Tape Measure Beam for Power Line Hunting - W1TRC.pdf
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Adapting a Three Element Tape Measure
Beam for Power Line Noise Hunting
Find that noise source to point your power utility in the right direction.
James T. Hanson, W1TRC
I
have been using an MFJ-852 power
line noise meter in conjunction
with my homemade ultrasonic
power line arc detector to track
down power line noise.
1
The MFJ-852 meter
uses a simple, built-in dipole antenna for
reception. In the process of hunting down
power line noise, I came across a situation in
which I just could not pinpoint the noisy pole
with my MFJ-852 power line noise meter. I
decided that there must be more than one
noise source in the vicinity. ARRL Lab RFI
Engineer Mike Gruber, W1MG, pointed out
the difficulty of using the MFJ-852 power
line noise meter in a complex noise environ-
ment in his November 2006
QST
review of
the meter.
2
Because of the problem I was
having at this location, I decided to replace
the simple dipole used in the MFJ-852 with
a directional three element Yagi.
Figure 1 —
YAGI-CAD
predicted tape measure beam patterns.
One characteristic that is common to both
RDF and noise hunting beam antennas is
ease of getting it in and out of a car. This fea-
ture is accomplished by the use of steel tape
measure elements. The elements can easily
be folded when fitting the antenna into a car
and yet, because of the short lengths at VHF/
UHF frequencies, they are self-supporting.
Another desirable design goal was to use
materials that were easy to obtain. The beam
uses schedule 40 PVC pipe and fittings
available at any hardware store for the boom
and element supports. This keeps the cost for
the antenna very low. The element supports
consist of PVC crosses and Ts. The elements
are cut from a steel tape measure.
program called
YAGI-CAD
written by Paul
McMahon, VK3DIP, was used to design
and evaluate the antenna.
4
A summary of the
theoretical beam characteristics achieved is
contained in Table 1. A plot of the E (elec-
tric field) and H (magnetic field) plane beam
patterns predicted by
YAGI-CAD
is included
in Figure 1. Notice that the beam has a
theoretical front-to-back ratio of 50 dB. I
have found the front-to-back ratio to be very
impressive in actual use.
An Easy to Make Yagi for the ’852
While doing some searches on the
Internet, I came across the Web site of Joe
Leggio, WB2HOL, titled “Tape Measure
Beam Optimized For Radio Direction
Finding.”
3
Joe developed a three element
tape measure beam that he optimized for
2 meter fox hunting. The characteristics that
have made his design such a success for fox
hunting include all of the desirable features
of an antenna optimized for power line noise
hunting. These features include a very high
front-to-back ratio, low cost, easy construc-
tion with simple hand tools and lightweight
portability.
The antenna evolved during a search
for a beam with a really great front-to-back
ratio to use in hidden 2 meter transmitter
hunts. The design exhibits a very clean pat-
tern and is perfect for radio direction-finder
(RDF) use. It trades a bit of forward gain in
exchange for a very deep notch in the pattern
toward the rear. These features, along with
ease of construction and low cost, convinced
me that this would be an ideal antenna to use
for power line noise hunting.
The Yagi Elements
The beam consists of a boom constructed
out of
1
⁄
2
inch PVC schedule 40 pipe and fit-
tings and elements cut from 1 inch wide steel
tape measure. I found a 25 foot, 1 inch wide
tape measure at the bargain table of a local
hardware store for under $5. This provided
more than enough tape to build a three element
beam at 135 MHz, the operating frequency of
the MFJ-852 power line noise meter. I have
since discovered that Home Depot also stocks
a 25 foot, 1 inch wide tape measure for under
$5. Inductive hairpin matching is used to
match the antenna to 50 Ω coax.
Antenna Design Details
A shareware computer aided Yagi design
Table 1
Three Element Tape Measure
Beam Performance Predicted by
YAGI-CAD
Gain
7.3 dBd
Front-to-back ratio
>50 dB
Beamwidth, 3 dB E-plane
67.5°
Coax Termination
In the original RDF application, RG-58,
Beamwidth, 3 dB H-plane
111°
1
Notes appear on page 31.
28
May 2007
Figure 2 — 135 MHz tape measure beam assembly.
T. This difference is 0.5 inch, which is mul-
tiplied by 2 to account for each end. This
is where the
1
comes from in Equation 2. If
you find the dimensions of your PVC parts
differ from this, an adjustment to the factor
“1” in Equation 2 should be made, so that the
resultant element spacing is correct.
L
N
= L
146.565
*(146.565/F
N
)
Table 2
Three Element Tape Measure Beam Dimensions (Inches)*
Frequency (MHz)
146.565 135 224 432
Refl ector length
41.38
44.92
27.07
14.04
Driven element length
35.50
38.54
23.23
12.04
1
⁄
2
driven element length
17.75
19.27
11.61
6.02
Director length
35.13
38.13
22.98
11.92
[Eq 1]
Refl ector to driven element center to
8.00
8.69
5.23
2.71
center spacing
Refl ector PVC length (see Note 1)
L
B
= L
E
– 1
[Eq 2]
7.00
7.69
4.23
1.71
where:
L
N
is the element length, element spac-
ing or hairpin match length in inches at the
new frequency.
L
146.565
is the corresponding 146.565
MHz antenna element length, element spac-
ing or hairpin match length in inches.
F
N
is the new frequency in MHz.
L
B
is the boom length in inches at the
new frequency.
L
E
is the element to element spacing in
inches at the new frequency.
The final dimensions for several differ-
ent frequencies are included in Table 2. I
found that the design scaled from 146.565
MHz to 135 MHz very well, and Joe Leggio,
WB2HOL, has informed me that it scales up
to 440 MHz just as well. It is recommended
that if a 440 MHz version is built, that a
1
Director to driven element center to
12.50
13.57
8.18
4.24
center spacing
Director PVC length (see Note 1) 11.50 12.57 7.18 3.24
U matching total length 5.00 5.43 3.27 1.70
*The length of PVC can vary somewhat depending on the PVC fi tting exact dimensions.
The critical antenna dimensions are the center to center element spacings.
of 146.565 MHz. Since the MFJ-852 power
line noise meter operates at a frequency
of 135 MHz, the dimensions must be
changed for this frequency. This is read-
ily accomplished using Equations 1 and 2
that scale the dimensions of the 146.565
MHz design by the inverse of the new fre-
quency. Equation 1 is used to scale the ele-
ment lengths, element spacing and hairpin
match length. Equation 2 is used to scale
the reflector to the driven element and the
director to the driven element boom lengths,
which are shortened to take into account
the length added by the PVC cross and T.
Equation 2 is based on a distance of 1.25
inches from the edge of the PVC cross or T to
the center of the corresponding beam element,
and a penetration of the boom sections of
0.75 inch into the corresponding cross and
50 Ω coax is connected directly to the driven
element and matching network. I decided
to create a balun by wrapping several turns
of the coax around the boom right after the
connection. This has also been done on a
number of antennas that have been built for
RDF use. The balun makes the transition
from the balanced antenna feed point to the
unbalanced coax. It should help generate
a symmetrical antenna pattern. There is a
measured antenna pattern plot on the Web
site referenced in Note 4 that shows that the
pattern is somewhat distorted. The pattern
was taken without a balun, and it has not
been redone with a balun.
⁄
2
inch wide tape measure be used in place
of 1 inch tape measure used in this model.
Remember that as frequency increases, the
tolerance of the dimensions gets tighter so
at the higher frequencies, accuracy becomes
more important. The antenna does not scale
well if small diameter element material such
Adapting to Other Frequencies
The original tape measure beam was
designed for a 2 meter fox hunt frequency
May 2007
29
as welding rod is substituted for the tape
measure elements.
Hooking it Up
Remove the paint from the corner of the
driven element tape with some emery cloth
and tin the steel tape. Temporarily tape the
driven elements to a piece of scrap wooden
board using electrical tape so the element
ends are
3
⁄
4
inch apart. Pre-form the hairpin
match into a U shape so the spacing is
1
⁄
2
inch.
Flare the last
1
⁄
4
inch of the hairpin match out
about 45°. Now solder the hairpin match and
the coax to the elements. Carefully remove
the electrical tape and position the assem-
bly over the PVC cross. Slip the stainless
steel clamps over the ends of the elements
and tighten the clamps. The final assembly
should look like the detail in Figure 3. After
winding and wrapping the balun with elec-
trical tape, the RG-58 coax was cut about
2 feet beyond the end of the reflector PVC
cross and a BNC male connector installed
on the coax. Figure 4 is a photograph of the
assembled beam. Fine-tuning of the final
match can be accomplished by increasing
or decreasing the gap between the driven
element halves. I checked the match with an
MFJ-259B SWR analyzer and found that it
was very close to 1.1:1.
Putting it All Together
A drawing that shows all of the 135 MHz
antenna construction details is included
in Figure 2. The PVC components are not
glued together. There is enough friction in
the PVC pipe to hold the beam together
and this allows it to be taken apart easily for
transportation.
The tape measure can be cut with a pair
of tin snips or a heavy pair of scissors. No
matter how you cut the elements, be very
careful. The edges are very sharp and will
inflict a nasty cut if you are careless. Use
some emery cloth to remove the really sharp
edges and burrs resulting from cutting the
elements to size. It is also a good idea to
chamfer the edges by cutting about
1
⁄
8
inch
off each corner. Put some vinyl electrical
tape on the ends of the elements to protect
from getting cut. Wear safety glasses while
cutting the elements. Those bits of tape mea-
sure can be hazardous.
When I cut the tape, I used the tape
measure itself to measure the length of the
elements. I started out by cutting the end
of the tape off at the 2 inch point and added
2 inches to the length of the first cut element
to determine where the second cut should be
made. I then trimmed the remaining tape to
the next whole inch mark on the tape, and
with simple arithmetic determined where
the next cut should be made. I continued
this process until all of the elements were
cut to size.
Figure 3 — Hairpin match and balun
details.
Taking it Apart
When it is not in use, and when transport-
ing the beam, disassemble the elements and
director portion of the boom. The boom sec-
tion that goes to the reflector is kept attached
to the driven element since it has the coax
balun attached. Each of the elements can
be folded back on itself and slipped into the
corresponding PVC cross or T to minimize
its size. The disassembled components are
shown in Figure 5. I carry the beam pieces
in a plastic shopping bag.
hairpin match is not critical. Hams have used
anything from 22 to 14 gauge stranded or
solid wire. I used 14 gauge solid copper house
wire. It appears to be a good choice from a
mechanical survivability point of view.
Holding it Together
The elements are attached to the PVC fit-
tings with stainless steel hose clamps. The
only tricky part of assembling the beam is
soldering the coax and hairpin match to the
driven element, since the PVC will melt if
you apply too much heat. One solution to
this potential problem is to do the soldering
before assembling the driven element to the
PVC fittings.
Keeping Track of What’s What
Notice that the driven element and reflec-
tor both use PVC crosses while the director
element uses a PVC T. This was done pur-
posely to make it easy to recognize the direc-
tor and reflector elements when assembling
the beam.
The wire size and type of wire used for the
Interfacing the Beam to the
MFJ-852 Power Line Noise Meter
The final part of the project is to inter-
face the beam with the MFJ-852 power line
noise meter. The MFJ-852 has a metal front
Figure 4 — Assembled 135 MHz tape measure beam.
Figure 5 — Disassembled beam ready for transport.
30
May 2007
panel but it is in a plastic box. I wanted to
shield the electronics from any stray pickup
to preserve the deep notch off the back of
the beam, so I removed the plastic box and
unsoldered the antenna coax connection
from the PC card. The plastic box, telescop-
ing antenna sections and coax balun are not
used. I then mounted the MFJ-852 power
line noise meter front panel and electron-
ics to a cutout in a 3 × 5 × 4 inch aluminum
BUD box (BUD part number AU-1028). I
mounted a BNC female connector to the box
and used a short length of small diameter
50 Ω coax (RG-174 or equivalent) to con-
nect the coax connector to the MFJ-852
printed circuit card. The holes in the MFJ-
852 printed circuit card have a very small
diameter, so I found it necessary to use a
piece of 22 gauge bus wire to make the con-
nection to the coax shield.
That Works.”
8
This article can be found on
the ARRL TIS Web page. The article also
describes how to use an attenuator when
tracking down a noise source.
As Mike Gruber pointed out in his review
of the MFJ-852, one of the unique require-
ments of a noise locating receiver is wide
bandwidth, since noise is a wide band signal
and the receiver sensitivity improves with
bandwidth. The MFJ-852 has an IF band-
width of 100 kHz. While most general cov-
erage receivers will have bandwidths much
narrower than this, the reduced sensitivity
will be partially compensated for by the gain
of the tape measure beam.
Notes
1
J. Hanson, W1TRC, “A Home-made Ultrasonic
Power Line Arc Detector,”
QST,
Apr 2006,
pp 41-45.
2
M. Gruber, W1MG, “Product Review — MFJ-
852 Power Line Noise Meter,”
QST,
Nov
2006, pp 75-76.
3
home.att.net/~jleggio/projects/rdf/rdf.htm
.
4
www.teara.org
.
5
See Note 1.
6
www.arrl.org/tis/tismenu.html
.
7
R. Littlefi eld, K1BQT, “A Simple TRF Receiver
for Tracking RFI,”
QST
, Mar 2001, pp 32-36.
8
W. Leavitt, W3AZ, “A Line Noise ‘Sniffer’ That
Works,”
QST,
Sep 1992, pp 52-55.
Jim Hanson, W1TRC, has been an ARRL mem-
ber for over 50 years. He received his General
class license in 1951, his Advanced class license
in 1952 and his Amateur Extra class license in
1984. His primary ham radio interest has been
working DX and he is at the top of the phone and
mixed DXCC Honor Roll. Jim has a BS degree
in electrical engineering and retired in 2004
from Raytheon Company Inc, where he worked
on radar receiver and low noise exciter designs.
He holds several patents relating to radar design.
You can reach the author at 8 Ethelyn Cir,
Maynard, MA 01754 or
w1trc@arrl.net
.
Figure 6 — Shielded MFJ-852 mounted in
metal Bud box.
Noise Hunting With the Three
Element Beam
The beam makes a big difference when
tracking down noise if there are two or more
noise sources in relatively close proximity. I
have found that the null off of the back of the
beam is a very good indicator of the direc-
tion of the noise. I have found that it works
best if I can get some distance (100 yards or
more) away from multiple noise sources. In
the complex environment I was investigat-
ing, I found that I could get more than one
null off of the back of the beam, and each
null pointed me to a noisy pole. Once I found
the area where poles were generating noise, I
was able to hear the arcing on the offending
poles using my ultrasonic detector, and I was
able to pinpoint the exact noise sources.
5
The MFJ-852 signal strength meter has
a dynamic range of about 50 dB. To extend
the range, I carry a small, homemade, 20 dB,
50 Ω
for the antenna. I purposely went hunting for
some power line noise (it is never very far
away) and was able to successfully hear and
locate a noise source with this receiver using
the tape measure beam.
There are also a growing number of small
handheld receivers and transceivers that
include general coverage receivers operat-
ing up to 1 GHz. Any that include an AM
detector are suitable. Some of these receiv-
ers include a signal strength indicator or a
switchable attenuator between the antenna
and receiver can also be used to determine
relative signal strength when pinpointing the
noise source. A suitable switchable attenu-
ator with 42 dB total range is described in
the
QST
article titled “A Line Noise Sniffer
-section attenuator in a shielded
enclosure with BNC connectors. The attenu-
ator can be connected between the antenna
and the MFJ-852 to extend the dynamic
range. I have found the attenuator to be help-
ful if the signal strength gets too strong.
π
New Products
MFJ 144/440 MHz YAGI ANTENNA
The MFJ-1760 Yagi antenna
uses 3 elements on 144 MHz
and 5 elements on 440 MHz
and requires only one feed
line. Power rating is 500 W,
the longest element is 40.5
inches, the boom length is
45 inches and the antenna
weighs 2 pounds. The MFJ-
1760 works with masts up
to 1.5 inches diameter. A
similar product, the DB-2345,
is available from MFJ’s Hy-
Gain product line. Price:
MFJ-1760, $79.95; Hy-Gain
DB-2345, $89.95. To order
or for your nearest dealer,
call 800-647-1800 or see
www.mfjenterprises.com
.
New Products
Using Receivers Other Than the
MFJ-852 Power Line Noise Meter
Any VHF or UHF receiver capable
of receiving AM can be used with a tape
measure beam to hunt for power line noise.
There are several homemade and modified
commercial receivers documented on the
ARRL Technical Information Service (TIS)
Web page.
6
One example of a simple receiver is a
tuned radio frequency (TRF) design by Rick
Littlefield, K1BQT, published in the March
2001 issue of
QST
.
7
Rick let me try out one
of his receivers that he had mounted in a
shielded box for me with an SMA connector
May 2007
31
Plik z chomika:
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A Simple TRF Receiver for Tracking RFI - K1BQT.pdf
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Chapter 13
Chapter 14
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