This antenna, in theory, receives a signal equally well from all directions in the horizontal plane.

There is a range of styles, usually resembling a flying saucer and mostly aimed at the marine market.

They have the advantage of being compact, robust, easy to install and weatherproof with trendy high tech appearance, but they are strictly limited to strong signal locations even though they incorporate a built in amplifier.   Because of their small size (most of them contain a circular folded dipole), they are more useful for VHF III and UHF IV & V reception.

If you have one of these you can improve low signal reception by rotating it for best signal. (The reception pattern for most of them is more cardioid than omnidirectional, there being a null point in the direction of the dipole termination.)

If relatively low cost and convenience is far more important than performance, then you could consider one of these.

They may also be useful where you were overnighting in a location where you didn't want it to be obvious you were doing so.

Due to their semi-omnidirectional characteristics, ghosting is usually a big problem when receiving analogue signals, however with the advent of digital TV transmissions, these antennas will find increasing usefulness.
Digital reception is completely devoid of multi path ghosting and noise, you either have a perfect picture or nothing, with only some "pixelation" at the changeover point.
Also, another redeeming feature for these antennas is the fact that digital transmission in Australia is confined the to the upper VHF band only and UHF bands, just where these antennas work best.  (But they are still only useful in a good signal area.)

Also, remember that these antennas by nature of their construction (with very few exceptions) are only designed for horizontally polarised signals.  Yes, you can tilt them 90^o but then you have a low gain directional antenna.
Moonraker claim dual polarisation in the UHF bands (but horizontal only in the VHF band) with their dome antenna.
There are dome antennas with a short vertical whip sprouting from the centre of the dome to provide for reception of vertically polarised signals but for this to work effectively the horizontal element and vertical whip must be combined with a correctly designed internal diplexer.
Of those I have seen, the vertical whip has been so short that it could only be effective at the high end of the UHF band, greatly limiting it usefulness in the reception of vertically polarised signals.

Very much a compromise antenna but depending on your location and the availability of digital signals, it may suit your needs

There is more on digital TV further down the page.

Half wave dipole antennas

This is a simple type of antenna that includes the 'rabbit ears'. The length of the antenna determines its suitability for receiving a particular frequency (TV channel) known as its' resonant frequency.

This type ranges from the commonly available grey or white PVC conduit 'T' shape type, with and without an amplifier, to the Winegard wind up-down-rotate model which lays flat on the roof and winds up to 1 meter above the roof.

Performance is improved on the omnidirectional type, there being a directional characteristic with corresponding increase in effective gain.   (A half wave dipole receives a signal broadside to its length and a deep null at the ends.  Because it receives a signal equally well from directly ahead and directly behind, it is said to have a 1:1 front to back ratio.)

However, it is still a long way from being a 'fringe area' antenna.
To give you a rough idea, it would need to be approximately 3100mm tip to tip for best reception of channel 0 and approximately 175mm for channel 69.

You can calculate this for yourself with the following formula (wave length is denoted by the lower case Greek letter Lambda - which really looks like an inverted "Y" but since some browsers may not be set up to display this character, I am using "W" instead).
 ^W/₂ = ( ¹⁴²⁶⁵⁰ / f_Mhz ) mm, or, Frequency (Mhz) approx  = 142650 / tip to tip length (mm).
In practice, the physical length is approximately 90% of the theoretical calculation.
For example, if your half wave dipole antenna is 1100mm tip to tip, then its' optimum reception will be approximately 130 Mhz or in the region of channel 5A.   Its' performance on other channels will gradually diminish above and below this.

Multi element beam antennas

The Yagi

This antenna consists of a dipole (called the driven element) and a number of other elements (called parasitic elements).

The number of parasitic elements (known as a reflector if it is behind the driven element and a director if it is in front) will be chosen by the designer to give the required compromise between gain, bandwidth, size and cost for a certain application.

The size of the antenna, both element length and beam length is governed by the frequency of operation and the required gain.

To increase the bandwidth and retain the gain, there may be two or more driven elements, each a different length, that is, tuned to a different frequency.

This design is commonly available with discrete VHF and UHF sections combined on the same beam, in effect, two antennas in one . This is a good choice for VHF III and UHF IV & V bands.

One antenna that is particularly suitable for RV use is a model from Tandy Electronics, Cat No 15-9706 and priced at $A95-95 (June 2001).   It is quite well made (and in Australia) and covers channels VHF 6-11 and UHF 28-69.   Unlike many, all tubing is seamless aluminium.   This is the largest antenna I would like to have to set up on a daily basis.

Photo on the right shows this model.
It can be easily assembled such that the VHF and UHF can be independently set for either horizontal or vertical polarisation.
The one pictured has both set for horizontal reception.

It is shown fitted with a GME Kingray MHW34G masthead amplifier which can be had for $A80 to $A115 depending on where you buy it (Oct 2003). This is a special version of the MHW34 that is modified by GME to operate from 12V DC for marine and RV use.

I am presently testing this antenna / amplifier combination and while first impressions look good, I will comment on its performance in the near future.


I consider a VHF version for VHF I, II & III bands to be too large for convenient use in caravan applications, not only cumbersome but too easily damaged.

A UHF only version for UHF IV & V would make an excellent choice for these bands.   Once again, Tandy Electronics have a well made (also Aust made) model,  Cat No 15-9811 for $69-95 (June 2001).   This one is quite compact, convenient and quick to set up.


The Log periodic

Another multi element antenna visually resembling the Yagi but giving a greater bandwidth is the Log-periodic antenna.   It differs from the Yagi in that all elements are driven and yet also act as directors and reflectors depending on the frequency of operation.

If correctly designed (and this is the problem), the log periodic with its' gain and greater bandwidth is the best choice for VHF III and UHF IV & V bands and may be a little more compact than the combined Yagi above.


Log periodic Yagi

This is a log periodic design that incorporates parasitic elements.

It offers the gain of a Yagi and bandwidth of a log periodic, but its' physical dimensions would generally preclude its' use with caravans etc.


Phased arrays

Put simply and in a TV antenna context, it is a series of dipole and reflector combinations, stacked vertically, usually two to four high to form one antenna.   It makes an excellent choice if designed for UHF IV & V band reception but a VHF version would be too unwieldy for caravan use.

You will also encounter antennas comprising Yagi / phased array combinations, which if correctly designed also make a good choice for VHF III and UHF IV & V bands .

Summary

Correctly designed these multi element beam antennas offer much higher gain and offer far better performance than so called omnidirectional and half wave dipole antennas.   This is governed by the law of physics, to use the light bulb analogy again, a bare 100 Watt light bulb will never, no matter how cleverly packaged and promoted, out perform the same reflector focused 100 Watt bulb.

I would avoid vendors who claim theirs is compact all band antenna that will perform as good (or better) than directional antennas dedicated to specific bands.    I have witnessed some outrageous claims for tiny 'all band', often 'omnidirectional' antennas.    They may look appealing due to their small size and ease of installation but unless you are close to the TV transmitter, my advice is to look elsewhere.

Polarisation

TV transmissions can be either horizontally or vertically polarised.

Without going into detail, horizontally polarised transmissions are received with the antenna elements oriented such that they are horizontal with reference to the ground, similarly vertically polarised signals require the antenna elements to be in the vertical position.
The above photo shows that antenna set up with both the VHF section and the UHF section set up to receive horizontally polarised transmissions.   However, it can be readily adjusted so that one section is set up to receive horizontally polarised transmissions and the other section set up for vertically polarised transmissions which can be very useful.

Fortunately, most TV signals in Australia are transmitted with the same (horizontal) polarisation although there are a number of vertically polarised transmissions.

(The theoretical separation between horizontal and vertically polarised signals is infinite, but due to the effects of propagation over the land mass, in practice it varies in the vicinity of 10-15 dB.)

Aligning the antenna at 45deg  to the horizon is useful if you are in situation of having to receive both, but you will loose 3dB or half of the available signal in doing so.

Amplifiers

Because we have had to compromise with our antenna design to keep it to a manageable size and also because we are invariably traveling to poorly serviced regions, a good masthead amplifier can be essential for analogue TV reception.

I have heard it said on a number of occasions, often by people that should know better, that an amplifier is not necessary in a caravan type of situation because an amplifier is only used to compensate for the loss of the coax cable which due to the short run is not significant in this case.
Whilst it is true that the amplifier does make up for the loss of the coax which isn't much in this case, it may be needed for another very important reason.
A major limiting factor in receiving a weak signal is the inherent noise generated in the front end of the TV receiver.   This inherent receiver noise masks the low level signals. 
A good masthead amplifier often has a lower noise figure than the TV receiver, some times considerably lower.  
^Note1The dominant factor in determining the system noise figure is the first amplifying stage and the lower the overall system noise figure, the lower the signal level that can be retrieved. 

For analogue TV, a good low noise masthead amplifier can often mean the difference between a good picture and nothing.

However, (in the context of receiver noise not feeder cable loss) the improvement achieved by an amplifier is a factor of the quality of the amplifier and of the receiver (TV, STB or VCR).
To achieve an improvement the amplifier must have a lower noise figure than the receiver, usually much lower to be worthwhile.
Typically consumer quality receivers have noise figures in the order of 8 to 15dB (although the front end of digital receivers seem to be generally better than this) and wide-band amplifiers of the quality usually available for this application in the order of 4 to 8dB.
If you have a receiver with a noise figure of 15dB and use an amplifier with a noise figure of 4dB, you will achieve a major improvement in receiving performance but if you have 8dB receiver and an 8dB amplifier there will be no improvement whatsoever.
Consumer quality receivers rarely specify this data so the question of whether to try an amplifier is, in the absence of dedicated test equipment, hit and miss.

In any case, a masthead amplifier should always be used with a good antenna, not as a substitute for one, always start with the best antenna possible.

However, don't use a masthead amplifier for digital TV reception unless absolutely necessary and if you must, only use one with variable gain and turn it down to minimum.

The amplifier will consist of two components, the masthead unit, and the power injector.

The mast head unit is the actual amplifier and should be mounted as close as possible to the dipole element of the antenna.

The power injector is mounted inside your RV and has three connections.
-	12 VDC input from caravan electrics.
-	Coax cable to the masthead unit (sends DC voltage to the amplifier and also receives TV signal from amplifier), preferably in one continuous unbroken run, i.e. no intermediate plugs, sockets or connectors.
-	Coax cable to TV or VCR, (preferably either one or the other, not both using a splitter).