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 “Senior and Junior” management for the day, they having decided that “Dad Taxi” is 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 can’t 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 let’s 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 won’t 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’ve 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 didn’t 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 attach 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 Tesco’s.
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 wire’s 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’ve done this using sticks from my son’s “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 degs 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’re 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’m not at the rally, as you’ll 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 tweek 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 numder of DX scalps will be increased in the next few years.