On 28th October we were delighted to welcome the return of Bob Burns G3OOU. In addition to being an active designer and builder of HF and VHF radio equipment Bob is a retired professional engineer with wide experience in antenna and equipment design, and is an excellent speaker. Bob describes his interest as “Amateur Radio - HF CW Operation, Aerial developments, Heathkit equipment restoration and upgrades. Giving talks on RF Communications topics”. Which brings us neatly to this evening’s event……..
HF Multi-Band Dipole Antenna
Bob started his presentation with a description of his stub-loaded HF band dipole antenna. Experiments had originally started some years back with a classic G5RV and a 10W transmitter, and had rapidly moved on to open wire feeders instead of coaxial cable, with instant improvements in contact rates and signal reports. To address 1.8 MHz matching, loading coils and extra end sections were initially tried with some improvements over the basic G5RV. The original idea for the aerial shown below was found in articles in QEX & CQ magazines and has been further developed for the current location. The length of the top started at 132 feet and has been adjusted to obtain a resonance on 1.84MHz with the stubs in place, and will change with aerial height and location - it is now 125 feet overall. The stubs exhibit an extremely high Q, have little or no effect on bands where they are resonant in odd multiples of a quarter wavelength, and increase the overall electrical length of the aerial on all other bands. A Z-Match or other balanced output ATU is required.
The open wire feeders and the two stubs are made of 16SWG (1/16 inch or 1.6mm diameter) enamelled copper wires spaced 0.90 inch apart with insulators approximately 7 inches apart. The insulators should be UV resistant for maximum length of use in high sunlight locations. The aerial can also be loaded up on 50, 70 and 144MHz with an appropriate ATU (a VHF Z-Match is fairly easy to construct) although more of the radiation occurs from each end than from broadside. All in all, a good multiband aerial.
The original polythene insulators lasted some 5-7 years before becoming brittle from UV exposure. They are being replaced with 8mm diameter acrylic insulators which should last rather longer.
A half size version would work well down to 3.5MHz with an overall length top of 66 feet and stubs of 16 feet 6 inches in length.
Bob then went on to describe the construction of his open wire feeders. These are detailed more fully on his website http://www.qsl.net/g3oou/. The use of such an antenna led naturally on to antenna matching units and here Bob had some interesting ideas, based on the classic Z-Match design. First was to extend the low frequency coverage from 3.5MHz down to 1.8 MHz by switching in an additional inductor below the 80m – 40m coil. The second was the housing of the unit.
Antenna Matching Unit
G3OOU has built a dual Z-Match covering 1.8 - 146MHz in a Heath SB series cabinet with the appropriate metering and switching for multiple transmitters and aerials. Two SWR bridges are used, one for the HF bands and one for VHF. The meter and small knobs are from a Heath SWR Bridge and the front panel has been painted and labeled to match other Heath units.
The VHF section uses a very similar circuit to that of the HF unit although the component values are significantly less, C1 is a single section variable capacitor and HF switch sections S1a and S1b and coils L5 and L6 are not required although S1c and S1d are required. C2 is 35 + 35pF. Note that the minimum value of C1 on VHF should be a few pF so some form of selection may be required and the wiring should be a short as possible. L1 = 2t 1.2inches ID spaced 0.75 inches in total, L2 = 3t 1.0 inch ID spaced 1.0 inch overall. L1 positioned concentrically over L2. L4 = 2t 1.2inches ID spaced 0.75 inches in total, L3 = 3t 1.0 inch ID spaced 1.0 inch overall. L4 positioned concentrically over L3. All coils wound with 16swg enamelled copper wire. The layout of the VHF section is much more critical than the HF section so some adjustments may be required for each constructors own version.
SB-101 and SB-301 Rebuilds and Upgrades
Bob on his website describes one of his passions as “Collecting, updating and rebuilding Heathkit radio communications equipment”. He has several ongoing projects to improve the Heathkit SB-101 transceiver and SB-301 receiver designs to add extra modes and take more account of modern band conditions whilst retaining much of the original architecture. The obvious areas to address included dynamic range, spurious responses, WARC bands, Frequency and Amplitude Modulation, RF and IF selectivity, CW break-in and RIT.
Having used the 7360 beam deflection mixer in an SB-301 rebuild he decided to use those venerable but still very high performance valves as mixers in both the transmit and receive paths and one as a transmit balanced modulator. Bob had brought examples of these projects, which created a lot of interest from the members.
Practical
An interesting feature of most of the equipment Bob brought down was the use (or re-use) of Heathkit cabinets, and Bob doesn’t seem to let an opportunity of acquiring one at junk sales and the like. Newly acquired second hand units have their cabinets washed in the bath to get them clean. Sometimes it being necessary to use a kitchen lemon cleaner and a soft nail brush to remove stubborn grime. Certainly the ones we saw looked new.
Front panel paints are usually Revell or Humbrol - their nearest to "Heathkit green" being No 48. A good primer is required on aluminium. Rub down painted panel with wet and dry paper and flat wooden block.
Front panels of the finished equipment are lettered with dry transfers to give that professional finish. Letraset has restarted the manufacture of dry transfer lettering in white and black. This type of product is also available from Decadry, a Belgian company with UK outlets. Bob not liking gloss finishes on front panels, uses an acrylic clear matt or silk protective spray on top of the lettering.
For new metalwork the chassis aluminium is known as NS4 half hard which cannot be bent as it is too brittle but extremely strong. The G3OOU local supplier is Outlook Stockholders Ltd, Woodcote Grove Farm, Meadow Hill, Coulsdon, CR5 2QQ. Tel: 020 8668 9656. Web URL: http://www.outlookmetalstock.com/ .They can supply a bending grade as well.
Space constrains dictate that this is only a very brief summary of Bob’s presentation and you are urged to visit Bob’s technical site: http://www.qsl.net/g3oou/ for very full details of most of the projects briefly outlined here, circuits of the ATU’s and much, much more besides.
John Longhurst G3VLH
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