Evaluating three CB base station aerials from the Sharman Storm range as well as some background information on the Ofcom EM Field Exposure Regulations
CB Storms and EMF Measurements
This month, I have had the opportunity to review three Sharman aerials, courtesy of Chris Taylor at Moonraker. The models under review are the Sharman Storm 100, Storm 200 and Storm 500 base aerials (Figs. 1 and 2).
Fig.1: The Sharman Storm 100 model on the ground.
Fig. 2: The Sharman Storm 500 CB aerial during assembly.
All three aerials are mechanically very similar and share a common design, with just the length of the vertical element changing between the models.
The supplied parts consist of the main radiating element, which is fixed to a sturdy base hub, three radial elements, one of which has a matching section fitted to it, a 200mm tube/sleeve that fits onto the base secured by an M6 A2 stainless steel bolt, two clamps for mounting the sleeve to a mast, and two 2mm and 3mm AF Allen Keys (Fig. 3). There were no assembly instructions.
Fig. 3: The hardware packs included
The aerials look well made. The vertical section consists of white GRP and the radials and fittings are made from stainless steel with the mast clamps made from aluminium extrusion.
Assembly is straightforward on the 100 and 200. Screw-in the three radials to the base and lock into place by tightening the nut; slide the mounting sleeve over your coaxial cable and fit a cable terminated with a PL259; then put the sleeve over the base and screw in the M6 which holds it in place; after this, fit the two aluminium clamps into place on the sleeve and tighten the Pozi screws; fit the two ‘U’ bolts and attach the whole array to your mast.
The 500 is slightly different, as the vertical element has been spilt into two sections, and there is a join where the two halves of the radiating element are fixed together, secured by a 2mm grub screw. Users of aerials such as the tri-band Diamond V2000 will be familiar with this.
The element in the top section may need to be pulled down the tube to expose the wire. It is a simple matter to tighten the grub screw once both pieces are together. The top GRP tube is then pushed into the ferrule; following this, the locking ring and compression washer on the top section are screwed into place to secure the two sections and make it watertight (Fig. 4).
Fig. 4: The joint on the Sharman Storm 500 CB aerial.
The next operation is to tune the aerials. This is done by loosening the grub screw at the end of the matching section and adjusting for minimum SWR by sliding the radial in or out. This can be done through ‘trial and error’, whilst raising and lowering the aerial as needed (Fig. 5).
Fig. 5: Mounting arrangements and tuning stub.
I do not have any equipment capable of transmitting on the CB bands. Nonetheless, I tried each aerial in turn and spent several hours listening, spread over a couple of months, making my evaluation with the aerials clamped to my temporary mast.
CB activity around here is not great but there were a few ‘regulars’ that could be heard.
I found that out of the three, the Storm 500 was, understandably, the better model. One operator, known to me only as ‘Mike’, was consistently around S2-3 on the 100, a steady S3 on the 200 and S5 on the 500. Another more local – and therefore stronger – station reading over S9 was consistently the same strength on each model.
I found all three to be very lightweight. The 100 and 200 models are certainly suitable for mounting on modest supports, while the 500 may need something a little more substantial.
Adjusting for best SWR was easily achieved on the 100 and 500 using my FA-VA5 Analyser. I found that the minimum SWR was as follows: 100=1.6:1, 200= 3.3:1, 500= 1.2:1.
As you can see, the 200 minimum was rather high (Fig. 6 [middle plot]).
Fig. 6: SWR Plots: Top: Storm 100; Middle: Storm 200; Bottom: Storm 500. Span 27-30Mhz.
Unless space or visual impact is an issue, I would go for the 500. However, whatever model you choose, all are well made, easy to assemble, and should last for years. I feel that the omission of any instructions needs looking at. Experienced hands will have no problems, but newcomers may be left wondering what to do.
Other than that, I think that all these aerials represent great value. The Storm 100 costs £39.95, the Storm 200 retails at £49.95, and the Storm 500 will set you back £69.95.
Below offers details of specifications and tools required when putting these together.
Table 1: Sharman Storm Aerials: Technical Details and Tools
• Frequency: 26-28MHz
• Max Power: 30W (100) 50W (200&500)
• Length: 1M(100) 1.5M(200) 2.2M(500)
• Gain: 100 0.5 dB 200 1.5 dB 500 2.5dB
• Max radial length: dependant on matching
• RF Connector: SO239.
• AF spanners: 8, 10, and 24mm.
• Pozi screwdriver.
• 2mm Allen key (supplied).
My (and the editor’s) thanks go to Chris Taylor at Moonraker, for the extended loan of the review items.
Readers may be aware of OFCOM's introduction of regulations limiting the amount of RF exposure to members of the public based on guidelines set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP).
This affects all users of the radio spectrum. Coming into force on frequencies above 10MHz in May 2021, and below this by May 2022, all users including radio amateurs are affected. CB is ‘exempt’ as power is (legally) limited to 4W. Ofcom states that, “All spectrum users that are subject to an Electro-Magnetic Field [EMF] condition, and whose radio equipment is currently authorised to transmit at powers higher than 10 Watts Effective Isotropic Radiated Power [EIRP] or 6.1 Watts Effective Radiated Power [ERP], are required to take this Guidance into account, in order to ensure they comply with the EMF condition. A failure to take this Guidance into account can constitute a breach of an EMF condition and may result in Ofcom taking enforcement action.”
This is not the place to go into the arguments or technical detail.
However, as the aerial type and its position will influence resulting EM fields, I think I would be remiss if I did not cover the subject, if only briefly.
First, for those of you who only listen, you will not be affected. I think, however, that it would be useful for listeners to be aware of the regulations so they are well informed should they have to face questions from a neighbour or other party that may believe that an aerial used only for receiving presents a radiation hazard.
For the licensed radio amateur, steps will have to be taken to ensure that the station is ‘compliant’. Remember that UK license conditions already state that licensees take safety precautions against RF radiation. The RSGB presently has a simple spreadsheet available, based on the OFCOM one (currently limited to 10MHz and above), to calculate compliance.
Details of frequency, transmit power, mode, per cent of transmit in six minutes, cable loss and aerial type are entered. Then a compliance distance will be calculated, and the amateur must ensure that no ‘members of the public’ can be within this distance from the aerial (the ‘exclusion zone’). ‘Members of the public’ includes family members, visitors, neighbours, workmen, idle passers-by – in fact, anyone who is not a radio amateur!
Calculations need to be made for each band/station configuration.
Once done, keep records of the calculations and hopefully forget about them, at least until a change in station configuration, at which point a new evaluation will have to be made.
At present, the calculator includes only a few aerial types. If an aerial type is not currently listed, an operator has two choices: either to change the aerial to one of the types listed in the spreadsheet or to undertake some calculations and modelling of their own aerial and enter the details into the spreadsheet.
I understand that models made with the EZNEC Engine software suite are acceptable.
If a station is found to be non-compliant, changes will have to be made.
As this was being written I have seen little guidance for mobile operation. Passengers in cars will most likely be within an exclusion distance, and how does an amateur operate when stuck in traffic jams? Mobile operation is essentially a ‘dynamic’ operation so how does an amateur demonstrate compliance?
I have seen concerns that the introduction of these measures may give local authorities more leverage to refuse planning applications for amateurs based on RF emission levels, and that it might make things easier for neighbours to take legal action against an amateur on the grounds of 'damages' caused by exposure to RF fields.
My view is that, by the use of a spreadsheet, an amateur is now able to demonstrate that their station is within the exposure limits set by Ofcom and so there should not be any issues. Ofcom is the UK regulator, and if a station is within the limits set by it, one would hope that there would be no case to answer.
RF field strength is not a local authority planning issue, but if an applicant can show that their station conforms to the regulator's requirements then it should kill off any objections to a planning application based on emission level. However, some amateurs may have to make alterations to their stations, to keep within the limits. To achieve compliance, this may mean reducing power, changing aerial type or position, setting up ‘exclusion- zones’, altering operating habits, or a combination of all of these things. For example, at the moment, my sloping 66ft doublet, ending at my boundary fence at 6ft high, is unlikely to comply when running 100W. However, raising it to a horizontal position will make it compliant (on 20m and above).
I imagine most amateurs running 100W or so on HF should be able to comply.
Another note is that, up to now, unless an amateur was pushing powers around the legal limit, accurate power measurement was not necessarily a concern. Now, however, demonstrating that you are running the power you say you are, can make all the difference if a ‘visit’ ever takes place.
Calculating EMF levels can be difficult because nearby objects can and do affect an aerial’s properties. See the extreme case model in Fig. 7, where power is concentrated in one direction from a 40m Vertical.
Accurate ‘real-world’ assessment, as would be made by Ofcom, requires expensive test equipment – probably using a calibrated antenna with a known gain curve. It will follow a predetermined measurement method, which, I imagine, is out of the reach of most amateurs.
Fig. 7: Extreme example of a 40m vertical next to the simulated screened metal wall of an industrial unit. Top: No screen with typical radiation pattern; Bottom: screened (deformed) pattern.
Avoid cheap handheld ‘EMF detectors’ from e-Bay, Amazon, and so on.
I hope that, by using the Ofcom/RSGB spreadsheet, amateurs can show they have acted in ‘good faith’.
If a station inspection did take place, and the levels based on the spreadsheet were found to be over by real-world measurement, we would hope, Ofcom would take the same ‘pragmatic’ approach which – if the statement on the Royal Yachting Association (RYA) is to be believed – is being offered to that group, in terms of potential enforcement action.