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The Mini-Whip at University of Twente's (Netherlands) Remote Receiver |
I think many hams believe that they need
big antennas and lots of space to operate on 630m or even to listen on the band. I can assure you that big antennas are definitely not needed to listen or for
that matter, even to transmit effectively.
You might be surprised at just how well
you can hear on 630m using one of your low band antennas, such as an 80m or 40m
dipole. Best results will be had with the coaxial feed's shield lifted from
ground and connected to the center pin. This, in effect, allows your dipole to
act as a top-loaded 'T', with the coaxial feedline now acting as the vertical
element and the dipole section as a capacitive tophat. With a few ground radials, such an antenna can even
be used for transmitting, if suitably resonated, but that's another topic
altogether. Even untuned, this system will allow you to receive suitably on 630m
providing your location is not inundated with high noise levels. High noise
levels can often be overcome by using loops, either
resonated or
broadband.
Loops can be very effective at nulling noise, particularly if it is coming from one
direction, and vastly improving LF receive-capabilities.
Another popular solution and one that can be quite effective
at lowering noise levels and improving overall signal-to-noise is the use of a
short active (amplified) whip antenna. One very popular and proven design is
that of Roeloff Bakker, PAØRDT. His simple-to-build active whip has been
employed around the world and is being used by many LF'rs for dependable
reception on the NDB band as well as on 630m. Roeloff himself operates from a
noisy urban area yet manages to hear amazingly well on LF and MF with this
simple antenna. Unlike a much larger wire antenna or large loop, the small
active whip can be easily positioned in the quietest part of your backyard or
rooftop. Often just moving such an antenna a few feet one way or the other can
make a huge change in the noise level if you are operating from a typical city
or suburb location....the PAØRDT whip may be all that you really need for
long-term, reliable LF reception.
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Courtesy: PAØRDT |
Common practice is to
mount the PCB inside a small weather-sealed PVC pipe and erect it atop a mast,
after taking the time to "sniff-out" the quietest location for its placement.
With very few parts needed, the whip can be built for just a
few dollars...even less with a good junkbox and the needed transistors.
I even see the parts and PCB available on e-bay. As well,
Roeloff himself, can provide finished, inexpensive, beautifully handcrafted units ... ready to mount in the
air.
There are plenty of informative Mini Whip articles and
construction stories on the web should you wish to learn more:
If you're stuck with limited space for an LF-receive antenna, this may be just the answer!