Monday, 24 February 2020

Ham radio-Off Centre Fed Dipole

Ham radio-Off Centre Fed Dipole

Ham radio-Off Centre Fed Dipole

Ham radio-Off Centre Fed Dipole



Ham radio-Off Centre Fed Dipole

The SWR curves from a Buckmaster 7 band aerial

Ham radio-Off Centre Fed Dipole.  A dipole is essentially a monoband aerial with an exception if cut for forty meters or the 7 megahertz, band, it will also operate successfully without an aerial matching unit on its third harmonic which is 21 megahertz or in our case or the fifteen-meter band, using this arrangement and attempting to feed the aerial at its even harmonic 14 megahertz with coaxial cable will result in a huge mismatch and induce excessive loss into your aerial system.

A Forty meter aerial cut for around 66 feet will work on odd and even harmonic bands, ie 14 Mhz 21 Mhz and 28 Mhz if fed with open wire feeder, I have used this very successfully over the years, but you will need a good quality Aerial Matching Unit, (A.T.U.) with wide-spaced capacitors to handle the high impedances generated in the system.

The advantage of using an off-center fed aerial eliminates to a point the need for a matching box that will handle high impedances. My aerial was cut for 80 meters 3.5 Mhz, the total length of the aerial is 133 feet, fed at 44 feet from one end via a 4:1 balun, I then placed a 1:1 choke consisting of ferrite beads slid over the coax.

Further experiments with the Off Centre Fed aerial involved a different percentage match, in my case, I have used a 20% measurement from one end or approximately feeding the aerial 26.6 feet from one end with around 106.4 feet on the other side of the dipole, I found a 6:1 ration balun was needed with this arrangement.

My SWR reading for the OCF was under 2:1 for 80 & 40 meters, 30 meters was excessive about 5:1, 17 meters 5:1, 15 and 12 meters were also well under 2:1 and 10 meters was 1:2:1 a very good match, length of feeder also plays its part, I started with 100 feet of RG-58CU 50 ohm coax, some adjustment is needed with the length for acceptable matching especially 17 meters.

Overall the performance of this aerial is acceptable if you are willing to accept some high SWR curves, the 44 feet 106 feet model gave better matches, taking into account the nulls on some bands, 10 meters, for example, I wonder how many stations I could hear.

Like all multi-band aerials, it is a compromise, propagation changes all the time and it may be good in the direction of one of the nulls on your aerial, if you have no preference and want a good compromise with one coax lead this aerial maybe for you.

40-meter OFF CENTRE FED aerial by M0ZPK

Ham radio-Off Centre Fed Dipole


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Ham Radio-articles

Ham Radio-articles. Thank you for your emails and comments about my Ham-Radio website which includes my other hobby of photography and Portrait painting, as requested a list of articles and pages are attached below.

Ham Radio-articles

John Allsopp G4YDM



  1. Portable operation
  2. Vertical aerials
  3. Loop aerials
  4. Cubical quad aerial
  5. The half-square
  6. A ham caught in oil
  7. Ham Radio aerials
  8. Ham Radio terms
  9. Ham Radio receivers
  10. Ham Radio propagation
  11. Ham radio digital modes
  12. Ham Radio fascinating hobby
  13. Ham Radio John G4YDM
  14. Ham Radio VHF UHF aerial
  15. Ham Radio SDR receiver
  16. Ham Radio mistakes
  17. QSL Cards
  18. How to make an efficient aerial
  19. From Ham Radio to H.N.D.
  20. Off Centre Fed Aerial
  21. Portable working near the sea
  22. Ham or Amateur Radio
  23. Using DX-commander aerial
  24. Half Wave End Fed Aerial
  25. What is a balun
  26. Ham Radio with no repeaters
  27. D-STAR Digital mode
  28. ICOM R-71E Receiver
  29. ICOM IC-7000 Receiver
  30. The Frame Aerial
  31. Ham Radio Noise



A great QRP rig Yaesu FT-7

A great QRP transceiver FT-7



I hope you find these articles useful in helping you start in the fascinating Ham Radio hobby, it's not expensive really as many believe, you can start off with simple homemade transceivers made from a book as I did years back or from one of the many kits available, kits have the advantage of being supplied with all you need to build the project without having to source components yourself.

Ham radio has many fascinating areas to explore from operating with low power QRP through to contest operating to building transceivers and aerial systems, there's something for everyone in this hobby, including airband receiving which is something I did when I first started building receivers. I converted a Pye Westminster AM for the reception of Usworth Airfield, Usworth an RAF base during world war two now is a car factory where Nissan cars are built.