11 - Results from my OCFD (SWR) - article written for CONTACT

Submitted to BDARS for inclusion in the monthly magazine "CONTACT"

My Offset Centre Fed Dipole

I finally got to tune the OCFD I talked about at the last club meeting. The antenna is suspended between three poles from 7m to 10m high.  My experimental work with MMANA-GAL had given me lengths of 8.3m and 12.6m – a Dipole with the 200 Ohm feedpoint at 39.6%. expected to give reasonable SWR but not perfect on 40,20,15 and 10m.

Above is a photograph of the necessary transformer and choke, all wound with FT240-43 cores. The pair of cores in the transformer are wired in series-parallel to give an overall turns ratio of 2:1 which means they act as an impedance transformer of 4:1, matching 200 Ohms to 50 Ohms, or anywhere from 400 Ohms to 100 Ohms if we accept an SWR of 2 to 1 or less.  MMANA produced predictions of the graph below. 

The SWR predictions for 40m is 2.6 although 20m and 15m are very good at 1.3 and 1.6 with the 10m band prediction drops to 1.7 but the bandwidth for swr <2 is only 600 kHz.

As for the measured plots, I took two sets with the wires at 8.3 and 12.6m using my Rigexpert AA-30 antenna analyser and my NanoVNA. Both figures agreed and showed the minimum SWR at 6.85 MHz. I wanted to move this to 7.1MHz and therefore had to shorten the antenna to 96.5% of its original length – I shortened the short side by 11.5 inches and the long side by 18 Inches and got the plots below.

Here is the overall plot – impressive how similar to the MMANA predictions, nice that the SWR is actually lower, particularly on 7MHz.  I now need to get WSPR going to see how well it actually works

Better than a simple dipole!