144MHz LFA Yagis

Low Noise LFA Yagis designed by G0KSC free to build for personal use.

70cms LFA Yagis

Twin-Boom G0KSC Quads

G0KSC Twin-Boom Quads

G0KSC Custom Dish feeds - Above installation @ HB9Q

Custom low-noise dish feeds

G0KSC Custom Dish Feeds

Above installation @ HB9Q

Read Time: 3 - 6 minutes

G0KSC SC6-4-9D 9el 50Mhz/70Mhz Dual band Yagi Antenna with a 3.5 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 5 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, as with all antennas on this site, if the SWR curve is wide and low, any small mistakes in sizing and spacign 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.

Built by Frank, IZ8DWF

'Hi Justin,

Thanks for putting your designs up on the internet!! I build 2 x SC6-4-9D ’s one for me and one for a friend, and I must say they work like a charm !!! They do exactly what you show in the graphs...'

'Here some links to show the SWR curve on 6. find the differences J'

antenna 1

antenna2

Wilfred ON4NDO

The 2 x SC6-4-9D at ON4NDO

One of the ON4NDO SC-6-4-9D's on test

The SC6-4-9D built by PA2M complete with XYL

The analyser at PA2M showing 2 dips exactly where they should be; 50/70MHz

The SC6-4-9D built by G4BRK

The SC6-4-9D by LA8AV and LA5ZK - More photos of this antenna at the bottom of the page

The SC6-4-9D installed at MM0GPZ

Antenna Layout

Sizing and spacing are as follows:

Dimensionsin Metres

Element spacing:

Ref 6 = 0
Ref 4 = .146
Driven 6 = 1.019
Driven 4 = 1.151
D1-6 = 1.693
D1-4 = 1.87
D2-4 = 2.462
D2-6 = 3.001
D3-4 = 3.493

Element sizes per element half:

Ref 6 = 1.481
Ref 4 = 1.035
Driven 6 = 1.423
Driven 4 = 1.007
D1-6 = 1.332
D1-4 = .95
D2-4 = .932
D2-6 = 1.268
D3-4 = .931

Performance figures @ 50.250Mhz:

Froward Gain: 9.13dBi free space
Front to Back: 14.44dB
Radiation angle at 10 Metres above ground: 10 degrees

Performance figures @ 70.250Mhz:

Froward Gain: 10.26dBi free space
Front to Back: 20.04dB
Radiation angle at 10 Metres above ground: 5 degrees

Element diameter:

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.

NOTE:

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.