I had always thought that an aerial mast or tower that could either be easily lowered or easily scaled would be great for experimentation… so when the opportunity to buy a second-hand 8.5 metre steel lattice tower arose, I couldn’t resist it! A huge lump of concrete, new paint job and five mates later, the object of my dreams was proudly erected! Within minutes I had scaled the tower to add the first of several wires and I have since been up and down over 20 times… but who is counting!
One of the objectives of putting up all sorts of wires was to find something that would lessen the amount of electrical interference and let’s face it, in modern households and neighbourhoods there is plenty of that! Firstly I did some reading on the Nordic DX Club’s web-page and come to the conclusion that it was time I sorted out my earthing and dabbled in baluns!
I was of the opinion that the mains earth connected via the three-pin plug to my receiver was as good a ground as you’d get… WRONG! A simple matter of driving a 2 metre galvanised rod into the garden near the base of my tower (which was also earthed to the rod) and running a heavy 6mm (or larger) cable from the rod to my receiver provided a very large decline in the amount of electrical noise. Most receivers have an earth stud on the back for this purpose, if not then one half of your aerial input will be an earth connection too. Across a wide range of frequencies, time and time again, intrusive noise all but disappeared. If you don’t have a separate earth rod, GET ONE! They’re available along with the 6mm cable and clamp from any electrical wholesaler. There is a whole raft of well-documented and accurate theory as to why this works, though I’m a practical type and am not writing a book (yet!) so I’ll try and keep the technical chat to a minimum and simply tell you what works!
On the subject of leads, using coax at low frequencies is very effective as losses are small, compared that is to the higher SW bands and anything above. If your tower/pole is some way from your receiver then the following tips are well worth consideration. Firstly bury your coax under ground for as far as possible – this stops the coax picking up unwanted noise signals. Secondly, earth the coax in as many points as practicable. Not only at the set but every 5 metres or so and to your main earth stake as well. Plan carefully so your coax cables are routed past your main earth as they run to your tower/pole. Earth your tower/pole as well, and if you want to get really keen, run your coaxes inside an earthed pipe up the tower, or up the inside of the pole itself.
On tower location. Sometimes you have little choice as to where you can site your tower though ideally it should be as far from electrical noise sources (i.e. houses etc.) as possible. Your wires should, whatever your tower situation run away from houses too… avoid having them running over rooves etc. or parallel with power pole wires… though running them at right angles may possibly yield low noise if they are sufficiently far away. My tower is only 1.5 metres from my house but with a good earth and a balun, things are working well!
What is a balun. A balun is a matching device that allows for maximum signal transfer between the aerial and coax cable into your receiver. I won’t get too technical here, but if you have a centre-fed dipole or a longwire and you intend to listen to a wide range of frequencies then you NEED a balun! And not the 1-to-1 ratio balun as used by the amateur radio fraternity, but rather something like the 9-to-1 balun as described in an article by Nick Hall-Patch and John Bryant. More on baluns later! To illustrate my point about maximum signal transfer, consider this analogy: imagine a 600mm diameter bucket of water held aloft with a 50mm diameter pipe to drain it. Consider that the bucket represents the aerial and the pipe is a coax cable that feeds the receiver, which has a 50mm socket for the pipe. To get maximum flow of water (read: signal) the pipe should be the same diameter as the bucket, and the receiver likewise. This isn’t practical as coax cable doesn’t come in 600mm and the receiver doesn’t have a 600mm socket! Now, one way of maximising the transfer of water (signal!) into the receiver would be to fit a pump below the bucket… and that’s where the balun comes in… it, in this analogy would become a pump and would quickly empty all the water into the set! Now, technical types will have realised this already, but when I have said 600mm I was really referring to an impedance of 600 ohms and 50mm is really 50 ohms. A random length wire, or rather a wire used at random frequencies will exhibit a wide range of impedances and with due deference to all the technical types steeped in tradition and most probably spitting the dummy right now, for DXing it is more practical to build an aerial to suit a wide range of frequencies than several or more to cover all bands of interest.
So, with an insight gained into these little devices (they look like a very small, dark grey doughnut with wire wound on them) I did some experimenting. My main aerial was a 25 metre V-beam fed by 50 ohm coax. One side of the V connected to the centre core, the other to the shield. As theory would have it, this aerial worked well at it’s resonant points, i.e. 49mb, 25mb & 13mb though as I had no point of comparison, I was unaware of how much the performance would drop off on non-resonant bands, i.e. all the others 90, 60, 41, 31, 19 & 16mb. So I strung a second wire under the main wire and fed it with coax and a balun then conducted a series of tests comparing the two across a wide range of frequencies from MW through to the 11mb. With the exception of the 49, 25 & 13mb, the balun-fed aerial was clearly better, and much quieter exhibiting lower pickup of electrical interference. Also, on the frequencies the coax-fed aerial favoured, the balun-fed aerial was only slightly lower in signal strength but more than made up for that in most cases by the reduction in background electrical interference. An added bonus I hadn’t considered was that our local radio station, Radio Waitaki on 1395kHz, on the coax fed aerial splashed all over 1386 and 1404 though with the balun-fed aerial both frequencies were audible! Even at MW frequencies, signals completely obliterated by electrical noise on the coax-fed aerial were audible on the balun-fed aerial! End result, now both of my aerials are fed with baluns!
My next experiment was to compare the balun-fed aerial with a open-wire feeder. The latter is basically two parallel wires running all the way from the top of the aerial to the set with a matching transformer at the set.. This took quite some setting up to run the wires off the mast, held apart by spreaders, under the house and through the floor to the receiver – so I was keen to make it work! Again the balun-fed aerial proved consistently better over a wide range of frequencies so the unsightly (according to my wife) open-wire feeder was removed. I figure that because the open-wire feeder passed through the house it was more prevalent to picking up nearby noise.
Now that I have optimised signal transfer from the aerial to the set, installed a good ground system and upgraded my coax, there is more experimenting to come.. I plan to compare other types of aerial with my dipoles. In the making is a terminated Delta loop, a T2FD (Tilted, Terminated, Folded Dipole), a sloper and a EWE, plus anything else that seems promising so more reviews will follow!
For more information on obtaining baluns & coax, please write to me. Let me know what sort of aerials you are using, even draw a diagram of your available space, house, showing me where your aerials run etc. and I’ll be able to give you some advice. Contact me at 13 Swift St, South Hill, Oamaru 9401, New Zealand Email: email@example.com