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The Versatile Icom AH-4 Auto Antenna Matching Unit

The Versatile Icom AH-4 Auto Antenna Matching Unit.

                                                                        By Tony Boddy, ZL3DQ, VK2ADQ, VK6DQ. 2015

Having moved to Mandurah during the first quarter of 2015 left me with no antennas apart from those strapped horizontally on the rack of our escape from NSW Trailer.

The whereabouts of my ham gear in the fifty or so boxes was unknown, the Wouxon hand held lost all functions the day before we left NSW and I can tell you that the withdrawal symptoms were really bad. Travelling with no radio was a traumatic experience. It took a month after our arrival to get an antenna off the top of the trailer up and a further month before I could find the Wouxon and do a factory reset. Now it talks to me in Chinese but it works.

I did find the IC-706 and the AH-4 as well as my Diamond x-50.

The first antenna to go up was the X-50. That and the 706 got me on VHF/UHF but what about HF.

During the time that I have owned my Icom gear I have really come to appreciate the AH-4. There is little information in the Icom AH-4 blurb sheet when it comes to the versatility of this excellent matching unit.

It is supposed to be for random longish wires. Well, I can tell you that I have used it on long and short wires, the longest 380 feet (a lot of meters) and the shortest about 2meters. It works well on dipoles, loops of all shapes and of course verticals.

My colleague, VK2AST, under my instruction, put a full size 80 meter horizontal loop up at about 25 feet, it is fed with 2 parallel RG58U coaxes taped together from an AH-4. The 60 foot feed-line runs from his underground shack to one corner of the loop which is only at about 25 feet. It works really well.

The first antenna up here was the X-50 on top of a 4m fiberglass pole. An extremely good, gain antenna on 2m and 70 cm but what was I to do for HF?

My mentor from my newbie days used to tell me that everything was possible and that we had the technology to do anything. Using that philosophy I wondered if the X50 could be made to perform on HF. If I were to short the braid to the inner at the bottom of the X50s coax and feed it as a long wire I would have a 25 foot vertical and the ground plane could be the metal RHS frame of the patio awning.  Hooked it up with the AH-4 and bingo I was on air. My first contact was with VK3CM in Tangambalanga Victoria, he gave me a 15 over nine report but he was using an optibeam at 100 feet. The contact into ZL was 5X7, more realistic, then a 40m contact into the states at 5X3. Pretty good going I thought.

The block I am on is a nightmare to stand up poles but stand one up I did. 30 feet of aluminum from my mate Brian at NBS Antennas. 3 sets of guys all in 1.2mm stainless MIG wire broken up with insulators to 10 foot lengths, non resonant in the ham bands, no harmonics here. Looking good already. Hung up a 100 foot doublet, non resonant on the ham bands, fed it with 50 feet of 450 Ohm ladder line from the AH-4 under the patio awning, checked out 80m to 6m and the AH-4 delivered a 1:1 SWR on all bands. My contact into the states was now 5X7 on 20m. I will be happy for a while.

There is no reason why you can’t stick the AH-4 up at the top of your pole, it is weatherproof but the overseas weather blokes have not encountered our Aussie sun which will kill most things exposed to it, even to the extent that it will decompose granite. You can make your own ladder line if you want to.  If you keep the coax feeding the tuner as short as you can you will have less feedline loss.

The AH-4 has a hot terminal on the top and an earth terminal on the bottom. Icom says that the earth terminal must be grounded. I only do that for vertical or long wires. For the rest I float the AH-4 above ground and remotely feed the antenna, albeit dipole or loop, with 450 OHM ladder-line.

Connect one side of the dipole to the hot terminal and the other to the ground terminal. Do NOT connect the braid of the incoming coax to this terminal at any time. The coax feeding the tuner will have losses which are directly proportional to its length. The SWR on that will be 1:1 after the tuner has done its bit. Make no mistake about it there is a relatively high SWR on the ladder-line but effectively at our frequencies and lengths we can consider it to be loss-less. I like to use it because it gives me access to the tuner at people level and keeps the sun off it. The AH-4 will tune into most things, don’t bother trying to feed coax with it. It is possible you may have to modify ladder-line lengths if you have a severe tuning problem on any one particular band. So far all of my feed lines have been random in length and I have had no problems. The control cable can be up to 120 feet long with no trouble at all. As for ladder-line, run it wherever you want, just keep it about 150mm away from steel and give it a twist every 2 feet or so. RF in the shack where I usually mount the AH-4, I just don’t get it.

Over the years I have used just about everything as a High Frequency antenna even to the extent that I used a metal framed kitchen chair on 80m on top of a dining room table from Albury to Tasmania with a 5X7 report. You can’t just chuck a Chair down, connect it up and have it work. In my experience if you have the right matching unit there is no limitation. The efficiency of short radiators is very suspect and I have to question the effects of the RF energy, enough to light up a hand held fluorescent tube whilst talking, that is very present during transmissions. Just between you and I, the home brew link tuner I used was a bit of magic stuff but I think the AH-4 would give it a run for its money.

Want to know more? Drop into a PARG club meeting and ask about antennas for tiny spaces. There will be someone there to help you get on air at your “no antennas” tenancy.