The DIY VHF Antenna Mk1

This article tells you how to make a cheap and effective DIY aerial for the Marine VHF Band. Just alter the length to make it suit any VHF band - outback TV on the old VHF bands, amateur radio 2-metre (144MHz) band, etc. We can't go into all that here as this is about boat radio. You just need to find the half-wave length for your band and tune it from there, ie make the antenna slightly shorter or longer till the signal is strongest. Tip: start with it longer!

Emergency aerials

There are many good reasons for having a portable back-up VHF aerial stowed away somewhere safe. In fact there are so many, it is hardly possible to list them all; which demonstrates the eternal optimism of most boat owners, since hardly anyone has one. The cable or connections to the masthead aerial might fail; or the aerial itself may fail; and if the mast is lost then the aerial goes too - just when you are likely to need it most.

If any of these occur you're up the creek without a paddle – though there is a more popular and pithier expression. For an item of safety equipment that might prove vital in some circumstances, such vulnerability seems rather risky. Not only that, but one must also add to this even more reasons for an alternative aerial: commercial masthead aerials are inefficient; a second aerial is handy for testing out VHF problems; a better external aerial is useful to connect a handheld VHF to; and, finally, some may not wish to have an aerial on display, for various reasons. Racing sailors, for instance, are likely to begrudge the extra wind resistance of the masthead aerial, and the weight aloft of the aerial and feeder cable...

All in all, you need a second aerial (or antenna – they're the same thing). You can indeed buy an emergency aerial, and one or two of these compact devices have tested as quite effective, in some cases better than the usual masthead type. However, the main factor affecting the efficiency of an aerial is quite simply size. The bigger, the better, and though the type of aerial and its design and manufacture do come into the equation, a simple way of estimating how good an aerial might perform is its size: nothing else is half as important. In ninety-nine cases out of a hundred, an aerial ten feet long will perform better than one a foot long, both for reception and transmission. Height also comes into the equation, for several reasons; the fact that your aerial is mounted at the masthead often compensates for the fact that it isn't of the most efficient type.

Aerials – myth and reality

There are a tremendous number of myths surrounding aerials, often promulgated by people with a little knowledge of the subject. One of them is that you can have a very efficient small aerial – you can't. You can have an effective small aerial, but that's not quite the same thing. A big one always outperforms a small one, whatever the frequency. An HF aerial is huge compared to a VHF aerial, because the frequency is lower and therefore the wavelength is longer, but the same thing applies: a big HF aerial is always going to be better than a small one. Just life I guess... Again, the same thing with the high frequencies used in radar; you will be well aware that a four-foot radar scanner outperforms a twelve-inch scanner by a very large margin. A big antenna out performs a small one.

If our backup aerial could therefore be rather larger than the usual masthead model, it should also have the potential to outperform it. This isn't a difficult aim, due to the small relative size of VHF aerials, especially compared to HF ones. If we can also improve on the design, then things are even better. All we lack is height – but something has to be sacrificed to expediency.

So we need an aerial of a decent size, preferably of a better design than is usual. In fact this is a remarkably easy specification to fulfill: a half-wave dipole fits the bill perfectly. To explain: the standard masthead style is what is known as a shunt-fed quarter-wave groundplane aerial. This means that it is a quarter of the required wavelength long, and uses an earth (or an electrical substitute) as the lower half of the radiator, to economise on length and simplify the mounting. Despite the advantages of compact size and easy mounting, it has two serious drawbacks: firstly, the use of a shunt coil instead of a real groundplane reduces its efficiency, and secondly its plane of radiation centres around an upward angle of around 30º, or worse. This means that most of the output power is dissipated up into the sky. For VHF purposes, (and most others, in fact) an aerial that puts its power out level with the ground, toward the horizon, would be a much better proposition. One type that satisfies this requirement is a free-space radiator, of which a dipole is an example. This type of aerial doesn't need a ground, earth, or groundplane, and is all the better for it – it is far more efficient.

A couple of points to note are that an aerial reproduces its transmission characteristics in its reception, so that optimum reception signal strength will parallel the transmission path. It is safe to disregard the effects of heeling for these purposes, since they tend to cancel each other out on opposite tacks, and there is little we can do about it; though it can be stated with confidence that any type of fixed radio antenna, including VHF, HF, and radar, will always work better in the ahead and astern directions when substantially heeled over. You can bear this in mind when experiencing difficulty in communicating, at extreme range, and turn your boat to point toward or away from the target; you may well get better results, since the majority of aerials are in practice directional, though not widely advertised as such.

A prime example of this is the popular insulated-backstay HF aerial, which can be fairly directional due both to its angle and to local effects on the boat. When poor reception and especially transmission results are encountered, then it can pay to turn the boat, 90° at a time until the best result is obtained. Insulated backstay antennas are directional, stern-mounted vertical whips are less so* - which is why they are superior.

* In theory they aren't directional at all, of course. In practice there are many local effects that often mean they are - but probably less so than a backstay. These influences include the nearby metal mass of the mast and rigging, in the same vertical plane and of a similar physical length; and whether or not the whip is a dipole or groundplane antenna. If it's grounded, then if the effective ground is mainly in one direction rather than distributed equally around the antenna, then the antenna will tend to be directional. On a boat, it's likely that the ground area will be contained within the hull of the boat, stretching away from the antenna in one direction, which is not ideal. This is yet another reason why free-space radiators are always superior.

The DIY VHF Aerial Mk 1

Returning to our VHF backup aerial, we have established that a half-wave dipole will suit admirably. It will outperform the usual type of masthead aerial, if hoisted to the same height; and should therefore perform quite reasonably at deck level. You can easily make one yourself for the VHF band, since it will be less than a metre long. There is also an excellent 'cheat' method that can be exploited, to make things even easier and cheaper; which is something of a change in the world of marine equipment. In fact it will cost you about £5, mainly for the cable – can't say fairer than that, squire!

The advantages of the DIY VHF dipole are several: it's cheap, lightweight, efficient, a free-space radiator so intrinsically a more powerful design than any groundplane type, is a sealed and waterproof tube, easy to hold up or suspend from the rigging, could even be hoisted higher if desired, is perfectly suitable for permanent fixing, can be stowed without difficulty, will take up to 100 watts power input, and will even work fine over a mile or so range when stowed below – perhaps more if stowed to advantage. Not if you've got a metal boat, though; tricky problems we can solve, but miracles are a bit much.

To clarify this point: with a dipole of this type you can leave it stowed below, perhaps up against the deckhead, and it will still function over short ranges. It does not need any physical connection to metal or whatever, since it is of a much better design than the normal type of groundplane aerial.

Aerials are often described in terms of their gain, or relative efficiency, such as for instance '3 db gain'. This is fine among experts but meaningless otherwise, since there are several ways of fudging this, and unless you know the score even an indifferent aerial can be made to look marvellous. Exactly the same situation exists with audio amplifiers, where a seemingly monster power output can actually be rather puny: 100 watts PMPO versus 30 watts RMS, and so on. Ignore the numbers unless you deal with them every day and know that the goalposts are in the same position; all you need to know is that a dipole is more efficient than the usual quarter-wave type.

Materials and tools needed

You will need a metre of plastic pipe, and plastic conduit of 20 or 25 mm (1 inch) is best; two corks to plug the ends; Sikaflex or similar for sealing it; duct tape or insulating tape; cable ties; two 1-inch or 1¼-inch screws of any kind (though stainless self-tappers are best); and a nice long length of ordinary transmission cable, normally referred to as RG 58, UR 76, or RG174. The longer, the better – it must reach from the site of your radio to up on deck, and then some; around eight metres should do. Note that you must not use TV cable for this, as it is the wrong type.

You'll need a drill, with 6 mm and 3 mm drill bits; a sharp knife; a length of string or wire about 1.5 or 2 metres long; and a pair of pliers.

The aerial itself is simply made by splitting the feeder cable and stretching the centre pole of the cable out away from the outer braid. A half-metre length split like this gives an aerial one metre long, which we will trim down slightly to the correct electrical length, and seal inside the pipe.

If you buy your feeder cable from a proper radio shop, have them solder on a plug at one end for you. For a standard fixed VHF set, this is a PL259 plug (often called a UHF plug, since that was its intended purpose when used in the 1940's, though it has long been superseded for that purpose). If connecting it to a handheld VHF, you will most likely need a BNC plug (the rubber duck aerial on the top unscrews, to reveal a BNC socket). If you have both, and may wish to fit it to either, the easy option is a PL259 fitted, with a PL259 to BNC adaptor plug for the handheld. Actually, a better option if available is a BNC plug on the aerial cable, and a patch lead with a BNC female to PL259 plug for the fixed set. A BNC female to PL259 adaptor may be available instead.

Build details

Metric measurements are used here, as they are best for this purpose, with an approximate Imperial value given after.

Cut the pipe 980 mm (38.5 inches) long. Drill a 6 mm (.25") hole at the centre, at 490 mm (19.25"), through one wall of the pipe only. Feed the bare end of the aerial cable into the hole and work it through so that about a metre projects from one end. The PL259 or BNC plug is on the far end of the cable.

Measure back 550 mm (21.5") from the cable end and fix a cable tie tightly round the cable, using pliers or a proper cable tie setting tool to get a very firm set. Use two cable ties, back to back, if necessary. Cut the free ends of the cable ties off flush with your knife – don't use wirecutters, as this leaves a razor-sharp cutting point on the cable tie (the proper tool cuts these off correctly). These act as a strain relief device to stop the cable pulling out of the tube. Now take a sharp knife and cut all round the cable insulation, 10 mm (.375") 'downstream' of the cable ties (toward the bare end) in order to remove the insulation. Be very careful not to cut into the cable braiding beneath.

Bend the cable gently around the cut, ensure the cut is complete, work the insulation gradually right off the cable and discard it. Now you should have a length of bare cable about 530 mm (21") long, with a couple of cable ties at the point where the cable is stripped.

Bend the cable slightly, and with a pin or a very small screwdriver, enter the braid at the point where it emerges from the insulation. Carefully work a gap into the braid, and pull the insulated centre core of the cable up through the hole. A fairly long gap is needed to do this, and you should try to create this by working the braid apart with care, and without damaging it or the centre core. Especially, do not damage the centre core insulation.

Now you have two lengths of wire starting at the point where the cable ties are: one is a loose length of braid, one a length of thin plastic core, with a centre wire within it. Fold the centre core hard where it emerges from the braid, to create a right angle bend at this point. The insulation on the centre core stays intact, and the braid is bare.

Using a 3 mm (.125") bit, drill a longitudinal hole through both corks. With a knife or a file, trim them so that they fit snugly into the pipe ends, and then remove them. The corks need to be a tight fit in the pipe ends, so that once fully inserted they are secure and will not move. Insert the centre core wire into one of the corks and pull it through so that there is 50 mm (2") projecting. Tape this end so that it can't pull back through.

Now take a piece of string or wire 1.5 m (5 feet) or so long, and drop it down through the pipe to emerge at the other end. Tie it to the end of the braid, and pull the braid back through, so that the braid emerges at the other end. You will need to pull the aerial feeder cable back out of the centre pipe hole as you do this. At this stage, pull on the aerial cable to ensure that the cable ties are snug up against the inside of the pipe wall. Then, fit a cable tie tightly on the cable where it exits the pipe, firmly up against the outside of the pipe wall, so that the cable can no longer move into or out of the pipe.

Push the cork with the centre core wire back down into its end of the pipe, and depress it well so that the cork lies 5 mm inside the pipe. At the other end, insert the braid through its cork and pull it through gently, to as far as it will go. Insert the cork into the pipe end and inside, until there is a 5 mm gap at the top.

Now gently stretch each cable end, the insulated centre core and the bare braid, pulling on them carefully so that each is stretched out straight between the cork and the centre point. At the braid end, where you will find that the braid projects much further since it elongates as it pulls out, take a screw and screw it into the top of the cork, into the hole from which the wire emerges, cutting into the wire and locking it firmly. Insert the screw well so that it is virtually flush at the top and not projecting. Trim off the excess braid that protrudes, with wirecutters.

At the other end, which has the centre core, wind some tape around the pipe about 100 mm (4") from the end; keep winding until there is a good solid amount on there that won't come off in the foreseeable future – this marks the top end of the aerial, the positive end if you like. Don't forget to do this before you go any further - later, you won't be able to tell which end is which and you need to know.

Stretch the centre wire firmly and insert the screw, bending over the centre core where it emerges, to make inserting the screw easier. Alternatively, you may cut the wire so that no more than 50 mm (2") projects, then strip it down to the cork, if you find this makes putting the screw in easier, since it will be easier to bend the wire over without the insulation. Cut off the surplus once the screw is home.

It is important that the wire within the pipe is stretched out absolutely straight, and tight; but of course you must be fairly gentle about the whole procedure. You now have a pipe 980 mm (38.5") long, with a cable entering it at the middle, and cable stretched between corks jammed in each end, 5 mm in from the ends. The aerial element itself, the half-wave dipole, is 970 mm (around 38.25") long. If you have a small knowledge of the formulae involved, note that this dimension is different in practice from the theoretical half-wave dimension.

To seal the aerial against water ingress, fill each of the pipe ends up with Sikaflex, then tape over them firmly. The tape is mainly to keep you clean while the mastic sets. At the centre, coat the cable entry with mastic, and again cover the area with tape. You can with advantage use sufficient tape to stabilise the cable, carrying the tape on 50 mm (2") up the cable, so that the cable remains square on to the pipe as it exits.

Make sure you have marked the top end of the aerial well (the end with the centre core or positive). The negative or downward end has the braid. The aerial works because the radio transmission cannot exit the coaxial cable until the braid is stripped away, which provides the positive and negative elements necessary.

In use

To use the aerial, take it out on deck for best range, hold it by the bottom end of the tube, and hold it up vertically. Make sure the positive end is always uppermost, and never invert it. If possible, keep the feeder cable out away from the aerial rather than holding the wire alongside it with the hand that holds the aerial. You can also use this aerial as a crude direction finder: no transmission or reception (or rather a microscopic amount) will take place out from the tube ends – it works in a plane at 90º to the centre of the tube. Therefore, if you hold the tube level with the ground, maximum signal strength on receive (and transmit) will be perpendicular to the tube length; there are nulls (zero signal) at the tube ends. Held vertically, the aerial is omnidirectional (all directions), and held horizontally, it is (in effect) bi-directional.

However, using the aerial like this is not usually productive, since all marine transmissions are vertically-polarised (the aerials are upright), whereas on its side, although directional, you will transmit in a horizontally-polarised mode. Reception will normally be weak for this reason, except in a marina situation, where signals will have been bounced around between reflecting surfaces so often that they won't know which way up they are.

You will find that it works very well indeed. In fact, you may feel it has made your masthead aerial redundant. If wished, you can suspend it semi-permanently, or even mount it at the masthead for maximum effect. To suspend it, try to keep it clear of any metal in the same plane, i.e. vertical metal items. This will of course be a little difficult on a sailboat, but in practice it will work if you hang it from a shroud by using some line to keep it clear; or even hang it from a line set up between two stays. Try to keep the feeder cable out away from the aerial if possible – running it a metre away straight is excellent, though half that or less will be enough.

To mount it at the masthead, if you really wanted to get the maximum range, then you need to fix a a mounting arm out at 90º from the centre of the tube, which the cable can run along. The arm needs to be a minimum of half a metre long, and could profitably be more.

A useful tip to know is that if you have a lot of extra antenna cable, don't have it in a nice neat coil when you are using the portable antenna. A pile of loops like this will form an electrical coil and will wreck the signal. There is also a risk of the transmitter blowing. If you have extra cable, then just lay the cable out on the deck; or pile it at random so that a series of parallel coils does not result; or if that's too sloppy for you, then simply take your nice coil, compress the opposite sides of the circle together, and tape it flat, into a flattened coil, with a band of tape in the middle. It may look iffy like this but it'll work fine for our purposes (though we wouldn't put a kilowatt up the line like this...).

Happy transmitting – and if you manage to contact your sailing club from 40 miles out to sea, you owe me a beer. In the trade, we sometimes add a bit of 'juice' to the signal to make that happen; but I would not corrupt you by elucidating further.

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