G0KSC SC2104S 4el 21Mhz Yagi Antenna with a 4.95 Metre Length Boom
This is a nice compact Yagi 21 Mhz beam. It has a short length boom which is made from a single piece 1.25inch square tube section. No boom support will be required.
This antenna shows good performance particularly Front to Back. The antenna covers all of 21Mhz to 21.45Mhz with good gains and SWR throughout.
This antenna uses elements of .5 inch diameter. As long as you buy thick wall, you should not see much sag and can avoid tappering the elements. I have a 3/4 inch - 19mm element thickness version too which is available upon request.
Dimensions in Metres
- Ref = 0
- Driven = 1.608
- D1 = 2.286
- D2 = 4.957
Element sizes per element half:
- Ref = 3.546
- Driven = 3.451
- D1 = 3.299
- D2 = 3.146
Performance figures @ 21.200Mhz:
- Froward Gain: 8.34dBi free space
- Front to Back: 21.1dB
- Radiation angle at 10 Metres above ground: 13.5 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.
Joining elements and insulator requirements
1/2 inch is available is 5 metre lengths and therefore, the elements will need to be joined. This join should take place in the centre where the element meets the boom and where it is mounted to the boom. the absolute centre of each element has no effect on the rest of the antenna so this is the best place for the join to occur.
Use a 1 metre length of 3/8 tubing in each piece of the element sleaving the 3/8 inside the 2 x 1/2 sections. The G0KSC insulators (if used) should be counter sunk in the centre and bolted to the boom WITHOUT going through the antenna element Make the G0KSC insulators to 400mm in length rather than 100mm length (described within 'G0KSC Insulators' which is linked from the main page) and bolt each side of the half wave element to each side of the insulator using 4 bolts in total.
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.