aerials

The subject of aerials is quite complex. The following lays out the basic requirements and principles of propagation plus links to sales information. Separate leaflets are available for the main items (follow links) and a selection of reviews on AOR products are also available.

Theory and practice
One interesting phenomena is that aerial theory and practice can be surprisingly different. Keeping common sense in mind it is one of the few remaining areas for listeners to easily experiment and often achieve fantastic results.

Whip aerials (TW500 RA8600)
If using hand held receivers, the standard supplied whip aerials should provide good results when using the unit portable. Flexible "rubber duck" whips are convenient because they are small in size and wont poke your eye out. A longer telescopic whip is usually a little better and the length can be varied for different frequencies.

Mounting location
If fitting an external aerial, it is important to mount the aerial as high as possible and in clear space although this is more important at VHF frequencies than for shortwave. If possible the aerial should have a clear path to the horizon. Results are usually disappointing when an installation is in a loft space.

Discone (DA3000)
For wide coverage in the VHF-UHF bands a compromise has to be met and the most popular aerial is a discone. Their appearance is like a large spider or umbrella without the covering material, the better models have about 16 elements... avoid discones with only a few elements (such as six). Typical usable coverage starts from about 25 MHz and extends continuously to 500 MHz, 1300 MHz or even 2000 MHz. The coverage peaks and dips throughout it’s range as the elements interact to provide the widest possible coverage. Due to their necessary construction discone aerials are a little prone to "wind noise" due to vibration and possible damage in severe gales. Unipoles are also gaining popularity such as SCANMASTER BASE.

Dipoles
For the very best results you should consider a dedicated aerial such as a single or multi-band dipole or similar aerial. The problem with a wide coverage receiver is that for ultimate results many dedicated aerials are required to cover the whole spectrum. This may involve complex aerial switching and reduces the ability to quickly monitor many bands. As a compromise it may be worth making up a dipole aerial for a band of particular interest and have a VHF-UHF discone plus second random wire for general listening. It is quite easy to make a dipole for shortwave, for that matter one can be easily made up for VHF or UHF too. If being made for VHF-UHF the centre connection of the coaxial cable feeds the upper element set vertically. Shortwave dipoles on the other hand are usually mounted horizontally.
It is worth noting that dipoles are also quite effective on two and three times their design frequency so you can cover a few bands at once. Reception using a half wave dipole is best at 90 degrees to the direction the aerial is laying, however if used at two or three times it’s fundamental design frequency, reception is best closer to the direction the aerial is lying. A dipole has two legs running in opposite directions and can be mounted vertically or horizontally (most VHF activity is vertical). One leg is connected to the centre conductor the coaxial feeder cable while the other leg is connected to the outer screen of the coaxial feeder cable. If mounted vertically, the centre of the coaxial feeder should be connected to the leg facing upward.
A simple formulae can be used to calculate the required length of each leg for a half wave dipole:

75 / Frequency in MHz = Length of each leg in metres
i.e. For 14.2 MHz 75 / 14.2 = 5.28 metres (i.e. the total length of the aerial is twice 5.28 metres)

L
Aerial Wire

L=75 / Frequency in MHz
Length in Metres

	Dipole Centre Piece (connection) close up view
Centre conductor of coax connected to aerial wire		Braid gathered and connected to aerial wire
	Coaxial cable leading to the receiver (30 OHM)

Coaxial cables
When constructing dipole aerials or connecting VHF aerials 50 OHM coaxial cable should be used. For shortwave or short runs of VHF URM43, URM76 or RG58U is ideal, for longer runs of VHF-UHF feeder, it may be worth considering a heavier URM67 or RG213 (or better!).

Active aerials
Active aerials are normally quite compact and combine a wide coverage aerial with a preamplifier mounted within the aerial its-self. They require power to enable them to operate. Not all designs allow you to switch the preamplifier off although some have a gain control. As with loop aerials they tend to provide good results on the lower bands when compared to poorly sighted short’ish wire aerials. Overload can be a problem on the busy 7 and 9 MHz bands. If you have a small garden, an active aerial may be worth considering.
VHF - UHF active aerials provide good results for low lying property in remote areas, this can be useful when an unobtrusive installation is of primary concern. However, such aerials usually provide very poor results in built-up areas due to the high number and close proximity of VHF transmitters... breakthrough can be a major problem.

Stub filters
When connecting hand held receivers to external aerial systems it is quite common to encounter breakthrough problems due to a large number of strong local transmissions mixing together in the receivers circuitry. Many words (some technical) are used to describe the different types of breakthrough: Breakthrough, Bleed-over, cross-modulation, over-load, reciprocal mixing etc.
Typical scenarios include music overlapping commercial airband transmissions, voice transmissions where they should not be located etc. Although annoying, such breakthrough will not cause damage to the receiver's circuitry.
Should you encounter "breakthrough" when using an external aerial (and the attenuator does not help) a simple stub-filter placed in the coaxial cable may help. This comprises of a `T’ connector with an open circuit 50 OHM cable length (the stub) attached to the `T’ piece.

A rough calculation for the stub length is as follows: (75 / Freq in MHz) x 0.67 = Stub length in metres
i.e. To reduce the strength of 88.3 MHz on VHF Band 2: (75 / 88.3) x 0.67 = 0.57m or 57cm

Commercial filters
Should you wish to purchase a commercial filter rather than "make one up" (such as a stub) then there are a few to choose from.

BANDPASS (ABF125): A VHF civil BANDPASS AIRBAND FILTER is now available from AOR called the ABF125. This will help minimise the possible effects of breakthrough when listening to VHF airband when located in BAND-2 and similar VHF high signal areas or when connected to external aerials. The filter PASSES JUST THE VHF AIRBAND signals and rejects all other bands - this is where the term BANDPASS comes from.
TUNEABLE NOTCH FILTERS: Other manufacturers are providing tuneable filters to notch out unwanted signals typically in the range of 75 to 175 MHz:

Mobile VHF - UHF aerials:
Hand held receivers are often used "on the move" from vehicles. Due to the metalwork of the vehicle and potential noise from the vehicle's electrical system, external aerials are desirable. When an aerial is mounted on the vehicle then the metalwork of the vehicle acts as a groundplane (presuming that it is not fibreglass) enhancing the performance of the aerial system.
The are now many ways to mount an aerial:

Long wire aerials
For shortwave reception a random length of long wire approximately 10 to 20 metres in length forms a good compromise. If using a hand held receiver, the wire should be connected to the centre pin of a BNC plug. If possible try to locate the receiver close to a window so that the wire has the shortest and most direct run from the receiver to the outside World. Never attach the wire aerial directly to a support or wall, instead attach a small length (one metre) of insulating material such as nylon to each support (house or tree for example) and then onto the aerial wire. Allow the wire aerial to drop diagonally into the window and receiver rather than straight down the wall. Keeping the aerial away from supports and building will reduce the loss of signal from the wire aerial and prevent unwanted noise from entering the aerial system.

Magnetic balun long wire aerials are becoming very popular as they allow coaxial cable to be used as the down-lead from the wire aerial into the receiver. The balun transforms the impedance to a low level suitable for 50 OHM coaxial cable. In this instance the path of feeder is unimportant and chances of noise entering the aerial system reduced.

ATU - AERIAL TUNING UNITS
Aerial tuning units (ATU) may improve the shortwave section of a wide range receiver in two ways. Firstly by improving the "match" between the aerial and receiver so providing better efficiency. Secondly by rejecting unwanted signals and only allowing a specific band of frequencies through. These ATUs are normally constructed in small boxes with about 3 controls on the front. The disadvantage is the need to constantly retune the ATU when changing frequency. An ATU of this nature is "passive", this means that no power is required to operate the ATU and no extra circuit-noise is introduced into the receiver.
An ATU of this nature is the GLOBAL AT1000 or (MFJ-965 passive preselector with magnetic balun).

PRE-SELECTOR
Usually a PRE-SELECTOR will provide better results than an ATU. The pre-selector is ACTIVE (usually being powered from 12V DC) and provides band selection, rotary pre-selector to "peak up" the signals, straight through mode, amplifier to add a little improvement to sensitivity (for quiet bands) and attenuation.
Note that the preselector adds some noise so that the overall system noise figure may deteriorate (when using a high dynamic low noise receiver such as the AR7030) so think seriously before adding a preselector.

Loop Aerials (LA320)
Short wave desktop loop aerials have the advantage of small size (such as the AOR LA320). They too have tuning controls to reject unwanted signals. As the loop is within easy reach of the operator it can be rotated to provide directivity and can be particularly useful for DX’ing the lower bands. Generally speaking they offer excellent portability but cannot compare on the higher bands with a well sighted long wire aerial.

D" layer During day time the lower "D" layer forms around 60 to 80 kilometres above the Earth’s surface. This "D" layer tends to absorb low frequencies reducing the distance covered by medium wave transmissions. In the night time when the "D" layer dissipates, medium and low frequency transmissions may propagate over much greater distances. If the transmitted frequency is too high for to be reflected by the ionosphere, or the angle too steep, transmissions will simply pass straight though the ionosphere without being reflected and will travel upward to the next ionosphere layer.

F1/F2 Layer		A - Transmitting Station B - First Receive Station C - Second Receiving station
	Earth Simple Illustration of the Ionispheric reflection of shortwave radio signals

“E” layer: Above the “D” layer is the “E” layer located at a height of about 100 kilometres. The “E” layer tends not to absorb signals as much as the “D” layer but refracts some signal back to Earth where it may be received some distance from the original point of transmission. Usually in Autumn and Spring “SPORADIC E” propagation consisting of dense pockets of “E” layer ionosphere, reflect even the higher VHF and UHF transmissions causing patterning on television sets. This is to the delight of Radio Amateurs who are then able to communicate for many hundreds and even thousands of kilometres on frequency bands usually capable of only local reception. Occasionally a similar effect can be caused by temperature inversion layers creating “tropospheric propagation” selectively “ducting” transmissions between two points. Tropospheric propagation is usually applicable to the higher VHF and UHF bands only.
“F1” & “F2” layers: During the day time there are two upper layers of the ionosphere, these being the “F1” layer at about 200 kilometres and the “F2” layer at about 400 kilometres. As evening falls, these layers combine to form a single “F” layer. It is “F” layer propagation that is largely responsible for shortwave propagation over great distances. The density of the ionosphere layers varies depending upon season, time of day and sunspot activity which is believed to follow an eleven year cycle of good and bad propagation conditions. You will note that large areas of the Earth’s surface lays between the point of transmission and reflection, in this area there will be little or no reception. For this reason “F” layer propagation is often referred to a “SKIP” and the reflected signal as “SKY WAVE”. Generally speaking only frequencies below 30MHz are reflected by the ionosphere. Higher frequencies pass straight through even the “F” layers and will continue outward into space for ever.

Choice of frequency
Depending upon the time of day and desired skip distance, different frequencies will be selected by Radio Amateurs and commercial users such as Oceanic Air Traffic. For instance the “MUF” (Maximum Usable Frequency) is often stated for a path between two locations. Choosing a frequency above the “MUF” will not produce results as transmissions will pass straight into space. Many propagation predictions and statistics are published and usually available from most country’s National Amateur Radio and Shortwave Listeners representatives. Various publications are produced giving transmission and contact details for World wide reception. These titles include:

Typical oceanic airband shortwave frequencies include (ALL USB): 5.505 MHz, 5.450 MHz, 11.197 MHz, 4.742 MHz, 5.616 MHz, 5,649 MHz, 8.825 MHz, 8.864 MHz etc. Remember, they do not transmit all of the time but only at longitudinal reporting points so be patient.

Questions & Answers - The most commonly asked questions about aerials include...

Q - My new hand held doesn't appear as sensitive on certain bands?
A - Most likely the cause will be the type of aerial supplied. Usually a small rubber type helical "rubber duck" is supplied as standard. While this is compact and durable, a larger telescopic aerial (AOR TW500 or RA8600) or good helical (AOR DA900) will provide better results. When using a telescopic aerial the length may be altered to peak performance. Some helicals are particularly poor on the VHF marine band.

Q - Can I mount a VHF/UHF aerial in the loft?
A - The obvious answer is yes - but, the performance particularly on VHF / UHF is affected tremendously when loft mounted. You may well be disappointed with the poor results especially when considering the financial outlay. The same is true for "testing" an aerial in a bedroom before erecting externally, don't be surprised if the results appear poor.

Q - Does a discone aerial really need to be on a chimney and will it help if it is higher?
A - Usually the answer is yes. The aerial has no gain so needs to be as high as possible and in clear space. The clearer the view to the horizon then the better the results. This also means that if the aerial already has a clear view to the horizon then adding another ten feet in height will not return great improvements.

Q - What can I expect from internal aerials of special types?
A - VHF / UHF active aerials provide acceptable results in remote areas but built-up towns and cities have too many low level buildings and materials in the way. Such aerials are usually fine for table-top use when camping, caravanning and from some hotels when sited close to a window... do not expect great results regardless of what the advertising claims! They are a LAST RESORT and are better than nothing at all.

Q - Can I use my discone aerial for short wave listening?
A - Yes - but don't expect too much. As discone aerials are not resonant below about 25 MHz, they will be acting very much as long wires. The AOR AR3000A receiver has an internal short wave switchable preamplifier so results can be surprisingly good on short wave with discone aerials. A long wire aerial and pre-selector is by far the better choice.

Q - Can I wrap the long wire round and round - as my garden / loft is only small?
A - No - as soon as you "double back" the wire, performance will be reduced. A shorter straight wire is better than a long wire doubled back. The rule of thumb is usually that you can use all three dimensions moving away from the receiver... along, up, twisted left or right once only.

Q - I can hear music on the VHF airband, what can I do about it?
A - This is quite common especially when hand held receivers are connected to external aerial systems. Try making up a stub filter or invest in a commercial filter such as the AOR ABF125 or similar. Sometimes liberal use of the attenuator can help but of course this will reduce the strength of the transmissions you want to hear too!

Q - My computer generates noise, what can I do?
A - Switching it off is the only CURE. However with careful planning: remote aerial, good quality coaxial cable, a good earth system, and if remote controlling the receiver (AR3000A for example) connect the screen of the RS232 serial lead (pin 1) at the computer end only to prevent earth loops.

Q - Will a preamplifier help?
A - Generally speaking preamplifiers are not recommended, this is because the strong signal handling of the receiver will be compromised... especially hand helds. If you are located well away from strong near-by transmitters and seek that extra little sensitivity then it may be worth having a go. Try to choose a preamplifier which can be easily by-passed and ideally with a gain control... reasonable results should be obtained.

Q - What about thunderstorms?
A - It is a good idea to remove AND EARTH an aerial when not in use, no not leave the plug lying around unearthed or "charge" will build up. Also, it is a good idea to EARTH an aerial prior to connection to the receiver just in case a charge has built-up following a storm or during / after snow etc.

Q - My discone whistles in the wind - what can I do?
A - This is a problem which seems to affect all discones. The problem is worsened if the aerial is NOT mounted in clear space (such as at the side of a chimney or between buildings) where a "jet" of air is forced passed. Try DAMPING DOWN the end of the elements with clear fishing line.

AR7030 and Aerials

Generally speaking, if your garden is long enough to errect a long wire of about 60 feet in a straight'ish line andas high as possible then this is the best choice.

It is most important to make the far end of the wire as high as ossible and to keep the wire away from buildings and supports... This is more significant than the length of the wire.

You may take the wire through three dimentions but do NOT double it back on itself or signals will be reduced, even if this extends the lengthof the wire!

On the higher bands above 14MHz, the direction the wire is pointing becomes significant... you must consider the great circle map when deciding which way to lay the wire should a choice be available.

Magnetic balun wire aerials are popular if the radio is not located right next to a window. This makes the run of coaxial cable less of a concer in terms of length and where it has to travel onits way to the radio. The coaxial cable will also help reduce interference to the radio from computers etc.

A good RF earth will help too, ther is further information contained in the AR7030 operating manual in this respect.

Where a long wire in clear space is not possible, it may be worth considering an active aerial. On lower bands belowe 7MHz such aerials are usually quite efective when compared to long wire stystems but they are prone to overload due to strong signals around 7 & 9 MHz, especially at night.

Selecting The Correct Aerial

Due to continuous developments, AOR reserves the right to make design and specification changes for product improvement without prior notice. The performance specification figures indicated are nominal values of production units. There may be some deviation from these values in individual units.

Aerials

Questions & Answers - The most commonly asked questions about aerials include...

AR7030 and Aerials

Selecting The Correct Aerial