Thursday, 3 September 2015

G0KSC SC6-4-8D 50/70Mhz Dualband Antenna.

SC6-4-8D 8el 50Mhz/70Mhz Dual band Yagi Antenna with a 2.08 Metre Length Boom.

About this Antenna:

This is a dual band Yagi for both 50Mhz and 70Mhz which has a single feed point. This is what is known as an 'interlaced' Yagi as both a 70Mhz beam and a 50Mhz beam (there are no common elements) are interlaced and sitting on the same boom.

The next interesting feature is the fact 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. This antenna performs as a 4 element beam on six metres and a 4 element beam on four metres.

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!

Design Considerations:

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 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 SC6-4-8D by G4PDS.
The SC6-4-8D by MW0CXH - Above is a 7 element 144MHz LFA.
The SC6-4-8D built by G4ZTR.
Sizing and spacing are as follows:

Dimensions in Meters.

Element spacing:

Ref 6     =     0
Ref 4     =     .04
Driven 6 =   .836
Driven 4 =    .935
D1-6     =     1.06
D1-4     =     1.129
D2-6     =     2.08
D2-4     =     1.981

Element sizes per element half:

Ref 6     =       1.458
Ref 4     =       1.052
Driven 6 =       1.45
Driven 4 =       1.027
D1-6      =       1.385
D1-4      =       1.014
D2-6      =       1.33
D2-4      =       1
Performance figures @ 50.250Mhz:

Froward Gain: 8.6dBi free space
Front to Back: 20.43dB
Radiation angle at 10 Metres above ground: 10 degrees

Performance figures @ 70.250Mhz:

Froward Gain: 8.54dBi free space
Front to Back: 18.76dB
Radiation angle at 10 Metres above ground: 5 degrees

Element diameter:

Each element is made out of single piece 1/2 inch (12.7mm) aluminium 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.

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