RFI: "Good RF Grounds" [Way too long!]
Cough cough... may I interject?
I am in an urban location and I am in no position to put up a dipole for
my SWL needs. The best options as I see them in order of preference are
(1) a broadband loop such as a Wellbrook ALA1530+ (2) an Inverted-L (3) a
random wire. I have about 5m up and 5m horizontal to play with, so for
most HF listening I'm probably going to have a high impedance for the
wires and a low one for the loop.
From what I've been reading, the random wire in an urban environment is
probably the worst of my options, since it will be e-field dominated and
pick up all manner of horrific local/near-field noise.
That leaves the next option, the inverted-L. The consensus is to use a 9:1
or 3:1 un-un at the base point with a common ground with a coax feeder
running back to the shack. I don't see how this would work without at
least an attempt at grounding the base of the antenna. Or should I ground
the high-impedance winding connected to the L wire and then float the
coax that runs back to the RX?
The loop is obviously by far the most elegant design, since as you say,
it obviates the need for any rf grounding. However, the Wellbrook designs,
however wonderful their IMD and nulling are, are a bit pricey for someone
who's just bought an SDR-IQ and is thinking about buying a Perseus next
month.
The inverted-L will cost me about nothing. However, can I emulate a
Wellbrook loop with a good Norton amp (either single-ended or push-pull)
and my own 1-2m diameter hoop of copper tube (I have loads of 1/2" lying
around here)? If I do this, what should I aim for as a middle-of-the-road
source impedance from the loop so a can wind the right kind of toriod? (I
want to listen from the bottom of the MW band to 10m).
Any practical experiences with inverted-L's or untuned loops would be most
welcome - since my SDR-IQ will be arriving on Monday and I do need to "get
some wire in the air"!
Cheers
Alex
|