BUILDING TIPS
As I have previously stated most of my antennas can be made from bits and prieces which you can buy from local shops and stores.
However I cannot guarentee that the shops I use are available in your town so you might have to go to a near equivalent and ask. I have put a few links to some of the bits and pieces I have used at the end of this chapter.
The next thing to bare in mind is that these designs are for mobile use. This means they need to be robust, yet light, espeecially as I'm using mid section loading coils. The antena need to be built to survive rain wind and snow and then be subjected to the forces caused by driving along at speeds up to the national maximum limit (or beyond!!).
COIL FORMERS.
Plastic Waste Pipes
When I first started these projects the idea was to use readily available items with little cost. I therefore started to wind coils on old PVC plastic wastepipe formers. These are usually 38mm or 43mm outer diameter and can be obtained from almost ant DIY store. problem here is that you will probably have to buy a 2m length which will make an awful lot of coils.
To overcome this there are a couple of other sources. Try a local plumbing shop where you may be able to get an off-cut about 300 - 500mm long which will do nicely for a ouple of coils. The alternative is to go along to your local recycling depot and have a scout around in the old appliance area and see if there are any bits of old pipe lying around. This may need some cleaning up.
Large Diameter Pipes
Making a self supporting load coil like the bug catchers strikes me as being extremely complicated. Heavy duty wire is also required so that the coil is self supporting, albeit that some type of former is needed to keep the coil in its shape.
For my high Q coil I managed to find a 100mm diameter, thin walled pipe from the DIY Store Wickes. The pipe is used as an exhaust pipe inside (not outside) a tumble dryer. They come in either black or white coloured. I've used the white version in case the balck one is carbon loaded and could therefore potentially be lossy. These tubes have quite a thin wall but this means they are extremely light which is essential for mid load mounting of a mobile antenna. This I have found as a very acceptable alternative to winding coils and makes them much easier to build. Slightly thinner guage wire can then be used which again makes for a lighter coil.
Ends and Braces.
When I first started making these antennas I used plastic water pipes throughout except for the top element which was a stainless steel whip. using water pipes proved very useful as there are fittings available from the plumbing shops which allow you to convert one pipe diameter to another. Using 22mm waste overflow pipe as my lower element meant that I could use commercially available reducers fom 38mm or 43mm diameter pipe to fit the 22mm pipe. I therefore had ways to connect load and matching coils to the long lower element.
I have also used these reducers for my later antennas where I have used aluminium pipe but they may need additional packing or again use the 22mm pipe which has an internal diameter of 18mm which can then be used with 15mm pipe plus some packing. For packing try cutting 30mm wide strips from plastic milk bottles.
My other trusted method for making plugs or braces is the plastic chopping board which you can buy from Tescos. Get the thich version which costs just over £3. This is made from polypropylene and is about 8mm thich. One board can be used for lots of plugs braces or insulators so it goes a long way. For the High Q coils I cut a cross brace about 40 to 50mm wide, shaped to the curvature of the pipe and held in place with 2.5 x 10mm bras wood screws which again can be obtained form any DIY shop.
I also make plugs from the board, by cutting out discs using a circular saw cutter used to cut holes in kitchen units for waste pipes. These are cut oversize then haved down using a sureform or cheese grater. You can do this using an electric drill. When cutting ut the original disc the cutter drills a 6mm pilot hole. Put a 30 to 40mm 6mm bolt through the hole and clamp it with a nut tightly. place the bolt in the drill, fix the drill firmly and then shave the disc down until it is the correct diameter to fit into the pipe.
Interfaces
On my vehicle I have a mounting bracket with a SO239 socket mounted on the horizonatal section. All my antennas have to interface with this socket. One useful way of doing this is to use commercial made antenna interfaces such as those found on some 2m and CB whips. I usually look for these at rallies then break the base open to get to the PL259 type interface. This is usually quite a robust plug far morre robust than the standar cable type Pl259
However you can also use a standard PL259, Pot this into one of the reducers using aradite. Alternatively you can make a mount using twp layers of chopping board plastic. Shave it down to size as mentioned above, then bore out the pilot hole until it is wide enough to take the barrel of the PL259. Fix it in place using araldite. One small suggestion here, if you intend to take the antenna on and off on a regular basis, try buying a really good qualit version. Cheap ones I have found they either corrode easily or strip their threads after a few changes.
Antenna Mounting Bracket
As I have already said all my antennas are grounded at the base and fed up from ground using the a matching coil. The base of each antenna uses a PL259 type adaptor which then mates with an SO239 plug which is mounted onto the vehicle bracket. As can be seen from the above photo I use a SO239 connector mounted onto a small mounting stub which I found one day at a rally but there is nothing to stop anyone just mounting the SO239 directly to the vehicle bracket.
The vehcile bracket is made from 30mm wide L shaped aluminium strip which again can be bought through a local DIY chain. I use 2 pieces one on top of the other so that the L section makes a U section. This part of the bracket should be long enough to go under the bodywork of the car, either on one side or as I have done in the middle, but until a good flat working surface is available. The U shaped sections can be seen in the photo below.
At right angles to this section is a similar constructed vertical section. This is about 500mm long and is attached where appropriate to the rear bumper> Modern car bumpers are usually made of plastic so securing can be done by means of a coarse threaded self tapping screw.
At the top of this vertical section is a second horizonatal section. This section is however made fom the 8mm thick polypropylene chopping board. here I have cut a piece about 50mm wide and 200mm long. At the end I cut out a U shaped recess of suficient diameter to take the lower element tubes. A second smaller U shaped section then forms a locking mechanism to secure the tube in place. being made of plastic isolates the antenna element from the earthed nature of the metallic bracket.
With this arrangement The antenna is now secured at its base and a a point 500mm abouve the base, thus reducing the load on the base mounting point. using a mount like this am able to travel at speeds up to th legal limit (and above) without detriment to the antenna.
Earthing
It will be noticed from the above photos that there are a number of earth straps connnecting the bracket back to the body of the car. The mount back to the bodywork underneath the car uses 3 M10 bolts drilled through the bodywork where the spare tyre is located. The area arount the bolts has been cleaned to expose bare metal. Good quality stainless steel bolts should be used, not only fro strength but also they will not corrode. Don;t use just steel screws as these will rust. However it should be noted here that by using aluminium for the bracket it will act as a sacrificial anode and hence should reduce the corrosion of the steel plate of your vehicle. My bracket has been on for over vtwo years now and there is no sign of rust aroung the fixing holes which have no other form of protection, yet the underside of the car is exposed to all the much and dampness the road can throw at it..
I have also put an earth strap near the top of the bracket where the tailgate lid opens. Here there is a good quality earth point where the boot latch is. For the earth straps I use 10mm wide tin copper braid. Where earh starps are used on the bracket two layers of braid have been used. I also use earth braid straps across all the door hinges, on the bonnet hinges, across the radiator, and on the tail gate.
Useful links
Telescopic Whips
http://www.maplin.co.uk/Module.aspx?ModuleNo=868
Also used for enamelled copper wire
Pipes and Reducers
http://www.wickes.co.uk/Round-Pipe/invt/713030
http://www.wickes.co.uk/Solvent-Weld-Waste-Pipe/invt/431987
http://www.wickes.co.uk/White-Overflow/invt/431934
http://www.wickes.co.uk/Overflow-Pipe-Connector/invt/424927
http://www.wickes.co.uk/Solvent-Weld-Reducer/invt/431990
http://www.wickes.co.uk/Solvent-Weld-Pipe-Connector/invt/431988
http://www.wickes.co.uk/Solvent-Weld-Pipe-Connector/invt/431991
Screws and Aluminium Strips
http://www.diy.co.uk
Rapidrive AVF-857638 Pack Of 100 (W)2.5mm x (L)10mm
Woodscrew AVF-861598 Pack Of 25 (W)1.5 x (L)12mm