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.
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.
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