A homebrew vertical for HF WSPR ‘Mobileers’
Since retiring to NZ in 2013 we have been travelling the country in our motorhome. Its an ex-rental 6 berth Kia Dreamtime Deluxe. Plenty of space for Maria and me. We both have our hobbies and the van is sufficiently spacious to allow us to occupy different parts of the living space without getting in each others way. Maria has the ‘lounge’ area with its picture window, and I have the 4 seater breakfast table. This doubles as my operating desk and construction bench. In the last 3 years this has worked very well.
When we first went mobile I was determined that I would have a radio station that was compact and easy to use. I had an Elecraft K2-100 and a QRP rig, the KX3. I have since acquired the latest Icom rig, the 7300, which is mentioned elsewhere on this site. I spent considerable time ( months ) scouring Ebay and Trademe looking for the LDG remote ATU, the RT-11. I had owned one of these back in the UK and never had any issues with it. I disposed of it when I purchased a rig that had an auto ATU built in. That was a mistake ! Not many integral tuners can handle such a wide mismatch as an external tuner can. So when we arrived in NZ I had no auto ATU. My patience was rewarded when I eventually found what I was looking for in USA. Rather than ‘peck’ at the bidding, I immediately bid what I was willing to pay. I think that was equivalent to £80GBP. The bidding didn’t get that far and I got what I considered to be a bargain. Exactly what I wanted for less than I was willing to pay. The total cost including shipping amounted to £78GBP if I recall correctly. The plan was to install the ATU inside a rear locker of the van and control it remotely using a homebrew button box. In the meantime ……..
One useful item that I brought with me from UK was a 8m fishing pole. I had planned to repurpose this as my mobile vertical antenna. After some trial and error I eventually mounted some 40mm waste pipe to the rear ladder using heavy duty cable tie wraps. I removed the bottom section of the pole as it was too large to fit inside the waste pipe. I also removed the top section as it was too thin to support the antenna wire without bending. The wire is silver plated ( I think ) and white Teflon coated. Courtesy of Westlake if I remember. The overall length is 35ft. No particular ‘magic’ length, just the longest piece of wire that fitted the installation. A few turns of insulating tape near the top and bottom of the outer section of the pole ensured a snug fit inside the wastepipe. The pipe has a stainless steel cross bolt near the bottom, to stop the pole falling straight thro, and its overall length is about 50mm shorter than the fishing pole. This is so that when the pole is collapsed inside the pipe, at least a small part of it is accessible so that it can be easily extended. The wire is loosly spiralled around the pole, down to the top of the white pipe, where it is secured with yet more insulating tape. The loose end then drapes down the back of the van and is routed into the rear locker. Here I had a balun mounted inside a plastic sandwich box. The coax ran alongside the van, and was routed thro a window to the radio. Erecting the antenna, deploying the counterpoise and running the coax to the radio took a few minutes, but was a real pain in wet weather. The auto ATU inside the rig would match this antenna setup on 40 and 15 with quite a low VSWR. However some of the other bands of interest were not so good. So about 14 months later ……..
I read some really complimentary revues of the Icom 7300, and subsequently ordered one. Despite being told by Icom NZ that the radio was in stock, I had to wait a month before it arrived. I assume they meant it was in stock in Japan, they simply neglected to mention that detail. Nevertheless when it arrived I was very impressed with the whole thing. My only minor criticism is the lack of a bandswitch. You have to use the touchscreen.
I soon dicovered that the 7300 had an auto ATU control port and to my delight, that it was compatible with the LDG tuner, which was still languishing in a storage box. Other than initially testing it I had never used it. I ordered a suitable Molex connector from a local on-line trader that would fit the ATU port on the 7300. The next thing to do was install a run of coax and some 4 way screened cable from inside the van to the back locker. This turned out to be considerably more difficult than I had imagined. I purchased the cables at a local outlet and was disappointed to find that the screening of the coax was nothing like as substantial as previous examples of RG-58 that I have used, and still have some short lengths of. Satisfied that the distance from the radio to the ATU was quite short I decided to use what I had rather than spend more time search for something better. Not a lot of choices in Northland NZ. Trying to purchase some flexible conduit was another problem. It was available from a number of places, but at a price that I considered too expensive. I eventually bought a couple of 3m lengths of 20mm water pipe. Being in a campsite with a hardstanding ( Waitangi ) and in dry weather, the day arrived when everything was ready for the installation. I squirmed under the van on my back and checked out the cable routing only to discover that I would have to cross a chassis member to get to the rear compartment, mainly due to having to avoid the double rear wheels. After much indecision and not a small amount of bad language, I fitted the water pipe along the chassis member as best I could, whilst avoiding the shock absorber, the wheels and various other bits of automotive hardware. This took far longer than I had planned on spending on the whole project. A few heavy duty cable tie wraps eventually held it in place. The cables would be exposed for about a meter at either end but at least the majority of the run was protected.
After very carefully measuring the floor area below the seating storage compartment inside the van, I drilled 2 holes of 8mm diam thro to the underside of the van. Similarly 2 holes were drilled into the sidewall of the rear storage locker. Feeding the cables down thro the floor then thro the length of pipe and into the locker was easier than I expected and was completed in a very short time. I tied both cable ends together inside the van end and pulled all the rest thro to the back of the van, out of the locker and then back inside the van thro a window. This allowed me to use my soldering iron to fit the PL259 to the coax and the DB9 to the multicore. I then pulled the excess cable back into the van, at the radio end, until there was about 150mm showing at the antenna end. This would be sufficient to reach the RT-11. Inside the van I dressed the cables along the inside of the seating compartment and then thro a gap in the seating hardware to a narrow shelf on the sidewall underneath the breakfast table. More cable ties ! On the shelf I mounted a small ( 100mm x 50mm ) plastic box and the cables ended there. On the lid of the box there are BNC, DB9, Red & Black 30A power terminals and 3A aux power spring terminals. The power terminals are connected directly to the house batteries. 2 x 85Ah deep cycle lead acid types. The spring terminals are in parallel.
The RT-11 is mounted inside the locker ( which is on the right hand side just forward of the light cluster ) and the antenna is connected directly to a terminal post on the ATU. Before installing the ATU, I constructed a heavy duty 4:1 balun and fitted it inside the ATU case along with some terminal posts for antenna and ground connections. See this easy design at http://qrznow.com/make-41-balun-cheap-easy/ I used a T-130-2 toroid.
The ground terminal connected to A, is also bonded to the metalwork of the chassis by a very short braided cable. The antenna and counterpoise wires are attached to the balanced terminals. I assume the antenna appears to be an ‘L’ shaped dipole as far as the balun is concerned. The ground counterpoise consists of 2 wires measuring 35ft and 16ft. These are 1/4 wavelengths on 40m and 20m. When travelling, the antenna is collapsed, of course, and the antenna wire is hung in loops around the top of the white pipe. The waste pipe makes the antenna almost ‘invisible’ when on the move. The counterpoise wires are stored in the back locker.
Does it all work ? Oh yes. VSWR was checked with the antenna analyser built into the Icom 7300. On all bands from 40m to 6m the highest SWR was no worse than 1.5:1. The antenna is not suitable for 80m and no attemp has been made to make it so. My best DX on 30m WSPR, is Portugal, using 1W ( the lowest power setting on the 7300 ). From my location in NZ that is just 60km short of 20,000km. Can’t get much further than that on the planet.