2001-11-25
Small Tuned Loop AM Broadcast Antenna
I threw this antenna together in 10 minutes to help out my dad. For the past month or more there has been a curious band of interference blanketing the lower part of the AM broadcast band in the general area where I live. More on the interference later.
The antenna is a simple coil of (red) wire wound on the box of an NRG hammer drill I purchased some months ago. When I looked around the room for a suitable former the only thing that looked about the right size was the old cardboard box sitting in a pile ready to be dumped.
The coil is resonated by a BC tuning gang from an old valve radio. Its meshed capacitance is about 400 pF. The coil was wound at random (well not quite totally at random, I made an educated guess at the number of turns I'd need for a flat solenoid of that area, but it was all pretty approximate mental maths) there are 15 turns, which resonate with the fully meshed cap at about 600 kHz. Adding another turn would offer complete BC band coverage but I haven't bothered to add an additional turn.
The signal is extracted via a single turn of (blue) wire. Here is a close-up of the windings near the feed point. Although the images don't show it, there have been some recent modifications, there is a knob on the tuning gang now, and I've added a 500 Ohm pot as an attenuator, the poor old VR-500 just can't handle the monster signals this baby can generate, if you tune in 2KY while listening to 2BL the overload will generate intermod and totally wipe out 2BL's signal.
Attached to the CRO the cause is evident, 2BL is an over 30mV signal (yes milli-volts) at resonance (the strongest on the band locally), but 2KY at a mear 2mV or so can overload the VR-500 front end and destroy this monster 30mV signal. This is perhaps not surprising, as the VR-500 is not designed for use with a good antenna at any frequency, in particular the low bands.
The nulls are deep, very deep. This was surprising considering the rush job of construction. The loop is not electrostatically shielded and will pull a bit in close proximity to conductive (or dielectric) objects. The pattern is quite symmetric, but the null on the tuning cap side is deeper. This may be more due to the pick-up loop on the other side of the coil than the metal frame of the tuning cap. A balun may help, but the effect is only a few dBs so I'm leaving well enough alone.
My homebrew regenerative receiver loves this antenna. While I built it with a different purpose in mind, I intend to use it for AM broadcast listening now, perhaps I'll build a bigger shielded (coax) loop. If nothing else it has kindled my interest in the lower bands, I've always been a VHF/UHF hacker, but the value of small loops for the long bands (which with my limited space in a home unit is an obvious blessing) is being hammered home.
Its main advantage is its ability to null out rubbish or co-channel during the evenings. Being able to completely null a local 50kW monster is fantastic, as soon as the local is into the mush the DX stations with some QSB come roaring in. At my Dee Why shack there is a local noise source that starts up in the evening, composed of heavy broadband impulse noise it comes in along the power lines it is tough to null, but I can generally find a position that knocks it down enough to make listening pleasurable. It is better than a noise blanker at least. A larger loop with a decoupled feed can only be better.
Back on the interference story; the loops original reason for being. While I've tried to DF it the buildings are scattering the signal around so much as to make it hard to 100% sure where it is coming from. What I do know about it is this; it is intermittent (the worst kind), it appears at multiples of 100kHz or so, and is broad, like a malfunctioning switch-mode PSU in some appliance or computer. Sometimes it is much worse than others, some channels are unaffected by it (the higher ones), but some, like 702 have annoying hets from their carriers mixing with the harmonics. A hissy 2kHz tone gets annoying really, really fast.
I've got a pretty strong hunch that it is actually coming from a unit in my building. New tenants have recently moved in there and probably brought a defective device with them. I'm not about to knock on their door making any accusations until I am pretty sure, neither am I going to waste $$$ on the ACA until I've nailed it myself. Besides it is a challenge.
The Dee Why impulse interference is another matter. I haven't studied it long enough to start to trace it, but it is _very_ strong, you can see it riding on the mains coming out of wall-wart style AC plug packs. I suspect refrigeration equipment across the road in the Spotlight building may be to blame. It must be causing the locals all hell, because it is audible over the entire AM band, and most PC speakers with their hopeless filtering will reproduce it softly too. Any TV with a poor antenna will suffer as well, the harmonics are well into the VHF region. When it starts up I may as well pack up and go home, HF is useless and even weak FM signals get clobbered. The noise blanker just turns it into an S9 hash rather than the buzz-buzz sound that peaks about S9+20dB
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Updates
- 2002-10-20: A new MW loop