Recent events have enabled me to return to New Zealand for a bit while Mum (my Wife in NZ Slang) spends time with our kids before they head off to University and are lost for good for a while. This has meant I have even more free time on my hands as there is not a lot for me to do down here. Besides it is cold (and I don’t like work anyway!) The flat we are living in does not lend itself to putting up any type of antenna so I have been going mobile to get my ‘on air’ radio ‘jollies’. I have not done much mobile work in the past as we were living in one of the most populous cities in the world, with horrendous traffic problems I needed all my wits about me just to drive there. Besides with a 6 element OWA Yagi at about 35m; if I wasn’t hearing anything, it usually meant there was nothing about. Much easier to work any DX from home.
NZ is a totally different experience and New Plymouth even more so. Much easier to go mobile.
I would like to share a few of my experiences going mobile and trying to make the best of what limited DX is about these days. The car is fitted with a Wilson 5000 on a smallish 3 magnet base. As the power goes up so does the SWR! Tried 6 radials but they didn’t help the SWR at all. Made it worse. Had to retune the antenna, but the SWR would not settle under 2 to 1. I then grounded the antenna coax at the base of the antenna and ran all the radials to to ground points around the car. That has helped but the SWR is very sensitive to anything within a few meters. Great when in the open. I have been using a 40ch President McKinley with a KL203 following, for about 80 watts PEP. Radio and Amp fed with a direct fused lead off the battery. If I am in the right spot I can work skip into Australia most afternoons on Channel 35 LSB.
One thing I have found with the car is that location is very important and being mobile we have the opportunity to explore and try out various places. I have found a few sites where the signals always improve. Almost without fail, any highpoint overlooking the sea has not only stronger signals but sometimes the only signals. Close to the sea is also good but the clifftop seems to have the edge, especially if facing the direction the skip is coming in from. The location seems to be more important than what ever antenna is being used albeit the better the antenna the stronger the signal, but a better antenna somewhere else still won’t hear the signals you can get from the seaside cliff top.
It is great to sit in the car and work DX but with a mobile antenna it is still difficult to work the weak signals with out bucket loads of power. Too much power in the mobile and you can run into all sorts of problems with newer cars. Often interfering with the security systems and setting them off. OK if its your car but freaks people out when its their car electronics that suddenly goes into melt down mode. Some of the modern cars computer systems can be ruined by placing them in a hi RF field. I would like to run 500w from the car but too scared. The wiring harness is all set up, but the little KL203 is all I got for now. It works off the same 10 amp lead as the radio does. It is protected by a 10 amp fuse that blew the other day in the middle of a QSO with NSW . Buggar. No spares on hand either. It now has a 15 Amp fuse in it! Hope the wire don’t melt next time!
But what we really need is an antenna we could easily deploy portable and get away from having an antenna that need the car shell to work?
I remember from an 11m activation in Bhutan a few years back that my simple inverted V atop a 10m fibreglass push up pole worked a treat. Over a thousand contacts in a couple of days and being omni directional did not have to worry about direction. If it was there and we heard it; we worked it. I was very impressed with the way it worked on 11m. Single mounting point and the string on the wires acted as guys for the pole as well. if there was a downside it was running the coax feed to the top of the pole.
I got to thinking that if we added some more wire we would have a quad loop and the feed point would come further down the pole and we would pick up some gain as well
But then what is gain? We only have so much power and antenna gain only means we have focused the power in a certain direction or angle at the expense of other directions and angles. When we are looking at DX we need to keep all our options (angles and directions) open.
With all these thoughts going through my head I went and bought a 100m roll of 300 ohm TV twin lead and with the help of the internet built an inverted V folded dipole, an extended double Zep also to be deployed in an inverted V configuration and a bi square quad loop fed at the bottom in a diamond shape. I was only intending to use these on on ch 35 so figured with a bit of pruning and an MFJ 941 tuner I could find a suitable match.
The folded dipole worked a treat. it tuned in at 1.5 to one using a 4:1 balun and a short length of coax into the car. No tuner required. Simple, light (made entirely of TV ribbon), Cheap, but as soon as the wind got up it blew the lead in all over the place. It did not seem to work significantly better than the wilson on the car, but I did not really give it enough time due to the wind. But it tuned up great with out needing the tuner. Fluke, good luck or good design? I am still wondering.!
Next in line was the zep, which in my language was just a dipole fed with ribbon with legs of 0.64 wavelengths long in inverted V configuration. 5/8 verticals work really good so maybe this will too. Received noise was considerably more than the wilson but unfortunately there was no skip to determine whether signals improved as well. I could not get it to tune with the MFJ no matter what so gave that a miss and went on to the Bi Square. Still need to experiment more with this and see if we can find a match.
The Bi Square. Wow. That was an eye opener. It really picked up the received signals. A lot. A few S points. Big difference. Once again using the balanced inputs on the MFJ was a waste of time. That tuner can not tune a balanced line. Once we put a decent current balun in line and a short length of coax back to the MFJ, the tuner handled it well and was making contacts into the East Coast of Australia easily when the 10A fuse blew and that was the end of that. It is a large antenna the size of a 20m quad, but no more difficult to build and erect than the inverted V dipole. The big issue with this antenna is how to simplify the tuning. If feed from the bottom then it really needs a proper Balanced Tuner or the correct matching stub, plus a balun back to the coax to the radio.
I also read somewhere it could be fed at a quarter wave off the bottom but then instead of two full waves in phase you would have a 3/4 wave on one side and 5/4 on the other. This would probably affect the gain and radiation pattern but if it simplifies the matching it might be worth a go. More wire in the air the better maybe? That would allow a direct feed with open line to a balun and back to the radio like the folded dipole inverted V.
But these wire antennas have two major disadvantages.
The first is the wind. The wind blows the shit out of them as they are essentially light weight portable antennas. They don’t like the wind.
The second thing is the polarity. They are primarily horizontally polarized antennas and don’t work well with long range ground wave vertically polarized signals. When we head up to a mountain car park to work local DX we need to be vertically polarized as the majority of NZ stations are running verticals and all cars will be running vertical.
Back to the drawing board. Just grab a station master and a 20′ pole? Easier said than done if we are looking for quick deployment. The tapered 10m push up telescopic fibreglass pole is so easy to raise and lower with the sections friction locking with a twist but it won’t take the weight of an Aluminium antenna.
Enter the coaxial end fed dipole. Simple. One piece of coax from start to finish. No matching sections required, Just need to choke the coax to prevent the RF coming down the coax and into the car. I have one built out of RG58 and will be testing it in the next few days. If you want to build one do your self a favour and don’t use coax with an aluminium sleeve under the braid. It is not worth the effort to remove it from the inner conductor. This antenna could be the answer. Cheap. Not too heavy. Simple to construct and tune and get up in the air. Doesn’t require ground planes. But will it work.? Fingers crossed. More next week.