G0KSC SC0405S 5el 70Mhz Yagi Antenna with a 2.55 Metre Length Boom
This is a nice compact good gain 70 Mhz beam. It has a short length boom which is made from a single piece 1inch square tube section. No boom support will be required.
This antenna shows good performance particularly Front to Back. The antenna covers all of 70Mhz to 70.5Mhzwith good gains and SWR throughout. with a nice short 2.55 metre boom, it should not be difficult to set up on a nice hill top somewhere!
This is a 5el Yagi that is shorter than 'ideal' and therefore a little radiation resistance exists. Although not the end of the World, for every 100 Watts at the feed point, 1.2 Watts will be lost. Nothing compared with non-OWA designs admittedly. However, this one has been deliberatley designed short to provide a compact antenna that exhibits good gain.
Dimensions in Metres
- Ref = 0
- Driven = .375
- D1 = .562
- D2 = 1.522
- D3 = 2.548
Element sizes per element half:
- Ref = 1.061
- Driven = 1.047
- D1 = .991
- D2 = .97
- D3 = .938
Performance figures @ 70.200Mhz:
- Froward Gain: 10.2dBi free space
- Front to Back: 26.2dB
- Radiation angle at 10 Metres above ground: 10 degrees
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.
For construction information see 'G0KSC insulators' and the UKSMG article from the main menu on the home page.