The schematic principle of a 3-El.-Yagi
| Theory of the DK7ZB-Match (new) | The "classic" DK7ZB-Match | Full symmetrical 28-Ohm-Match by DF9IC | "Sperrtopf" by DF7DJ |
Bandwidth of some
Amateurbands
|
Band |
Frequency Range |
Bandwidth
in % |
|
70cm |
430-440MHz |
2,3% |
|
2m |
144-146MHz |
1,38% |
|
6m |
50,0-50,3MHz |
0,6% |
|
10m brought |
28,0-29,7MHz |
5,9% |
|
10m narrow |
28,0-28,8MHz |
2,81% |
|
15m |
21,0-21,45MHz |
2,1% |
|
20m |
14,0-14,35MHz |
2,5% |
For a 3-El.-Yagi the following parameters are interesting:
1. Gain: High gain means narrow bandwidth, bad F/B-ratio and low impedance
2. F/B: High F/B means lower gain
3. Bandwidth/SWR: Great bandwidth means low gain and high impedance
4. Impedance: - High impedance means great bandwidth, but low gain (50 Ohm)
- Low impedance means narrow bandwidth, but high gain (12,5 Ohm)
- Medium Impedance means medium bandwidth and medium gain (28 Ohm)
|
The schematic principle of a 3-El.-Yagi |
|
|
50 Ohm |
28 Ohm |
12,5 Ohm |
||||||
|
SWR
<1,5 6,7% |
SWR
<1,5 2,9% |
SWR
<1,5 1% |
|||||||
|
Gain
5,0dBd, V/R 20dB |
Gain
5,6dBd, V/R 24dB |
Gain
6,25dB, V/R 26dB |
|||||||
|
Pos. |
Length |
Diff. |
Pos. |
Length |
Diff. |
Pos. |
Length |
Diff. |
|
|
Ref |
0mm |
1041mm |
+4,8% |
0mm |
1026mm |
+4,6% |
0mm |
1022mm |
+4,5% |
|
Rad |
350mm |
993mm |
- |
275mm |
981mm |
- |
220mm |
978mm |
- |
|
Dir |
600mm |
890mm |
-10,4% |
600mm |
900mm |
-8,25% |
600mm |
923mm |
-5,6% |
|
SWR 50-Ohm-Type |
SWR 28-Ohm-Type |
SWR 12,5-Ohm-Type |
As you see ist the best impedance for the bandwidth of the most bands 25-30Ohm and gives you a good balance of gain and F/B. This is the reason for the 28-Ohm-constructions.
But what is with long Yagis? To understand that we look at the pictures below. A very good indicator for a Yagi structure is the element current profile.
Element currents in a 5-El.-28-Ohm-2m-Yagi with 2,00m-Boom
The highest current is in the radiator, from the center the currents in the wave structure of the yagi are falling to the end slightly. If you have elements with lower currents in the structure, there is a failure in the contruction! This is the case for many older designs and the reason for bad patterns.
Now we try to change the design to an impedance of 50 Ohm. Therefore we need an additional "match-element" very close to the radiator. This element is not working as a classic director, it is an "open-sleeve-element", which has now the highest current in the yagi system.
We have replaced the one radiator (2) by two new elements ( 2 and 3). This system is very critical for the impedance trend and the pattern of the yagi. Why do that? My conclusion is the 28-Ohm-principle. All high-gain-long yagis with 50Ohm-impedance have a significant lower bandwidth with slighly less or equal gain in comparison with the 28-Ohm-Yagis.
Element currents in a comparable (good designed!) 6-El.-50-Ohm-2m-Yagi with 2,00m-Boom
A lot of 50Ohm-Yagis have a bad element current profile!
 |
One of the best yagi designs for 2m I know is the 12-El.-28-Ohm-Yagi. Please look for the extrem clear pattern (no big backlobe and good sidelobes), gain 14,28dBd, bandwidth for SWR < 1,5 is 2 MHz! You will not find any comparable yagi with the same boomlength, gain, pattern and bandwidth. It is impossible to match the antenna with the described match-element for a 50Ohm-direct feed. This is the reason why a lot of contest groups, EME-amateurs and severe DXers use homemade DK7ZB-yagis. |
Horizontal pattern of the 12-Element-28-Ohm-Yagi at 144,3 MHz
You can earn a box with 20 bottles of good German beer if you reach that aim to construct any comparable 50-Ohm-Yagi with 8m-boom, 14,28dBd gain, 2MHz bandwidth and such a clean pattern!
