ProCone Discone Antenna's
By Jason Reilly.....
To the uninitiated, some scanner antennas look quite weird, while others are more conventional looking. The more conventional looking antennas are similar in appearance to a UHF two way radio base station antenna; a fibreglass 'stick' mounted on a mast. These antennas often utilise a dipole design, and are made to provide reasonable reception on a range of frequencies. However, you can only go so far with a dipole-type design, and sooner or later, you will run into a limitation.
That's not to say that these type of antennas aren't good; they are fine for scanners operating in what I call the 'basic' bands of 70-85 MHz, 118-176 MHz and 400-520 MHz. In fact, this type of antenna may well exhibit a useful gain in some of the bands. Conversely, it may well be a bit 'lossy' at another. This is the inherent compromise that must be made with such an antenna. The idea is to provide reasonable reception over a very wide bandwidth, which is no mean feat. This is in stark contrast to a normal two way radio antenna which will resonate well on one particular frequency and will perform exceptionally well there, but move away from the design frequency and performance will suffer greatly.
Today's scanner can cover huge amounts of turf - maybe up to 1300 MHz or beyond. No other user places such a heavy and unique demand on an antenna than does the scanner user. The only exceptions I can think of is the military, where SIGINT (Signals intelligence) or COMINT (Communications intelligence) may cover large amounts of spectrum, or communications using frequency hopping or spread-spectrum techniques (both of these being attempts to keep the communications secure, undetected or unjammable) demand antennas that are broadband in nature. The demand that a scanner user places on an antenna for the scanner of today couldn't possibly be met by any single antenna, could it? Enter the discone. This antenna is definitely one of the weirdest looking antennas that you will see in use for scanning. David Flynn had described them in one of his reviews as looking like a randy pair of anorexic spiders caught in the act. It is called a discone due to its construction; a disc on top and a cone underneath. The disc and the cone can be made of solid sheet metal, which realises the greatest performance, or can be approximated by using a 'skeleton' of metal rods.
So, why use a discone? The advantages of the discone are many. Firstly, the discone has a phenomenally wide bandwidth; a good discone can receive with a very even response from its lowest design frequency up to a frequency 10 times higher. Some discones can have a frequency response even higher than that! The discone has a radiation pattern that is omni-directional and is the same as a half-wave dipole over its bandwidth. If you need to transmit on or receive over a number of frequencies, the discone is ideally suited to you. The need to provide a ground plane for the antenna is eliminated, since the discone carries its own groundplane in the form of the cone. But all these advantages come with compromises, however. The discone isn't suited to all frequencies; at HF frequencies, discones can become unmanageably large, and have a practical upper frequency limit as well. The discone itself needs some room to mount, since it is an odd shape. And because of its odd appearance, your ever-curious neighbours may start blaming you for all sorts of things that is happening to their TV/radio/toaster etc. The discone offers absolutely nil gain over a dipole, so don't put a discone up expecting it to drag in signals from far away exotic locations. The fact that it offers no gain may actually be an advantage, especially if you live in a high location or in a metropolitan radio-interference saturated environment.
Let's consider the perfect discone. It should cover 100 KHz to 2000 MHz (the coverage of today's super scanners/receivers), be well constructed, be strong, be made of a solid cone and disc to realise maximum performance, be made out of rust and corrosion proof materials, and have the cone end up in an infinitely fine point, and have the disc sit right on top of the cone as close as possible, without actually touching so as to ensure the best possible performance at frequencies above 1000 MHz. There, the perfect discone. But, there's a few problems with this 'perfect' discone: at 100 KHz, the discone needs to stand about 750 metres high! Hardly practical... Having a discone made with a solid cone and disc at the top of a ten metre mast will act as a sail very nicely in a high wind, which could result in damage. So, to minimise wind loading, and at the same time make the discone easy to package, assemble and mount and reduce weight, the discone should be of the 'skeleton' type, with sufficient elements to retain acceptable performance. Being well made and strong goes hand in hand. If the antenna isn't well made or designed, it may well bend or break in a strong wind, or the elements might vibrate loose, or any number of problems that commonly afflict antennas under adverse conditions. Engineering and material limitations prevent the cone ending in an infinitely small point, and the same applies to the disc sitting as close as possible to this point. So, you can see that the 'perfect' discone can never exist, but the 'close to perfect as practically possible' discone IS possible. Are you still looking for a 'practically perfect' discone? Well, stop right here. I think that I've found what you're looking for!
A company called ProCone are marketing two versions of their ProCone discone, called the ProCone 16 and 32. As you may have guessed, the number indicates the number of elements. The ProCone 16 is much the same as your average 16 element discone except that every other element in the disc and cone is shortened to half length. The ProCone 32 is based on the ProCone 16, but with twice as many elements. The shorter elements are designed to realise some extra performance above about 150 MHz. It's construction is superb throughout, with each antenna being virtually hand made. The antennae have been engineered to survive wind speeds up to 120 Kilometres per hour and to survive in harsh environments like salt air or icing on a mountain top. Marine grade stainless steel is the order of the day for the elements, and each element comes with a locking nut to help prevent the elements from vibrating loose. I've heard stories of other discones without such locking nuts having their elements fall out after the wind had vibrated and worked the elements loose. Who wants to hear thunk-thunk on their roof during a wind storm as the elements decide to part company with the rest of the antenna? Let me tell you, this antenna is VERY strong. The manufacturer showed me the antenna sitting on the ground, supporting a 10 kilogram bag of cement!
As a side story, I've also heard a story of one fellow who had his discone rendered useless by magpies perching on the elements, working them loose, even after using lock nuts and liberal application of loctite! Maybe the answer to this one is to weld the elements in place. I'd like to see the magpies try to shift an element then!
The hub of the ProCone range is unique. It is made of aluminium, nothing unusual about that. The aluminium is also coated to prevent corrosion, which only makes sense. What makes the hub unique is that the coax cable comes already attached to the hub, and it isn't easily removable. In fact, the actual method of attaching the coax to the hub is almost identical to the method used to attach RG58 to a high quality mobile antenna base (you know the ones - like what most CBs and two way radios use). The coax that comes supplied with the ProCone is RG58. The manufacturer also claims that this arrangement causes less loss because the need for a lossy RF connector is negated. The hub also has provision to add a stainless steel whip on top of the discone to help with low frequency response, if needed. The top whip is included as standard. I've heard that the addition of such a whip can compromise some performance in the higher frequency ranges, so if you are considering adding your own whip on top, the best idea is to experiment, see what suits your needs best, to come to the best compromise that suits you. As stated before, the hub comes pre terminated with coax, ten metres of it, and this can be seen as a bonus for the beginner, since you won't have to go out and buy coax to get this antenna 'on air', nor will you have to worry about buying and terminating an RF connector, at the antenna end at least. Herein lies a problem. If you wanted to have a run of coax longer than ten metres, or you wanted to use extremely high grade coax like Heliax, you won't be able to overcome this problem unless you add another RF connector into the chain. Still, this is my only single complaint about the antenna.
All of this sounds impressive, but how does it actually perform, and how easy is it to assemble? The manufacturer says that anyone who follows the instructions can have the antenna assembled in thirty minutes. Now I might be a technician by trade but I still have trouble following instructions written by Japanese which have been translated into a rough approximation of English, and I still have trouble with anything mechanical. One fine example of Japanese to English translation that I've seen went something like: 'once all the verticals have been installed, the horizontal plates are next to be installed. Each other plate has it's upper surface under the lower surface of the other plate, with the left overlapping the right on each alternate plate'. Follow that? Neither did I, so I followed the pictures in this particular case. However, this won't happen in the assembly of your shiny new ProCone. I took delivery of a ProCone 16 and set about to assemble it. Like all technicians, I decided to ignore the instructions and rely on my intuition on how the thing should go together. I wasn't disappointed. Assembly time took about 20 minutes, and all that was required was a small shifter-spanner. Not bad, huh?
So, how does it perform? I decided to give the ProCone an on-air test side by side to my faithful discone (of well known brand and quality with 16 elements) that has sat on my roof for about 3 years. The antennae were mounted at similar heights, and with 4 metres of RG58 coax on the ProCone, this approximated my 8 or so metres of Belden 9913 coax on my usual discone nicely, so that the discones under comparison had a fairly level playing field. I tested a range of frequencies ranging from HF to above 1300 MHz, and the results were interesting. On HF, the two antennae performed pretty much the same. That was only to be expected. Other frequencies I tried are tabulated below.
| Frequency | ProCone Without Top Element |
ProCone With Top Element |
Tandy 16 Element |
| 49.830 MHz | 5 | 7 | 5 |
| 112.600 MHz | 6 Flickering 7 | 7 Flickering 8 | 3 |
| 250.450 MHz | 1 | 1 | 1 |
| 467.500 MHz | 0 Quite Audible | 0 Barely Audible | Barely Audible |
| 945.800 MHz | 1 | 1 | 4 |
All these frequencies are 'on air' all the time at my home, using the signal strength meter on my receiver as a reference. As such, they make good test frequencies. From the above, you can see that the ProCone was equal or better in performance to the more well known discone, no doubt helped by the top element at the lower frequency. Up at 900 MHz, however, the other discone gave the better performance, probably as a result of it's cone ending in a smaller point. As we'll see, the more well known antenna pays more attention in it's construction in providing a more constant impedance. Yet, over most of the test frequencies, the ProCone had the better receive performance.
Having completed the 'on air' tests, I then went off to do a more scientific test on the two discones. By 'sweep testing' the two antennae and comparing the plots side by side, I would have hard, scientific evidence of how much better or worse one antenna is at a given frequency (or so the theory goes). The process of sweep testing involves a small signal generator that sweeps up and down between two nominated frequencies. This swept signal is sent to the antenna to be radiated. All antennae aren't perfect in radiating a signal, and some signal is reflected back to the source. The same happens with a discone, just like any other antenna. If the signal is sent to the discone with a 'directional bridge', any reflected power can be picked off and fed to a spectrum analyser, to see just how much signal is being reflected back from the antenna, which gives a good idea of how efficient the antenna is radiating across a wide range of frequencies, as well as give an idea of how well the impedance is matched. The display shows frequency versus amplitude.
The peaks show that at that particular frequency, energy is being reflected back from the antenna, showing a loss in efficiency, while a valley in the display means little energy is reflected, meaning that most of the energy is radiated at this frequency. I decided to do a plot of my own discone, the ProCone and the ProCone with a 80 cm stainless steel whip attached on top to show the effect this can have. These plots took place between 0-1500 MHz and for a closer look at the areas we are interested in, 50-1000 MHz. From the plots, we can see that the more well known antenna has an average level lower than the ProCone, meaning that the ProCone may not be exhibiting an exact 50 ohm impedance across the range. The well known antenna also has more pronounced valleys, meaning that it probably has more 'spot resonances' where the antenna will radiate very well. The ProCone on the other hand, has fewer valleys, and are shallower. Yet, from the on air tests above, the ProCone would seem the better antenna for receive. An interesting effect of the top element being added to the ProCone is to lower, in frequency, the lowest resonant 'valley' and to make the first few valleys below 150 MHz more pronounced.
So what antenna would I choose? It's all very subjective. It would seem that the ProCone is the better receiver, and the Tandy is the better for impedance matching, and hence, transmitting. Both are still pretty darn good, though. At worst, the ProCone is probably only 6 dB down on the return loss measurement, which equates to maybe one S-point. The ProCone would be the ideal choice for those who spend most of their time monitoring the lower end of VHF (say, below 150 MHz), with no real winner apparent from this frequency up to maybe 800 MHz, where the performance begins to fall away a bit. I also decided to check the transmit VSWR on a few UHF frequencies. In the past, I've found discones to be very good at transmitting, and across a whole swag of frequency ranges, albeit with no gain. Again, comparing my own discone to the ProCone at a few select UHF frequencies revealed that the ProCone is quite suited to being employed for a transmit antenna (it certainly made an improvement over my handheld's 1/4 wave "rubber duck"), so long as you don't try to load up the antenna with kilowatts of RF energy. No transmit power rating is given, however, judging by the design, I'd say that 25 or 50 watts could be handled, no problem. Remember all this time, that the ProCone was only intended to be a receive only device. Really, however, a well designed discone SHOULD be able to transmit as well. The ProCone didn't disappoint me on this side of things.