137MHz Lindenblad construction details

Building a Lindenblad Antenna for 137MHz.

I like Lindenblads because they're easy, cheap, robust, and good performers. 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 trick below.

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 coax, one of 50-ohm RG-58 and another of 75-ohm RG-59, both measuring 117mm in 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 velocity 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).

Although it's expensive and unnecessary for the average Lindenblad constructor, an antenna 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.

Connectors

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', with 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 coax cables.

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, but 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 (ironically) as 'UHF Connectors'. These are horrible performers at VHF frequencies and above, and not in the least bit waterproof. But strangely, one commonly finds SO-259 sockets used on VHF Ham Radio equipment. Using these connectors is a historical habit (dating back to WW2) that is way past it's use-by date, and when I buy equipment that comes with these connectors installed, I strip them off and replace them with Type-N's. For outside cable connections exposed to the weather, it is most important to ensure that rainwater cannot seep into the inside of the coaxial cable. Once inside the coax, there is no way for water to get out again, and over time the coaxial braiding corrodes and becomes electrically 'lossy' and a good absorber of the RF signals flowing down it. If non-waterproof connector are used, they need to be thoroughly shielded from weather, and/or wrapped in self-amalgamating tape or the like.

Antenna location

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 and wire, and with a reasonable view of the sky, then this is usually fine for NOAA 137MHz reception. Also being placed lower down reduces the risk of lightning damage.

The problem of radio interference should not be underestimated, particularly if one lives within a large city. The advent of TVs, VCRs, PCs, microwave ovens, and a host of microprocessor controlled equipment, means that the VHF radio spectrum within a city is usually filled with electromagnetic crud, formally known as Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI). In general, national RF spectrum regulatory authorities protect the electromagnetic spectrum around the 137-138MHz space downlink band, but regrettably not in Australia. Most distressing of all in Australia, is VHF TV Channel 5A, which has its vision carrier located smack-bang in the middle on the 137-138MHz band. Tens of kilowatts of broadcast TV radio signal, easily swamps the 5 watts of radio signal from a 1000km distant spacecraft. In such cases where local broadcast TV interference is a problem, placing one's antenna low down in a back yard, or behind a large building, or in a small valley, may partially shield the antenna from the offending TV signal whilst maintaining a good view of the sky.

Antenna orientation

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 this as the region where the antenna is most sensitive.

The Lindenblad's overhead deaf spot may also be put to use. My friend Colin has his house located in an area which is blasted by Australian TV Channel 5A (which has the channel's FM video carrier dead-centred on 137MHz). After 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 APT imagery.

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, located 8.7km 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 kilometres away, and always find it near power lines. My suspicion is that it's being transmitted by power generation utilities, to remotely control and monitor their equipment. Finally, I'm sorry to admit, there is my own home-grown 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 image. Poor reception may 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 antenna's 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. Moreover, 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 followed Colin's lead and simply tilted my tried-and-true Lindenblad by 90 degrees (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 which conspicuously out-performs a 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).

Filters

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 inserted between the antenna and the coax. The trick is to identify the frequency of the transmission, and then buy the appropriate TV-grade high/low pass filter to block the noise. Interference on the 137-138MHz band is a common problem, and there are commercial band-pass filters available which are designed specifically for 137MHz space-downlink band. Or one can 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 forced 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 performance data.

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.

References:

ARRL Handbook for Radio Communications (current edition). Available from ARRL publications.

ARRL Antenna Handbook (current edition), edited by Dean Straw. Available from ARRL publications.

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.

Images:

Joining 50 and 75ohm coax to make a coaxial impedance transformer