Saturday, 21 April 2018

Revisiting the I-AM End-Loaded Vertical.

I hate to think how much time I've spent building antennas over the years.  But one thing I don't have to worry about is having spent too much money on them.

A long time ago, I wrote about a simple multiband end-loaded vertical that has, by a very wide margin, become the most read article of any in this blog.  I'm also glad to say that a number of people have contacted me to tell me how they built their own and found it very effective.

A 20-6m vertical for £50 or less!
Spending more time at my 'weekend' shack these days, I decided to finally deploy my all-aluminium version of the I-AM antenna in the field.  This has been lying around under various stages of development for around two years, so it was nice to get it away from the garden and into practical use!

The main advantage to me at this site is the ability to reliably and quickly disassemble the antenna into smaller parts that can be stored when not in use, and the rapid deployment due to there being no radials or much by way of a support mast, etc.

The antenna in this version is also very physically strong, and can withstand the kinds of accidents and occasional encounter with cows and sheep that operating in the field inevitably brings!

To recap the basics:
  •  The antenna is a vertical dipole, except it is made physically much shorter by cutting each 'leg' in half, and joining the other half on top of the first in a 'T' arrangement.  
  • Aluminium (and many other materials) are often sold in 2.5m lengths, which makes them readily car-transportable.  This is very useful, because the electrical length of each leg of a 20m dipole is almost exactly 5m.
  • Using simple 'wing' nuts that can be operated easily without tools, the horizontal end loads can be attached and detached from the vertical sections for easy transport/storage.
  • Insulation between the dipole legs can be achieved by several methods.  The original version I built was simply timber end load supports with wires running along them, then some vertical wire strapped onto a fibreglass (insulating) fishing pole.  For the current version, I used Delrin plastic inserts both for inter-leg insulation and as the physical support at ground level, which also needs to be insulated.  
  • Feed point contacts in my case are metal straps secured to the poles with stainless steel hose ('Jubilee') clamps.   A better arrangement that I will use shortly is a bolt through the pole, and a ring connector from there to the feedline.

The commercial version of the end-loaded vertical is the I-Pro Home, currently sold by Nevada at £259.  There has recently been talk of Pro Antennas returning to sales in their own right, but their website is currently (21/4/18) reporting a '404'.

Easily split into two halves for storage/transport.

The cost of building your own I-AM antenna will only be roughly £50 in all-new materials, made up mostly of the aluminium tubes and Delrin rod.  Other bits you will have lying around.  Ebay is a good place to find all the materials if you choose your sellers carefully.

The I-Pro Home commercial antenna uses a 4:1 transformer at the feedpoint of the vertical.  Whilst this works just fine, it is not the best arrangement.  Firstly, it adds substantial weight to an already rather top-heavy antenna.

Secondly, for an antenna that must be matched with an ATU on most or all bands, it makes much more sense to use a direct feed with 300 Ohm twin wire running either to a balanced line ATU, or directly to your rig or unbalanced line ATU via a 4:1 balun on the floor or a wall near the transceiver, and a very short length of coax to finish the connection.

Full HF sweep of my I-AM antenna in an open field setting.  From 20m up (which is the design intention), the SWR remains within easily matched limits.
The reason for this is that the antenna works at relatively high SWR, where losses are very much higher with coax - up to 20 times higher - than if twin wire is used.  The higher SWR itself is not a problem and should not be viewed as such, because it is, of itself, a lossless phenomenon.  Those reflected waves do ultimately get radiated.

End load attachment method.  Wing nuts permit easy mounting/dismounting.

Last night I confirmed again that this antenna matches up on 20-6m, although the internal ATU was getting towards its limits at 6m.  With an external ATU, you will probably manage a match on 30 and possibly 40m, but it probably won't be very good on the latter band.

One solution for feedline attachment.  A bolt with a ring connector is a better one!

My first call last night was to A41NN in a pile-up on 14.200MHz.  I was rather pleased with that!  I look forward to using the I-AM as my main operating antenna from this /P site from now on.


I had a very quick chance to run WSPR on 20m using a very old and simple manual ATU to my WSPRlite recently.   Free time for a longer run will come at some point.

I compare my antenna with a GI station running 1W as opposed to my 200mW, who is known to be a very good performer and a consistent reference station for me.

As you can see, whilst the data is very limited, and mindful of the more than 6dB greater output from the GI station, the WSPR results do clearly show this vertical end loaded dipole is performing very well indeed.

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