Balanced or Unbalanced...

Another widespread misconception is the impression that radiators of Mosley's type cannot be fed with an unbalanced line such as coaxial cable and still maintain a balanced radiator and radiation pattern. This is a fallacy as can be seen by the directional pattern reproduced in Fig. 4. This pattern holds essentially true for any beam at the frequency of maximum front-to-back ratio. However, the same beam at other frequencies may show characteristics of maximum forward gain. This, of course, will result in slight variation of the overall pattern - but pattern symmetry is still maintained!

In other words, a beam can be tuned for maximum forward gain at its resonant frequency and still show maximum front-to-back at another frequency within the band or bands. This is a fact not generally known to the Ham Fraternity.

MOSLEY TRAP MASTER beams do not require baluns or other balancing devices to provide a balanced radiation pattern. Baluns and similar devices only introduce loss and limitations. At the same time, the need for hard-to-handle open-wire or other special feed-line types is eliminated. Ordinary RG-8/U coax makes a highly satisfactory feed-line.

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Checking Mosley Antenna Systems...

One of the wishes an Amateur Radio Operator has for his/her antenna system is that it work forever without repair. MOSLEY antennas are well designed, but, as with all electro-mechanisms, failures do occur. When they do occur, the trouble must be located and repairs made as soon as possible. It is hoped that the test procedures set forward here will help you maintain and repair your MOSLEY trap-type antenna system. The procedures described will apply to any MOSLEY dipole, beam, or vertical antenna system.

Your MOSLEY trap antenna consists of a series of 1/4 wave elements. In the case of a vertical antenna, there is basically only one 1/4 wave element. In the case of a dipole or beam, there are two 1/4 wave elements joined together to make a 1/2 wave element.

In the vertical, beam, or dipole, the 1/4 wave element may be effectively broken at predetermined intervals. This may be done with insulators, by terminating the tubing, or by installing parallel resonant circuits. Only in the last case can you construct an antenna system that will automatically change bands. In MOSLEY antenna systems these resonant circuits are called "Trap Assemblies". These highly efficient assemblies usually have two parallel resonant circuits within them. The resonant circuit operating on the highest frequency, i.e. 28.0 Mhz. is nearest the base of the vertical. In the case of a dipole or beam, it is nearest the antenna center or closest to the boom In the opposite end of this assembly another resonant circuit is found that operates on the next lowest frequency. This circuit usually works at about 21.0 Mhz. Individual traps cannot be checked by a grid -dip meter.

Due to the circuitry of trap-type antennas, it will be found that any malfunction of the antenna system on the highest resonant frequency will also cause the antenna system to operate incorrectly on the lower resonant frequencies. It is possible that the highest frequency portion of the antenna may work properly and the lower frequencies may not work properly. In very rare cases, we find that only the two highest resonant frequencies will work properly and the lowest resonant frequency will not work properly.

A very good check of the antenna is to check the standing-wave-ratio on all the bands on which the antenna is designed to operate. This check should be done every 100 kcs. in and out of the band. A malfunction on any of the bands will be indicated by higher than normal SWR readings. When checking SWR, be sure of the accuracy of your measuring instrument and that the proper connection is used. Nothing should be installed between the SWR instrument and the antennas except the connecting transmission line. Filters, relays etc. may cause incorrect readings when installed between the SWR bridge and the antenna. Be sure the transmitter is operating on the correct frequency and no serious harmonics are present. When you are making these tests, use low power. Some SWR instruments do not indicate minimum SWR at the true resonant frequency.

In cases where a high SWR is indicated on all bands of operation, and if all components, other than the antenna, are known to be functioning correctly, check all the following:

Transmission line and its connections. Check the overall antenna lengths up to the the highest frequency resonant circuit, which is the portion of the trap assembly nearest to boom on a beam or dipole or the portion of the vertical nearest the ground.

In the cases where a high SWR is obtained on the lower resonant frequencies and a normal SWR is obtained on the highest resonant frequency, the indications are that something is wrong with the outer resonant circuit. This portion of the trap assembly is located on the outboard ends of the antenna, furthest from the center on a beam or dipole or furthest above ground on a vertical.

In the rare cases where only the lowest frequency of a tri-band element is operating incorrectly, it is advisable that the element lengths be checked. This length would consist of the element from the outboard end of the trap assembly to the tip of the antenna element.

In vertical antenna systems, it is possible that more than one trap assembly will be used. This could also be true of beams and dipoles. These systems incorporate even lower frequency resonant circuits. These traps may not necessarily be in the same assembly as noted for the higher resonant frequencies. Use of the traps is the same and defective assemblies may be located by noting at what band a high SWR is first located. The frequency band at which the high SWR is first located (going from highest band to lowest band) indicates that the defective component is used on this band and is affecting the lower bands.

Trap assemblies can be visually inspected. Before disassembly, not that the color coding is still on all components or devise some method to insure reassembly of the trap onto the antenna in the correct position.

We have been asked frequently by BEAM users how to correctly install the traps when the color coding has weathered off. The following system of color coding is used on all trap assemblies used on BEAMS:

Both coils used to make on trap assembly are color coded with the SAME color. On the coil, which is placed closest to the boom, the code will appear on the tubing and will be visible when the plastic trap seals are installed.

On the JUNIOR series of beams, the outboard coil is coded on the short end of the tubing extending beyond the coil form. To inspect for this code, it is necessary to remove the coils from the trap assembly.

On the SENIOR series of beams, the outboard coil is color coded on the plastic coil form. It is not necessary to remove this coil for inspection. This color code will be covered by the trap seals. The outboard color code is not used for assembly when assembly is performed as stated in the assembly instructions.

To disassemble the trap assembly, begin by pulling the trap seals from the ends of the trap assemblies. These seals can be removed without damaging them. The removal of these seals will expose a wire which is terminated to the end of the outside metal cover. Remove the outer screw terminating this wire. Place one hand on the cover, or largest tube. Place the other hand around the small tubing and the thumb of this hand on the wire. Pull the assembly apart using the thumb to keep the wire from unwinding or loosing any tension. Tape this unterminated wire to the plastic coil from and inspect the coil form. Damage may be noted by shorted turns, deformed coil forms, melted plastic form and other abnormalities. It is important that these coils have the correct number of turns. The instructions pertinent to the antenna being inspected usually give these turns and overall antenna lengths.

When reassembling the antenna, note that the wire on each coil form is tight because loose wires will detune these coils and cause additional troubles. Be sure screw is pulled down tightly and wire and screw are clean.

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Feed Systems - The Simpler, The Better...

Despite popular belief, linear radiators, normally employed in 2 and 3 element parasitic arrays, have a driving point impedance of close to 52 ohms when open at the center. To feed such a radiator it is only necessary to connect a 52 ohm line at this point to achieve the best possible match of line to antenna.

However, because of certain design characteristics, some beam antennas require elaborate and, sometime, unstable matching devices such as Gamma or T-Match systems or variations of these systems. Such devices are usually difficult to adjust and to maintain in adjustment when used in multi-band beams.

MOSLEY TRAP MASTER beams, however, are so designed as to not require any unwieldy matching arrangements. Mosley beams are fed by connecting the line directly to the open center of the radiator. Thus, an excellent match is achieved over the entire width of each Ham band resulting in extremely low SWR near resonant frequencies of each band and the ability to range from one end of the band to the other without excessive SWR.

By eliminating such matching devices, MOSLEY TRAP MASTER beams provide their users with stable and dependable operation without the necessity of frequent trips to the roof or up the tower to make readjustment.

MOSLEY TRAP MASTER AND POWER MASTER SERIES...  A Discussion of Beam Antenna Feed Systems

Among those Hams interested in beam antennas, many are concerned with the feed systems employed.  It is for these Hams that we shall attempt to explain the wonderfully simple -  yet highly efficient - feed system used in both the Trap Master and the Power Master series of Mosley beam antennas.

The beliefs that a balanced radiator element cannot be fed with an unbalanced line and that the impedance at the center of the element is not of suitable value to permit direct connection of a 52 ohm coax line are not always correct.  We will show, with authoritative references, that antennas can be designed to take advantage of the simplicity of such a system and still provide low VSWR over a broad bandwidth and a symmetrical radiation pattern.  Page numbers will refer to the ARRL Antenna Book, seventh edition.

The voltage distribution over a half-wave radiator is shown in Fig. A.  Since voltage is zero at the center, a ground may be placed at this point. (Page 26)

There are a variety of methods for introducing energy into the antenna.  A balanced line may be connected directly to suitable points at each side of the grounded element center - a method commonly called the delta match or, with slight modification, T match.  To connect an unbalanced line to a grounded un-split element, the gamma match from grounded center to a suitable point at one side of center may be used.

However, the element may be split at the center and fed with either a balanced line without a ground at the center or an unbalanced line with or without a ground at the center.  With an unbalanced line and ground on both the outer conductor (coax braid) and antenna center, a voltage introduced just off center in the position of the power source (Fig. B.) will introduce a voltage in this excited side, as shown.  With the other half of the antenna element an integral part of the circuit, voltage will appear as indicated by the dotted line.  Since the end of the feed line is fundamentally the same as a power source it may be replaced in the circuit, (Fig. C.), resulting in a balanced antenna fed with an unbalanced line.  The ground at the center helps to minimize stray feed line currents to achieve the balance pattern. (Pages 98-100) (See, also, page 224, Fig. 10-10).

Curves on page 169, Fig. 4-51, show how a three element beam, by correct tuning and element spacing, may present a feed point impedance of from 10 to 70 ohms.  Of course, MOSLEY beams are tuned and spaced to present 52 ohms at suitable tuning points with low VSWR over the entire bands of operation and to achieve proper voltage distribution for a balanced radiation pattern.

THE CLASSIC FEED SYSTEM...   By W.E. "Barney" St. Vrain, W0PXE 

Since the introduction of multi-frequency beams, the method of feeding such antennas has been a subject of much disagreement.  When the Mosley Trap Master and Power Master beams were introduced, Mosley Electronics ran a series of advertisements in the technical magazines explaining the method employed.  Since that time we have tried a wide variety of feed systems endeavoring to improve on the original system.

Testing Other Feed Systems

In testing, we found a three-band gamma system ineffective without isolation networks which resulted in a feed system cost about equal to the antenna cost; with a system using hairpins, the cost proved low but the system did not provide a better match that the original Mosley matching system.  It became quite clear to us that the Mosley system was hard to beat, for we had found only one slight disadvantage: the elements needed to be stagger tuned to raise the feed point resistance from about 30 to 50 ohms.  This slight detuning, which proved advantageous in increasing bandwidth, brought about, in turn, a slight gain loss of about 0.5 to 1.0 db. at resonance.

The Classic System

In order to give hams a new choice in beam matching systems and an antenna featuring maximum gain with increased bandwidth, we devised the matching method used on our Classic antennas - Balanced Capacitive Matching (Patented) - a method which takes advantage of the principle that antenna resistance at the center driving point increases s the antenna length increases.  Figure No. 1 shows the radiator element of a three-element beam at resonance having an impedance at the driving point (Z_A) of about 30 + J0 ohms.  If the element is made longer, Z_A can be raised to about 50 +J50 ohms (Figure No. 2).  Since the reactance is inductive, it can be cancelled with a series capacitor of 50 ohms reactance, leaving 50 ohms feed point resistance (Figure No. 3).  Series capacitors used on the Classic antennas are made by inserting a suitable length of heavily insulated wire into each half of the element tube at the center.  The wires are terminated in a plastic tube enclosure with a type 'N' or type 'SO-239' connector for connection of the coaxial cable.  To isolate the outer coax conductor from ground, the coax line is coiled for a few turns near the antenna end.  This is designed to prevent the very unlikely effect of "Feed Line Radiation."

Conversion to Other Bands

Classic beams are not designed for 40-meter or other conversion.  (Except the CL-33-M WARC).  The Classic Feed System has a fixed capacity which is not easily changed.  This capacity is not high enough for the antenna to operate on 40 meters without making the element excessively long.  The engineers at Mosley designed the Classic Feed to give the ham increased bandwidth and extra gain on all bands.  It is our conviction that discriminating DX'ers will find the Classic Beams specifically suited to their needs. (NOTE:  The CL-33-M can be modified to add 12/17, then 40-meters is possible.  The Classic Feed System is replaced with the Mosley Matching System).

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Maintenance of Your Mosley Antenna System...

Frequently it is asked, "How can I clean my MOSLEY antenna system?", or "How can I remove MOSLEY Weather-Guard from my antenna?" MOSLEY Weather-Guard can only be removed with sandpaper or steel wool. When the trap type antennas are sanded or steel wool used, care should be exercised to see that small bits of aluminum dust or wool bits do not get into the trap coils and short them out. MOSLEY Weather-Guard, when properly applied, is a permanent protection and seldom needs to be removed. Usually a good cleaning will remove dirt.

To remove oxidation and corrosion from the aluminum components, Mosley recommends using either a double 00 or a triple 000 grade steel wheel.  With a little elbow grease, the aluminum can be brought back to its original luster.  Also, a good cleaning agent that can be used is Tri-Sodium Phosphate, (1/2 cup per gallon of water). This is a high detergent cleaning agent.  A solution of vinegar and water can be substituted as well.  Care should also be exercised to keep this out of the trap assemblies. When finished, thoroughly rinse with clean water ALL portions of the antenna. BE CAREFUL TO KEEP SOLUTION OUT OF THE TRAP ASSEMBLIES. 

A MOSLEY anti-corrosion compound is available and should be used on all electrical connections except on terminating wires on the trap assemblies. It is very effective and, when used properly, will prevent corrosion or oxidation of metals.

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The trap design has been under fire since its inception.  Mosley Electronics, Inc. invented the metal enclosed trap.  We have been using the same design for over 50 years.  Why?  Because it works!   Don't be mislead by misinformation.  Gain is a function of element spacing and boom length and not what constitutes an element.  The published gain figures for our products are dBd, actual measured gain.  Antennas which use other forms of trapping such as linear loading, poor trap copies, baluns or matching devices have inherit loss.  Mosley antenna systems are efficient and effective through proper element spacing, actual resonant elements and no insertion loss from matching devices or unnecessary connectors.  Any measured loss from trapping is already accounted for in the specification measurements.  Mosley trap designs are second to none.  Click for more information on our TRAPS or FEED SYSTEM.  Be sure to check out the vintage CQ Magazine ad to the right.  That ad is older than most of our competition.  Just click on the button!

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Repair Parts for Your Mosley Antenna System...

Mosley stocks replacement parts going back 65 years for any antenna we ever produced.  Some items may be limited in quantity, but replacements can be fabricated.  Instructions for all MOSLEY antennas may be obtained by writing directly to the factory. It is recommended that all replacement parts be ordered by part number, color coding, description and the form number of the instruction manual used. This will insure that you receive the proper parts. Prices for replacement parts will be given on request. All replacement parts are purchased direct, f.o.b. our factory.  MORE INFORMATION ON ORDERING

PDF files for select antennas will be available in the coming months.  For now, please call or write for documentation on any Mosley antenna for a nominal charge.

Mosley Electronics, Inc.
1325 Style Master Drive
Union, MO 63084

(636) 583-8595

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The advent of low-cost SWR bridges and analyzers has enabled ever-increasing numbers of Hams to become acquainted with so-called reflected power. This has become such a popular conversation piece on the Ham bands that an entirely unwarranted degree of importance has attached itself to the subject of SWR.

Many Hams - who for years had experienced excellent results with their beam antennas - suddenly found that their feed-lines possessed something less than perfect unity match to their antennas. Such is human nature, that, irregardless of past performance, this newly discovered "revolting development" became entirely intolerable!

This unjustified mental attitude has for some years provided nice incomes for chiropractors and broken bone specialists who have reaped far more profit from the roof and tower excursions than have the Hams who restlessly seek perfection.

Competent professional antenna engineers generally agree that in most applications a standing wave ratio of up to 5 to 1, or somewhat higher, is satisfactory and acceptable.

For example; referring to the ARRL Antenna Handbook, we see that a SWR of 5 to 1 in RG-8/U coax will result in a signal loss at 28 mcs. of only 1 db. This decrease in signal strength cannot normally be detected on an S-Meter!

This is the "introduced loss", due to SWR, and has no bearing on the normal 1 db. line loss inherent in 100 feet of the line which is present in any case. Furthermore, these losses decrease as frequency decreases!

Sometimes - and particularly with Ham antennas which must operate over wide ranges of frequencies - it is not at all desirable to have a feed-line/antenna combination that offers perfect unity SWR. Such combinations usually are very critical and while providing unity at one frequency will often have a poorer response curve than other antennas designed to give optimum performance over a greater band-width.

In actual practice, most Hams will be far better off with beam showing 1.1/1 SWR at tuned point and increasing but little as frequency is changed; rather than with antennas that show unity at resonance but must be retuned for operation at other frequencies.

MOSLEY TRAP MASTER beams do not claim perfect unity match at any point in the bands but are so designed to provide optimum performance over the full width of each band without tiresome roof or tower climbing.

As can be seen in Fig. 5, the frequency response curves show a very low SWR at the minimum tuning points and very slight and inconsequential rise in SWR to the band limits.

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The Importance of Good Trap Design...

An antenna trap can be described as being an application of a resonant circuit. Such a trap, utilizing a parallel resonant tuned circuit, offers very high impedance at or near resonance and simply acts as an insulator to effectively cut off the element to a length resonant to the band being used. See Fig. 1.

A trap can properly be considered the "weak link" in the chain of components that comprise a multi-band antenna. Any failure or malfunction of one of the traps will put the antenna out of operation or, at least seriously effect its performance capability. Thus, it is of paramount importance that the trap design be such as to assure stability of resonance in wide extremes of temperature and humidity and to exclude or inhibit dirt and moisture formation which could cause malfunction or breakdown.

Any variation in capacity or inductance in the trap's tuned circuit will cause the resonant frequency to shift. Therefore, each of these components must be firmly fixed and this can best be accomplished by winding the coil on grooved forms and by making the capacitor sections immovable with respect to each other. Traps used in MOSLEY TRAP MASTER antennas follow this good basic design as can be seen in the cut-away model photo, Fig. 2.

HOW THE TRAP ANTENNA WORKS

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