By Jason Reilly.....

 

‘Necessity is the mother of invention’ goes an old saying, and how true it is! I recently wanted to get my hands on a dedicated 800 MHz antenna for my handheld scanner. 

 

 

While my UHF rubber duck had served me well, and had done so for some time, the time had come for me to experiment a little bit to see if I could improve on the old faithful antenna. 

 

 

Looking through an old, 94 vintage RFI catalogue, I noted that there was not one 800 MHz hand portable antenna terminating in a BNC plug. Pondering the predicament for a minute or two, the solution became clear: why not make my own? Surely it couldn’t be all that difficult?

 

 

While my need was to create an antenna specifically for 800 MHz, the idea could just as easily be applied to 460 MHz, or even 160 MHz, if you don’t mind the length that results. Given a little bit of careful tuning, the finished product could even be used as a transmitting antenna.

 

 

I decided to experiment with three different types of antennae: the standard ¼ wave type, a stubby type, and lastly a 5/8 wave type.

 

 

The Full ¼ Wave Antenna:

 

So, lets get stuck into making the simplest of the antennae - the full length ¼ wave rubber duck. Firstly, you will need a crimp style BNC plug for RG 58 size cable. You will also need the appropriate crimper. I prefer the crimp style connectors simply because with a little practice, they are much quicker than their solder counterparts and just as reliable. A good crimper will make all the difference, too. Just squishing the centre pin onto the coax inner conductor and squishing the crimp tube with pliers is definitely not conducive to good RF performance, nor will it look terribly professional! You could use a solder style BNC connector instead, but the end result will not look as appealing.

 

 

You will also need to purchase a rubber ‘boot’ that is sometimes supplied with crimp style connectors, but can be bought separately. Dick Smith sell a BNC connector that has a rubber boot with it, and Jaycar sell rubber boots individually. I found mine at an electrical wholesaler, costing about $2 each.

 

 

For antennas to be made for 400 MHz and up, a short scrap length, about a foot, of RG 58 coax cable is required. If you want to make the antenna for 170 MHz or less, then you’ll need about one metre. For VHF, I’d recommend that you use an air-core dielectric TV cable, which is a fair bit stiffer than RG 58. Yes, it is of a different impedance and it will require a different BNC connector (75 ohm, instead of 50 ohm) to match the slightly larger size, but that is really of no consequence.

 

 

You will also need a similar length of black heatshrink tubing. The tubing should be fairly close in diameter to the coax you are using. I found that heatshrink with a diameter of 4.8mm is perfect for the RG58, while the next size up at 6.4mm is used for the RG59 cable. Try to obtain a length that doesn’t have any folds or kinks in it, which could mar the end finish. That is pretty much all you will need in the way of materials. The tools you will need include a sharp safety blade or knife, cutters, and a pair of scissors, as well as the tools needed to terminate your choice of BNC connector. A heat gun or hair drier will come in handy, too.

 

 

In my example, I’m going to make an antenna suitable for 867 MHz. Start by terminating the BNC connector on to the coax. Don’t fit the boot at this stage, though. Trim the coax so that it is about 12cm in length overall, this measurement isn’t critical, so you don’t have to get too precise at this stage. If you wanted to make your antenna for 400 to 500 MHz, then trim the coax to 20cm, or for an antenna for 118 through to 176 MHz, trim to 65cm. Next, take a sharp razor blade and carefully cut through the coax outer sheath and the braid right where the coax goes into the crimp connector at the crimp tube. Work slowly using a sawing motion, carefully following the lip of the crimp tube as a guide. Try not to cut into the coax dielectric. Once you think that you have cut through the outer sheath and braid, slowly ease both off. If all the braid doesn’t come off, any loose strands can be trimmed using the razor blade. Once this is done, what you should have is a coax inner conductor and dielectric terminated on the crimp BNC connector, with the coax outer and shield removed except what is actually left in the crimp tube of the connector itself.

 

 

Next, take a length of heatshrink and slip over the coax, so that it covers the entire length of the exposed coax dielectric. Using a heat gun or hair drier, very slowly shrink down the heatshrink. Use a gentle heat, and keep the hot air constantly moving over the length, so that the heatshrink shrinks down evenly, without undue deformation. If too much heat is used, or you rush the job, the result can look a little ugly, or if you have used the air cored RG59, the heatshrink can actually crush the air dielectric down to a much smaller diameter and loose the desired stiffness.

 

 

Don’t forget to turn off the heat gun or hair drier and place it somewhere safe so that you won’t accidentally burn yourself or anything else while it cools down.

 

 

After letting the heatshrink cool, you can cut the coax to a more precise length, to tune it to the frequency of which you are interested. Measuring from the top of the metal crimp tube, mark your desired length on the heatshrink and cut at that point with a good strong pair of scissors or flush cutting side cutters. As a general guide, to determine the length of your antenna from metal crimp tube top edge to tip of the dielectric where you cut it, use the formula: 7200 divide by desired frequency in MegaHertz = length to cut in centimetres. This is not precise, but it will get the antenna tuned to within 5% of your desired frequency.

 

 

Now the time has come to fit the rubber boot. Slide the boot down to the base of the BNC connector. Notice how the boot does not make a real snug fit over the antenna if it is pre-sized for RG59 cable? There is a gap at the top of the boot, because the heatshrink and the dielectric is smaller than the whole RG58 overall. To fix this, we will slip a short length of coax sheath into this gap. Using your razor blade, cut a small length of coax outer sheath so that it forms a short tube of about 10mm, and begin to work it on to the antenna. It will be a tight fit, but once it is on, you can slowly work it downwards towards the rubber boot using your fingernails, so that it eventually fills the gap between the heatshrink and the rubber boot.

 

 

If the whole rubber boot is loose once fitted, you can take the boot off, and wind a few turns of electrical tape around the crimp tube to increase it’s diameter. Begin by placing too much tape on, making a test fit of the boot, and begin taking some tape off until the desired fit is achieved. The boot should not be too hard to fit on the connector which is indicated by deforming the tape underneath, nor should it ‘give’ any under the pressure of a two finger grasp & pull.

 

 

Now that the rubber boot is on and snug, and any gap between the neck of the boot and the heatshrink over the coax dielectric has been filled, the antenna is almost complete. All that is required to give the antenna a professional finish is a small rubber cap over the end. This will probably be the hardest item to find. You could retrieve a suitably sized rubber cap from another worn out handheld antenna, or even a fibreglass whip antenna. Another source of these caps can be found sealing the ends of cable bought on a drum or reel. For this, you could ask at your local electrical or electronic supplier if you could remove a cap or two from any of their cable reels, and maybe offer a few dollars as an inducement. My local Dick Smith Electronics store retrieves such caps from various items that they sell, and place them in a bowl for customers to take, gratis. I believe that you can also buy the caps direct from various manufacturers or distributors, but the quantities involved would not make this an attractive proposition.

 

 

Once you’ve found a suitably sized cap, glue it on using a little contact adhesive. You only need use a drop, since too much glue will squeeze out and spoil the appearance.

 

 

The finished product that you now have is fairly professional looking and reasonably strong. While it certainly doesn’t fall under the category of rugged, it should last for many years given a little respect.

 

 

The Stubby Antenna:

 

Making a stubby antenna is a little more fun than the run-of-the-mill ¼ wave antenna. The materials you will need will again consist of a BNC crimp style connector, heatshrink, and suitably sized rubber cap. You’ll also need to find some enamelled copper wire of about 0.6-1.0mm diameter, and some clear, flexible plastic tubing of about 7.5mm outside diameter / 4mm inside diameter, the sort that you might use for fish pond fountains or fish tanks etc.

 

 

Begin by cutting your materials roughly to length. Take the tubing and trim it to your desired overall length of the antenna. For UHF, a length of about 5cm is convenient, or for VHF 10 to 15cm, but you can change this to your taste. Remember, however, that the shorter the antenna the less efficient it will be, so you need to arrive at a compromise between length and performance. Having said that, I find that my 5cm long UHF antenna works 95% as well as a full length ¼ wave one.

 

 

If your tubing has been sitting in a coil for some time, it will need to be straightened out. Of all the suggestions to get the tubing on the ‘straight & narrow’, my wife came up with the best solution so far: lay the tubing out in the sun, using two heavy items at either end to keep the tubing from rolling up again. Two full days of sun did the trick for me (yes, we do actually get sunny days in Tasmania). Not to be outdone, I can suggest a blast of hot air from a heat gun on the tubing to soften it, and then allow it to cool laying perfectly flat.

 

 

The enamelled wire can be cut, too, using 30cm for a UHF antenna, 110cm for an airband (118-136 MHz) antenna, 85cm for a VHF high band (144-176 MHz) antenna, or 180cm for a VHF mid-band (70-85 MHz) antenna. Yet again, these measurements aren’t critical.

 

 

Straighten the enamelled wire of any kinks and twists by hand. A good way to do this is to take the wire and under hand tension, pull it over a right angle edge, such as a piece of scrap timber. Any remaining kinks and irregularities can be gently corrected using the smooth section of jaws of a pair of pliers.

 

 

Next, scrape the enamel off one end of the wire, ready for soldering. You’ll need to remove about 5mm of enamel. I find it best to use a razor blade to scrape the majority of the enamel off, and use the heat of a soldering iron to finish the job. Don’t try to use a soldering iron to do all of the enamel removal for two reasons: the enamel used in some wire is corrosive to your soldering iron tip when burnt off, and the fumes produced can be rather nasty. Using some solder, tin the copper wire where you have removed the enamel. While the wire is still hot, use the heat in the wire to bore a small hole just big enough to let the wire pass through easily in the plastic tubing about 10mm from one end. Only make one hole - don’t go too far and bore a second hole in the other side of the tubing.

 

 

Re-scrape any residual plastic off the tinned end of the wire, and attach the BNC connectors centre pin to that end of the wire by soldering, not by crimping in this instance. Bend a very small right angle in the wire at the very other end, and use this right angle to pass the wire through the hole in the plastic tubing from the inside to the out. A pair of fine needle nose pliers will help feed the wire into the hole from the inside, and pull the rest of the wire through from the outside. Leave enough length of wire with the attached centre pin so that you can use the needle nose pliers to push the centre pin into place inside the BNC connector body. This will take patience and care not to kink the wire as you push the centre pin home. Choosing a thicker wire will assist you in doing this, as will using the pliers to carefully grasp and pull the centre pin into it’s final resting place, or you can use a really fine jewellers screwdriver or even a needle to push down on the centre pin to set it home.

 

 

Now push the tubing over the crimp ferrule of the BNC connector, pulling the enamelled wire as you go, but being careful not to let the wire eventually end up touching the metal of the BNC connector. The fit of the tubing will be firm, but not too tight. I’ve found that fit is enough to hold the antenna together, but if you are in the habit of picking your radio up by the antenna, then I’d suggest some sort of very strong glue to make sure that the tubing will not pull loose.

 

 

If you are tuning the antenna for UHF, trim the wire to 25cm in length measured from where the wire exits the tube, and spread this out over 4 or 5 cm of plastic tubing. These figures will produce an antenna that is roughly resonant between 420-470 MHz once completed.

 

 

Here comes the tedious bit: winding the enamelled wire. Begin by winding the wire slowly and firmly, but definitely not too tight, with each turn sitting snug against the previous one. Be as neat as you can at this point; a kink or even slightly out of place turn here can spoil the look of the finished product. Once all the wire has been wound onto the plastic tubing, lightly pull the tightly wound coil of wire up so that the turns evenly spread over the entire length of the tubing. If the wire does not want to spread out, you will need to loosen the tension of the windings by wriggling the coil slightly as you pull. You may need to adjust by hand each turn of wire slightly to ensure an even distance between each to achieve a suitably pleasing effect to the eye once finished. Be hyper-critical on the even distance between each turn of wire - it may not look too bad now, but once the heatshrink is in place, it really accentuates any imperfections or irregularities in the wire that you have wound.

 

 

Once the wire is wound and looking neat and tidy, place a length of heatshrink over the wire and tubing, allowing some extra length of heatshrink since it gets shorter as well as smaller in diameter with the application of heat. Again, using a low heat, slowly and evenly shrink the heatshrink down. Trim off any excess heatshrink with a razor blade or really sharp pair of scissors.

 

 

The only finishing touch you need to complete the stubby antenna is a rubber cap. Finding a rubber cap of this larger size is a little more difficult, but the end result looks great, especially on the micro sized scanners of late. Mine looked just the part on top of my old Icom R1 scanner.

 

 

The 5/8 Wave Antenna.

 

This antenna offers the added performance of slight gain offered by the 5/8 wave radiator. It also acts as a bottom loaded ¼ wave antenna at 1/3 of the design frequency, so it is ideal for scanner users who wish to have a high-VHF band (160 MHz or so) and UHF antenna in one. It also works very well as an amateur HT antenna on 2 metres and 70 cm given some real careful tuning.

 

 

Begin by taking some solid copper core, air spaced TV type coax, and cutting it to roughly 650mm long. Take a razor blade or Stanley knife and carefully slit the outer sheath lengthways the entire length, and remove the sheath. Also remove the braid shield and any aluminium foil that may be present, so that all you are left with is the inner air-spaced dielectric and centre conductor. Using the Stanley knife again, make a 100mm lengthways slit in one end of the dielectric, being careful not to damage the centre conductor. Using a pair of fine needle nose pliers, grab the centre conductor where you have made the slit in the dielectric and bend it out at a ninety degree angle, so that the bend is right up against the top of the slit. Take the conductor and wind it around the dielectric for four turns, just as if you are winding a coil over the slitted part of the dielectric. 

 

 

Leave about 3 to 4mm between each turn. The next bit is tricky and will require some good judgement, a pair of fine needle nose pliers (again) and a fine blade screwdriver. On the fourth turn of centre conductor, you need to have it bend at right angles back into the slit, and sit back in the dielectric as the conductor normally would. The end result is some 500 to 550mm of coax with the outer sheath and braid shield removed, with the inner conductor neatly exiting the centre of the dielectric, making four turns around the dielectric, and entering back into the centre of the dielectric towards one end. You can cut off the bit of dielectric that doesn’t have any centre conductor in it (the result of using some of that conductor to wind the coil)

 

Now install either a standard or crimp style 75 ohm BNC connector on the coax at the end where you have made the coil. This will be just like terminating the connector on a standard bit of RG59, except there is no braid to worry about. You may find it necessary to build out the dielectric using heatshrink or electrical tape where it enters the connector if using a standard type (non-crimp) BNC connector. On the other hand if you are using a crimp connector, you may find the air core dielectric is too big to fit inside the crimp connector, this can be fixed by squeezing down the air core to make it fit. This isn’t an elegant solution, but it does work quite OK.

 

 

Keep the coil close to the connector, say about 10 or 15mm from the top of the connector. This can be a real challenge, especially if using a crimp connector, but it is necessary to make the coil of the correct impedance for the job at hand. I would recommend a trial fit of the connector first, decide what needs to be done to make the termination of the connector snug, and then go ahead and do it for real. After that, you can place some heatshrink over the coil, or over the whole antenna if you think it is warranted (be careful of the heatshrink crushing the air dielectric), and one of those little rubber caps to top it off. You can even slide on a rubber ‘boot’ to give extra protection to the coil and give the whole antenna a professional finish. To get the antenna somewhere near resonant to the middle of the UHF band (and as a bonus near the middle of the VHF high band) trim the antenna to about 430mm to 450mm in length.

 

 

If You Want To Make A Transmitting Antenna…

 

There are some things to take note of if you wish to make your antenna suitable for transmitting on.

 

 

For the full sized ¼ wave antenna, the cutting formula given above will be fairly close, but to tune the antenna precisely, you will need some sort of VSWR meter or directional bridge. There are all sorts of variables that will affect the tuning: the final thickness of the heatshrink when applied, the sort of coax used, etc. Even the static cap can have an effect, so when you are testing the antenna and taking your readings, make sure you hold the antenna being tested up to your face as if you were really using it to transmit with, and have the static cap on. Begin to tune the antenna by using a pair of very strong scissors, shears or side cutters to shorten the antenna, a little bit at a time. Keep trimming the antenna and replacing the static cap until you reach minimum VSWR or return loss.

 

 

The stubby antenna is much more critical to tune, and to get it right really does need the VSWR meter. Make the antenna up as described, and to tune, use a pair of pointy nosed sharp diagonal cutters to slowly remove about 5 mm of wire at a time. Cut through the heatshrink and wire together, or you can use a razor blade to remove some heatshrink and expose the wire for cutting. If you find you need to remove more than one turn of wire, you can use a razor blade to trim down the plastic tubing as well. Don’t worry too much about neatness here, the rubber cap you put on will cover this. As with the ¼ wave antenna, factors that affect tuning can be proximity to your head while transmitting, the thickness of the heatshrink, how much of a ground plane your hand provides for the radio & antenna and lastly the static cap; only all these effects are magnified threefold as compared to the normal ¼ wave antenna. As an example, I found that on UHF, a rubber cap can alter the centre frequency of the antenna by as much as 20 MHz.

 

 

In addition to these factors, the overall spacing between the windings also has a dramatic effect - the tighter the windings, the lower the frequency the antenna will resonate at. Also note that the closer the windings, the less efficient the antenna will be. I’ve found that a spacing of the windings of 4 or 5 mm will produce quite an acceptable result, while only sacrificing a minimum of performance, at least on UHF; to achieve a similar result on VHF requires the windings to be spaced considerably more, but of course you loose the convenient size. It’s all about arriving at a compromise that you are happy with.

 

 

The 5/8 wave antenna isn’t quite as critical to tune as the stubby, but it is not easy nonetheless. Again, have the antenna with the static cap on and heatshrink shrunk in place, with your hand providing a ground plane of sorts etc when tuning. If you are tuning for both VHF & UHF (ie 2 metres and 70 cm), you will need to trim the antenna to about 450mm in length. Start by measuring the VSWR or return loss on VHF, and trim the antenna, little bits at a time, until the VHF match is pretty good, then check the UHF match. Trim very little bits off at this stage to optimise the match at VHF and UHF. Of course if you are designing the antenna for purely UHF or VHF only, then trim for minimum VSWR or return loss for your desired frequency, pretty much as you would for a normal ¼ wave antenna.

 

 

Performance Comparisons:

 

All my testing was done at one frequency in the UHF band. Surprisingly, the antenna that ended up giving the best return loss measurement was the stubby ¼ wave antenna. I had made up all three antennae described here, and then in turn set each of them atop a receiver and tuned into a weak signal, and wandered around outside noting how badly and how often the signal dropped out during my wanderings. I then connected the antennae to a transceiver and asked a fixed station to give me reports for each antenna.

 

 

The results were as expected; the stubby UHF antenna was only marginally worse than the full ¼ wave antenna for both transmit and receive, and the 5/8 antenna was only marginally better than the full ¼ wave. I was a little disappointed that the 5/8 didn’t give more improvement than I expected - I put this down to the fact that a HT with a hand wrapped around it does not provide a good enough ground plane for this antenna to work to it’s full potential. A ½ wave ground independent antenna would have been better, which was confirmed by testing a commercially made ½ wave design. I did try to construct such a design but found the matching coil difficult and fiddly to make. Maybe this will be the basis for a future project.

 

 

As a final confidence check, I compared the stubby ¼ wave antenna and full ¼ wave antenna against commercially made stubby and full ¼ wave designs, and noted no difference between the home-made antennas and the shop-bought antennae respectively.

 

 

Variations Of course, if your scanner or HT uses a different connector, such as the SMA connectors now finding favour due to their space saving dimensions, you can adapt these designs to that style of connector very easily. Just substitute the BNC crimp connector for whatever suits your radio. Just make sure you get the crimp version of the connector that suits. So there you have it. Given some good materials to begin with, and a little time and patience, you can make your own scanner or HT antenna that is custom made to your requirements, with an appealing appearance that rivals that of a commercially made product, at a fraction of the cost.