G0KSC SC0604Q 4el 50Mhz OWA Quad Antenna with a 2.998 Metre Length Boom

Some have said there are no benefits to be seen from Quads over Yagi's with more than 3 elements. This model is set to prove this statement wrong. Over 10.4dBi has been achieved with respectable front to back ratio on a boom length of less than Three metres in length. This is around one metre less than would be required to see similar gain from a Yagi. In addition, I have been able to model the antenna without the narrow bandwidth characteristics normally associated with an antenna of this kind.

If Quads are your type of antenna, this one maybe for you.

As with all my antennas, this is a non-critical design and therefore, any small errors in you calculations when building the antenna WILL NOT have a great impact on the antennas final performance.

Dimensions in Metres

Element spacing:

• Ref =      0
• Driven =  .942
• D1 =       1.668
• D2 =       2.998

Elements lengths - full quad length:

• Ref =       6.344
• Driven =   6.144
• D1 =        6.032
• D2 =        5.936

Performance figures @ 50.250Mhz:

• Froward Gain: 10.44dBi free space
• Front to Back: 20.09dB
• Radiation angle at 10 Metres above ground: 10 degrees

Element diameter:

This antenna has been models with 12# copper wire. You will need to mail me if you wish to use a different gauge as the model will change. Isolated spacers will be needed. fiberglass would be a good option.

NOTE:

Build orientation is as per the layout image below with the feed point on the bottom section of the quad indicated by a red circle.

Feeding the Antenna

Whilst OWA is not normally a term used in quad antennas, I felt it appropriate due to the low SWR and wide bandwidth of this antenna. Like all of my antennas, this has a 50Ohm impedance so can be fed directly with 50Ohm coax, no matching is required. However, a balun or RF chokes should be used in the coax as close to the feedpoint as possible.

G0KSC 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, 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 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 Metres

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) 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.