This is a revised article about a the ominous loaded 80/40m Dipole. Ominous? Well, it should not be seen as a wonder antenna. It will however, be a very potent antenna on 40m which will, as an added bonus get you onto parts of the 80m band. Which parts? Well that depends.
So the design is based on a 40m dipole with loading coils for the 80m. Because the overall length of the antenna is about 26m (give or take) she is rather short for 80m. However, this is an antenna for the city dwellers like me, who only have limited space available. If you are able to put up a 40m full size beam at a height of 30m then you might move on as I believe that you do have enough space for a more potent 80m band antenna e.g., this antenna might not be for you.
So the design is based on a 40m dipole with loading coils for the 80m. Because the overall length of the antenna is about 26m (give or take) she is rather short for 80m. However, this is an antenna for the city dwellers like me, who only have limited space available. If you are able to put up a 40m full size beam at a height of 30m then you might move on as I believe that you do have enough space for a more potent 80m band antenna e.g., this antenna might not be for you.
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| Loaded dipole for 80/40m |
These dimensions are the result of a model at a height of 7m above poor ground. So if you decide to build one you most likely need to adjust those values.
Lets go through the model, showing 40m details first.
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| 40m VSWR Plot |
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| 40m Gain Plot |
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| Gain of 40m Full size dipole vs gain of 80/40m loaded dipole |
As we can see from the above, the 2:1 VSWR bandwidth is more then 200 kHz and even at the end (7.3MHz) of the amateur allocation (VK) the VSWR is below 3:1. Most modern rigs with a build in Antenna
Trans Match (ATM) will be able to keep the radios finals happy. And of course the old boat-anchors
with their Collins-filter should have no problem to transfer all of
their RF-power into this antenna on the 40m Band.
The next pictures show what we can expect from the 80m part of the antenna.
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| 80m VSWR of loaded dipole |
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| Gain plot of the 80m loaded dipole |
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| Z of the 80m loaded dipole |
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As we can see from the above the antenna will get us on 80m however, as the antenna is rather short the 2:1 SWR bandwidth is rather small with about 40kHz and the 3:1 SWR bandwidth is only about 80kHz. This is not the only limitation, with a loss of about 2.5dB vs a full size 80m dipole at the same height of 7m. Now an 80m full size dipole at 7m height is not a great antenna, the only benefit I see is that there is 2.5dB more gain in NVIS coms. However, 2.5dB is not even 1/2 an S-Point as such it will do quite well in a confined space such as ours.
So, the antenna will give us a slice of the 80m band albeit with reduced/compromised performance. But on 40m the situation is not that bleak as the antenna will perform the same as a full size 40m dipole at he same height.
NOTE: These dimensions are for the specified height of 7m above ground!
Those length will vary and will be quite different if modeled in free space i.e., 1/2λ on 40m = 10.56m.
What this means is, if you build this antenna YOU have to MEASURE and CUT.
The tails to the left and right (1.55m) would need to be adjusted for best 80m VSWR at the frequency you would like to operate at. The coils (L1) are 120µH which is enough inductance to isolate the 40m part of the antenna. I used the mini Ring Core Calculator to calculate the mechanical requirements for the coils.
Below is a quick sketch of the antenna with details for the coils.
Remember, these are modeled dimensions, which means, if you build this antenna YOU have to MEASURE and CUT.
Now one thing I'd like to stress and that is the use of a 1:1 current BalUn at the feed point of the antenna.
NOTE: The antenna can be setup in an inverted V configuration, i.e. a 7-10 m center mast and 2 sky-hooks of approx. 2.5 to 3 m height, this will make the radiation pattern more even and as such might give the impression of improve performance. One could also set the antenna up in an angled configuration; however I would recommend not to make the apex smaller then 120° for an inverted V installation and not smaller then 60° for an angled installation.
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