G0KSC SC6-4-6D 6el 50Mhz/70Mhz Dual Band Yagi Antenna with a 3.2 Metre Length Boom
About this Antenna
This is what is known as an 'interlaced' Yagi as both a 50Mhz beam and a 70Mhz beam (there are no common elements) are interlaced and sitting on the same boom. This presents a huge advantage over beams which use common elements with traps a coils. Firstly, these traps and coils present a loss within the antennas elements. In addition, the spacing of elements is a compromise to make a 'best job' for both bands.
None of the 70Mhz elements are physically connected to the feed point. The 70Mhz antenna is feed by it's closeness to the 50Mhz elements. i.e. the driven element of the 70Mhz Yagi sitting closely to the 50Mhz driven. It is a little more technical than that but so as the masses can understand how it works. The antenna works as a 3 element beam on each band.
The Interlaced OWA design removes these restrictions from the mix. With no losses from matching and just one feed point you have a winning formula!
Interlaced, single feed point Yagi's are hard to model and there is some interaction between the elements. As a result, I have had to compromise a little more than one normally would have to in order to achieve my performance directives. In this case, I have sacrificed a little Front to Back ratio in an attempt to maintain as flat an SWR curve as possible. There are a couple of reasons for doing this. One, of course is to give as much bandwidth as possible but mainly, as with all antennas on this site, if the SWR curve is wide and low, any small mistakes in sizing and spacing by the builder will not make a tremendous impact on the antennas overall performance.
The image below shows how the antenna looks. The 50Mhz and 70Mhz antenna elements are easily seen apart. These images are produced with EZNEC+. The red circle indicates the point of feed for the coax connection. one feed for both bands.
The SC6-4-6D Built and installed by PA0VAJ
Sizing and spacing are as follows:
- Ref 6 = 0
- Ref 4 = 1.325
- Driven 6 = 1.908
- Driven 4 = 2.028
- D1-6 = 3.135
- D1-4 = 3.186
Element sizes per element half:
- Ref 6 = 1.454
- Ref 4 = 1.041
- Driven 6 = 1.408
- Driven 4 = 1.001
- D1-6 = 1.293
- D1-4 = 1.01
Performance figures @ 50.250Mhz:
- Froward Gain: 8.05dBi free space
- Front to Back: 12.03dB
- Radiation angle at 10 Metres above ground: 15 degrees
Performance figures @ 70.250Mhz:
- Froward Gain: 8.44dBi free space
- Front to Back: 12.43dB
- Radiation angle at 10 Metres above ground: 10 degrees
Each element is made out of single piece 1/2 inch (12.7mm) aluminum tubing Each element half length needs to be doubled in order to gain your total element size. No difference is length needs to be calculated for the 1inch or 1,1/4 inch boom as the elements sit high enough above the boom for the boom to have no influence.
You can build this antenna with 13mm diameter tubing using the same parameters above. Slightly higher front to back ratio and forward gain will be seen along with a narrower SWR curve.
For construction information see 'G0KSC insulators' and the UKSMG article from the main menu on the home page.
Below are two images showing current distribution through the antenna on both 50Mhz and 70Mhz. This current is represented by the line lines overlaying the elements. The higher the line is away from the element, the higher the current. Following these are performance images including plots and SWR sweeps.