HF MOBILE ANTENNAS

Remember that an HF mobile antenna is only a fraction of a full size vertical. This applies particularly to the lower bands. This means efficiency is going to be a lot less than your base antenna system. For this reason it is essential to keep losses as low as possible.

Of prime importance is earthing and base matching. Avoid roof bars, they can cause major resonance problems. And remember that car panels are not always earthed to other panels. Indeed, a lot of cars have plastic body sections.

The best two options are a 3-way magnetic mount on the roof (the largest metal section on most cars) or a special welded braacket that attaches to the car chassis. The latter can often be made up for you at a car repair shop.

The 3-way magentic mount (W-300S) offers good RF transfer even down to 80m and it is just as good as mounting your antenna directly onto the car roof. It gives good omni-directional radiation. However, you must remember it is there when parking the car in an underground car park!

The metal bracket idea is great if you do a lot of mobile operation. The normal arrangem,ent is to place the bracket at the rear bumper level. This enable you to put one of the larget mobile antennas on the car. Two points to remember: make sure the antenna does not mask any rear lights and also that it does not prevent the rear door or hatch from opening.

The base of the antenna has a very low feed impedance and so will often benefit from a matching transformer. MFJ do such an item. Check out MFJ-908 & MFJ-909 under RF Acce3ssories section.