The 'I' vertical dipole is perhaps best known at the moment in the form of the very well-made and favourably-reviewed I Pro Home by Pro Antennas. Even the price of the I Pro Home is very good indeed, and for those who don't like to build their own stuff, or want a more professional finish, then I think the following experience should give you comfort that, if you buy the I Pro Home, it's likely to work very well indeed.
But it would be wrong to suggest that an end-loaded dipole is in any way complicated in form, and can't be knocked-up in half an hour if you've all the materials to hand.
So, with my homebrew version with its wood dipole centre, a wooden mast fixed to a fence post, and with the bottom end load pipe at less than a metre above ground, I connected up using 300 Ohm twin, although a vertical dipole is not exactly balanced, by virtue of one end being up in the air whilst the other is face to the ground.
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| Heath Robinson? Absolutely! Effective antenna? Well, it got into Indonesia in the first two QSOs attempted. |
In case you want to build your own, the vertical part of each half of the dipole is 2.5m long, the end loaded section at the end being 1.25m each end of the 'tee', a total end load of 2.5m (repeated for the other half of the dipole). It is in practice just an 'I', cut in the middle to feed it.
The result? Well, of course an ATU is necessary for this system, but that's pretty run-of-the-mill for any ham these days. The antenna loads up nicely on every band 20-10m. My first contact on 100W SSB on the 17m band took me, rather incredibly, to Indonesia with a 55 (QSB) into Dani, YB2TJV, with plenty of other stations calling. That made me smile. Much bigger stations were getting 55-57 with QSB. Not that is means much for an antenna test, I was getting 59+ into Tomislav, the very active 9A2AA in Croatia, on the hunt for Olympic callsigns.
I've not modelled this antenna, but according to Pro Antennas, their version has a low radiation angle of about 20 degrees, which is just what we need for DX. The contact I made with YB-land tends to confirm the low DX angles modelled. Wholly independently, blogger EtherGeist finds precisely the same result:
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| Low angle DX pattern. Excellent! |
So, although copper tube isn't exactly cheap, it's also not that expensive, so I guess the total cost of my homebrew end-loaded 'I' dipole to be about £30 in new materials. Many of us already have plenty of scrap pipes about, and if they're too short, just join them with soldered fittings, which are remarkably expensive for what they are.
Best of all, the total height of this antenna is just 6m, including a 1m ground clearance. That is very useful indeed from a visual impact point of view and its ability to withstand the very heavy winds we get here at all times of the year.
The future? I'm certainly going to be using this antenna more, not least because it's likely to keep me transmitting during the most severe gales of autumn and winter. It's quite low noise, though not as good as the delta loop. It will benefit from being made of 1-inch or 1.25 inch aluminium pole, using normal pole-to-pole clamps to fix the end loads. This will make it much more windproof, as copper pipe does bend a bit after a few months of battering, especially with the end load making it top heavy. The end loads could probably stay as copper, which also lends itself to portable operation in that you could have a compression fitting or two at either end (you'd still need to fit and compress the olive to the pipe, which is what does the fixing), where you could then attach and detach the two halves of the end loads.
Excellent! Another superb DX multiband antenna for the price of a few days' shopping, if that.
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