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**Let’s Design An Antenna VK3PY**

Episode 1 A 40m Vertical

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**The design brief: High efficiency (i.e. low losses)**

Suitable for working DX Covers the entire 40m band, preferably without an ATU Suitable for a permanent home installation or portable operation Fully self-contained, i.e. does not rely on existing supports (trees etc.) Can be erected by one person Minimal space requirements

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**Some options: Half-wave dipole Inverted “L” Random wire**

Vertical of some sort Of these, only the vertical appears to fit all the requirements of the design brief.

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**Any 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 DX Dipole Radiation Pattern

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**What 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 pattern Needs an extensive earth radial system (not really practical) 40m Ground mounted vertical

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**Ground mounted radials are a deal-breaker……..**

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**A ground radial system needs lots of wire to achieve a reasonable efficiency……..**

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**…….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.

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Radiator Elevating the radial system reduces the available radiator length to 8.4m. l ≈ 8.4m Feed point Elevated radials h ≈ 1.2m Ground

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**The antenna can be brought to resonance with a loading coil.**

The coil must have very low losses (High Q) Radiator Loading Coil Radials Ground 40m Base loaded GP

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**The 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 narrower The 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

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Loading Coil Radials Ground

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**I chose to place the loading coil so it would be 2m above the ground.**

This keeps the coil within reach for adjustment The radiator length (8.4m) is not very much shorter than a natural quarter wave (10.5m) so little loading is required

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**But 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 desirable That’s a lot of wire to put up. The radial system alone would occupy a circular area of over 21m in diameter!

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**Could 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?

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But of course……… Shortening the radials has the effect of raising the antenna’s resonant frequency Since we already need a loading coil, we can include some additional inductance to bring the entire antenna system to resonance However, the feed point will be RF “hot”. Some means of isolating or de-coupling the transmission line will be required

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**A shrunken 40m Ground Plane**

The final design comprises: a vertical wire radiator 8.4m long with a loading coil 0.8m above its feed point Up to 8 radials, each only 5.25m long, elevated by 1.2m above ground An impedance matching/isolating transformer at the feed point

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**40m Loaded GP 8 Short Radials**

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**The loading coil 10 turns of 2.5 mm dia. enamelled copper wire**

Coil dia. = 65 mm L ≈ 5.2 μH Q > 800 Coil Calculator

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**Matching 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.

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**Designing the transformer**

Primary winding NP Secondary winding NS The 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

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**The impedance ratio is the square of the turns ratio.**

𝑍 𝑃 𝑍 𝑆 = 𝑁 2 We require an impedance ratio of 50 : 23 = 2.174 Transposing the equation above we arrive at: 𝑁= 2 𝑍 𝑃 𝑍 𝑆 = = 1.474 or near enough to 1.5

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**So 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.

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**For the core we can use ferrite sleeves.**

A suitable type for 100 Watts at HF is the Jaycar LF1258.

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**Primary winding (red wire) 3 turns**

Secondary winding orange wire) 2 turns

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**No electrical connection between feedline and antenna**

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A 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

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Test Results Measured feedpoint impedance: 25Ω at resonance (7.150 MHz) The transformer raises this to 57Ω giving an SWR of 1.14 Bandwidth remains under 1.5 across the entire 40m band It gets out! First QSO was into EA8, Canary Islands with a 5 X 9 report

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**Impedance 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. Trees Garage Tower Neighbour’s antenna Etc.

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Homework Re-design this antenna for use with an 8m long squid pole as the support Modify your design to allow operation on both the 40 or 30 metre bands

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