A Very Simple Broadband Receive Loop for the Broadcast Band
and HF Spectrum - W1AEX
Over
the years, my location has been fairly quiet across
the RF spectrum, but like most places, it has its
moments. In my own house, the paper shredder,
microwave oven, furnace ignition system, eliptical
exercise machine, dvd player, and other assorted
appliances can cause quite a bit of momentary noise
that is picked up by my main transmitting wire antenna
for the 160 - 40 meter bands. In addition, noisy
powerline hardware in the area can cause random noise
now and then. Fortunately, the layout of my yard
allows me to place a few receive antennas about 100
feet away from the house and the most obnoxious noise
generators.
I chose to build the triangular loops because they are
cheap, easy, broadband, and exhibit a bi-directional
pattern on the lower bands. They also require only one
support line at about the 25 to 30 foot level. There
is nothing really critical about the construction or
symmetry of the antenna, but it is helpful to take the
time to orient the loop in such a way that the receive
nulls, which are broadside to the loop, are facing the
most likely sources of noise. I ended up with a loop
that is oriented to favor reception from the north and
south on one edge of my property, and another loop
oriented to receive from the east and west in another
area of the yard. I have found that these receive
antennas are quite directional from the bottom of the
AM broadcast band through 40 meters. They receive
quite well up through the 30 meter band, but at the
higher frequencies they behave more as omnidirectional
antennas. At 20 meters and above they don't hear much,
but I believe that if a preamp were to be placed at
the feedpoint they would probably respond well right
up through the 10 meter band. The loops are nearly
invisible, and fairly indestructible, even through the
wind, snow, rain, and sun that we experience up here
in the northeast. The total length of wire used in the
loop is 85 feet. The schematic of the loop is shown in
the figure below:
If you are uncomfortable with the thought of winding
the 9:1 transformer, don't let that put you off. It is
a very simple task to construct it, and there is
nothing really critical about its fabrication. The
core material is mix 43 and the core only needs to be
large enough to let you pass the 30 primary turns and
the 10 secondary turns. Note that I have also wound
transformers using 21 turns/7 turns as well as 15 turns/5 turns and they
seemed to work fine, so experiment if you like. I made a few
transformers with
cores that I recovered from an old crapped out
computer power supply. I have no idea what mix
material they were, but they worked fine. I also
frequently find mix 43 core material at most ham radio
flea markets that sell for about a buck per core. The
pictures below will give you some ideas about how to
make a nice weatherproof transformer.
(Note: Solder one lead of
the transformer to the F-connector center conductor and
solder the other lead to the shield connection.)
The
picture
shows a 9:1 transformer built into a Radio Shack
project box.
A 4 foot length of conduit is
used as a mounting post for the transformer on the
north-south loop.
Tie
wraps hold the transformer in place against the
pipe.
The same mounting arrangement is
used for the east-west loop with the preamp mounted
underneath.
Tie wraps support the transformer and broadband
preamp in the Bud box underneath.
Initially, I simply tied the balun for
the east-west loop to the large tree seen near the
feedpoint in the picture above. Things went well with that
configuration for about a year, and then it got messy as a
squirrel developed an appetite for my RG-6 feedline. He
managed to chomp through the shield in about a dozen
different places that could be reached from the trunk of
the tree, and he actually exposed the center conductor in
one spot. The picture below shows where a few tasty bites
were munched out of the feedline. Fortunately, RG-6 is
relatively cheap, but if it happens again, I may need to
declare war. At any rate, moving it off the tree seems to
have made it a less convenient target, but only time will
tell if the rodents have actually lost interest.
Typically, I find that signals are roughly 20db lower
than what is received on the full-size transmitting
antenna, however, the signal to noise is vastly
improved. To see how the loop performs while listening
to a QSO on the 75 meter band take a look at the video
below for a short demonstration. The video starts out
with the transceiver using the receive loop. Keep an
eye on the spectrum scope as it gives a very good
indication of when the 256 foot center-fed antenna is
switched in by the visible increase in noise across
the baseline. At one point, the operating station
reduces power and the signal to noise advantage
becomes very apparent when the loop is switched in.
Loop reception on 75
meters with an IC-756 Pro III
The video below demonstrates reception of the AM
broadcast band with one of the loops. It does very
well across the entire medium wave band as it picks
off stations up and down the east coast during a late
afternoon listening session. As the receiver is tuned
you will see several IBOC signals on the broadcast
band. They are characterized by the sound of white
noise as you tune through the flat-top digital streams
on either side of the analog signal when viewed on the
panadapter. The most prominent one in this area is
WTIC at 1.080 MHz. The bandwidth of the Flex 5000 is
set to 9 KC to prevent hetrodynes from adjacent
stations as it is tuned up the band. At 1.480 MHz the
bandwidth is briefly widened to 16 KC to enjoy a brief
segment of an old song I like. I have found that if I
pause for more than 30 seconds on a song that is being
played on the air, the YouTube "copyright auto-bot"
will flag the video and inform me that the audio will
be disabled until the video is reviewed by their
staff. Apparently, less than 30 seconds complies with
the public "fair use" policy under our legal system's
definition of copyright infringement. Note
that if you select a viewing resolution of 720p you
can view this video in high definition at full screen.
Loop reception on the broadcast band with a Flex
5000A
To install the feedline I slit the soil and run
the RG-6 a few inches under the ground to avoid
mowing over it with the lawn mower in the summer.
The 300 foot length of RG-6 for the north-south
loop has been out there for over 20 years and it
still works fine! At any rate, these loops are
very cheap and easy to make and give me options
when the station transmitting antenna is picking
up more noise than I wish to tolerate. Once you
gather the materials together, it only takes about
an hour to get a loop in the air and connected. As
a final note, after running the two loops for
almost 2 years, I
built and installed a W7IUV
broadband preamp at
the feedpoint of the North-South loop.
The performance increase was nothing short of
amazing. The video below shows a gain of roughly
four S-units on the 40 meter band with the preamp
engaged. In the video a weak signal right at the
band noise threshold becomes fully readable with
the preamp inline. It's fairly obvious when the
preamp is "in" and "out" when viewing the video,
but the picture below the video shows the
difference in recovered audio under both
conditions. Needless to say I added a preamp to
the East-West loop as soon as the parts came in
from Mouser the following week!
Loop reception on 40 meters with a broadband preamp
installed
The audio waveform of the video above makes it pretty
obvious when the broadband preamp is engaged.
Comparison of the 190' double extended zepp vs. the
north-south loop with the preamp active during a 40
meter AM QSO with Rich-K1ETP and Tim-WA1HLR.
For complete construction details visit the page
that Larry W7IUV
has put together and download his W7IUV Preamp
article which is in PDF format. Larry has provided
everything you need to know to easily put together
one of these outstanding broadband RF preamps.
Print out and study his article and any questions
you might have will be answered. Normally I would
build something like this in dead bug style, but I
really liked the way Larry utilized a Dremel tool
to make the board so I added a brand new Dremel
tool to my collection of things to play with here.
As he mentions in his preamp article, it's really
easy to create a board for this project and in
fact, once I laid it all out it only took about 10
minutes. After looking at a bunch of layouts in
Larry's article I ended up with the pattern of
isolated pads in the picture below. The picture of
my layout is quite a bit larger than the actual
board which measures 2.5" x 2.0" so that it fits
snugly inside a standard Bud box as in the bottom
photo. Note that this design is based upon Larry's
2009 design of the preamp. You should always check
Larry's web site for the most up-to-date version
of his preamp design as components and the layout
may change.
My board layout for the 2N5109
preamp
Obviously, I drank too much coffee before making
these two boards, but they worked fine.
My first attempt at the W7IUV preamp shown with the
cover removed. It works perfectly.
I liked the first one so much that I built two more!
The one on the left definitely needs a bigger
heatsink before seeing regular use.
To power the preamp I feed
the 13.8 VDC through the coax using a little power
injector that I built into an old F connector type switch
box. The voltage travels to the preamp through the center
conductor of the RG-6 feedline and passes to the +VCC
point on the circuit board through a 10 mH choke. The
choke is visible in the upper left hand corner of each
preamp in the photo above. I added a normally closed
bypass relay which can be seen mounted upside down in the
upper right part of the two preamps pictured. When no +VCC
is present the relay connects the antenna input jack to
the preamp output jack through a .1 uf capacitor. This
allows me to use the loop with the preamp turned off. When
power is applied, the relay opens the normally closed
bypass path and the preamp is then functional.
If you are not interested in building a preamp but still
want the advantages of all that gain at the feedpoint you
might consider looking at the Advanced
Receiver Research broadband HF preamps. They are
priced very reasonably and I believe they can be powered
through the coaxial line using the DC
Injector/Bias-T that is sold separately.