ANOTHER MULTIBAND WIRE ANTENNA

Transcription

1 ANOTHER MULTIBAND WIRE ANTENNA Above The multiband long wire with balun (cover is off) by Ron VK3AFW. I wanted to build a simple wire antenna dedicated to 30 m and 17m for operation during the 2015 ILLW activity at McCrae. I knew that the relationship of 10.1 MHz and 18.1 MHz was not as convenient as say 7.1 and 14.2 MHz which would enable a basic OCFD to be used. A 10 MHz doublet fed with open wire line would work well but coax cable is required for entry to the operating area as access is via metal framed sliding doors. VK3AFW V1.0 August 2015 Page 1 of 9

2 300 ohm ribbon has proved awkward in the past and been a source of rf in the shack. No combination of n times 10.1 and m times 18.1 will coincide exactly. A compromise was required. I figured a long wire has fewer impedance fluctuations so going to a 3 half wave antenna on 30 m which is only a bit longer than a 5 half wave on 17 m might be able to be tuned to both on my ATU. After fiddling around with a computer simulation I came up with a wire length of m (139 ft) fed at 5.30 m (17 ft 4 inches) from the end. An optimized compromise. This gave the adjacent SWR plot for 200 ohm feed VK3AFW V1.0 August 2015 Page 2 of 9

3 The feed resistance was chosen to be 200 ohms as I could use a 4:1 balun. VSWR minima were at MHz, MHz, MHz and MHz. The SWR at 10.1 MHz was 3.68 and at it was So long as the ATU could handle 4:1 SWR we were OK. A bonus was the good match for 20 m and 15 m. So what about lower down? A new scan was done with the following result. The resonant points were 3.42 MHz and 6.92 MHz. At 7.2 MHz the SWR had climbed to 4:1. So 40 m should be matcheable but 80 m might be hard because the SWR would be about 13:1 at 3.6 MHz. A later check showed no resonance around 160 m. Now what about the rest of the HF spectrum? VK3AFW V1.0 August 2015 Page 3 of 9

4 Well there are SWR dips at MHz and 53 MHz but the ATU would need to match up to 10:1 SWR for 12 m and 6 m and 17:1 for 28 MHz. When the antenna was erected the MFJ tuner achieved a very good match on 3.65 MHz, MHz, MHz MHz, MHz, MHz and MHz. Matching on MHz and MHz was not as good but was less than 2:1 as seen by the IC m could not be matched although connecting the coax in parallel and feeding against a ground was not tried. The near end fed arrangement was very convenient for the location. VK3AFW V1.0 August 2015 Page 4 of 9

5 A 7 m mast was erected next to the building and the short end of the antenna tied off to a stair landing. The long end went to a 6 m mast which lef about 10 m to be run down to a fence. However before cutting wire it was decided to see what (free space) pattern could be expected? On 10.1 MHz the azimuth pattern at the right was obtained. Some gain about, 1.8 dbd, and a couple of nulls. Bending the ends of the wire would reduce the depth of the nulls considerably. VK3AFW V1.0 August 2015 Page 5 of 9

6 The SWR and radiation patterns were all plotted for an antenna height of 8 m. On 10.1 MHz the elevation pattern at the right was obtained. Good for short haul near vertical incidence propagation (NVI) and good for the 500 to 1,000 km range. VK3AFW V1.0 August 2015 Page 6 of 9

7 On 18.1 MHz the azimuth pattern at the right was obtained. A gain of up to 6 dbd in the favored directions and most nulls not too deep to be reduced considerably by a bent installation VK3AFW V1.0 August 2015 Page 7 of 9

8 On 18.1 MHz the elevation pattern at the right was obtained. There is some vertically projected radiation for NVI contacts. Most of the energy is directed out to 600 to 900 km for the first hop. At least the first 3 hops could expect to give good results. VK3AFW V1.0 August 2015 Page 8 of 9

9 The patterns are reasonable as in the expected set position there would be a major lobe toward VK5 and VK6 and one toward VK1, 2, 4 with a useful lobe toward ZL on both bands. There is useful gain of 2 to 6 dbd in the main lobes. On 10 MHz the antenna is a good NVI antenna for local contacts as well as for interstate contacts. With the ends being sloping and one bent, the nulls would have been filled in to a significant extent. A 4:1 current balun was built and this time there was no problem with RF in the shack as has happened with other near end fed designs. The signals on 10 MHz were good from both local and interstate. This was as hoped for. There was little activity on 18 MHz so it was hard to draw any conclusion as to the effectiveness of the antenna. But contact was made into ZL under adverse conditions so the simulation results seem to be supported. The pattern on 20 m resembles that for 17 m and the results were good. Conclusion: The design objective was met, a simple wire antenna covering the 30 m and 17 m WARC bands was built and found to work as expected with the bonus of being useful on adjacent bands. VK3AFW V1.0 August 2015 Page 9 of 9

10 The wire can just be made out running to the light pole. The end was tied off to the fence to the right of the picture. VK3AFW V1.0 August 2015 Page 10 of 9