40m Antenna
Following the success of the 80m antenna I then decied to make a 40m version. This was built on the same lines as the 80m version using a base matching coil, a lower elemnt using the helical winding technique, a load coil based on the 43mm water pipe and a top element using a stailess steel whip.
The resulting antenna produced a number of European contacts. However following discussions on 40m with my friend Walter DK5DR, he persuaded me to look at High Q coils. Walter had undertaken a lot of research work back in the 60's on such coils and suggested they would work much better han the version I was currently using.
I then undertook my own research looking into Bug Catcher coils but decided these were far to heavy for the type of antenna I was trying to construct. fate interviened whilst looking around my local DIY Store Wickes, in the shape of a 500mm long plastic tumble dryer tube. This I thought would make a good coil support and with the large diameter make a good High Q coil. The antenna described below was therefore born.
The 40m version
This version is for 40m and was inspired by an interest in high Q loading coils. High Q coils have the advantage that they are more efficient, but the disadvantage that they have very narrow bandwidth.
The advantage of 40m over 80metres is that short antennas have a higher radiation resistance (between 5 and 10Ohms) compared to about 1Ohm for the same sized 80metre version. This antenna has been designed so that the loading coil has a Q of about 1000 and the antenna has an efficiency of about 55%.
Thus compared to a full sized quarter wave means that it should be no more than 2 S points down. So lets get going with the construction
1. Base matching coil.
This was based on the original waterpipe antenna. This consists of 14 turns of 18swg (16awg) (approx 1.25mm dia. Wound on a length of 42 -43mm diameter plastic water pipe. The coil length is approx 45mm long so the spacing between turns is about a wire thickness. The coil is tapped at turn 7 up from the base although this may depend upon the vehicle on which it is mountd and the proximity of the nearest metalwork, earth point etc. So be prepared to move it up or down during the tuning phase.
The base of the coil is at ground. The PL259 interface is made from the base of an old 2m whip, but a PL259 glued into a plastic support will work equally as well. The centre pin of the Pl259 is attached to the tap point and the outer soldered to the base of the coil.
2. Lower Element
This is 1.2metres long and made from 20mm diameter aluminium tube. The diameter is not too important but should be strong enough to take the loading coil weight and bending force when used mobile. A plastic plug is fixed into the top of this tube drilled a tapped to take a 6mm bolt which is used to attach the loading coil to the lower element.
3. Loading Coil.
This consists of 15 turns 18SWG (16awg)(1.25mm dia) enamel copper wire wound on a 150mm diameter plastic former and is 45mm long. The spacing between turns is between 1.5 a 2 wire diameters.
The former was made from a pipe which is used in the exhaust section of a tumble dryer. You can buy this cheaply (approx £4) from a Wykes’s DIY. Use white plastic as black plastic can be carbon loaded and therefore lossy. This is a rigid tube used inside the tumble dryer and is about 500mm long, not the flexible stuff that goes from the back of the tumble dryer to outside or vent.
The cross braces for the coil again were made from the plastic chopping board available from Tescos.
The former length is about 80 to 90mm to allow fixing of the cross braces. Attach the lower coil end to the lower element. I have drilled and tapped a 3mm dia hole into the aluminium vertical element tube and attach the end of the wire around it then screw it up tight. It can be later covered in PVC tape pr heat shrink to stop corrosion.
The upper end of the coil is wrapped round the 5mm bolt used to connect the load coil to the upper element. Once the connection is made, coat the end of the bolt with RTV, or again a waterproof sink/bathroom sealant, available from local DIY stores.
Laterly I have used 10mm wide tinned braid as the connection between coil and lower element and coil and interface bolt for the upper element. This I have found is more flexible than the enammelled copper wire used to wind the coil. It can be spraked with clear varnish or RTV compound during the final finnishing.
4. Upper Element
I have tried 2 upper elements both of which work well. One is a 1.3m telescopic element bought from Maplins, the other is a 1.3m stainless steel whip section from an old 5/8th 2m whip. The advantage of the telescopic whip is that its length can be altered to tune the antenna across the band. It also has a pre-drilled 4mm dia screw thread to attach it to the loading coil. However I have drilled this out and re tapped to take a 5mm screw which is stronger than the 4mm screw.
Again strength being essential for good mobile operation.
5. Tuning
Assemble the component parts of the antenna together and mount it to the antenna fixing bracket. Carry out tuning checks using an MFJ 259 analyser. I suggest you add a few extra turns to the loading coil at first as check at what frequency it tunes. It should go off low. Then remove turns to bring the antenna up to the desired frequency. If you are using the telescopic whip you can alter its length to bring it to the desired frequency.
6. Proof of the Pudding
So having got the antenna operational how well does it work. Well the simple answer is exceptionally. Most stations either inter G or on the continent can't believe they are woking a mobile. Signal strengths are well above other mobiles particularly the commercial ones and is not prone to QSB. I can often be heard rag chewing as I make my way to and from work. My best contact is to W9 in Washington state but hope to make a VK or ZL contact soon one morning.