Off-Centre-Fed (OCF) Antennas
I’ve now decided to use a ZS6BKW antenna as it fits better in an inverted V configuration in my garden but for Multi-band use without tuning, the off-centre fed antenna has a lot going for it (and lots of happy users), so I thought I’d put up some of the useful links I’ve found.
These links describe very clearly how it works (in summary – it’s better NOT to feed at the low impedance point like a dipole as when you double the frequency the same point is now a VERY high impedance – it’s better to accept a little mismatch and get lots of bands instead.
For more detailed explanations (with pictures :-) ) see:
* Good OCF Introduction
http://www.radioelectronicschool.net/files/downloads/ocfdipole.pdf
* Clear explanation but shows the difference using a 20% feedpoint can make -it includes 10MHz)
http://www.w8ji.com/windom_off_center_fed.htm
* A more extensive article with various shortened versions
http://www.stroobandt.com/cl-ocfd/windom/index.html
or without the frame:
http://www.stroobandt.com/cl-ocfd/windom/windom.html
OCF Antennas are sold commercially from
http://hamcall.net/7bandocf.html
and the user reviews on EHam.net do seem very positive:
http://www.eham.net/reviews/detail/5838
These are still full length antennas (e.g. 40m/130ft if you want 80M without using an ATU) but without the anti-resonance that makes a 80M dipole hard to feed on 40M (for example), and having the feedpoint at 20% or 33% can sometimes be more convenient.
Cheers
Giovanni – ZL2BOI
