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!

10 - Antennas: Experimenting with MMANA - Dipoles and my OCFD Offset Centre Fed Dipole.

Having been inspired by M0MCX, Callum's youtube videos on MMANA-GAL I decided to give it a try. Callum owns DX Commander and makes many videos - his DX Commander Vertical antennas look very interesting, if I had room for the radials I would build one for the shack. (You want a 30 foot circle for his 40m and up vertical made from a fishing pole.

MMANA-GAL is a free package for modelling antennas, I experimented with a simple dipole - at several heights, with drooping ends, with a run of wire near one leg to see what effect it had.

I give most of the details in the previous post - an article I wrote for BDARS's magazine "CONTACT"

After playing with 40m dipoles at 7m and 9m I moved on to consider Off Centre Fed Dipoles;

These are traditionally feed at the 33% point along the wire instead of the 50% in a simple dipole. At 33% you get a feedpoint impedance of 200 Ohms and meed a 4 to 1 transformer to match to 50 Ohm coax cable feeder. You also have some asymmetry and hence RF flowing down the outside of the cable; You need a choke to reduce this to zero.

I researched OCFDs at length, and was very much impressed with the website of Rick DJ0IP (https://www.dj0ip.de/ ). and also the groups.io page on OCFD (https://groups.io/g/ocfd )

I also used excel to plot some diagrams to see how other % splits would match - and then used MMANA to see what SWR was achievable (with a 4:1 transformer and choke)

The various sinewaves and parts of a sinewaves are plotted assuming a start position on the left hand side - in practice it doesn't matter if you start with the opposite polarity as the sinewaves reverse every cycle anyway - so  I show two plots below which makes it clear how different bands can be matched on the same antenna - if the currents (and volatges) are at a similar point the different waveforms can be matched. The ratio of current to voltage tells you what impedance you match to.

On the graph above you see that the traditional 33% gives superb matching on three bands, I wanted 4 bands so choose 41% (actually 39.7% after playing with MMANA) to get a compromise solution. The previous diagram shows how 14 to 19% would also give 4 bands but I was afraid that the gross asymmetry of a 14% antenna would create RF in the shack, and RF on the coax could (a) create interference locally - my name would be mud if my rf switched on the coffee machine in the house! and (b) pick up local interference and reduce my receive performance.

After much experimentation with MMANA I made an OCFD with legs of 8.3m and 12.6m but after measuring SWR (with a RigExpert AA-30 and a nanoVNA) I reduced the short leg by 11.5 inches and the long leg by 18". This was to move the SWR minimum point from 6.85MHz to 7.1MHz (a reduction to 96.5% of initial length)

The final result was a four band antenna that didn't need an ATU - apart from extremities of the 10m band. I could fiddle a bit more but I actually have an ATU inline with my transmitters - an MFJ-969 and I use the SWR meter and dummy load. Adding a tiny bit of loading will not waste Tx Power so I am now setup and will use it as is.

I have reported the tuning performance in another post here - the contects of another article I wrote for CONTACT, for my local club BDARS.

Here is my combined Transformer and choke; I now know that I didn't need to wrap tape around the FT240-43 cores but its hard to take off! All cores FT240-43. I include some downloaded schematics from the web that should help you understand the manufacture of this. I drilled a couple of 1/16" holes in the bottom of the box to let heat and condensation out.

(8 turns - I used more)

Next stage is to setup WSPR and see how it gets out - I have worked a couple of stations and it seems to receive well enough.

My next project is to get on 80m I hope to use an End Fed Half Wave (EFHW) that's 132 feet long - see the appropriate post for that when it gets posted!

Three masts the centre one supports the blue box.

9 - Using MMANA-GAL – Free Antenna modelling software

Using MMANA-GAL – Free Antenna modelling software

(Copy of article given to Bangor and District Amateur Radio society for their members magazine "CONTACT" )

Antenna modelling software can be complex but you can use it in a very simple way to get useful results. The free package MMANA-GAL is well supported with Youtube videos listed at the end of this article but in practice you only need 4 or 5 commands to use it effectively.  This article makes more sense if you download and install the software and run it whilst reading!

As an example of its utility, what if you were thinking of putting up a dipole for 40m, what's the difference between having it at 7m or 9m. What if it was made of thicker wire. What happens if you droop the ends to make an inverted V? You can "test" all this in software. You can specify the metal the antenna is made of (e.g Copper or Aluminium) and what thickness. You cannot specify a velocity factor in MMANA which means insulated wire ends up a bit longer (95-97%) but that is no real hardship. You can specify different types of earth, although I have only ever used the default "normal earth".  The results are useful for comparison purposes, don’t assume they are 100% right but they are a reasonable estimate of the truth.

Of course once you start fiddling (experimenting) with MMANA you might not find time to actually make antennas - so you do need a bit of discipline to avoid wasting time (having fun).

To use MMANA quickly note that the opening screen has 4 main tabs - you only need the "Geometry", "Calculate" and "Far Field Plots" tab. You also need to access the wire editor

To draw a dipole you need to run the "Wire edit" tool using the 6th icon in from the right or hit Control-W.  You also need to know that the view you will see has XYZ directions where Z is the height. You can think of X as pointing East and Y pointing North as the default directions. Once in the wire edit window click on the button marked XY. The diagram below is a screen dump of the Wire edit window.

The wire edit window has an icon tool for editing a wire and another one for creating a wire. These are the only two you need. You could instead enter all data into a table - I usually create a simple wire model, with approximate lengths, then nip into the table and adjust the lengths to what I want. Depends how good you are with a mouse!

A couple of tricks; you can specify height, either when you draw the antenna, or later when you go to calculate its performance. I usually leave it at zero when drawing.

Example: A dipole, one wire 20 m long and fed in the middle; in the Wire edit window, (XY view) draw one line that crosses the corner where the blue Y and X axes cross. It should turn red. Click OK which takes you back to the main geometry window, listed below

Edit the X1 and X2 values above to the lengths you want and click on the box marked PULSE under the Sources 1 window (bottom LHS) and enter the text w1c – this puts the source at the centre of wire 1

Switch to the calculate tab and Set 4 things; the frequency, the type of ground (“Real”), the height and the material of the wires e.g copper. You are then ready to click the “Start” button at the bottom LHS. Your window should look like that below;

Don’t worry about the SWR if it is below 2. – in real life it varies with your type of earth and antenna height.  You can also look at where the best directions are and the relative signal strengths at various angles. Click “Far Field Plots” up at the top, beside the Calculate tab. (if you click on the “Plots” button at the bottom you get a static display.  If you click your mouse on the antenna plots you can get various gain values in different directions. The left hand plot is a bird’s eye view – but you are best to specify a particular angle of elevation. Antennas send signals out at the take off angle to bounce off the Ionosphere – google TOA MUF Calculator for a useful tool. (I found mine at http://www.kolumbus.fi/pekka.ketonen/TOA MUF calculator.xls ) Energy leaving your antenna at 5 degrees will go 2000 km (DX!), an elevation of 15 degrees will give ranges of 1000 km (Inter-Europe) and 30 degrees for 500 km for UK use.

 – click the “Elevation” button at the bottom and set the value to 5 degrees.

So at 5 degrees you get a “gain” of -10.7dBi. At 15 degrees you get -1.6dBi and at 30 degrees you get 3.3 dBi.  When using dB (or dBi) you need to think in terms of ratios and factors, 3dB is a doubling, 6dB is a factor of 4 and 10 dB is a factor of 10. You add the dBs but multiply the factors. Anyway – for this article I just want to show the relative “gains” at different heights. At 7m this antenna puts 20 times more power into UK stations as DX ones. From -10.7 to +3.3 is 14dB – 13dB is (10dB +3 dB so 13dB is a factor of 20 to 1 (10:1 times 2:1)

If we go back to the calculate tab and change the height to 9m, click start and use the far field plots we can get the relative “gains” at various angles, and we can repeat this for 12m to get the table below.  

Elevation angle/likely distance reachable Height of antenna	5° (2000km) Best/Worst Directions	15° (1000km)	30° (500km)
7m (SWR=1.07)	-10.7/   -16.3	-1.6/   -9.7	3.3/   -3.8
9m (SWR=1.45)	-10.1/   -17.8	-1.2/   -10.9	3.5/   -4.3
12m (SWR=1.91)	-8.9/   -18.8	-0.1/   -11.7	4.2/   -4.4

Note I also show the relative gain in the X direction (the worst direction) to the right of the ‘/’

Note also the SWR changes as we are close to the ground – if your coax is poor quaility you will see better values in the shack (try https://www.qsl.net/co8tw/Coax_Calculator.htm to see how SWR changes.

The summary is, height matters! Unless your antenna is a vertical (which can pick up local noise). All antennas work and all real antennas are compromises.

I expect to demonstrate MMANA-Gal at the BDARs March talk. It comes with many, many examples of quite sophisticated antennas – you can have traps and feeders other than the default 50 Ohm coax. There are examples of cobwebs, hex beams and more.

Google for MMANA-GAL(Basic version) or use http://gal-ana.de/basicmm/en/

Good YouTube videos on MMANA are available from Callum, M0MCX – who makes the DX Commander Verticals – all his videos are good! – his channel is at https://www.youtube.com/user/m0mcx/videos and if you search for titles such as

“Part 1 - Idiot Guide to Antenna Modelling - Vertical and Dipole” or

“COBWEB Ham Radio Antenna 6 feet off the ground”

He also has much older videos in a series, for example

“MMANA Tutorial Part 1 M0MCX 20m dipoles”