wire element.

If we cut a dipole and tuned it for self-resonance at the lowest frequency, and shorted it, and then added a driven element, the reflector would be near perfect as a reflector at reasonable spacings. A reflector is typically within about 1% of self-resonance when at maximum gain, 5% extra makes it too long, gain is rolling off pretty fast at that length.

The reason the reflector element sometimes can wind up about 4-5% shorter is mostly because the driven element is mis-tuned by the reflector, and the driven element has to be shortened slightly from self-resonance to be made resonant. It isn't because the reflector has to be made 5% longer.

If I add a director and make it 5% shorter than true self-resonance, it is mis-tuned also. The director barely improves pattern. It actually needs to be around 2% shorter than self-resonance.

So the old 5% rule makes a pretty sloppy Yagi. Using the 5% rule, a three element Yagi has about the same gain as a properly tuned 2 element Yagi and a F/B of about 20 dB. The user would think it was working OK, because he would see F/B, but the gain would not be better than a two-element.

If we make the director about 2% shorter than resonance, and space it much closer than the reflector, the system has more gain. Not only that, we can get 30 db F/B, or more, at the optimum frequency.

The spacing also does not wind up the same. The director is closer, but the reflector is typically at .15 to .2 wavelength.

If we look at Yagi's now, we will see a definite improvement from old rules of thumb. We might feel like people were smarter in the 50's or 1960's, like we do with building cars, but it is mostly nostalgia at work. We can do things a lot better now. That's why I take my old designs and usually wind up throwing an element away, like my 5 element 20 meter 205BA Hygain beams, that I was always happy with in the 70's, are now all 4 elements and work better.

The real optimum tuning of a Yagi is about 1% or less low for the reflector than self resonance, and about 2% higher than self-resonance for the director at optimum spacings. The driven element needs to be slightly shorter than self-resonance.

73 Tom

Wow, I had a friend Bud Murphy, who is now a S/K , absolutely Rave about his Force 12 Tribander. I looked at that thing, and saw it was only 3 - 2 element full size beams on a common boom.
He swore by that antenna, and Bud had previously had other Tribanders.
That is very interesting that Computer Modeling really has drastically improved Yagi design so much.

I used to own B&W 801 Matrix series 2 speakers. The crossover of the Matrix 801 series 2 has about 55 parts in it.
The B&W Matrix series 3 has the same speakers in it, but a computer designed crossover that has less then 1/2 the amount of parts!
The Matrix Series 3 sounds a lot better, because the audio signal is running through less parts.
Audio is one of my other hobbies, and computer design has made "cut and try" crossover design pretty much obsolete.

What we need now is a way for the human brain to directly connect to a computer, Hi Hi !
I went to a lecture at the University of South Florida here in Tampa once. The lecturer was talking about combining human and artificial intelligence.
He was asked if he believed in almighty God ?
The guy replied "Yes, I believe in Almighty God, and I intend to build him"

[N8WWM]

I'd got to MFJ's site, look up the manual for the Cushcraft 10-3, and go from there. It may not be the IDEAL and PERFECT antenna, but a lot of QSO's have been worked via the 10-3.