Benelec 24 Element Discone

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Benelec 24 Element Discone

Post by RF-Bot »

Submitted By: Jason.


My belt erupts with an insistent high pitched ringing sound. Well, actually it's my mobile phone making the sound, pleading me to stab a button on it's face to answer a call. When I am away from the office, my mobile phone becomes my office. "I wonder who this is?" I mutter as I answer it. It's Len, editor supreme of Radio and Communications. He wants to know where I am in Melbourne, can I pick up a discone, and would I like to review it.

Would I? Does a fish need water to live "Yes, that would be great, thanks, Len". I'm not able to collect the antenna myself, so I'll have to leave it to a courier to send it to me. That's always the worrying bit. I have much trouble getting anything delivered to me with reliability. A lot of my mail goes missing from time to time, and as luck would have it, very few of the bills go astray. Some days later, waiting in eager anticipation, it arrives. Good, test one passed. I've actually got the review antenna in my hot little hands.

Opening the package, I see many metal rods, some locking nuts, a mounting tube, a discone hub and some instructions. The instructions proclaim that the discone is made by Benelec, and made and designed in Australia. Looking a bit further, I notice that the package contains 12 long and 12 short elements, a total of 24 elements. The picture on the instructions show a 12 element discone, just like the one my neighbour has on top of his roof. Hang on, something funny is going on here, why do I have 24 elements? Read the instructions, silly! I have received a 24 element discone, it's just that I saw the picture of one with 12 elements, which threw me off course for a minute.

A quick read through the instructions tell me that I should have x number of screws and nuts and elements etc. Good, test two passed, all the bits are there. If you are going to mount this discone on a mast of some sort, you will need to get a few antenna-to-mast clamps or you might be able to get away with a couple of worm-drive stainless steel hose clamps. The instructions also give the reader a bit of information on the discone, how it works, some of it's advantages, and gives some specifications. Speaking of specifications, the manufacturer claims a transmit power handling capability of 250 watts, a bandwidth of 60-1000 MHz (transmit with VSWR <1.5:1 above 100 MHz). The elements are all made from stainless steel, with the hub made of chromed brass, and the mounting tube also of stainless steel.

The instructions are very clear and concise how to put the thing together, which is great for someone who has never assembled a discone before. The instructions say that you should put the short horizontal elements on first, then the long, droopy elements (the 'cone'), and then put on the connector and cable, then the mounting tube. I've always found it easier to put the connector and cable onto the hub first thing, then put the mounting tube on and secure that, then tackle the elements, preferably the cone elements first, tightening the lock nuts as you go, and then fix the whole affair to your mast. But, do the installation however you feel best. However, it always helps to do a trial fit first, just to see how everything goes together.

So how does everything fit together? On the review antenna, everything went together very smoothly indeed, if you do not use the locking nuts. With so many elements sitting so close together at the hub, the locking nuts can be difficult to do up, so the trick here is to use the smallest possible spanner that you can find to do up the locking nuts (of 7/32"size), or even consider using some strong needle nose pliers. If you really want to make sure that the elements will never come loose, apply a bit of loctite to each element where it screws into the hub, tighten each element with a pair of multi-grips close to the hub (not too tight or you will break or fatigue the element), then do up the locking nut.

I noticed that the hub appeared to be, in a previous life (so to speak), a hub for the 12 element discone version, and has had the extra holes drilled and threads tapped for the 'extra' 12 elements. These new holes weren't drilled with over-great precision, and as a result, when the elements do go in, they don't all sit in the perfect geometric shape that you would hope for. Some of the horizontal elements weren't truly horizontal, and some other elements weren't evenly spaced, at least on my review antenna they weren't. Still, it's nothing that cannot be fixed by a bit of judicious bending and re-aligning with the pair of multi-grips that you used to tighten the elements up with.

The finished antenna is not as strong as some discones I've seen, but so long as you do not install the antenna where it is likely to suffer extreme icing or windy conditions, I do not see this as a problem.

Connecting the antenna to some cable is no drama, since the hub of the discone has a SO-239 socket, which really looks the part with it's gold-plated centre pin. The choice of cable is up to you, but since the discone is a base-type antenna, you may as well use at least RG213. I favour Belden 9913, which has some really astonishing performance, better than RG213. Unlike the famous Heliax, it is readily available to the hobbyist, and you can use standard connectors with it. Oh, choose your connectors carefully, too. If you buy $2 PL259s, then $2 performance is all you will get. If you are installing the antenna outside, the application of some silicone waterproofing sealant or some vaseline around the connector and cable will help with waterproofing. The PL259 series of connectors really weren't designed to be fully waterproof. You may want to consider earthing the antenna, via the mounting tube, to your mast, and earthing the mast to ground if you think electrical noise is going to be a problem, or if you live in a lightning prone area. Of course, if you choose to mount the discone in the loft of your roof space (under a tile roof only!) then you will not have to worry about waterproofing too much. Come to think about it, you wont have to worry about earthing the antenna, or mast mounting worries, or nosy neighbours asking silly questions as to when you think you will make contact with aliens.

Ok, lets take a look at what doesn't come with the antenna. First, which I've already mentioned, you will need some mast mounting clamps or stainless steel hose clamps, since these aren't included with the discone. I also notice that the elements do not have those little static-caps on the ends of them. These serve a purpose other than 'looking right' or preventing injury if you poke an element in your eye; when you put your antenna up into the open, as the wind blows past your antenna, static electricity builds up on your antenna due to the friction of the wind passing it. When the static charge builds up to a point, it will want to discharge. And since the static electricity charge will want to discharge from a sharp point, the tips of your elements on the discone will be where it will discharge from, causing noise on and possibly damage to, your receiver connected at the other end. Covering this sharp point with a static cap will prevent this discharge, and hence, help reduce the noise experienced on your receiver.

There are two ways around this. Make sure your antenna is earthed, which will help prevent the static build up in the first place. In this case, you can only earth the 'cone' part of the antenna. The other thing you might like to do is to find static caps and add them on to the antenna, or if you have the patience, file each elements end into a nice smooth rounded end with no sharp points or edges that just invite electrons to discharge from. The Benelec 24 element discone also does not have provision for a top-mounted whip. I don't see this as being a great problem unless you want greater performance below about 100 MHz. The discone itself is big enough to resonate, theoretically, down to about 70 MHz due to it's dimensions. It's interesting to note that the manufacturer specifies a VSWR of less than 1.5:1 above 100 MHz. I cannot see why this antenna could not be used to transmit efficiently down to, say, 75 MHz, given it's dimensions.

Having gotten the construction, the design and the mounting considerations out of the way, it's time to place the metal skyward and see how it performs. Like my review of the ProCone, I will split the test into two parts. First, the on-air comparison between my faithful Tandy discone, and the more 'scientific' sweep test to show up any particular strengths or weaknesses.

First, the 'on-air' test. Simply put, this antenna had comparable performance with my Tandy 16 element discone, maybe a little better at the lower frequencies due to it's larger dimensions. The table shows relative receive performance as shown on my receivers signal strength indicator (RSSI) between the two antennae over a select range of test frequencies.


Just before going over the 'sweep' test of the Benelec, I shall quickly recap how to interpret the results of the printout. A small signal generator is connected to the antenna, which is 'swept' up and down between two frequencies that I nominate. This signal is sent to the antenna, and because all antennae aren't perfect, some of this signal is reflected back towards the signal generator. If this reflected signal is picked off and fed to a spectrum analyzer, we can see just how much signal is being reflected by the antenna at whatever frequency. Looking at the printout, a valley shows that little signal is being reflected, hence the antenna must be radiating well at this frequency. A peak shows more signal being reflected, thus showing a possible loss in efficiency. It can be seen that the first resonant valley exists around about 90 MHz, with another nice wide valley at about 200 MHz. The average level after this second valley is not too high, either, which shows a good 50 ohm match (or thereabouts) across the frequency range of this antenna. Generally, all the desirable characteristics that a well made discone should possess are found in the Benelec 24.

A question that might be posed at this point is: what real advantage does the extra elements offer over an equivalent discone with fewer elements? Certainly, if your discone has too few elements, you begin to run into bandwidth, impedance and directivity problems. The bandwidth would be limited, the impedance matching would not be very inspiring, and the radiation pattern (both for receive and transmit) would not be exactly what you would call omnidirectional. As more elements are added to the discone, the bandwidth and impedance limitations take care of themselves, and directivity also improves. As with anything however, the law of diminishing return begins to creep in, so if you add twice as many elements to a discone, it will not make it twice as good. Design and build quality considerations will yield more performance increase than extra elements, especially at really high frequencies. I've seen performance specifications of military SIGINT discone antennae, with an upper limit beyond 2 GHz. But oh boy, were they expensive! For my money, little advantage is gained beyond 16 elements in a discone for our purposes. But, this antennas price tag is similar to many other discones price tag, so you may as well go for as many elements as your money can buy.

So, in summary, the Benelec is a bit tricky to assemble with those locking nuts being in close proximity, a little off 'true' with some of the elements, is not particularly strong or heavy duty, yet isn't over heavy and is rugged enough for most situations, produces good RF performance and is engineered very well, and hence should last a very long time gave a little regular maintenance. At $145 it represents great value, too, given that most other 16 element discones cost this much or more.

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