General article on antennas, polarity and masting for the Australian Caravan Industry.

19 Apr, 2021, 5:50 AM

Written by Richard Henry

Some basic term definitions are required to keep things clear as you read this.

VHF = Very High Frequencies channels 0 to 12.
UHF = Ultra high frequencies Channels 28 to 69.

Element: any electrical conductor on the antenna, such as an aluminium tube, designed to collect signal. Antennas are often described by the number of elements for example a 13-element antenna.

Reflector: A directional antenna has as part of its design a reflector behind the active element. This blocks or reduces signals from behind and the sides the antenna reduces the noise in the signal and increases the gain of the antenna.

Director: as in the case of a Yaggi type antenna is a smaller length element in front of the active element whose function is to narrow the angle over which the antenna will pick up signal and thereby focus like a lens resulting in a higher gain of signal over a narrower frequency range.

Active element: This is the element that is the final collector of signal and that the cable is attached to and that feeds the signal onto the coax cable.

Phased array: By phased array is meant, that two or more single antennas are connected in such a way that the signals they independently collect are channelled together to make one signal with a resultant increase in signal strength, they generally have no directors in front (the smaller bars that extend forward on the yaggi type) which makes them less frequency specific and therefore will work over a larger range of frequencies than the yaggi type. They also tend to have a relatively even gain of signal across the whole range of UHF frequencies – important for digital signals and are therefore very suitable as a caravan antenna.

Please also see the attached educational series for further term explanations and pictures.

Polarity: This direction of the transmitted signal. The antenna is fixed in either a vertical or horizontal plane to match the transmitted signal. Failure to do this seriously reduces the performance of the antenna sometimes below the useable range.

Summary of what an antenna should be able to do for Australian conditions:

1. Be able to mount vertically or horizontally. Best case scenario it would be able to mount the VHF and UHF at different polarities to each other as well.
2. Have very good directionality and high raw gain before amplifying for the best results on digital signals. It should therefore have good reflectors (shields) that protect the active elements from       noise and bounced signals that confuse digital set top boxes or and noisy signals and pixilation in your pictures.
3. Be a complete system that provides good connections for high frequency use and good quality cable. All parts should work together to function as a whole to achieve the best results.
4. Good quality and flexible in its use masting options that allow for good height adjustment and rigidity against swaying in the wind and convenience of use allowing easy rotation of the antenna to peak it in to the signal.
5. The antenna should have a very high performance on UHF and have very good performing reflectors (shielding).

It can be seen from the above summarised points that the explorer Product Range is designed to meet the above needs well.

Australian Signals.

87% of all transmitted signals in Australia are UHF and of these 52% are from channel 52 to 69. These high frequency signals are usually from small power transmitters. So the antenna should be able to work efficiently over the full range of signals and should have its performance biased towards the UHF range. Quality of cables and connections are important here at these higher ranges as the higher the frequency the greater the losses in travelling through the cable and connections.

Polarity: 40% of all transmitted signals in Australia are vertical.

The less directional antennas such as the omni-directional antennas and basic dipoles are less effected by this as being not very directional they pick up signals from all directions or in the case of a dipole from the front and back as efficiently. Therefore, being a less sophisticated antenna they are less efficient with a consequent increase in noise and reduction of signal gain.

Mounting for Polarity matching of the transmitter.

The more directional the antenna the more important it is to match the polarity of the transmitted signal. However, while a reduction in signal and picture quality can produce a picture on the analogue system it will fail quite often when using the digital system. This means that it is more important than ever to match the polarity of the transmitted signal. Just check the antennas on the houses in the local area to check directional and polarity of the local transmitter and adjust your antenna to match.

So always check to see if your antenna can be mounted vertically before buying.

The explorer Product Range of Antennas can be mounted so that both the UHF and VHF are mounted vertically.

Antenna performance (gain), Differences, Pros and cons:

There is gain of the antenna and gain from an amplifier.

These two gains are not the same, by far the most important is the gain from the antenna. Why?

Because the amplifier can only amplify what it gets from the antenna so having a huge amplifier on a low gain antenna is of no use. It will not work if there is not enough information there in the first place to amplify. Digital in particular does not benefit from large amplification and in fact is usually made worse. The highest amplification of a digital signal you would want is no more than 25 db.

The basic dipoles will be limited in antenna gain before being amplified. They therefore rely heavily on the performance of their amplifier.

The yaggi type antenna for UHF is high gain for a certain number of frequencies and then its performance drops off. It gets its higher gain by virtue of making itself specific for a certain number of frequencies and is therefore more efficient within those frequencies. This means that they will not cover well all of the frequencies from 28 to 69.

The basic dipole used on VHF will function adequately into secondary areas but not fringe areas.

The Omni-Directional antenna is a basic dipole, but it is bent into a circle to make it Omni-Directional. Their performance is reasonable up to secondary areas on analogue when amplified but are not fringe area antennas and may fail much earlier on digital. These antennas are reasonably good for the East Coast of Australia but will have problems with ghosting and on digital at some locations due to not having proper reflectors. They are poorly protected from noise and bounced signals.

The explorer Nomad or East Coast Antennas use a high gain Phased array antenna for the UHF signals and a basic dipole for the VHF signals. This means that it is a fringe area antenna for the UHF range and suitable for secondary reception areas in the VHF range. As 87% of signals are UHF then this is the most important area to have signal gain. This is an excellent choice if space is at a premium in a small van or for along the East Coast of Australia.

Component and Cable Quality

Quality of cables and connections are very important and that is why the explorer range uses screw on F connector type connections designed for the higher frequencies. The explorer range comes with an exclusive through the wall connection kit that bypasses the often rusty and faulty push in type wall fittings. The push fittings called PAL fittings, are actually skirting board fittings and therefore corrode especially around costal areas. It also reduces the number of connections needed for an amplifier system and performs better at higher frequencies. It is important that all parts are matched as good quality items as the weakest link will limit the performance of the whole system this is why the explorer range is sold as a system and not just an antenna. It is why explorer has the reputation of being the best.

Importance of masting options and flexibility.

Masting is often shrugged of as unimportant as it is believed that “any Stick will do” in practice it can make a big difference in the eventual performance of the antenna system.

A plastic mast if it is short and reasonably wide will provide sufficient rigidity to hold the antenna well provided it is short or very light antenna. When mounting the antenna vertically however the longer VHF elements can hit the van if the mast can not be raised high enough. When raised a plastic mast can bend easily in the wind shaking the antenna around and resulting in jumping pictures or altered reception badly affecting your picture quality.
A mast often used is an aluminium pool handle type that has a twist lock to lock it in place for height adjustments. These masts tend to be cheaper and are usually able to be raised to a good height. However, I have found that when the mast is extended the twist lock is insufficient to hold the extended length of the mast and it waves around in the wind a great deal causing the same problems as the plastic mast.

A steel mast is fine so long as the connections are strong and prevent the swaying of the antenna when raised.

The explorer three-piece telescopic mast is a lightweight aluminium mast that can be extended to 4.5 meters and collapses down to 1.65 meters sufficient to fit into the boot of the van. It has thumb screws to secure the height adjustment and supports the raised sections completely along the joint length giving excellent rigidity even at maximum height.

Explorer also produces a couple of two stage masts for side mounting on the side of the van when access to the draw bar is limited. The quick release version; part number QFPC-P and the more permanently mounted version the SM+TM28. The first one uses special brackets to allow the mast to be clipped on and off in seconds. The available height extension is 2.5 meters plus the amount off the ground from the mounting position on the side of the Van and the second one extends to 2.8 meters. The larger unit is good for any van where the beds pullout over the draw bar.

Comparisons of Mounting types and explorer Product Range options:

Currently you have available the plastic type masts and a set of side mounts which are fine for very lightweight antennas that have small frontal areas that will not be pushed around by the wind.

That is the basic dipole type antennas such as the straight plastic rod antennas.

You have also the gold type draw bar mounting bracket, which if you have ever used, are fiddly and tend to damage the mast as people keep screwing the bolts up tighter to stop it from falling over in the wind. It is difficult to adjust the antenna position to line up for the best signal with this type as it jams the mast against the draw bar to hold it and to also mount it against the draw bar. If you have ever used an explorer mount, you would hate using this other type.

The explorer Draw Bar Mount (DBM-P or MK-P with mast) is a permanently mounted Stainless steel mounting bracket that you simply drop the three-stage explorer mast into; adjust for height and rotation and lock with the two large thumb screws. Vertically mounting the antenna is simple with our new quick polarity change mount. It is very convenient and offers the most flexible antenna positioning of any masting system on the market. This means greater options when looking for that weak signal. Some people get two mounts one for the front and one for the back of the van. Giving a much greater range of location for the antenna, which is the most important option for any antenna system as you may have to move the antenna into the signal area just like any professional installer would do when installing an antenna for your home. They do not just put it on the chimney because it is convenient, they will test for the best spot on your roof – if they are professional – and use that location.

A limited height plastic mast does not lend itself well to the Yaggi style of UHF antenna it as this style does not pick up well above or below where it is pointed.

Roof mounts should be able to be rotated and the antenna must be able to be turned vertical for Australian Transmitters. It may be said that an omni-directional antenna does not need to be rotated however no omni-directional antenna is truly omni-directional. Therefore, they still benefit from being rotated. Where the omni-directional antenna has been turned vertical then it is now directional to a larger degree and has a front and back. This then acts more like a dipole antenna and would definitely benefit from being rotated. The advantage maybe that they do not need to be turned for vertical signals as they will pickup both. Of course, this maybe a disadvantage where there are also horizontal signals coming into the same area and may defeat the reasons they chose a vertical transmitter in the first place.

The Roof mounted Antenna such as Winegard is very convenient for the user and is the main reason why the Winegard sells well. At least at the van manufacturing stage. The mount can be rotated while watching the TV and raised and lowered without leaving the van and it means you don’t have to store the antenna.
The latest version for Australia has two basic dipole antennas on the same length of aluminium at opposite polarities. These are then split into a single output to the TV. It does work but the nature of the antenna is that it has low gain and so is weaker to begin with in its performance. In addition, sometimes the two signals will interfere with each other and add much noise to the signal making it non-functional.

So, It does have serious drawbacks however to balance these conveniences.

The maximum height is limited and so you cannot move the antenna into the signal area. This can easily be the difference between getting a good signal or no signal at all.
The mount is expensive to install and has caused much damage to vans when the owner has forgotten to lower the mast before driving off. Also, the constant rotation of the antenna tends to damage the short cable length. It would be better if the outside cable was made easily replaceable to maintain performance.

This being said it is still very poular with manufacturers and is often requested by van buyers due to its convenience – which shows what a sales tool convenience can be.

Mast Head Amplifier:

Most Caravan antennas sold specifically for this industry are fitted with a mast head amplifier necessary for weaker signals but is not critical for digital signals provided sufficient raw gain from the antenna is available. As time goes by and more digital comes in then an amplifier will be much less required, and a good higher gain basic antenna would most likely be better unless feeding more than one point in the van. However, for the more fringe areas or the van with multiple points the amplifier will still be needed.