By George Beardsmore and Jim Searle
As a result of an earlier article on aerials, baluns and earthing I have had contact with quite a few members. Many have used the material to improve their aerial system and others have given the benefit of their experience in these matters. I thought it worthwhile that these ideas be shared further, so the following anthology of tips comes to life. Credit goes to New Zealand Radio DX League members George Beardsmore of Mosgiel and Jim Searle from Dunedin… these items aren’t verbatim though I hope my editing conveys the correct impression. Firstly, the “good oil” from Jim…
– It is interesting to check for receiver internal noise. No matter what you do with antennas and grounds, this level of noise (if any) will always be with you. Checking this is quite simple: disconnect the aerial and turn the volume way up… ideally you should have total silence. Noise remaining in the way of hiss indicates circuit noise… i.e. the noise generated internally by the receiver. Today’s digital read-out sets with their synthesisers and fluorescent displays are culprits in this respect. Jim says that on this test his older JRC NRD-515 is totally dead on this test while the more up-to-date NRD-535 has some internal noise. To quote Jim “The latest is not always the greatest or best”
– Earth Rods: Electrical earths (i.e. the house mains earth) and signal earth (i.e. the rod for your receiver/aerial system) should always be separate and the rods should be as far apart as possible. The signal earth rod site should be as close to your receiver as possible. Short, heavy wire is mandatory. The mains earth is not as critical to the length of wire. Jim’s signal earth rod is 2 metres from the receiver and mains earth 3 metres from the signal rod, he considers this too close together and plans to move it to the other side of the house shortly. Mine is 4 metres from the receiver and 9 from the mains earth. Jim has plans to run buried wire radials from his mast earth.
– Interference-zone: Jim advocates keeping all of your connections and aerials at least 5 metres away from your house in order to avoid picking up electrical noise from within. In days gone by, it was quite normal to put the mast beside your house and have open wire running down to your receiver… back when all the electrical appliances in your house created as much noise as a bee rubbing his legs together! It also seemed to make sense that you would have your feed-wire as short as possible… uh uh! Read on! Now days we are dealing with all sorts of electronic hash emanating from our abode, Sky TV decoders, washing machines, microwaves, light dimmers, even electronic power-meters and a whole lot more.. So if you’re looking at a site for your mast, find the point on your section as far from yours and your neighbours house as possible… run your coax from your receiver (underground if possible as this will also prevent pick-up of noise) all the way to your receiver… remembering that at SW frequencies and lower, coax doesn’t have anywhere as much loss as VHF and UHF. And while on the coax theme, if it’s a particularly long run of 20 metres or more, it might pay to buy the more expensive cable. Right, now we’ve got our mast at the end of the section… and need somewhere to attach the other end of the aerial… but the only other high point is the house… and you don’t want to wander into that 5 metre “interference zone”… easy… just use nylon rope for that last 5 metres… hey, your antenna may be a bit shorter but the improved signal-to-noise ratio will more than make up for that.
– Triple-earth/isolation balun set-up. Jim’s idea of the ultimate: an isolation balun as close to the receiver as possible, another on the mast and a third earth connection. Isolation baluns, such a balun (transformer) has two windings completely separate from each other… one winding is connected to the aerial, the other to the receiver. Then an earth is attached to one side of the aerial winding and connected to the signal earth, i.e. an earth rod closest to the receiver. Then coax is run from this point all the way to the top of the mast where it is connected to a second isolation balun, with the aerial connected to one side of the aerial winding and the other side to a third earth rod, the mast earth … again the distance between the mast earth, the signal earth and mains earth should be as far as possible, and with your mast some distance from the house as in Jim’s case, this will not be a problem.. The receiver is now earthed via the mains as would normally be the case. It is important to only earth the coax at one place… preferably at the isolation balun beside the receiver. Never earth the coax at both ends otherwise you will have problems with earth loops and stray interference currents. In my situation I use isolation baluns on two-wire aerials and instead of earthing one side of the antenna balun, I connect one leg of the dipole on each side of the balun… seems to do the trick here as I can hear Yanks on the X-band even with the computer on… and if I had read Jim’s letter first I wouldn’t have put my mast right beside the house! Though at 8.5m tall it must be getting far enough away from the interference zone. Still, I’ll regret not having it down the end of the section and be forever wondering just how quiet it is down there!
And some sage advice from George…
– Earths: If you are using a metal pipe always check with a sensitive ohmeter for good, sound connections and seal the joins to prevent corrosion. A pipe driven straight into the ground is OK but dry conditions do not make for good electrical contact to earth. To assist against this happening, drill small holes into the pipe around the circumference in random fashion before driving. Then place a small funnel into the neck of the pipe and fill with water from time to time to prevent the ground drying out. George also tells us of a commercially available salt compound used by electrical companies who need good earth connections. I can also think of a gel-type solution sold by gardening shops for retaining moisture in pot-plants that may be worth a go as well. And also on the gardening theme, how about one of those variable drip water heads as used in those micro watering systems? My wife uses one to keep the goldfish pond topped up during summer when transpiration is high. Alternatives: You can also use copper wire radials buried as deep as practical – preferably 90 degrees horizontal spacing from each other in circumference. Solder joins and check for near zero resistance between them. If using a metal tower, pole etc, earth it to avoid any spurious noise-making capacity that may carry to the antenna.
– Antennas: One source of copper wire (for ground radials and aerials) is old electrical wire from buildings being demolished. This can usually be readily stripped if for use as radials, will solder well and is very strong…. George’s aerials have withstood 120-130kmh winds… and can be used unstripped, easily sealed etc… which is especially important if you are in a salt laden atmosphere.
– If you are short of room for two masts and have one pole at the centre, you can drop the ends to say 1 meter from the ground if you can maintain a 45 degree angle to the horizontal. This is an “Inverted Vee”. Such an aerial is omnidirectional unlike a dipole which works side on resonating frequencies and end on between resonance.