| The "classic" DK7ZB-Match and principles | Full symmetrical 28-Ohm-Match by DF9IC | "Sperrtopf" by DF7DJ |
The DK7ZB-Match for Yagis
It is a well known phenomenon, that the radiators resistance in a Yagi-structure drops down by adding parasitic elements to a dipole as a radiating element.
For the VHF-Bands (50-50.5MHz, 144-146MHz, 430-440MHz) a radiation resistance of 25-30Ohm has the best balance for gain, back- and sidelobes, bandwidth and SWR at tenable losses.
The classic match for that resistance/impedance is the Gamma-Match. The Beta-Match is a good choice as well, but realisation on VHF is difficult for homebrewing. For that reason a new simple, easy to built match was developed in 1995 for Yagis with resistitive loads of 12.5, 18 and 28 Ohm.
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The basic principle is a quarter-wave line of two coax-cables in parallel. One end of the line ist connected to the radiator, the other to a socket, which is grounded at the metallic boom and has two functions: 1. It transforms the resistance of the radiator to the 50Ohm of the feeding line. 2. This line is a simplified coaxial sleeve balun to avoid sleeve-waves on the braid of the cable running to the station. |
Antennas which that match have some advantages:
- You do not need a folded dipole, the length of a normal dipole can be corrected easier.
- The radiation pattern is very clear.
- Simple mechanical solution for the match with the coax-cables.
- For shortwave and 6m you can wind the cable to a choke with an additional suppression of sleeve-waves
With usual coax of 50, 60 or 75Ohm the following cases are interesting:
| Radiation resistance Z of the Yagi |
Lambda/4 Coax-cable |
Impedance of the line |
|
12.5 Ohm |
2x50Ohm parallel | 25 Ohm |
| 18 Ohm | 2x60 Ohm parallel | 30 Ohm |
| 28 Ohm | 2x75 Ohm parallel | 37.5 Ohm |
12.5-Ohm: Good for high-gain yagis with small bandwidth in the 6m- and 2m-Band, losses tenable
18-Ohm: Can be used on 6m and 2m, medium bandwidth
28-Ohm: Best balance for all antenna parameters on 2m and 70cm
Important hint:
The 2x75-Ohm-coax-cables for the match 28/50Ohm must be absolutely parallel. If you mount them above the boom, take a piece of insulated material (uncritical) of 2-3mm thickness between the cables and the boom.The high voltage at power >300W RF can cause a short circuit or arcing between screen and boom!
What can happen, you see below (the occasioner will not be mentioned, hi....):
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A lot of antennas were built for the 6m, 2m and 70cm-Band. The performance of the Yagis is excellent. Meanwhile stations in many countries are using Yagis with the DK7ZB-match.
Left: The vertical 3-Element-Yagi is built by Peter, PA4PS for use on a 202,5-MHz-link to the FM-Radiostation Havenstad FM in the Netherlands. The Yagi is a 28-Ohm-type with the DK7ZB-match. Seems to be the first time that a DK7ZB-Yagi is used by a commercial station.... |
1. Long-Yagis for the 2m-Band with 28-Ohm-technology got the first price in the technical competition of the German Ham-Magazine "FUNKAMATEUR" 1996.
2. See the VE7BQH-List for 2m-Yagis, the DK7ZB-Yagis have good gain and pattern.
3. The DK7ZB-Match got the first price (part "ANTENNAS") in the construction-competition at the German VHF-Convention in Weinheim 2000.
4. Meanwhile a lot of Yagis with the DK7ZB-Design are used in several countries, reports about the yagis can be found in many HAM-Magazines and the internet. Many VHF-/UHF-contest-crews are working with stacked arrays of these Yagis.
The DK7ZB-Match is working quite well in practice: DO1IR built up the 6-Element-144MHz-Yagi and corrected only the length of the radiator-element.
With an HP-Networkanalyzer HP 8713-C he tested the antenna. Look for the result: The reflected wave is damped >50dB! Compare that with other commercial built Yagis.....
