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.
 |
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.
With 2x75Ohm-cables 28 to 50Ohm.
With 2x50Ohm-cables 12.5 to
50Ohm.
With 3x93Ohm-cables 18 to 50Ohm
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 or any exotic radiators like loops,
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,
75 or 93 Ohm the following cases are interesting:
| Radiation resistance
Z of the Yagi |
Lambda/4 Coax-cable |
Impedance of the line |
|
12.5 Ohm |
2x50 Ohm parallel |
25 Ohm |
| 18/19 Ohm |
3x93 Ohm parallel |
31 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
The 28-Ohm-match with
2x75-Ohm-CATV/SATV-cable shown above will handle 750Wtts RF on SSB/CW on 2m and
>1KW on 50MHz. Do not bend the loop with a to small diameter! The V (Velocity
of propagation) with these cables is in the range of 0,8-0,85, most cables have
a V=0,82. Use a good quality, in the last time a lot of
inferior cables with a steel braid are on the market.
With 2xRG59 (good MIL-quality!)
you can handle 400Wtts on 2m and 1 KW on 50MHz. The
given or calculated lengths is related to the shield with short leads.
There are people criticysing the
medium (28 Ω) an low
(12,5 Ω) impedance Yagis and have speculations about any mystic losses in
the match for that feedpoint impedances. But the same people use stacking
harnesses and splitters exact with such impedance points of 25 Ω and 12,5
Ω at the connections for matching. Nobody believes that this has any
disadvantage.
Paul,
GW8IZR, has made an independent measuring of the losses in such a match
12,5 Ω to
50 Ω with the following test circuit:
With two of such transformation
sections back to back he measured the losses against a halfwave piece of
straight coax. His results for 2x RG58 (a more lossy cable you will not find...)
and 40 MHz:
Upper graph: A piece of 50-Ω-coax
with the length of lambda/2
Lower
graph: Two matching sections parallel back to back
The difference is -0,025 dB. Do
you think we should talk about that? If you use better cables the difference
will be less. If you construct a 50-Ω-choke
for feeding a 50-Ω-Yagi or a choke with 2x75-Ω- or
2x50-Ω-cables in parallel makes no difference in the losses!
Important hint:
The 2x75-Ohm-coax-cables (or 2x50-Ohm) for the match
28/50 Ohm (or 12,5/50 Ohm) 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!
|
|
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.....
