Presentation on theme: "Let’s Design An Antenna VK3PY"— Presentation transcript:
1Let’s Design An Antenna VK3PY Episode 1A 40m Vertical
2The design brief: High efficiency (i.e. low losses) Suitable for working DXCovers the entire 40m band, preferably without an ATUSuitable for a permanent home installation or portable operationFully self-contained, i.e. does not rely on existing supports (trees etc.)Can be erected by one personMinimal space requirements
3Some options: Half-wave dipole Inverted “L” Random wire Vertical of some sortOf these, only the vertical appears to fit all the requirements of the design brief.
4Any horizontal wire antenna will require at least two supports Any horizontal wire antenna will require at least two supports. More significantly, at 40m a horizontal wire will have a very high angle of radiation unless it is at least half a wavelength above ground (impractical in our case).This contravenes our design requirement of working DXDipole Radiation Pattern
5What about a vertical then? A full-sized quarter-wave would be about 10.5 m high.Almost practical with a squid pole support (mine is 9.6 m high). A loading coil would be required.Has the desired radiation patternNeeds an extensive earth radial system (not really practical)40m Ground mounted vertical
7A ground radial system needs lots of wire to achieve a reasonable efficiency……..
8…….but we can get around this by using only a few elevated radials It turns out that just 4 radials elevated a metre or so above ground give the same performance as 64 radials lying on the ground.The downside is that for a given height, the length of the vertical radiating element is reduced by the height of the radial system. It will resonate at a much higher frequency.
9RadiatorElevating the radial system reduces the available radiator length to 8.4m.l ≈ 8.4mFeed pointElevated radialsh ≈ 1.2mGround
10The antenna can be brought to resonance with a loading coil. The coil must have very low losses (High Q)RadiatorLoading CoilRadialsGround40m Base loaded GP
11The loading coil may be placed anywhere along the radiator’s length The higher up the coil is placed, the better the antenna efficiency will be, but:The operating bandwidth will be narrowerThe coil will need to have greater inductance (i.e. it will be bigger)Adjustment will be very inconvenient if the coil is out of reach
13I chose to place the loading coil so it would be 2m above the ground. This keeps the coil within reach for adjustmentThe radiator length (8.4m) is not very much shorter than a natural quarter wave (10.5m) so little loading is required
14But what about those radials? Each radial is also of a resonant length (1/4 wavelength, or 10.5m)At least four radials are required, and more would be desirableThat’s a lot of wire to put up. The radial system alone would occupy a circular area of over 21m in diameter!
15Could the radials be shortened? Indeed they can. But they would then not be resonant.Each radial would need its own loading coil. DON’T GO THERE!Is there another way?
16But of course………Shortening the radials has the effect of raising the antenna’s resonant frequencySince we already need a loading coil, we can include some additional inductance to bring the entire antenna system to resonanceHowever, the feed point will be RF “hot”. Some means of isolating or de-coupling the transmission line will be required
17A shrunken 40m Ground Plane The final design comprises:a vertical wire radiator 8.4m long with a loading coil 0.8m above its feed pointUp to 8 radials, each only 5.25m long, elevated by 1.2m above groundAn impedance matching/isolating transformer at the feed point
22Matching the antenna to 50Ω The antenna’s feed point impedance is predicted to be 23Ω. This requires transformation to 50Ω to achieve an acceptable match.A good option is a ferrite-core transformer. Being a broad-band device, it does not require adjustment.A transformer also provides the electrical isolation we need between the antenna and feedline.
23Designing the transformer Primary windingNPSecondary windingNSThe ratio of the number of turns on the primary winding to those on the secondary winding is called the turns ratio, N.𝑁= 𝑁 𝑃 𝑁 𝑆
24The impedance ratio is the square of the turns ratio. 𝑍 𝑃 𝑍 𝑆 = 𝑁 2We require an impedance ratio of 50 : 23 = 2.174Transposing the equation above we arrive at:𝑁= 2 𝑍 𝑃 𝑍 𝑆 == 1.474or near enough to 1.5
25So our transformer needs to have 1 So our transformer needs to have 1.5 times as many turns on its primary than its secondary winding.We can’t do “half” turns on a transformer – only full turns.We can use 2 turns on the secondary. That would then require 1.5 X 2 = 3 turns on the primary.
26For the core we can use ferrite sleeves. A suitable type for 100 Watts at HF is the Jaycar LF1258.
28No electrical connection between feedline and antenna
29A clamp-on ferrite block “choke” with 3 turns of feedline passed through it was placed near the antenna to further suppress current on the outer conductor.Tests proved this to be unnecessary.Ferrite block choke
30Test ResultsMeasured feedpoint impedance: 25Ω at resonance (7.150 MHz)The transformer raises this to 57Ω giving an SWR of 1.14Bandwidth remains under 1.5 across the entire 40m bandIt gets out! First QSO was into EA8, Canary Islands with a 5 X 9 report
31Impedance measurement at the connector. The antenna was set up in my backyard. I suspect the slightly higher impedance is due to interaction with nearby structures.TreesGarageTowerNeighbour’s antennaEtc.
32HomeworkRe-design this antenna for use with an 8m long squid pole as the supportModify your design to allow operation on both the 40 or 30 metre bands