Since any aerial used for reception on a caravan is usually required to work at different locations it is a given that a wideband aerial is required, with all the inherent disadvantages that this can bring in poor signal locations, compared with a grouped aerial. Much as I dislike wideband aerials (wideband Yagis anyway) it must be admitted that the requirement for one is the least of the problems for a boater, or caravanner. Many sites are in poor locations (especially for boaters as waterways tend to be in valleys ! see importance of line of sight) and furthermore space, for storage of the aerial, is usually at a premium on a caravan, so an aerial of large physical size is problematic. Unfortunately, as a general rule, larger aerials give more gain (i.e. more signal) and smaller aerials give less gain. Just using an aerial amplifier to “boost” the signal from a low gain antenna is unlikely to be effective. I certainly found this to be my general experience, particularly when used with an Omni directional aerial.
Actually all of the above is only the start of it, the biggest problem is that getting the best signal involves juggling with not one, not two, but three variables :The direction of the transmitter is the most obvious variable, your aerial must be pointing at it (unless you're right next to a main transmitter!) or your picture will be poor.
The tuning of the transmitter(s) which you`re trying to receive. Even if you`re right next to Emley Moor your screen will be just a snowstorm if your set is not tuned into it !
The third variable is often overlooked, and that is the polarity of the transmitter(s). If you get the wrong polarity you could be losing up to 90% (or more) of your signal, which isn`t to say it won`t still work if the signal strength is high enough, but if it isn`t, well, no Coronation St for you.
It is possible to eliminate two of the variables mentioned above, namely direction and polarity, by using an omnidirectional or "Omni” aerial. The problem with this strategy is that, how can I put this, Omni`s are crap...... In fact the gain of an Omni is actually negative, theoretically it`s minus 3 dB, and that`s without taking into account the additional loss of signal that results from an incorrectly polarised antenna. Furthermore an Omni`s beam width (to eliminate unwanted signals), is, well, 360° !
The DM Log is pretty strongly made and because it`s nearly all metal in construction it`s eminently fixable if it gets damaged, as any touring aerial is bound to be...... Furthermore, Log aerials are flat, thus making storage much easier and lowering the risk of the antenna being damaged. At a conservative estimate the gain of the DM Log would be about 6 to 7 dB and since we use it as a control aerial for all our antenna testing its relative gain to all our other aerials can be seen on the relevant graphs. Now it must be admitted that 7 dBd is not "high gain", but since all high gain aerials are large, and as such are unsuitable for boats or caravans anyway, I wouldn`t worry about that too much, particularly, as we`ve just discussed, when an Omni has minus 3dB of gain. So, compared with an Omni, a DM is high gain !
Cable -Because the cable runs on caravans are usually pretty short cable quality is not quite as important as it is with aerial installs on houses, but since the price difference is so small anyway it really is a false economy to use cheap crappy cable. “Low Loss” coaxial cable is not recommended, I`d always use decent quality satellite grade lead. It`s not so much for the difference in signal loss (which would be relatively small on the short runs), it`s more for the suppression of impulse noise which can be very annoying with digital pictures. It`s probably an even better idea to go for copper/copper type cable (as opposed to RG6) on a bost than it is on a house install because damp might be a problem and copper/copper is less susceptible to corrosion than alloy foil RG6 type cable. Finally when it comes to cable colour it is often tempting to go for white but remember that it soon gets dirty, so I`d go for black myself !
Caravan Aerial Pole Installations - It`s all very well having your aerial(s) and knowing how to get the best of them, but how do you install the pole that it`ll mount on ? Permanent installations require a bit of thought and are outside the scope of this article but there`s a picture below left of a neat method of attaching the pole to a canal boat.Incidentally, you`re probably better not choosing white cable. It may look nice when brand new, but it`ll start showing up the dirt and then you`ll wish you`d gone for black cable !