2001-10-27
UHF Log-Periodic Array
Presented here is an experimental LPA for the Australian UHF broadcast TV bands IV and V. They span about 520 MHz to 820 MHz. The original design was started when a friend wanted to build a cheap and effective TV antenna for his girlfriend's TV.
The boom/feeder is constructed out of PCB material, two strips of single sided fibreglass substrate, with about '1 ounce' copper cladding. The material is available from DSE in 300mm square sheets, so one fundamental constraint on the design was a 300mm boom length.
This constraint puts Tau at about 0.8 and sigma at 0.11, giving approximately 6 dBi of gain and a fairly flat VSWR profile across the design range. Originally the PCB/transmission line was carefully designed for 75 Ohms, but the dimensions required were mechanically challenging owing to the high dielectric constant of the substrate. 20mm wide strips, glued back-to-back was the final choice, mainly for its excellent mechanical properties rather than any real Zo design point.
Here you can see the boom marked with element spacings and reminders about the phasing of alternating elements. The two largest elements are also seen, like all the elements, about 20mm extra length was cut, and trimmed as the final construction step. The boom is already laminated at this point, common super-glue was used for this step.
| Element | Tip-Tip Length (mm) | Spacing (mm to next element) |
|---|---|---|
| 1 | 343 | 75 |
| 2 | 275 | 60 |
| 3 | 220 | 48 |
| 4 | 176 | 37 |
| 5 | 141 | 31 |
| 6 | 112 | 25 |
| 7 | 90 | 20 |
Eutectic Tin/Silver solder was used, again not for any particular design purpose, it was what was plentiful and on hand. The higher melting point of this alloy compared to normal Tin/Lead solder made holding the shorter elements while soldering them to the boom quite a painful experience. The elements themselves are 1.5mm diameter solid copper wire, from the centre of heavy-duty mains cable.
The array is fed directly by 50 Ohm coax (DSE branded RG-58CU in this case). The feed point is as the 'sharp' end of the array, the braid simply going to one side and the centre conductor to the other. A sleeve bead choke of VHF/UHF ferrites helps maintain balance, as does an air-wound choke coil in the coax. Some experimentation was performed with the placement of the sleeve choke, the final decision was to take the coax along the braid connected side of the boom/feeder and place the chokes beyond the end of the boom. There is no terminating stub. This arrangement produced the cleanest pattern
A short piece of wood dowel extends the boom and provides a mounting surface. Nylon cable ties are used to hold the feeder/boom to the dowel and dress the coax from the feed-point back along the boom.
Performance testing is work in progress. So far the antenna shows excellent balance for such a simple feed. The first null is outstandingly deep (full) and symmetric off either side of the main 60-90 degree lobe. Polarization purity is also excellent, with full nulls being achieved by cross polarization. Tests have were carried out by ear, eye and S-meter, using the local North Head TV translator as a signal source. The antenna appears 'useful' well above and below the design range. No transmitting tests have been performed yet.
Once the feed VSWR has been assessed a new prototype will be constructed out of hobby store brass tube and box stock. The pattern and bandwidth seem excellent so far, so only the matching really needs validation/tuning. The extra expense of the brass material may not be worthwhile, the cheap (but somewhat easily bent) wire is doing an excellent job.
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Updates
- 2009-01-10: Mathias Katzar Builds One
- Mathias Katzar builds an all-brass UHF Log-Periodic antenna.
- 2003-04-14: Tjerk Schuringa PE9ZZ writes