Aerials

A radio is only as good as the aerial it's attached to

Rubber Duck or Helical
Telescopic
Dipole
Four Stack
Co-linear
Discone
Yagi Beam
Log Periodic
Corner Reflector
Mounting your aerial
Selecting Co-ax
HF Antennas
Wellbrook Loop
Joke Antennas

When you got your scanner, it probably came with a supplied portable antenna. These are generally convenient, portable but not outstanding performers. Most of the reason for this is the position in which they tend to be used. Often, this means being clipped to a belt, or lying flat on a table and quite often inside a building or metal vehicle

If you were to stand on your rooftop with a handheld scanner, you'll find that your performance improves dramatically. Not a practical solution for many but this illustrates a point.

If you were to mount your rubber duck on your chimney, and connect it up to your scanner via a length of coax, your antenna would always be in an optimal position and you could site your scanner wherever you wanted it in the house and the performance would not waver. You would not need to constantly adjust the antenna to get the best signal.

Better than this is to mount a base aerial on you rooftop. The reason for this is as I hinted that rubber ducks are not very good performers. Base antennas can be either directional or omnidirectional. Directional antennas tend to have a lot of gain but of course they need to be pointed at the transmitting aerial and that usually takes a rotator.

One frequently asked question is "what is the best sort of antenna". There is no straightforward answer to this as each situation is different. Here are a few antenna types and let's discuss their relative merits.

UHF / VHF Antenna selection guide
Usage	Physical	Gain Characteristics	Type	Comments
Portable	Chose BNC or SMA to suit your radio	Inefficient	Helical (Rubber Duck)	Lossy but convenient
		Inefficient	Compacted Helical	Very short but corresponding performance
		Unity Gain	Telescopic quarter wave	Tuneable
		Unity Gain	Flexible quarter wave	Optimised for given band
		Gain Antennas	Telescopic 5/8th wave	Tuneable, awkward to carry around
		Gain Antennas	Flexi co-linear	Good performance, cumbersome but robust
Mobile	Choose magnetic mount, hole mount or fixed mount to suit your vehicle	Unity Gain	Quarterwave	Monoband
		Gain Antennas	Base loaded 5/8th wave	Monoband
			Centre loaded 5/8th wave	Monoband
			Base loaded 7/8th wave	Monoband
			Centre loaded 7/8th wave	Monoband
			Dual band co-linear	Usually covers 2m & 70cms
			Tri band co-linear	Usually covers 6m, 2m & 70cms
			Quad band co-linear	Usually covers 10m, 6m, 2m & 70cms
Base	Omnidirectional	Inefficient	Discone	Very wide coverage but poor performance
		Unity Gain	Dipole	Simple, cheap and easy to make
		Gain Antennas	Co-linear	Very discreet, can be multi-band
			Stacked Dipole Array	An array of co-phased dipoles
			Base loaded 5/8th wave	A decent gain compared with a quarterwave
			Dual band co-linear	Good for civ-air and UHF
			Tri band co-linear	Good for 50MHz civ-air and UHF
			Quad band co-linear
	Directional	Gain Antennas	Stacked Dipole Array	Can be arrayed to give an offset or bi-di pattern
			Yagi Beam
			Log Periodic

Rubber Duck or Helical

This consists of a number of turns of copper or copper coated steel wire on a plastic or fibreglass former. They are of course often seen on commercial portable two way radios. When they are used as such, they are cut to resonate at a particular frequency. If you were to use the lower part of VHF, say 2 metres, air band or mid band, you would order one type of antenna. If your radios use the upper part of VHF, called high band, your helical would be cut slightly shorter. For Band II, it would be even shorter still. So you can see from this that the helical antenna is not ideal for a wideband scanner because it only works well within a small frequency band.

Even when a helical is perfectly tuned, it is quite lossy but popular because it's break resistant, compact, cheap to manufacture and hey, it looks the part.

Telescopic

This of course is another popular, cheaply made antenna which is commonly supplied with scanners, both portable and desktop. Performance is slightly better than the helical but not as good as a dipole, the reason being that in truth, it's only half an aerial.
It works as a quarterwave, but in order to do so, it should have a counterpoise of some sort and all it has is the body of the scanner which there days can be quite small. This becomes less of an issue at higher frequencies, they are generally good as a quarterwave at UHF. They have a wider bandwidth than a helical as by adjusting their length, they can be made to resonate at differing frequencies.

Dipole

This is a keystone of antenna design and if you want to have a go at making your own antenna, this is the best place to start. Essentially, a half wave dipole is a quarter wave driven element, mirrored by a quarterwave counterpoise.

Here you see a typical half wave dipole. The thickest upright tube is the support mast. The horizontal tube section is the boom, and the remaining two small tubes are the elements. Where they join is called the feed point. This needs to be a waterproof box if the antenna is to be used outside to stop water seeping back down the co-ax which ends inside this junction box. The centre of the co-ax connects to the upward pointing element and the outer braid is connected to the downward pointing element.

As you can see, it is very simple and if you were to make one yourself, most of the work goes in making the support structure and making it able to withstand the elements.

If you want to make a dipole for indoor use, it's a doddle. Get hold of a suitable length of RG58U co-ax and put a plug on one end to suit your scanner. This will usually be BNC. Some scanners use an SMA socket and it's probably best here to use a BNC anyway with an adaptor. SMA plugs will not fit directly on RG59 co-ax.

At the other end, strip off the outer insulation to just over a quarter wavelength. Then, tease open a hole in the braid until you can see the inner insulation. Hook out the whole quarter wavelength of the centre core complete with its insulation. Pull the braid enough to flatten it slightly and tidy it up. You can use bamboo cane to hold its shape and plastic plumbing materials to waterproof it and support it outside.

If you want a running start, you can buy a dipole which is meant for domestic radio reception and adapt it slightly. This involves nothing more than shortening the elements for frequencies above 100MHz. If you want to use it on low band then you need to extend the elements. This can be done by inserting a suitable length of 10mm aluminium tube so that it telescopes into the existing elements. First you need to castellate the ends using a hacksaw. Having done this, you can use hose clips to secure each element. This has the added advantage that the antenna can be tuned by sliding the elements in and out.

I have used a kit made by a company in Kings Lynn called Maxview. With this kit, you get everything you will need apart from the BNC plug on the end. Incidentally, the coax which comes with it is 75Ω and as you may be aware, coax used for comms such as CB, HAM and PMR has a characteristic impedance of 50Ω. In practice, the difference is so slight as to be not of any real concern. In actual fact the dipole will present an impedance of around 72Ω so it's really just fine. As it happens, some of the lowest loss coax you can get at a reasonable price is satellite TV cable. This has foil as well as braid and generally works well up to 1.2GHz and beyond. One thing to bear in mind here is that often the braid is stainless steel which can't be soldered.

Here's a slightly different dipole.

As you can see, the elements carry on, folding back on themselves and join at the boom.

This is known as a folded dipole.

Folded dipole at UHF

Masterplug Omnidirectional FM Dipole Aerial

Four Stack

A dipole as I said earlier is a bit of a yardstick in the antenna world. It is considered to have a gain equal to a theoretical or isotropic antenna which neither has a gain nor a loss. We therefore say that it has unitary gain or 0dB gain when compared with the isotropic antenna. Some antennas will work less well than the dipole and can be described as inefficient. Others receive better than the dipole and are said to have gain.

In order to give a dipole some gain, what we do is stack a number on top of each other. It's not at straightforward as using a 'T' connector to achieve this.

If you did this, the impedance would be wrong. Commercial stacked dipoles usually come as an array of four and are supplied with a phasing harness which make all four elements work in unison.

One of the nice things about a four stack is that it can be arrayed to be directional in one of three different ways. The four elements face out in four directions, all at 90˚. If all four elements are set facing the same direction, the antenna becomes slightly directional in that direction. We call this an offset pattern and it's useful if your on the outskirts of a town as you can slightly bias it towards the centre of the action but still receive other signals from all around. The third pattern you can set up is a figure of eight pattern by putting two elements each side of the support mast. This would be good if you were between two towns or cities.

Co-linear

in a similar way to the four stack, a co-linear is made up of two or more dipole arrays but instead of the support and feed coming up through a separate mast, it goes through the middle of the elements to make a very compact and discrete antenna.

You will commonly see them in the form of a 'white stick' which is usually a fibreglass tube, sometimes with radials at the bottom but not always. So here we have an antenna which has a significant amount of gain, doesn't look like a chimney sweep's broom and is durable.

There are a few co-linears made for the scanner market but many more for the amateur radio market. They are quite often dual band being meant for two metres and 70cms. In practice, they will work as a receive aerial very well outside the amateur bands although performance drops off the further you go away. In short, they are good on airband, Marine VHF and VHF midband. Highband is ok too but performance tends to drop off towards the top end. With UHF they have lots of gain and great for PMR446 all of UHF1 including UK general, similarly, performance drops off towards the top of UHF2 but performance is still quite acceptable.

Discone

The discone is favoured for wide band reception as it works over such a wide spread of frequencies. The downside of this type of antenna is that its performance is not that special. It's not the sort of thing that you'd want to use for pulling in really weak distant signals but it's fine a jack of all trades for strong local signals. Other considerations are that it is physically complex, has a high wind loading and has a reputation for shaking apart. Plus points are that it makes a nice perch for our flying flu carrying friends

Yagi Beam

A yagi beam is a dipole which has been made directional by adding a reflector behind the element and a number of directors in front of it.

The Yagi gets its name from the Japanese doctor who invented it.

A yagi beam can be mounted with either vertical and horizontal polarisation.

It is a directional antenna. The more elements it has, the more directivity it has, and hence, more gain.

Log Periodic beam

The Log periodic antenna comes in a few variations but one common one is the Log Periodic Dipole Array. This can be considered as a set of various half wave dipoles arrayed in such a way that it acts as a directional beam and offering an array of different resonant frequencies. The signal travels down the beam and transduces through a cluster of elements which suits the received frequency. This is called the active region.

Corner reflector

Another dipole, this time with no directors but instead, a large angular reflector.

Mounting your aerial

Outdoor aerials

Mounting an antenna in your loft space seems like an attractive option. You don't have to get a ladder out or drill through walls, there's plenty of dry space, it's high up, discrete, accessible etc. etc.

By all means give it a go, there are a lot of things in it favour. It's dry and wind free for a start and so your aerials won't get as corroded, wind damaged or bird limed. It's worth considering for experimental homebrew aerials.

The trouble is that in many cases it does give very disappointing results. Obviously it's not quite as high up as it would be as if it were on the roof but the main problem is that it is partly shielded by the roof structure. This situation is usually at its worst when the roof is wet and affects higher frequencies more than low frequencies.

Sometimes, you will find that your roofing material might have an aluminium layer in it so that's really not going to help matters much.

There's no substitute for mounting your antenna on your roof as high and clearly visible from all directions as possible.

Selecting co-axial cable

Looking at 50Ω coax, the immediately visual difference is the thickness. Generally, thicker cables are less lossy than thinner cable. All coax cables will loose some of the signal you want to convey from your aerial to your radio. The better your cable, the less you will loose over a given length at a given frequency.

Loss through coax = Loss per metre x Length in metres x Frequency factor

As you can see, all three factors are at play here and it's worth noting that you can get away with cheaper, thinner cable over a short length at a lower frequency

In fact, you may find that you need a selection of both for practical wiring reasons. It's hugely impracticable to plumb stiff thick cable into a scanner which is small enough to fit into a shirt pocket. Further than this, it is very possible that damage can be done to a receiver by exerting too much mechanical strain on its antenna socket. Also, it may be difficult to find certain connectors, an SMA plug to fit on 10.3mm cable for instance.

Satellite co-ax is a cheap alternative. Much easier to get hold of and good right up to many gigahertz. Look for the double wrapped stuff with a foil layer just below the braid.

If you've got a length lying around, you can do a lot worse than give it a go. The main problem with this is that most 50Ω connectors won't fit onto 75Ω cable and quite often, sat cable has a stainless steel braid so it can't be soldered.

Don't worry about the difference in impedance, a wideband aerial is barely ever in a good match and the difference between 75Ω and 50Ω is too small to get greatly concerned over.

RG58U

Your regular small flexible 50Ω cable. It's about 5mm in diameter. Not recommended for higher frequencies over long lengths, but convenient and cheap over short lengths. Note that RG58 defines just a basic form factor and that not all RG58 cables are equal. Look for a good thick braid as an indication of quality.

This cable is also used for computer networking, in which case it is known as 10 base 2

RG213

This is proper low loss cable. It's usually around 10.3mm in diameter. If you need to do a long run of cable, this is a very good co-ax to use.

Typical properties of RG213

Part

Material

Size

Centre (Inner conductor)

Stranded Copper

7x 0.75 mm

Dielectric

Solid PE

7.25 mm ± 0.15 mm

Braid

(Outer Conductor)

Bare Copper

Diameter screen: 8.0 mm ± 0.25 mm

Coverage braid: 92 % ± 4 %

Sheath (Jacket)

PVC according the European Standard HD 624.

Diameter: 10.3 mm ± 0.2 mm

Note that some co-ax has a solid core and some is stranded. Solid core is easier to attach plugs to, but should not be used where the cable will be flexed about continually.

HF Antennas

The reception of HF, by which I mean frequencies below 30MHz, enters a whole different ball game when it comes to antennas.

Because of the longer wavelengths involved, passive antennas are generally physically much larger. Rubber ducks as frequently supplied with a scanner are usually rubbish at HF. A long telescopic antenna may bring in some interesting broadcast stations but you may like to try something a little more effective for utility listening.

Random wire

The most simple thing you can do to bring in those short wave frequencies is string out a length of copper and attach it to your scanner. It doesn't need to be that long, in fact, many scanners will get overloaded if the antenna you make is too efficient. The main thing is to mop up as much signal as you can get without receiving too much noise.

Man made noise, or QRM comes from everywhere in the urban environment. If you live out in the sticks, you probably won't get much action on VHF/UHF but you are very much at an advantage when it comes to HF receiving.

G5RV

The G5RV is a multiband antenna used primarily by amateur radio enthusiasts. It's performance is not fantastic, but it is very cheap and

Wellbrook loop

I have no personal experience with this product but I have read much acclaim from respected sources

Joke Antennas

You may have seen the odd device on eBay claming all wonderfully absurd things about how you can turn your TV antenna into the best scanning antenna you could ever find.
These things are fraudulently poor

Firstly, you can damage your scanner with a piece of rubbish like this. You may have experienced getting a small electric shock from your TV or cable coax when unplugging it. The reason you get a 'tickle' is that TVs, DVD players, VCRs, satellite receivers, cable boxes etc are not usually earthed. This means that any leakage internally, which there inevitably is, means that very often hundreds of volts are present at the antenna socket. They are all commoned together, usually with SCART leads, and together, this adds up to a not inconsiderable current.

The impedance is another matter. It's not something in itself which will cause damage to the scanner. Most scanners will be unharmed by anything from a total short to infinite impedance (*note) but these eBay things are a complete abortion. Firstly, a television antenna is a very good antenna. It's very good at doing exactly what it's meant to do. That is, to receive a sub-band of the band IV/V TV band from one direction only. It's already pointed at what is probably the most powerful transmitter in your area, that's your local TV transmitter, so if you ever want to swamp and block your front end with RF, then this is the way to go. Otherwise, it's practically useless at picking up frequencies outside the TV band, which, unless you like listening to analogue TV sound subcarriers in their dying throws, is about as much use as a chocolate teapot.

You may hope to pick up a bit of JFMG, but if you're doing this, the last place you want it to be pointing your antenna is towards the local TV mast. You've also got around a 50% chance of it being the wrong polarisation for anything in any case.

If you go up to the roof, remove the antenna and chuck it in your neighbour's garden, and then short out the coax at the top, you will probably find that this oversold item works just as well. The truth is that the outer braid of the coax is probably picking up far more signal than the TV antenna ever would.

And on top of this, and back to the impedance thing, as far as I can from the manufacturer / bodger / seller of these things, there is no matcher / splitter circuit, it just puts the two receivers in parallel, so it will probably screw up your TV picture too.

*Note: Some scanners (ie, those which are designed for an external down-converter or pre-amp) can be permanently damaged if they are plugged into an antenna which is a DC short. Most TV antennas do present a DC short.