IW5EDI Simone - Ham-Radio

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About Loop Antennas

Posted: 27 Mar 2018 03:33 PM PDT
http://www.iw5edi.com/ham-radio/2599/about-loop-antenna

Loop Antennas

The Principles of the Loop Antenna article was in a newsletter of the MDXC.
The next three were from a talk that Mike Bates and James Dale gave to the
Northland Antique Radio Clubs Radio Workshop at the Pavek Museum of
Broadcasting.



Principles of the Loop Antenna

A loop antenna is an antenna primarily for the AM broadcast and the
Longwave bands. There are two different types of loop antennas, one is the
ferrite bar (as in your am radio), the other is wound on an air core form.
A loop antenna is very directional. The pickup pattern is shaped like a
figure eight. The loop will allow signals on opposite sides to be received,
while off the sides of the loop the signal will decrease or be nulled out.
The nulling feature will allow you to remove a local station on a frequency
and pick up another on the same frequency by removing the local signal. A
loop may have an amplifier or may not.

Air core loop antennas come in many sizes. The larger the loop the more
gain there is. A small loop will actually lose part of the signal. That is
why most small loops will use an amplifier. There are two ways a loop can
be wound, box or spiral. In the box or solenoid loop the plane of the
winding are wound perpendicular to the diameter of the loop, so each loop
is the same size. In the spiral loop the plane of the windings are wound
parallel with the diameter of the loop, so each loop gets smaller as you
wind into the center of the loop. A loop needs to be able to rotate to null
out a station. And a loop also needs to be able to till from vertical. This
also helps in in nulling of a signal (altazimuth feature).

The number of turns the loop needs is determined by the size of the loop,
the frequency range that you want to tune and the value of your tuning
capacitor. The larger the loop the fewer turns you will need. A 4 foot loop
needs 8 turns and a 2 foot loop needs 18 turns. The capacitor that is used
is the standard AM tuning capacitor with a range of 10 to 365 pf. The
tuning capacitor is used to tune the loop to the frequency that you want to
listen to. When you are tuned in to the frequency the signal will peak. You
may not be able to tune the full frequency range that you want to tune. So
you will need to use a 2 section capacitor and switch the second section
in. (more capacitance)
There are three ways that you can connect your loop to your radio.

One way is not connecting it at all. (This requires a portable radio with a
internal loop antenna.) The field of the loop will radiate the peaked
signal and you will be able to pick it up with no connection to the radio.
You can move the radio around to get the best reception.

You can also direct couple to the loop. This way you connect to each end of
the loop and also to the center tap of the loop. Using this method you will
need to use it with an amplifier.

The last method is to use a pick up coil. This consists of one turn of wire
that is placed inside the loop on the cross arms. This is then connected to
the radio. The distance from the main tank coil can be determined by using
a pocket radio and moving it inside the loop to find the place were the
signal is strongest, and were it peaks sharpest.

In the past loops were made from wood. I have built them and found them to
be heavy, clumsy, and flimsy. The mounting system were not very stable. In
talking with Mike Bates, he came up with the idea of using PVC to build
loops. PVC is easy to cut and because you use PVC molded parts, the loop
that you make are is stable. By using PVC cement for some gluing and small
nylon screws to connect parts you have no metal parts except the wire and
tuning cap to throw the pattern of the loop off. Using PVC it helps to have
a drill press, but if a person drills very carefully there should be little
problems.




What is a loop and why use it?

1). A loop antenna is a small multi turn loop of less than 1/10th
wavelength in length. The loop is wound on a form, which may be either box
(solenoid), or spiral (pancake) wound. The core material can either be air,
or a powdered iron compound (Ferrite). The gain of a loop is much less than
a longwire, but it has much less noise pickup. A properly designed Loop
primarily responds to the magnetic component of the radio wave. Note that
noise resides primarily in the electrical component. A vertical antenna
responds mainly to the electrical component.
2). Why use a loop?

A). No available space for a longwire antenna

B). To eliminate unwanted signals, and noise

C). Radio Direction Finding

D). To improve the performance of a simple receiving system, by providing
pre-selection which improves image rejection, and adjacent channel
selectivity.
3). History

A) 1915-1920’s Early receivers used loop antennas, until they were
discontinued in favor of long wire antennas, prior to 1930.

The loop antenna appeared again about 1938. This time it was used to
eliminate the need for a longwire antenna, and to provide for safer
operation of the small midget AC/DC sets that came into wide use at that
time.

B). The first known use of a high performance loop antenna is the box loop
made by Ray Moore in the mid 1940’s(1) This antenna was written up in DX
Horizons in 1960. The Moore Loop was wound on a 40 square box frame. Note:
Ray Moore is the Author of the book on the history of Communications
Receivers, and a new companion book on Transmitters.

C). The next major advance in Loop Antenna design came about as a result of
advances by Gordon Nelson of the National Radio Club. The NRC Loop
Antenna(2)was designed by Nelson in the Mid to Late 1960’s time frame,
Nelson was at M.I.T. at the time. The major advance that Nelson made was
allowing the loop to rotate in the vertical as well as horizontal plane.
The addition of the Alt-azimuth adjustment allows for the elimination of
the effects of wave tilt and allows for much deeper nulling of certain
stations. This loop was a 35 on a side and wound on a wood frame. In one
form it utilized another Nelson first, a direct coupled Balanced amplifier
using 2N4416 J-FET’s with the outputs fed to a balanced feedline. The other
version was link coupled to the receiver.

D). Sanserino Loop (1970-1985) This is a 2 foot Air core box loop designed
by Ralph Sanserino, and later marketed by Radio West. This loop antenna
used a Differential Amplifier similar to Nelson’s except the output is not
balanced. This antenna also has the Alt-azimuth feature. (available as a
kit) The amplifier was later used in the Radio West Ferrite Loop Antenna
(see below) .

E). Joe Worchester (1970-1977) a retired GE engineer developed the Space
Magnet , a small 12 ferrite rod loop antenna using a Bipolar Junction
Transistor amplifier(3). Nulls were not as deep as with the Nelson Loop.
This is also probably the first loop antenna commercially available to the
hobbyist, at a cost of about $45.00 if I remember correctly. Later versions
utilized the Nelson Alt-azimuth feature. This antenna also used a Faraday
Shield around the Ferrite Bar.

F). Mackay Dymek (1974-Early 1980’s) , Palomar Engineers (1977-current).
These are small ferrite antennas made by larger commercial concerns. The
Mackay Dymek was primarily for the Broadcast Band, where the Palomar has
plug in coils for ranges from 10Khz to 15Mhz. Note that both of these
antennas incorporated alt-azimuth design.

G). Radio West(1979-1985) 23 ferrite rod assembly using Sanserino
Differential Amplifier, direct coupled, Has Alt-azimuth feature, $160.00 in
1979.High performance for its day, quieter than the Space Magnet

H). Quantum Loop (about 1990) by Gerry Thomas is a small ferrite rod less
than 1’ in size (length), with a high gain 40Db amplifier. has Alt-azimuth
feature, in current production in various forms $135-$200.00.

I). KIWA Loop 1992 First Air core available since Nelson/Sanserino. Uses IC
amplifier Opto isolated regeneration and varactor tuning. High performance,
solidly built, in current production. $360.00.

J). RSM Communications (Ray Moore) RSM-105 (1994) A high performance
transformer coupled, non amplified antenna described by Moore in Dec 1994
IRCA DX Monitor, Later in March 6 1995 issue of NRC DX News. Still in
production? Price?? 35 spiral wound.
4). Electrical Design Characteristics

A). Two main types of Loops available 1). Directly Coupled and 2).
Indirectly coupled (Transformer coupled) The Directly Coupled Loop has its
windings directly attached to an Amplifier. Usually the main Tank Coil
(parallel tuned circuit that forms the loop primary) in the loop is
grounded at the center of the winding (center tapped), to allow for
electrical balancing. The Amplifiers are usually but not always J-FET’s,
with 2 FET’s in a Differential configuration, where the ends of the tank
winding go to each FET gate. The Transformer coupled version uses a link
winding to couple the signal to the receiver. This version can be amplified
or non amplified.

B). The pick up pattern of a properly designed loop should be a figure 8
pattern. The null should be of the same depth, if the antenna is rotated
180 degrees horizontally (loop should not be adjusted for alt-azimuth, but
left vertical 90 degrees from the ground). The 180 degree symmetry should
be the same + or one degree. If this condition does not occur the Antenna
is not properly balanced. In a transformer loop balance deals with the
signals being equal on both lines of the feed line (equal potential to
ground). The feed line should preferably be shielded with the shield being
grounded to the receiver chassis. If the line is affected by an electric
field signal, a metallic object, or some other imbalance to ground, the
loop will become unbalanced, resulting in a distortion of its pick up
pattern. Balance is critical to getting the best nulls, and for precision
Radio Direction Finding. The use of a broadband balun allows for better
balance, but thought should be put into the design of the link winding, and
receiver feed line, as well as the mechanical integrity of the coil.

C) The transformer coupled loop is the easiest to balance, especially if it
is an air core loop. Ferrite loops are not as easy to balance due to the
compression of flux lines in the ferrite. These antennas seem to be
somewhat more prone to pick up electric fields.

D). In a directly coupled loop, the balance is affected by the gain of the
amplifying devices on either side of the center tap being equal. If they
are not very close to, or equal, they will cause the voltage in the tank
coil to be imbalanced with respect to ground causing the same undesirable
effects that the feed line caused in a Transformer Loop.

E). Some loops utilize a Faraday shield to maintain balance (4) Usually a
one turn loop. these are usually circular, and are used on ships and other
areas where direction finding is necessary. An example of this antenna is
the 160 meter loop wound out of coax described by Doug DeMaw (5) Using a
Faraday Shield will affect the pick up gain, as well as the Q of the tank
coil(3) Another variant of the shielded loop is the Mike Hawk Loop(6)

Also note that imbalance is sometimes referred to as Antenna Effect(4) Also
please note that a balanced loop antenna can be spoiled to a cardioid
pattern by putting a vertical sense antenna within its field.(4)

F). The amount of coupling (placement of the link turn) is critical to the
performance of the Transformer Coupled Loop. The placement can vary
depending upon the load that the antenna sees. The best way to obtain
optimum performance is to experiment with various distances from the Tank
Coil. Most designs call for this to be wound amongst the tank coil
windings, however this coupling is much too tight for most uses, and allows
for tuning to be too broad, Q to be too low, and sensitivity to be not
quite optimal.

G). The physical size of the Loop Tank Coil affects the overall pickup
(capture ability) of the loop. The larger the winding size the greater the
pickup. Larger loops will also be easier to balance than smaller ones.

H). The Tuning Sharpness Q is determined by the size of the wire (surface
area). The lower the resistance the higher the Q will be. The loading of
the Tank Coil also affects the Q. This more than wire resistance affects
the Transformer Coupled Loop. In a Transformer Loop, the placement of the
Link Coil in relation to the main tank (distance) determines the amount of
coupling, and hence the loading of the tank circuit. The point of critical
coupling can be found by varying the coupling link distance, while
comparing tuning sharpness and gain. the critical coupling point will be
found at the sharpest tuning before the gain starts to drop. Tuning will
continue to sharpen (slightly), but gain will fall off more rapidly, as one
couples more loosely (moving the link physically farther from the Tank
Coil). Further improvement can be had by matching the load impedance to the
link coil with a matching transformer. This can be done as part of a balun,
or following the balun (lead-in side). For optimum performance all
impedance’s in the system should be properly matched.

I). The L/C ratio and mechanical design of the coil should be considered
when looking at a good design for a loop. The loop should be mechanically
stable (wires not flopping loose) The distributed capacitance between turns
should be kept low by proper design to allow for wide tuning range, but not
too wide to degrade the length to diameter ratio of the coil. Note that the
best null performance occurs with the best length to diameter ratio of the
Tank Coil. A spiral wound coil affords the best performance in this regard,
but does not afford as great a signal pickup as a solenoid coil of the same
diameter. (A Trade off)

Also note that the L/C ratio should allow for one 10 to 500pf variable
capacitor to tune the whole Medium Wave Broadcast Band.(530-1700 KC)

J). Performance can be further enhanced if the amplifier following a
transformer coupled loop is tuned. This provides still better image
rejection, and adjacent channel selectivity. It is important that the
amplifier be isolated from the loop by a transformer to maintain balance
and pattern integrity.

K). Note that the spacing of the windings determines the inter-electrode
capacitance. The wider the spacing between windings, the lower the
capacitance, and the higher in frequency the loop will tune. the use of
interlaced spreaders further reduces this effect (solenoid loop) provided
that the spreaders are of sufficient width. Also note that the winding
spacing is a compromise with the length to diameter ratio.

Construction Principles
5). Mechanical design

A). Up to now, loops were made from wood. It was used because it was
readily available and easy to work with. Wood does have disadvantages. They
are, finding good wood, making accurate cuts, and heavy weight.

B). In a wood loop, the alt-azimuth tilting mechanism does not work very
well. Wood loops use a bolt for the alt-azimuth tilt. It uses an arm that
goes from the loop to a clamping setup on the mounting post. This does not
work very well, as you need to tighten the bolt every time that you change
the vertical tilt. The bolt will become loose, and on high angle tilts does
not hold very well.

C). Most loops use a pipe mounted vertically with a dowel to do the
horizontal rotation. This does not work well, as it allows the loop to move
on it own. Wood will also wear after some use. This allows the loop to lose
its square ness which can affect the loops pick up pattern.

D). The first loop I built was the Harley Loop(7). It is small spiral loop
that was easy to build, but had no Alt-Azimuth feature, so the loop would
not vertically tilt. It uses two cross arms with saw kerfs part way through
to hold the wire.

E). I then built the 4 foot NRC Loop(2). This loop worked well, but the
Alt-azimuth tilt needed work so I did some modification on the tilt
mechanism. I wanted to design a tilt that would be easy to tilt and would
stay in place. I tried different ideas and in my design I used a 3 inch PVC
pipe for the mast and the loop head would tilt off that, this did work
better but was not perfect.

F). While talking to Mike Bates(1995) about loops, He had the idea to build
a large spiral octagon loop(5ft), out of PVC pipe and Alt-azimuth tilt it
with a tripod. We built the loop, and this got my interest in using PVC for
designing and building loop antennas. The tripod did not work very well due
to the heavy weight of the loop head, but the performance was quite good.

G). My next design was a 4 foot PVC spiral loop that is collapsible. New
features added to this loop was the use of a lazy susan for the horizontal
rotation of the loop, however, you need to use a liberal amount of grease
to give it tension. For alt-azimuth tilt I took a 3/4 inch PVC tee, reamed
out the inside smooth, and cut a slit length wise. Through this I ran a
piece of PVC pipe and with the use of elbows and tees attached it to the
loop head. I then used plastic hose clamps to adjust the tension. This
worked better, but still did not work very well. It is hard to get the
angle just right, it does not move smooth enough.

H). Then I built a 4 foot Loop modeled from the NRC plans(2) out of PVC.
For this loop I added the use of PVC in the base. I used the same type of
Alt-azimuth tilt mechanism as the earlier spiral. To mount it to the loop
head, I used a hole in the crossover of the loop to attach to the
alt-azimuth mount. This was done to allow for having the ability to build
different loop heads, like one for the longwave beacon band. This allowed
the loop head to rotate on the mounting mechanism which made the loop
unstable, and not very easy to use. I decided to make the mounting like the
spiral loop, but to add the tees, I needed to cut part of the tee to mount
it on the loop arms. When I assembled it for fit, I found out that cut out
tees worked much better for tilting, and hose clamps are no longer needed.
The alt-azimuth tilt mechanism now works very smooth and holds well at all
angles.

I). The 2 foot loop was also built based on the NRC plans(8), it employs a
gimbal mount for alt-azimuth tilt.

J). These loops are made entirely out of PVC except for the base plate that
employs a lazy susan to rotate the loop. I built jigs to drill the holes in
the cross pieces. A drill press helps a lot, but with very careful
measuring and drilling, a hand drill may work. In my first loops I used PVC
cement to glue the loops together. This cement sets up very fast, so you
have to be very careful assembling it. I found out that some parts can be
glued, but on some it is better to use a small nylon screw. This allows for
you to align the pieces right on. To do this drill and tap a hole for the
screw and run it though both pieces of plastic (PVC).

K). It helps to make a jig to wind the tank coil onto the frame. To
accomplish this I mounted a Lazy Susan to a board, and ran a board with two
vertical pieces of PVC Pipe The loop frame slides over the pipes, allowing
the loop to be rotated while the wire is brought off of the spool in the
same direction, while being laced through the holes in the frame. This
helps greatly, in minimizing twisting of the wire.

L). Reasons for using PVC in loop construction:

1). Readily available, at low cost

2). Easy to work with, Saw and drill are main tools required, however, a
miter box saw allows for clean perpendicular cuts.

3) Very symmetrical loops can be built, because the fittings are identical,
and pre made.

4). Very low weight

5). The ability to come up with modular designs

6). The ability to design a collapsible loop that can be mechanically
strong, allowing for easy transport

7). The use of spacers will tighten up the wires, so that they do not flop
around, and distort the pickup pattern, as well as reduce inter-electrode
capacitance. This makes for a very stable loop.

M). Notes:

Loop shapes: Triangle (Wedge), Square (Also called box, this is the most
common shape) Octagon, Circular.

Note that the box loop is used because it is the simplest to build. The
circular loop provides the nearest to the perfect shape electrically, but
it is very difficult to fabricate a multi turn loop of this type. The
octagonal loop is the practical compromise. Also note that the Octagonal is
more difficult to fabricate due to it having 8 arms instead of 4, for the
box loop.

What Can I use a Loop For
6). Using the Loop practical applications

The small loop is a versatile antenna, and can be used for many different
applications, here are a few.

A). The loop can be used for improving the performance of a poorly designed
broadcast receiver. Depending on the type of antenna that is in the
receiver determines how the loop can be attached. It may be attached via a
transmission line if the set has wire or screw binding posts for the
Antenna, or it may be inductively coupled (transformer) for a receiver
using a very small loop. In the case of the receiver with a small loop the
coupling rules apply as if the receivers loop is the link turn in the
Transformer loop. (Note that the link coil is not needed for the loop to
work for this, as the internal loop in the receiver is receiving the signal
from the main tank circuit). The distance from the receiver to the larger
loop will determine the amount of coupling, and tank loading. One can vary
the pick up pattern by varying the angle of the receivers internal antenna
to the external loop. The antennas provide maximum transfer of signal, and
closest to a figure 8 pattern. when the pick up angle of the antennas is
opposite parallel (90 degrees) (Beams of the antennas aimed at each other)
Minimum pickup occurs at 180 degrees. The pattern can be spoiled to a
cardioid (null in only one direction) by varying the angle. Please note
that the pattern will probably be somewhat spoiled from a figure 8 at the
maximum signal 90 degree points. Please note that there are 2 commercially
available products designed to inductively couple to the receiver, and
improve its signal. These are the Select A Tenna, and the Q Stick by Radio
Plus (Gerry Thomas). However a 2ft or larger loop provides for much better
performance provided one properly adjusts the coupling. A Large loop (4 ft)
can cause a poorly designed receiver to overload. Loosening the coupling
will allow for the overload to be eliminated. One must also be sure for
proper operation that the loop is tuned to the same frequency that the loop
is tuned to, or unwanted overload effects will likely be noted. All tuned
circuits should be aligned to the same frequency. Also note that high Q
tuned circuits can sometimes be touchy to adjust spot on, some practice
will probably be necessary. One can be amazed at the improvement in
performance when using a properly designed loop. Stations can be brought
from out of nowhere on a poor set. Images at the low end of the broadcast
band will be cut down significantly or completely eliminated. As stated
previously, adding a properly designed tuned amplifier further improves the
performance of the system. The amplifier can be fed by feedline to a
coupling link that couples to the receivers internal loop, or can be direct
attached to a receiver with antenna connections.

B). A loop when properly balanced can be used to null down AC Line noise,
TV Sweep Harmonics, or other locally generated interference. The
Alt-azimuth feature helps greatly reduce, sometimes totally eliminating the
noise. This feature is also quite useful for nulling of co-channel, or
adjacent channel broadcast band stations. If properly balanced, nulls of
over 60 dB may be attained by using the Alt-azimuth feature. Deep nulls can
be difficult to find and maintain. A larger antenna allows for one to find
the null more easily due to the larger pick up(field) created by the loop.
Loops of 2 ft and smaller in diameter, can be quite touchy to null, and
electrical balance can be quite hard to attain. Hand capacitance can also
affect the null in these small loops, causing the null to move as ones hand
is moved away. This effect is minimized when using a large loop, as your
whole body is within the pick up pattern of the loop, and it will be less
likely to distort the pattern. One needs to be 6 to 1’ away from the small
loop(2ft and smaller) to avoid the hand capacity effects. It is also
notable that nulling works best on local ground wave signals. Distant sky
wave signals can be more difficult to null. It is difficult to get a null
of greater than 30 dB on a sky wave signal at the top end of the broadcast
band at night. For Sky wave, phased antennas provide for much better
nulling, but are much more complex, and difficult to operate. Also note
that the higher the Q of the tank coil, the sharper the null. Sometimes the
null will be excessively sharp, and difficult to find, or the null will be
so narrow in bandwidth that the carrier of a station will be deeply nulled,
but the sidebands will be well received as slop(splash). This affect is
more noticeable in small, or amplified direct coupled loops.

C). Radio Direction Finding

One can accurately direction find signals provided that the antenna is
properly balanced as described above. The general concept is that the
deepest null will be in the direction of the signal being checked. You
cannot use the Alt-azimuth feature, you must keep the loop perpendicular 90
degrees to the ground. An accurate compass, and a marked 360 degree circle
can be used to pin point the exact bearing that the signal is coming from.

Bibliography from the three articles above can be found at the bottom of
this page.

Loop Antennas Another Look

The last 30 years have brought about much refinement in the design of loop
antennas. Starting from the basic box loop described by Ray Moore, major
developments over this time are; The NRC 4ft Alt-azimuth loop, the Space
Magnet, Sanserino Loop, Palomar, Mckay Dymek, Radio West, Quantum,
Lankford, Kiwa, and RSM 105, and 103. As time has progressed, so has the
design of Receiving Equipment, from the R390A, HQ-180, and SX-122 and
Zenith Trans-Oceanic to the Sony 2010, Drake SW-8, Drake R8-B, and the
AOR7030 Plus. Antenna needs have changed, with today’s broadband front
ends, and synthesizer phase noise a concern, a high performance loop, or
other means of pre-selection is more important than ever.

We will show a slightly different twist on the same basic loop antennas of
the past, with a couple of refinements, as well as construction details of
our antennas.

To explain our antennas we want to start with the design criteria necessary
to improve the modern communications receiver, as well as consumer grade
radios such as, the Super Radio III, the Radio Shack Optimus, and most
other portable short wave/broadcast receivers.

Important loop criteria have been explained before in the pages of DX News,
and the NRC Antenna Manuals, however, a review is in order. It is our
opinion that there are 4 basic parameters that loop performance should be
based upon; 1). The loops signal to noise ratio. 2). The electrical
Balance. 3). The selectivity or Q of the loop. 4). The mechanical
rigidity/integrity of the coil assembly, and Alt-azimuth mechanism.
Signal to Noise Ratio

Most efforts during the last 30 years have dealt with making loops smaller,
to allow them to be used by the DXer who has limited space. S/N ratio has
suffered as a result. The use of direct-coupled balanced FET amplifiers,
and smaller, and smaller loop coils means that the bulk of the work in the
system is being done by the amplifier. If you capture a very tiny signal,
and add some amplifier noise to it, you have degraded what signal that you
have to the point that you may bury a signal slightly above the noise
floor. A rule of thumb would be to use no more amplification than is
necessary. It is better to make the coil larger to enhance the capture
area, and insure that what amplification is used is as low gain, and low
noise as possible. The small loops are probably OK for most uses, but when
you want to extract the last decibel out of the ether, a larger loop that
is properly designed will be best.
Electrical Balance

The electrical balance of the antenna insures that the current at the
termination of one end of the loop tank coil is of equal magnitude and
opposite polarity to that at the other termination of the loop coil. (A is
equal and opposite B) When properly balanced the deepest possible null will
be obtained, with the loop. Please note that balance is quite difficult to
attain. Anything connected to the tank coil (or other metal brought
physically near it) other than the resonating capacitor, can throw this
balance (equal and opposite) off. This distorts the theoretical figure 8
pattern of the loop. If a link turn is used to couple the loop to the
receiver, this link and the transmission line must be balanced, or coupled
to an unbalanced line (Coax Cable) using a Balun. The link coil should be
balanced, as well as the main tank coil. Using a balanced FET amplifier on
the tank coil will throw off the balance if care is not to insure that the
FET’s are not exactly matched in their gain and transconductance. Not to
mention that an amplifier not properly balanced, and running at excessive
gain will be prone to create intermod products which degrade further the
performance of the system. If a loop antenna seems to have a problem with
hand capacitance it is a pretty good bet that it is not properly balanced.
Refer to Nelson’s article for detailed hints on how to attain balance.
Selectivity

The Q or quality factor of the tank coil will determine its selectivity at
resonance (the tuned frequency). If the Q of the tank coil is loaded
(reduced by the effect of a load on the coil), the Q will decrease, and the
selectivity of the loop will decrease, or broaden. In the late 60’s Nelson
devised the balanced FET amplifier as a way to minimize the loading of the
tank coil. This allowed for selectivity that was so sharp that a loading
potentiometer was added across the tank coil to reduce the Q so that the
loop could be more easily tuned. Prior to this, loops were of the
transformer variety, with the signal being coupled to the receiver via a
link turn, wound amongst, but not attached to the windings of the main tank
coil. This process did not take into full account transformer theory, as
the loop is now a transformer due to the link coupling. The main drawback
of the early designs is that they did not use the coefficient of coupling
when designing the loops. Moving the winding away, somewhat from the main
winding allows for less loading. Maximum energy transfer from the tank to
the link would occur at critical coupling. The load impedance also affects
the loading, and should be matched to the impedance of the link turn, as
well. As stated before the link should be balanced. A balun, and matching
transformer should be used with a modern receiver with a 50 ohm coaxial
input. The distance that the link turn is from the primary tank coil
greatly affects the performance of the loop. The Q can be greatly improved,
as well as the S/N ratio if the link turn is placed at the critical
coupling point from the main primary tank winding. This distance from the
main winding can be approximately determined prior to winding the link turn
using a pocket radio. Tune the pocket radio into a station that is within
the tuning range of the loop. Start out with the pocket radio placed facing
the plane of the loop (see fig 2) right against the loop winding, rotate
the loop capacitor for a peak (maximum signal) on the pocket radio. Now
move the pocket radio ½ from the winding and re-peak the loop. Observe how
sharply that the loop tunes. Move the pocket radio away from the loop in ½
increments. Observe how sharply the loop peaks on the pocket radio, as well
as the signal strength at each point. The loop should peak more sharply,
and increase its gain as the radio is moved away. The point of critical
coupling is attained when the signal is at maximum, the sharpness may still
increase somewhat, but the gain will fall off more rapidly as the radio is
moved away. Note the critical coupling point, and wind the link coil this
distance from the main winding. This process can also be used when
passively coupling the loop to a radio using a ferrite antenna, or when
using a device such as a Select-A-Tenna, or a shotgun loop , or Q stick
with a radio with a built in loop.
Mechanical Integrity

Another consideration when building a loop is the mechanical design. This
is often overlooked, and can affect the loop balance if the mechanics are
sloppy. There are two different types of loop coil designs, The solenoid or
box wound, and the spiral or pancake wound. Each type has several
advantages, design trade offs, and dis-advantages. The box loop has a
higher gain for the same diameter, but is more difficult to balance. With
this type of loop there is a trade off between the inter-electrode
capacitance and the length to diameter ratio. Spreaders can be used to cut
down on the inter-electrode capacitance, and to maintain coil rigidity.
Note that the better the length to diameter ratio of the tank coil, the
nulling ability is enhanced and balancing is easier. The spiral loop has
the advantage of almost perfect length to diameter ratio, as well as being
easier to balance. The main drawback of the spiral is its lower gain than
the box loop. Also, if an amplifier is used, it is much more difficult to
tap the tank coil at its center. An amplifier can be used more practically
with the spiral by attaching it following the balun and matching
transformer. This way the amplifier is isolated from the loop, and the
undesirable unbalancing effects can be avoided. Note that an amplifier
should be used only when necessary, and should be as low in gain as to
improve signal strength on very weak signals, where not using it would
yield them unreadable. It is important that the loop antenna be well
constructed mechanically to insure that the wires do not flop around, to
distort the balance, as well as to prohibit the need for re-winding. We
have switched from wood to PVC in our designs, with a minimum of metallic
objects near the tank, and link coils. Note that metallic objects within
the near pick up field of the loop will induce a voltage into the coil
unevenly and throw it out of balance.








Bibliography

1). NRC DX News March 6, 1995, IRCA DX Monitor Dec 1994

The Full size Full Performance Loop Antenna Ray Moore

2). NRC Antenna reference Manual Vol 1 Dec 1995 Edition

NRC Alt-azimuth Loop antenna P6

3). NRC Antenna Reference Manual Vol 1 1985 Edition

HQ tests the Space Magnet P40

4). ARRL Handbook 1988 Edition P39-3

5). QST Magazine July 1977 Beat the Noise with a Scoop Loop

Doug DeMaw

6). NRC Antenna Reference Manual Vol 1 Dec 1995, The Hawk Loop

7) NRC Antenna Reference Manual Vol 2 Dec 1995 The Harley Loop P 11

8). NRC Antenna Reference Manual Vol 1 Dec 1995 P21 The NRC 2 Foot Loop

Suggested Reading:

1).National Radio Club Antenna Reference Manuals Vol 1 and 2 1995.
Available from the National Radio Club P.O Box 164 Mannsville, NY 13661-0164

2). National Radio Club Loop Antennas Design and theory 1995

Available from the National Radio Club P. O. Box 164 Mannesville, NY
13661-0164

3). ARRL Antenna Book 1994 American Radio Relay League, 225 Main St.,
Newington, CT 06111-1494

4). Joe Carr’s Receiving Antenna Handbook 1993, High Text Publications P.O.
Box 1489 Solana Beach, CA 92075



Article originally available at
http://www.frontiernet.net:80/~jadale/Loop.htm



The post About Loop Antennas appeared first on IW5EDI Simone - Ham-Radio.


///////////////////////////////////////////
2.4 GHz Cubical Quad Antenna

Posted: 27 Mar 2018 03:12 PM PDT
http://www.iw5edi.com/ham-radio/2587...l-quad-antenna

2.4 GHz Cubical Quad Antenna Introduction

The Cubic Quad antenna is a commonly homemade antenna in the range of about
150 odd MHz. Our little project was to design one of these for use in the
2.4GHz range for 802.11 wireless LANs. The reason these are seldomly used
for 2.4GHz is the size.

The picture below is a 4 element cubic quad for the 147MHz range.Â* Large
isnt it.



The one we are going to build for 2.4GHz will only be 6cm long!


The Design

I scratched together an initial plan on how I was going to set about
putting this together. The measurements came from the second (or third)
link above. While each element was made the same as in the design, the
support structure was changed to a much easier one. This was about the only
advantage of building a really small antenna.

Materials

1 hot glue gun
1 soldering iron
1 soldering god (enter ChrisK)
short length of coax with connector
60cm of builders wiring (stripped to get one solid copper wire ~ 2mm thick)
3 cotton buds (hehe Ill get to that bit later)

Construction

After stripping the copper wire, we constructed the four elements as per
the measurements I pilfered from the java application on the previously
mentioned page. We bent the wire with a pair of pliars against a small
anvil. The reflector and director elements were soldered closed by ChrisK,
the driven element left open for connection to the coax.



From left to right. Reflector, Driven element, Director element 1, Director
element 2

The white sticks are cotton buds with the cotton crudely removed.

Each element differs in size from the next. From the reflector through to
director 2, the sides of the squares get smaller by only ~0.1mm. Human
error can really screw this up. As this is only really a prototype we are
not overly concerned. However, when it comes to building the real deal, we
have decided that getting a computer driven robot to cut out some copper on
a fibreglass board with some precision in length and squareness would be a
goer.

The next step is to solder the driven element to a nice thick and chunky
bit of LMR400 We did this on an angle to prevent the direct short.



Here lies problem number two. The space created by the gap between coax
core and outer is huge in comparison to the size of the element. We decided
that keeping the length of wire for the element was more important than the
shape, so it is also not really square anymoreprototype. This would also
ideally feed into a balun rather than directly onto the coax. We just need
to figure out how.

We then built the rest of the elements onto the driven element with the
assistance of a hot glue gun and some cotton buds. When you put cotton buds
in the microwave for one minute next to a glass of water, they do not get
hot. Ideal antenna construction material! The elements were distanced
according to the java application. However, it should be noted that
increasing the distance between the elements will increase gain at the
expense of bandwidth. The final version will hopefully be totally
adjustable for tuning.

We used three cotton buds and the hot glue gun to hold it all together. It
is messy prototype but it is also very small. Hehe. You can still see the
leftover cotton wool on the ends of the sticks



OK. So once all done, we did some very quick testing.

It worked! We didnt keep logs of the test as we intend to do it properly
soon, but it gave a dramatic increase in signal, S/N and reduced noise. I
will add test results to this page when they are ready.

Here is a closer look at the prototype.


Test Results

Two laptops with wireless cards were moved apart at such a distance where
signal could be improved. One of the laptops was then given a balaxy dish
(a galaxy dish that ChrisK modded to have a different balun and dipole).
The balaxy dish was then replaces with the prototype cubic quad. Results
were logged and the peak of all results were as follows;

RX Signal Noise SNR TX Signal Noise SNR

Internal -78 -97 20 -78 -94 17
Galaxy -61 -99 38 -61 -94 34
Cubic -70 -99 28 -70 -93 25

I am very encouraged by these results. The prototype cubic quad was a
complete bodge job with very little precision. More precise elements may
give better results. It was not adjustable due to the hotglue used to stick
everything together. With tuning these results may be better. And there was
no balun used, on account of my having to figure out how to make a balun
for this little beasty. A balun would hopefully give me another 3db
Future Directions

The elements need to be more precise. Having them properly machined would
be ideal. The support structures should be threaded. This will allow us to
put plastic washers at each bend with some plastic nuts, giving us the
ability to tune it for maximum gain/bandwidth.

A balun is required (perhaps). The signal is skewed about 15 degrees to the
right (guestimate). We also need to figure out how to design the connection
to the balun/coax in such a manner that will cause the least hassle to the
shape and length of the driven element.

v1.2 is under way. v1.1 was scrapped before I put it together because I am
still unhappy with the elements.

We have some good ideas on where to go from here, so watch this space for
developments over the next week or so.



article originally available at
http://members.iinet.net.au/~stygen/Quad.html

The post 2.4 GHz Cubical Quad Antenna appeared first on IW5EDI Simone -
Ham-Radio.