A Bare Bones 'I-AM' End-Loaded Vertical Dipole

Spurred on by the compact size and excellent, near delta-loop performance of the copper pipe end loaded vertical dipole, and with a warm evening on offer, I set about to make this antenna something manageable by a real human.

The aim: because copper pipe is heavy, not very wind resistant and expensive, I wanted to make the antenna very much lighter, much more wind resistant and made of stuff likely to be in most hams' sheds.

So, I had lying about:

• A 10m glass fibre fishing pole 

• Some unused 2.5m long moulded wood edging strips (a coving shape is strong and lighter than square section; round profiles are not very strong and less easy to work with in general).

• Random bits of kevlar wire and enamelled copper wire

• A good length of 300 Ohm twin feed and dipole centre (homebrew, of course!)

What I did:

Take out the top three sections of the 10m pole, as they are not needed for this design and anyway are too flexible to carry any weight.  This leaves you with a sturdy pole about 6m or so long.

Use cable ties ('ty wraps') in a cross fashion to fix the timber edgings to the pole.  I've changed this now to use simple plywood clamps using bolts and wing nuts.  This allows the whole thing to fit into the smallest of cars, with minimal set-up/down time or complexity.  Try to keep the clamps as small as possible, to keep the weight down, especially for the top end load.

First attempt at a lightweight clamp so the whole thing can go portable in a small car and not be a headache to set up or down.  It works a treat, but do keep the top clamp as light as possible, to avoid top-heaviness.

I took about 2.5m or enamelled wire, bent it in two, and scratched half an inch of the enamel off at the centre point.  I then made a small connector, initially out of flexweave (anything will do), but now recommend, for durability, you use solid copper, with a spade connector attached to one end.  I soldered the other end to the centre of the copper wire.  The whole wire was taped onto the timber edgings (cable ties are better for more permanent versions, and you should lightly exterior-varnish the timber).

Solder a small wire and spade connector to the middle of the end loads.  Makes for easy disassembly.  After a year of testing, I now recommend you use solid copper for the tag, not Flexweave or similar.

I did exactly the same for the bottom end-load.

I then took some old kevlar wire, which is very light, robust and collapses into a manageable lump that doesn't readily get tangled when you collapse the antenna.  The length was just over 5m (this isn't that critical).  Both ends of the kevlar had spade connectors put on, so that the whole antenna can be dismantled easily if needs be.

Finally, I connected the dipole centre to the kevlar wire, and used one cable tie to attach the centre to the fishing pole.  Again, I'm going to make an easily detachable clamp for this, as I don't like to clutter the environment with wasted nylon ties, and it's less convenient if you have to cut ties all the time.

Nearly down to car size.  The collapsed end-loaded vertical dipole, just a fishing pole and $2 timber edging (with wires!)  Performance of the fully-deployed antenna is just under that of a full wave delta loop on the 20m band.

Connect up the 300 Ohm twin to the ATU and: voila!  A fully-functioning low angle DX vertical dipole that collapses into a 1m-long package that easily fits in a small car.  Total deployment and take-down time: about 1 minute apiece.

Proof that this is light: very light!  An adult can easily balance it on one finger.

For mounting, I use my trusty cement mixer tripod (actually, it has four legs!), onto which a standard 2" aluminium pole of about 1m is put onto the spigot, and then the fishing pole sleeves over the pole.  It's very sturdy even in heavy winds, though some simple rope guying is best, as is putting a couple of heavy cement blocks or stones on the feet of the tripod.

This is probably going to be my permanent portable antenna, and the one that will get deployed when it's blowing an 85mph gale at home.  It performs almost as well as the full wave 20m delta loop, which is no mean feat.

The antenna working clifftop 'graveyard portable' from Llanbadrig Church, Anglesey.   On this evening, conditions were not good on HF, but I did manage the US west coast and Japan on 50W SSB.  Not bad for a made-for-peanuts antenna!

Thanks to the late Les Moxon, who inspired this design from a 2-element wire vertical beam in his excellent book HF Antennas for All Locations.  The more you read this book, the more you realise how good it is.

Incidentally, the 2-element beam is just 5m in height, needs only about 2m ground clearance, and for me, worked ZS and VK easily on the first outing, suspended - badly - from two bendy fishing poles!

UPDATE:

During a visit to the toilet, I belatedly had the brainwave of trying telescopic whips for the end loads, so that the whole thing could become even more compact and portable.  Finding good quality whips of the required length isn't that easy, but very reasonably-priced units are sold by Buddipole.

Update (2018 July):

I've started operating at a rural shack that really brings out the best in this I-AM.  I also want to start working at beaches and other places more often, as the domestic environment continues to worsen in terms of RFI for most of us, and will hit me, sooner or later.

To that end, I find that the end loads, which are effectively capacity hats, have too much mass and result in either 'head nodding' for a flexible mast and timber/fibreglass arrangement, or an antenna that is too massive overall in the case of an all-aluminium version.

The answer is to build disc-like capacity hats instead.

In early tests, using ARRL Antenna Book formulae, I could just about match the antenna with an internal ATU on 14MHz, but only above 14.260MHz.  The discs are obviously slightly too small.   I could only match-up on all bands 20-6m when the lower end of the antenna was about 30cm from the ground.  Normally, that means excessive ground coupling - and unacceptable losses.  But this was during excpetionally (as in 50+ year exceptionally) dry conditions where the water table has lowered substantially over normal levels.  In wetter conditions, the antenna would probably match at a greater height.

Even so, I did manage good contacts with, for example, A41 land on FT8, with decent (equal) signal reports both ways. A quick WSPR test for about 30 minutes also showed very good comparison with other stations.

All that said, I am going to make end discs that are a bit bigger. For this, to provide physical robustness and self-support, I've ordered some 2mm copper wire for the disc radials, and some 0.8mm wire for the outer ring.   Each disc simply slips over the fibreglass mast, making for an antenna you can roll up and carry in your hands or backpack.  More details when I complete the exercise...

And here's a wind test, when I was on-air to PC1H, DL1HA and ON9CGB, at 40mph (64km/h), gusting about 48mph (77km/h) with one rope guy to a concrete block, just mounted on a pole placed loose on a cement mixer stand (two concrete blocks to secure). I'm happy this could take significantly higher winds :

DSCN7844 from John Rowlands on Vimeo.

Following discussion on QRZ.com's forum, and a tongue-in-cheek suggestion from G3TXQ, this antenna shall now and forever be known as the 'I-AM' (!)  

Go build!