Körner 19.4

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k6sti
Posts: 63
Joined: 20 Oct 2014 17:27

Körner 19.4

Post by k6sti » 15 Jun 2016 13:05

Peter Körner has revised his 19.3 Yagi design. The 19.4 further reduces backlobes over the rear quarter-plane. The boom is 0.44 meters longer and forward gain is slightly lower above 98 MHz.

I've added the 19.4 to my 19.3 writeup. I include several performance comparison plots and graphs.

http://ham-radio.com/k6sti/korn193.htm#19.4

Brian

Peter Körner
Posts: 33
Joined: 21 Oct 2014 10:05

Re: Körner 19.4

Post by Peter Körner » 20 Jun 2016 18:25

To give some background to the new 19.4 antenna design, it is the combined result of trying several ways of improving the 19.3, and often sharing and comparing modeling results with Brian Beezley. Being curious, I like to try various design ideas, and also learn how the design works, or could possibly be improved. One such idea was adding dipole tails or wings to the ends of the horizontal folded dipole. The best result was achieved if these tails were arranged rear facing and the dipole was retuned. This idea also provided an interesting opportunity to choose which back lobes to suppress the most, the main 120°-240° part, or 90°-110°+260°-270° parts. There turned out to be two alternatives:

First alternative: Priority on suppressing the back lobes within 120°-240°, but tolerate small remaining side lobes at 90°-110°+260°-270°. For the antenna design this means a “too long and too widely separated” horizontal folded dipole and the rework of directors and reflectors. The end result is a combination of high gain and wideband F/R around 40 dB within the main rear angles - like the 19.4.

Second alternative: Suppressing the lobes within 90°-110°+260°-270°, but with slightly less well suppressed back lobes between 120°-240°. This would basically mean keeping the horizontal folded dipole of the 19.3, and mainly altering the directors.

Since the angles 90°-110°+260°-270° are very close to the base of the front lobe, it makes little sense to make a big effort to suppress tiny lobes within these angles, while leaving remaining back lobes in the main rear directions between 120°-240°.

Adding rear facing dipole tails to the ends of the horizontal folded dipole, which as I said, is one interesting way to enhance F/R, but it tends to make the side lobes bigger, especially at the upper frequencies. I decided not to proceed further with these rearward dipole tails, since I could achieve better performance by widening the separation of the horizontal folded dipole parts. It took lots of interesting tweaking, often looking more at the patterns than the calculated numbers, before arriving at the 19.4 design. Incidentally, the 19.4 side lobes in the vertical plane are also better suppressed than for the 19.3.

The 19.3 and 19.4 (like the 15.11 and 15.12) share the same wideband design philosophy: widening the gain maximum and placing it as much towards the middle of the band as possible. The 19.4 gain curve is somewhat flatter than for the 19.3, with the maximum gain occurring at 100.5 MHz. This is clearly lower than for designs with less actual bandwidth. The end result is higher average gain.

The reflector rework was very interesting, and luckily resulted in a rather easy-to-build design, since four of the reflectors are positioned in the same vertical plane. Due to moving the middle reflector to the rear, the total boom length for the 19.4 is somewhat longer compared to the 19.3. Adding more reflectors to make the “reflector wall” bigger would just make performance worse.

For the optimization work, I used 90° rear angle (135-225°) to be able to separate the different back and side lobes. I allow some F/R drop close to the band ends, since the small remaining back lobes are narrow, and I could get a performance benefit for the remaining >90% of the bandwidth.
On http://ham-radio.com/k6sti/korn193.htm, scroll down to find the 19.4, and you can click repeatedly on the last plot to see F/R development as the rear angle is changed…

Peter

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