20m Antenna

This started as a project to see if I could take an old 2m mobile whip aquired at a local rally and convert it to a 20m mobile antenna, using as many of the original parts as possible. The design and making of this antenna is detailed below.

From2m to 20m

Or how to make a 20m mobile antenna from a 2m whip

I don’t know about you, but what do rallies mean to you? If you,re like me then it,s obtaining a travel pass from a Senior and Junior management for the day, they having decided that Dad Taxiis not needed for a few hours.
Next it funds. Having squirreled away a few quid for the entrance fee and a bacon sarnie the next big challenge is what to buy with what you’ve got left. Are you just like me; another plastic badge to replace the one you bought last year, but can’t find, and a bag full of PL259 connectors, just in case you need them to make up some cables for that project that never gets built. Having gone through this same ritual for many years I decided that one time I would break the mould and embark on another antenna project. After wracking my brains for some time on how I was going to achieve this, the answer suddenly proffered itself in the form of an old 2m antenna and magnetic mount for �2.

Now what whip do I mean, not the modern dual or tri band versions, but a more, old fashioned one from the days before 6m and 70cms were invented. These are likely to be more available anyway, since has everyone subsequently gone out and bought the new tri band antennas and confined the old one to the back of the garage, loft, or rally bring and buy stall.

Hopefully, if you find one, like I did, it should consist of a base loading coil, a lower whip section about 90 - 95cms long, a centre phase matching section and a second whip section on top again about 100cms long. The big advantage of using an existing antenna, is that it has some useful bits of metal such as the whip interface sections, and a SL239 base mount.

Now don’t despair if you cannot find one, as you can still make this antenna out of some stainless steel whip sections and some plastic waste water pipe

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1. Dismantling the Old Antenna

Ok, so lets get started. Using a suitable allen key dismantle the whip sections from the each other. Next; ( and this can be painful to some people), using a pair of mole grips or plumbers pliers disassemble the SL239 connector at the bottom of the base matching section from the rest of the matching section body. You may need to use a bit of brute force as they can be quite stubborn to remove. Check there are no screws or grub screws to hindering you in the task. Once you have removed this section you should be left with two brass sections separated by the loading coil support. You should now be able to remove the outer cover of the coil.

You should now have revealed the 2m matching coil and it central support. It should consist of about 5 or 6 turns of 18 or 20 gauge copper wire with a tapping point at about 3 turns which goes through the central support and connects to the centre pin of the SL239 base connector. Cut the coil from the top ferrule and the base and unsolder it from the feed-through. Discard this wire we will not be using it.

2. The Matching Coil

Using a reasonably heavy duty soldering iron, tidy up the top and bottom sections. Now wind a new coil using 20 turns of 20swg enamelled copper wire close wound. Solder the ends of the coil to the top and bottom brass end pieces. Then clean a short section of the coil around the area where the feed-through comes through from the SL239 and solder it to the wire. This should be around turn 8 or 9. Check you have made a good connection using a resistance meter connected to the centre pin of the SL239 and the top brass ferrule.
Now for those of you who did not managed to find an old 2m antenna, you can make the same coil by winding the 20 turns on to a 13 to 15mm diameter plastic water pipe or similar tube. Centre-tap the coil at 10 turns and attatch this to the centre pin of the antenna connector.

You can make a new outer sheath using some 20mm OD waste water pipe or, use as I have done, a short piece of tubing made from one of the extension stalks used in those mushroom type garden lights sold at nearly every garage, DIY store or garden centre. The black colour makes the antenna look just like a professional base coil section so no-one would know. If you need to make any packing washers I suggest you cut them from my old faithful polypropylene chopping board which you can buy from Tescos.

3. The Load Coil

Next we need to wind the loading coil. For this I have used a 42/43mm ID waste water pipe about 70mm long. Wind on 18 turns of 20swg enamelled copper wire spaced at least one wires thickness apart, to make a winding length of around 60mm One other thing I have tried (but is not absolutely necessary, but does add to the final bandwidth), is to stand the windings off from the PVC pipe. I have done this using sticks from my sons Connex building kit, but other plastic sticks could be used. Drill three holes 1.5mm to 2mm diameter 10mm from the top and bottom edge of the coil. Weave the wire in and out of the holes so that the ends end up inside the tube so that they can be attached to the end bosses, which we will make later.

Using the polypropylene chopping board, make two 42mm diameter discs to fit inside the water pipe. I use a pipe cutter used to bore holes in kitchen cabinets to take water and waste pipes. If your cutter is like mine this will also drill a central hole approx 6mm in diameter through the disc. Don’t worry if the disc is oversize as it can be shaved down using a file Sureform or cheese grater. The best way to do this is to put a 50mm or similar size 6mm bolt through the central hole and lock it in place with a nut. Tighten this securely. Place the end of the bolt into your electric drill, turn it on so that the disc is spinning and can then be shaved down using one of the above tools. Check the diameter regularly, until it forms a snug fit into top or bottom of the pipe.

Using a shorter 6mm bolt (suggest 12 - 15mm) attach the wire from the end of the loading coil around the bolt using a washer to spread the load. Secure in place with a nut on the top of the disc and tighten. Fit the discs into the top and bottom of the pipe coil and secure in place using either UHU All-Purpose adhesive, or small wood screws.

4. The Top Element

Take the phase matching section discarded during the original dismantling of the 2m antenna and cut it in half using a junior hacksaw. Drill a 5.5mm hole down through the plastic centre, until the brass part is reached. Drill through this to a depth of about 10mm. Then using a suitable tap, tap the hole to take a 6mm bolt depending on the diameter of the original drilled hole.
If you didn’t have the phase matching section you can make similar bosses using 8mm diameter solid brass rod which you can buy from the larger B and Q DIY stores.

5. Construction and Tuning

Now construct the antenna using the lower and upper sections of stainless steel whip with the matching coil at the bottom and the loading coil in the middle. Use an antenna tuning meter such as the MFJ 259 and ascertain at what frequency the antenna tunes best at. If it is close to 14MHz then cut down the upper whip length until a frequency of around 14.2MHz is achieved. Do this by cutting off about 1cm at a time. The antenna should now tune across the 20m band from 14.1 to 14.3 to the VSWR 2:1 points.

If the antenna goes off much lower than 14MHz then reduce the number of turns on the loading coil. Do this one turn at a time. until a frequency close to 14.1MHz is achieved, then adjust the length of the upper whip section to get it to tune to 14.2MHz.

Don’t worry if you make a mistake and over cut the length, just add a small capacity hat to the top section and tune the antenna back to the required frequency.

To make a simple capacity hat, get hold of a 13amp choc box strip. Cut off one section and remove the plastic outer cover to reveal the brass inner section. Cut 4 length, 10cm long of 20swg ( or thicker) enamelled copper wire . Clean off 1cm from each end and solder them around the brass fitting. Bend them at 90 degrees to the brass fitting. Slide the fitting onto the whip section and secure about half way up. Trim the wire lengths to bring the antenna back into the required frequency.

So there you have it an antenna from 2m to 20m with a reasonable bandwidth and should have reasonable efficiency. I have managed to work all of Europe and the Near East, The States and South Africa just using my ALINCO DX70TH with 100watts output.

6. Adding 17m and 15m Bands

Now as an added bonus, if you have some stainless steel whip section left over or you got some odd lengths kicking around, then you’ve got the makings of alternative bands. Using a length of whip about 10cms shorter than the one tuned for 20m should give you a tune point close to the 17m band . Adjust its length to bring the frequency to around 18.10MHz. Its bandwidth will be wide enough to cover all the band.

Cut another section about 20cms shorter than the 20m top section and tune this for around 21.2MHz. This version will cover the whole of the 15m band. Cut a final length about 300mm long and tune this for 24.7MHz. Finally a length of about 10cms will tune around 28.5MHz.

You now have one single base section and 5 interchangeable whips that will cover you for all the higher HF bands.

So there you have it a cheap and simple 20m mobile whip antenna made mostly from an old 2m whip. So next time you are at a rally and see some old stainless steel 2m whips just think what you could do with them. Oh just one thing, just check I am not at the rally, as you have probably have to climb over me to get at the goodies.

The 20M Mobile Antenna Mark 2

Following the success of the 2 to 20m antenna and the lessons learned from the 80 and 40m high Q coil antennas I have then embarked on a high Q 20m mobile antenna.

1. Base Matching Coil

Based on the 20m antenna above the base matching coil remains the same.
Lower Element

The lower stainless steel whip is replaced with a 15mm diameter aluminium tube 1.2m long. The tube diameter is not overly important but needs to be strong enough to survive the environmental conditions See the notes form the 80m and 40m antennas.

2. Load coil

This like the 80m and 40m antennas is made from the 100mm diameter tumble dryer tube. The coil is about 40mm in length wound with 6 � 7 turns of 20 swg enamelled copper wire with 2 wire width (approx 2mm) between each turn. You will need to secure each end of the coil windings by drilling three holes through the tube about 1cm apart and weaving the wire ends through the three holes.

Clean off a short section of the coating to connect to the lower element, I suggest tinning this with solder to reduce the problem of dissimilar metals. As an alternative I have used 1cm wide tine coated braid if you can hold of it, have a look at the rallies. I have used a 10cm length to connect to the lower element and a similar length one to connect to the bolt on the top of the coil which interfaces with the telescopic upper element. You can see how this works from the photograph.

The braces have been made from the polypropylene supermarket chopping board. It isn�t necessary to fill the whole of the diameter of the tube, gust a brace wide enough to support the bolt and lower element and keep then firm whilst under load when mobile. My cross braces are about 50mm wide. You can lighten them by drilling a few lattice holes through the brace but be careful not to compromise the strength.

3. Upper Element.

As before the upper element is a 1.3m telescopic whip from Maplins. I have used this as a common element on all of my latest antennas so I only need one regardless of the band. You could use one on each band if you don’t mind the expense. Again a 5mm diameter bolt about 30mm long is secured into the top cross brace of the coil.

4. Tuning

As with the 80 and 40m elements check for the tuning point of the antenna using a suitable analyser. First find the frequency for best VSWR. If this is on the low side then remove a turn from the coil. However be careful here as the diameter of the coil means that one turn can change the tune frequency quite considerably. If you are using a stainless steel whip as the top element it might be easier to reduce its length about 1cm at a time until a tune point of around 14.2MHz is achieved. If you are using a telescopic whip as I have done then you should only need to reduce the length by pushing the 1st but 1 lower section into the lower section until a frequency of 14.2MHz is achieved.

At a frequency of 14.2 and starting with a VSWR of better than 1.3:1 the antenna should tune between 14.15 and 14.3MHz for a VSWR of <1.5:1. If a VSWR of <1.3:1 cannot be achieved at the required centre frequency, then move the matching coil tap point by one turn until a good match is achieved.
One of the big advantages of the telescopic whip is that by altering the length this antenna can be used on the 17m, 15m and 24m bands. Using the analyser reduce the number of telescopic sections until a centre frequency of around 18. 13 is achieved and note or mark the position on the elements. Do the same thing for a centre frequency of 21.2MHz and 24.5MHz. At these points the antenna should tune over each of the bands being matched.
The antenna can be used on 28 to 30MHz by removing the telescopic whip and adding a short stub of an antenna to the top of the 5mm dia bolt.

5. And Finally

The dimensions for coils, lengths of elements and matching points should only be used as general guidelines. Factors such as the size of your vehicle, the goodness of the earthing, the antenna position on the vehicle and many other things will ultimately affect the way the antenna matches to the outside world. You will need to experiment, but hopefully these guidelines should put you close enough to where you want to be, after which you will have to tweak the system to match you specific installation.

This antenna has worked the world from Europe to most of the States from East to West North to South. It has also worked down to South Africa and the Middle East. With the sunspot activity begining to improve I hoping that the number of DX scalps will be increased in the next few

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40m Antenna

Following the success of the 80m antenna I then decieded to make a 40m version. This was built on the same lines as the 80m version using a base matching coil, a lower element using the helical winding technique, a load coil based on the 43mm water pipe and a top element using a stainless 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 than 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 mounted 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 enamelled copper wire used to wind the coil. It can be spraked with clear varnish or RTV compound during the final finishing.

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.

80m Antennas

Up until a few years ago mobile to me meant working on 2metres through a repeater or occassionally simplex. However having spent a portion of my professional life working mobile albeit on X band through a satellite, something which is not for the faint hearted or small bank balance, I decided it was time I tried working HF mobile. My first attempt was a base loaded 80m monopole. This was only 1.3m long and mounted on top of my vehicle.

This as can be expected did not prove to be very successful. In fact, although I did get one report from a short wave listener, I made no real contacts with it. This lead to further reseach. Having read an article on the internet about a centre loaded 20m antenna, I decided to try the same techinque for a 80m antenna. The details of how this antenna was made is detailed below.

The First Antenna

In the begining

At the start let me say that the antennas that I have designed and built, using components that can be readily purchased from the likes of Maplins and your local DIY shop and can be made for a few pounds. See building tips

The antenna is made in four sections which consists of a base matching coil, a helical section, a centre loading coil and the top whip section with its capacity hat.

1.The Base Loading Coil

The matching coil is 20 turns of 20swg enamel copper wire spaced 2 mm apart between windings, wound on a 42mm OD PVC water pipe. The coil is tapped at 10 turns which acts as the 50 � feed point. The bottom of the coil is attached either to a mounting stud such as a M10 bolt or as I did an old SO 239 (Male) connector, potted into the tube.

A wire is then attached from the 50 � feeder to the tapped point on the coil. Using a 37mm to 13mm pipe adaptor the middle section can be fitted into the bottom section.

2. The Helical Section

The lower verticle section is something of an experiment. I thought about how I could make the load coil appear to be electrically higher than the physical height. I therefore came up with the idea of a helically would element. By winding the helical with a wide spacing between the turns would mean mean that it had very little inductance so was not adding to much to the load. It also meant that I could use some ordinary plastic water pipe to make the element.

The helical section is a 13mm OD plastic overflow tube 940mm long with a helical winding of 14swg enamelled copper wire wound up the tube with 33mm spacing between each winding. This is soldered to the base matching section.

3. The Centre Load Coil

The centre loading coil attaches to the helical section using the same 13mm to 37mm pipe adaptor as was used on the bottom section. The coil is wound again on 42mm OD PVC water pipe, about 200mm long. The coil consists of 80 turns of 20swg close wound on the tube. The top of the coil is attached to the whip using a ferrule and screw arrangement. This was an old 2m whip interface which I adapted fro my purposes.

4. The Top Whip

The top part of the antenna is a standard 1.3m stainless steel whip. I used an old 3/8 2m whip, but stainless steel sections can be picked up from rallies quite cheaply. This will need adjusting to bring the antenna to the frequency area desired.

5. Tuning

One piece of essential equipment is the MFJ 259 Antenna analyser. This last part must be carried out on the vehicle you wish to mount the antenna and with all the necessary earth bonding wires in place as these will all affect the resonant point of the antenna.

What I found best was firstly to check at what frequency the antenna was initially resonant. This wants to be about 4MHz. If the antenna goes off low e.g. 3.2MHz then take a few turns of the centre loading coil until a tune of about 4MHz is reached. Then construct a simple X shaped capacity hat about 150mm across from two pieces of 16swg tinned copper wire and fix this about 2/3 of the way up the 1.3m whip. This should bring the tune down from 4MHz to about 3.5MHz. Carefully trim each arm of the capacity hat until the desired operating frequency is reached e.g.3.66MHz. Minor adjustments to the antenna can be made with a suitable ATU. I use my antenna from 3.6 to 3.79MHz using an MFJ mobile ATU.

Once this has been completed then check all the sections are well soldered, that the pipes are glued together and any other fixtures are well secured. Cover the exposed sections of the coils in PVC tape and spray a clear lacquer over the coils to improve the water-proofness. Then your antenna should be complete and happy mobile.

6. First Contacts

Following the lack of success from the original base loaded antenna I was very dubious about the outcome from this antenna. Receive signals seemed good and the antenna tuned up well. I might add that I did not at this time have a suitable mobile transceiver so used my base station rig the FT 102 with my car parked on my driveway.

My first station worked was in Northern France, the second was in Holland and may more followed that. Having managed to prove that the antenna worked I then managed to borrow an AlLINCO DX70TH ( which later I purchased) and then started being truely mobile. Many contacts both truely mobile and static mobile folllowed over a period of a year.

7. The Future

The success of the 80m antenna then generated 40 and top band versions. All worked very well with contacts mobile on top band all over the UK. The 40m version worked well into both interUK and the near continent. More research followed where I looked into the idea of high Q coils similar to the Bug Catcher.

The problem that I could see with the Bug Catcher was the size and weight. The weight meant that if used mobile then a stong lower section the the antenna was going to be required and possibly some guying support. I needed to find a way of winding a high Q coil in the same way as I had made the load coils for the first antennas.

Whist on a trip to my local Wickes DIY store, I noticed that they sold a wide diameter tube which is used inside a tumble dryer. The tube is 100mm diameter, made of thin plastic and I thought ideal for winding a loading coil.

The construction of this antenna is explained below.

The Second Antenna

1. Base Matching Coil.

This consists of 20 turns of 18swg (16awg) (approx 1.25mm dia. Wound on a length of 42 -43mm diameter plastic water pipe. The coil length is approx 60mm long so the spacing between turns is about a wire thickness. The coil is tapped at turn 10 up from the base. This point however is very dependent on the mounting position on the vehicle which can be affected by local metalwork. During final tuning it may be necessary to move this point up or down.

The base of the coil is at electrical ground. I have glued a PL259 connector into a plastic former which fits into the inner diameter of the water pipe I often cut pieces out from a plastic kitchen chopping board which I can buy from my local super market. This board is made from polypropylene and is about 8mm thick. I use it a lot especially for making end caps and discs for the pipes.

So the centre pin of the Pl259 is attached to the tap point and the outer soldered to the base of the coil. In this picture you can see the slight bulge where the tapping pint is. The upper end of the coil is attached to the lower element

2. Lower Element

This is 1.2metres long and made from 15mm 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 30 turns 18SWG (16awg)(1.25mm dia) enamel copper wire wound on a 100mm diameter plastic former and is 65mm 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. 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. 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 in the tube to attach the end of the wire around it then screw it up tight.

In late versions I have used tinned braid about 1cm wide as can be seen in the photo below. This is more flexible than the copper wire used on the windings and so is easier to fix to upper and lower element.

The coil can be covered in PVC tape or heatsgrink 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 as shown in the photograph.

4. Upper Element

I have tried 2 upper elements both of which work well. One is a 1.3m telescopic element bought from my local electronics store, 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 it�s 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

I have a bracket already attached to the underside of my car. I have made sure there are good earth straps across the door hinges and bonnet hinges, as well as a good earth strap to the antenna base fixture. I use 1cm wide braiding straps but you can use good high amperage electrical house wire.

I assemble the component parts of the antenna together and mount it to the antenna fixing bracket. I 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. I also use a small ATU unit to give me wider band coverage but the basic antenna is tuned to approx 3.650MHz.

6. And Finally.

Once the antenna matches at the desired frequency then make sure all the joins and brackets are secure a tight, use glue to add strength where necessary. Wrap exposed connections with PVC tape, and if necessary around the coils themselves. I also usually overspray the coils with clear lacquer varnish. This helps glue the wires in place on the former, as well as adding additional waterproofing. .

Like the original 80m antenna this has made many good contacts both inter UK and to the near continent.

Other Antennas

20m,15m,30m Minibeam

This antenna started off trying to make a minibeam antenna from an old Band 2 VHF antenna. However the rods were not long enough so new elements had to be bought. These I purchased from a metal tube supplier called Metals4U. The driven elements are each 1.5meters long. The director is made from a single length 3metres long.

The two elements are mounted onto a central pole so that they are 2m apart. This is a compromise distance for all 3 frequencies, none of which is ideal.

The load coils at the end of the elements are made from 22mm waste pipe. The coils are wound with 20swg enammeled copper wire.

Winding lengths are as follows

Inner coil 10m 25turns

Middle coil 15m 45turns

Outer Coil 20m 85turns

The feed point is made from plastic chopping board and the remains of the cable entry from the VHF antenna. One of the only pieces to survive from the original project. Across the feed point but not shown is a balun made from 10 turns of low loss caxial cable wound on a 100mm diameter ferrite ring which you can buy from rallies.

Between and beyond each coil is a series of capacity hats. These were made from 3mm diameter aluminium rod which can be bought from some local DIY stores. Each capacity hat is made from 2 rods each 400mm long formed into a cross shape. There is a capacity hat after each coil so three at the end of each element. To increase the length of the reflector an additional 500mm of aluminium rod was attached to outer point of the non driven element.

Tuning needs to be carried out with the antenna as high as possible before it paced into its final resting place on top of the mast. At a low height the antenna will go off low so this should be kept in mind. Tuning is carried out by clipping the length of the capacity hats until a frequency about 50 to 100KHz is achieved. When mounted up at about 30ft (10m) or more the antenna should tune about 100KHz higher

These pictures show my minibeam mounted on my antenna mast on top of my house, in the picture you can see the balan close to the feed point and the capacity hats at the end of each coil section.

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 guarantee 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, especially as I'm using mid section loading coils. The antenna 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 couple 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 black 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 thick 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 halved down using a sureform or cheese grater. You can do this using an electric drill. When cutting out 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 horizontal 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 more robust than the standard cable type Pl259

However you can also use a standard PL259, Pot this into one of the reducers using araldite. 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 quality 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 vehicle 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 horizontal 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 sufficient 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 above the base, thus reducing the load on the base mounting point. using a mount like this am able to travel at speeds up to the 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 connecting 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 around 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 two years now and there is no sign of rust around 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 earth straps 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

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