Building a Lindenblad Antenna for
I like Lindenblads because they're easy, cheap, robust, and good
Section 10 of the ARRL
Satellite Handbook presents good practical details for the
construction of a Lindenblad antenna. I have built my last couple
of Lindenblads from lengths of 4mm-diameter galvanised steel fencing
wire, some standard 300-ohm television ribbon cable, and a few bits of
wood and plastic. Another useful feature of a Lindenblad is that
its 75-ohm impedance nicely matches into standard 75-ohm TV coaxial
cable. Keeping things 'TV standard' helps to keep costs down.
The only catch with using 75-ohm standard coaxial
cable, connectors and masthead preamps, is that most 137MHz radio
receivers have a 50-ohm input impedance. A receiver should be
connected to a coaxial cable with the same characteristic impedance
as the receiver's input impedance. One cannot just
connect 75-ohm coax into a radio's 50-ohm antenna socket and expect
optimal performance. To mismatch impedances throws away a large
proportion of an already weak signal, as well as providing a conduit
for radio interference to enter the receiver. Fortunately there
is a simple transformer that can be made from a couple of short
sections of coax, which can nicely match 75-ohm coax into 50-ohm coax,
which can then be fed into a receiver's 50-ohm antenna socket.
More on this
The dimensions for the Lindenblad presented in Figure 10.15 (on
page 10-14 of the ARRL Satellite Handbook)
are for the 2-metre and 70cm wavelength Amateur bands. For a
137.58MHz-resonant antenna, the s,l,w and d dimensions
presented in Figure 10.15, are 42, 926, 893 and 654mm
respectively. Note that 137.58MHz is half way between the two
main frequencies used by NOAA satellites, 137.50 and 137.62MHz.
NOAA 137MHz NOAA radio transmissions are Right Hand
Circularly Polarised (RHCP), meaning that the four
folded dipole elements of the Lindenblad, need to be tilted clockwise
from the horizontal by 30°, as viewed by a butterfly perched at the
centre of the antenna.
Matching 75-ohm coax into 50-ohm coax
I first came across this
useful technique on page 8.8 of the RSGB
VHF/UHF Manual. This type of
impedance matching transformer is also known as a 'twelfth-wave
matching transformer', and many references to it may be found on the
Web. For the technically minded, this
article by Darrel
Emerson is quite good. But, all you really need to know is
for 137MHz, the match can be achieved by soldering
together two short sections of
one of 50-ohm RG-58 and another of 75-ohm RG-59, both measuring 117mm
length (pictured right and below, are a couple of 137MHz 50:75ohm
transformers that I have made). It is important to note that this
match only works for a
single frequency, and using standard RG-58 and RG-59 coax with a
factor of 0.66. In fact any 50- and 75-ohm impedance coax
must be used, but allowance must be made should the coax have a
velocity factor different from 0.66 (see references
below to standard coaxial cable specifications).
expensive and unnecessary for the average Lindenblad constructor, an
analyser such as the MFJ-269
is very handy for optimising the dimensions of a new antenna, as well
as measuring coax velocity factors etc. For those who own an
MFJ-269, an RF noise bridge, or some other RF impedance measuring
instrument, and who really want to understand antenna electrical
behavior, The ARRL Antenna Handbook
incorporates an excellent brief introduction to the use of Smith Chart paper
for making antenna measurements, matching antennas to feedlines etc.
Anyone who tells you that 50-ohm connectors can be used with 75-ohm
coaxial cable, has a great future selling used cars. Connectors
are important, and particularly matching the correct connector to the
coaxial cable type. Of course the best kind of coaxial connector
is no connector at all, so try and keep one's main coax feeder in one
long unbroken piece. For inside-the-house coaxial connections,
Cable TV style 'F56' and 'F59' connectors (for RG-56 and RG-59 type
coax), are good, cheap and low-loss. Likewise so-called
PAL/Belling-Lee types. They come in many varieties such as 'twist
on' and 'crimp on' types, for various coax cable thicknesses, for
indoor and outdoor application. Common 75-ohm cables go by the
names 'RG-6', 'RG-56' and 'RG-59'. When buying coax for NOAA
satellite reception purposes, definitely buy "the good stuff".
The sales people at you local Radio & TV store will know what
you mean. "The
good stuff", usually means 'quad-shielded low-loss RG-6 or RG56/59',
all-metal 75-ohm connectors to suit. Specifications
of 'RG' Series coaxial
cables can be found in this
useful chart, and similarly for British 'UR' Series
The ultimate connector for this type of application is the
'Type-N'. They are very low-loss and very waterproof.
Type-N's are usually found in professional applications where
expense is less of a concern. Type-N's come in 50-ohm and 75-ohm
versions, the 75-ohm being somewhat rarer and more expensive than the
50-ohm. So if one should come across some Type-N's and would like
to use them on 75-ohm cable, make absolutely sure they really are
75-ohm connectors (this is usually stamped on them in small letters
somewhere). Even rarer and more expensive and than Type-N 75-ohm,
is Type-N 75-ohm specifically made for RG-6 cable. We use
Quad-shield low low-loss RG-6 coax, terminated with 75-ohm
Type-N connectors manufactured by Huber
& Suhner. These are outstandingly well made connectors,
rather expensive. They are overkill for normal applications.
With 50-ohm cables, such as RG-58, Type-N's or BNC's are the best
(but note that BNC does come in a 75-ohm version, although is somewhat
rarer). Most scanning-type receivers use a 50-ohm BNC connector
for the antenna socket. Under no circumstances do I recommend use
of so-called PL-259/SO-259 plugs/sockets, also sometimes known
as 'UHF Connectors'. These are horrible performers at VHF
and above, and not in the least bit waterproof. But strangely,
commonly finds SO-259 sockets used on VHF Ham Radio equipment.
these connectors is a historical habit (dating back to WW2) that is way
it's use-by date, and when I buy equipment that comes with these
installed, I strip them off and replace them with Type-N's. For
cable connections exposed to the weather, it is most important to
that rainwater cannot seep into the inside of the coaxial cable.
inside the coax, there is no way for water to get out again, and over
the coaxial braiding corrodes and becomes electrically 'lossy' and a
absorber of the RF signals flowing down it. If non-waterproof
are used, they need to be thoroughly shielded from weather, and/or
in self-amalgamating tape or the like.
Placing an antenna for receiving LEO satellites is a little different
for placing antennas for TV reception. With TV reception, the TV
station transmitter is usually placed atop a distant hill, in which
case a TV receiver antenna is best placed as high as possible, with a
good clear line-of-site view toward the transmitter tower. But an
overflying satellite is usually at a considerable angle above the
horizon, and the receiver antenna can easily 'see' the satellite even
when it is placed relatively close to the ground. As long as the
antenna is at least a couple of metres
above the ground, and likewise clear of electrically conductive objects
wire, and with a reasonable view of the sky, then this is usually fine
NOAA 137MHz reception. Also being placed lower down reduces the
of lightning damage.
The problem of radio interference should not be underestimated,
particularly if one lives within a large city. The advent of TVs,
microwave ovens, and a host of microprocessor controlled equipment,
that the VHF radio spectrum within a city is usually filled with
crud, formally known as Electromagnetic Interference
(EMI) or Radio Frequency Interference (RFI).
In general, national RF spectrum regulatory authorities protect
electromagnetic spectrum around the 137-138MHz space downlink band, but
regrettably not in Australia. Most distressing of all in
TV Channel 5A, which has its vision carrier located smack-bang
the middle on the 137-138MHz band. Tens of kilowatts of broadcast
radio signal, easily swamps the 5 watts of radio signal from a 1000km
spacecraft. In such cases where local broadcast TV interference
a problem, placing one's antenna low down in a back yard, or behind a
building, or in a small valley, may partially shield the antenna from
offending TV signal whilst maintaining a good view of the sky.
Lindenblad antennas have a
deaf spot located directly above the antenna, and when a satellite
passes directly overhead, the signal may be lost in the Lindenblad's
'cone of silence' for a few seconds.
All antennas have directions in which they are more sensitive or
less sensitive, and designing an 'omnidirectional' antenna involves
compromises, as one cannot truly make it equally sensitive in all
directions. A Lindenblad maximises it sensitivity
towards the horizon, where a rising/setting satellite is relatively far
away and it's signal correspondingly weak.
Lower elevations are also where LEO satellites spend most of
their time during a typical satellite pass, so it makes sense to have
the region where the antenna is most sensitive.
deaf spot may also be put to use. My friend Colin has his house
located in an area which is blasted by
TV Channel 5A (which has the channel's FM video carrier
weeks of trying every filtering and interference-reducing trick in the
book, Colin tried tipping
the Lindenblad antenna on its side (pictured right) and
deliberately pointed the
antenna's deaf spot toward the offending TV transmitter. To our
surprise and satisfaction, the offending TV signal was largely removed,
and now Colin receives acceptable NOAA
Our receiving site at Siding Spring Observatory (NSW, Australia) has a
serious problem with radio interference at 137MHz. For starters
there's nearby 22kV power lines, transmitting a broadband 100Hz
'crackle' across the entire VHF radio spectrum. There's also a
very powerful broadcastTV/Radio transmitter tower (Needle Mountain,
away). There's a
mysterious broadband digital telemetry which shifts backwards and
forwards in frequency, and which appears to be propagating along the
electrical mains. I have measured this interference many
away, and always find it near power lines. My suspicion is that
transmitted by power generation utilities, to remotely control
their equipment. Finally, I'm sorry to admit, there is my own
noise from our insufficiently EMI-shielded telescope control
electronics. In short, we're swamped in a sea of radio noise.
In locations with high radio noise, and
during times when a satellite's signal
is low, interference patterns will usually appear on the APT
be due to a large distance to the satellite (i.e. times when the
satellite is near the horizon), a relatively weak signal transmitted
from a satellite (as
is usually the case with NOAA-12), or a satellite is passing through an
deaf spot (for a standard horizontally oriented Lindenblad, elevations
> 80 degrees). If the
local radio interference should dominate the satellite's signal, a
strip of 'salt and
pepper' noise pattern will usually appear across the APT image.
when a NOAA satellite is passing directly overhead through a
Lindenblad's 'cone of silence', this noise stripe will likely
pass right through the observer's location displayed on the APT
image. Since we use our APT
imagery primarily for local cloud cover forecasting, this interference
stripe is particularly inconvenient. On rare occasions a noise
stripe will occur across an APT image, but due to extraterrestrial
noise rather than any shortcomings of the Lindenblad.
After experiments with different antenna types, I finally
Colin's lead and simply tilted my tried-and-true Lindenblad by 90
(April, 2004). This orientation gives reduced reception of
satellite signals near
the horizon, but considerably improves the reception when satellites
are passing overhead. For our particular application, this is a
more desirable outcome. I have made and tested a few different
designs of fixed polarised VHF antennas, but I am yet to find one
Lindenblad, whether horizontally or vertically aligned.
Incidentally, note that the tilting trick can only be performed with a
non-conductive mast (a wooden one in my case).
If you are being driven to distraction by the presence of nearby
powerful FM-radio or TV transmitters, and these signals are so powerful
that they are breaking through the front-end filtering of your
receiver, there are
a number of commercially available in-line filters which may be
between the antenna and the coax. The trick is to identify the
of the transmission, and then buy the appropriate TV-grade high/low
filter to block the noise. Interference on the 137-138MHz band is
common problem, and there are commercial band-pass filters available
are designed specifically for 137MHz space-downlink band. Or one
build one's own, such as this example of a 'helical
filter notch resonator'.
With our receiving station, we use a rather
special kind of band-pass filter known as a cavity resonator.
These are the ultimate for filtering
out noise, but rather expensive for a home constructor. We are
to use it because of our rather close proximity to a powerful TV and
telecommunications transmitter tower (at Needle Mountain, located 8.7km
away). Our model C2A-6
single-stage cavity resonator was manufactured Polar Electronic Industries in
Melbourne, Australia, and has these
Another way that unwanted radio noise can get into a receiver, is for
it to be picked up on, and flow down the outside of the coaxial
cable toward the antenna, where is it received, amplified, and fed back
down the inside of the coax back to the receiver. At our
receiving site we are particularly susceptible to this type of noise
due to the presence of many computers and poorly EMI-shielded
electronics which are also located where our receiver is located.
Fortunately the signals flowing on the outside of the coax my be
blocked by feeding the coax through cylindrical shaped ferrites, much
the same as is usually done on computer video monitor cables and the
like. In Australia, cylindrical ferrites from Jaycar (stock no. LF1258)
can be recommended.
ARRL Handbook for Radio Communications (current edition).
Available from ARRL
ARRL Antenna Handbook (current edition), edited by Dean Straw.
Available from ARRL
Radio Amateur's Satellite Handbook (1998), by Martin Davidoff.
Available from ARRL
publications. In Australia on sale at Dick Smith's.
RSGB VHF/UHF Manual (fourth edition, 1994), edited by G.R.
Jessop. Published by the RSGB
- Radio Society of Great Britain. Since superceded by the VHF/UHF
Handbook, edited by Dick Biddulph
Page last modified - 2005-03-17. This page is still under
development. Please feel welcome advise me of any typos, broken
web links, clumsy language, and/or suggestions for improvements.
Joining 50 and 75ohm coax to make a
coaxial impedance transformer