Antenna. NO5V Rick Bono
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1 Portable End Fed Half Wave Antenna NO5V Rick Bono October 15, 2016
2 Overview Develop a Portable End Fed Half Wave Antenna Portable and easy to deploy Multiband capability Resonant Antenna No Tuner Needed! Power handling limited by components Ideally run it on 40m through 10m bands For theory and discussion try Steve Yates (AA5TB) website at: Many thanks for Steve for publishing his work. Much of this presentation is based on his findings.
3 Voltage Current Distribution in a Dipole A center fed dipole has it s feed point at highest current and lowest voltage leading to a low feed point timpedance of around d72oh Ohms. A dipole fed on the end has a low current and maximum voltage leading to a high feed point impedance typically around 1800 to 5000 Ohms
4 End Fed Wire Antenna Deployment Deployed as any wire dipole Simplifies feed line arrangement and support requirements Pattern & gain is same as the equivalent center-fed dipole configuration.
5 Two Approaches to the End Fed Antenna Non-resonant Approach Match the high impedance at the feed line with a 9:1 UNUN. Use a NON-resonant length of wire i.e. the wire must not be a length that is a half-wave multiple of any frequency you intend to use. Operates multiband with a tuner (tuner required!) See for the definitive homebrew instructions by the Emergency Amateur Radio Club in Honolulu Resonant Approach (EFHW) Match the impedance at the feed line with a 9:1 transformer. Use a ½ wave length of wire on operating band Operate single band at a time Use a variable capacitor to tune to resonance No external tuner needed as antenna is resonant (i.e., zero reactance) Focus of this presentation
6 Components of a End Fed Half Wave Antenna Radiator length is 1/2 λ of the operating frequency Radiator length is 1/2 λ of the operating frequency. Requires a 0.05 λ counterpoise Transformer is a toroid with N:1 turns between primary and secondary Capacitor should be in range to resonate at the operating frequency with the inductance of the secondary windings. Capacitor should be a high voltage type if running more than QRP levels
7 A Little Math We need to find the Capacitance needed to tune the band of interest. The formula needed is based on a tuned LC tank circuit:
8 Basic EFHW Design Spreadsheet EFHW Matchbox Design Amidon Toroid Parameters Al (T94 2) 84 Min(pF) Max(pF) Al(T80 2) 55 Capacitor (full) Al(T50 2) 49 Inductance (uh) Resonance Frequency (MhZ) Primary Turns Primary Turns Primary Turns Secondary Turns T94 2 T80 2 T50 2 T94 2 Core 2Z 3Z 4Z , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Selected 18 turns primary and 2 turns secondary for 9:1 ratio with T94-2 toroid
9 Capacitor Selection Capacitor must be selected to meet voltage requirement and allow tuning range. A fixed capacitor can be used for single band operation A variable capacitor in the 10pF-320pF range can be used for a multiband version Needs to handle ~650V for 100W operation I am using a NOS Alps from Ebay. Shipped from Thailand for $12.50 (surplus from my magnetic loop experiments) Measures 10pF to 310pF Handles 650V - good for 100W
10 More on Capacitor Voltage Ratings Tablebasedona47KOhmImpedance based a 4.7K
11 Toroid Transformer Amidon is your friend: com/specs/ Tons of data on inductance, sizing etc Good source for Amidon cores is Buy from their website or Ebay Store For a fixed number of turns, the larger the core diameter the greater the inductance. Recommend using a type 2 powdered Iron core for the match Iron Cores such as type 43 will NOT work as inductance of primary will be to high to tune! For 9:1 ratio on a T94-2 core use 18 turns of #22 magnet wire for the secondary and two turns for the primary. Inductance will be ~2.7uH
12 Basic Components & Construction Wind 18 turns of #22 Insulated Magnet wire as primary Wind 2 turns of #22 insulated wire as secondary Connect secondary to SO- 239 Connector Wire primary across Capacitor Terminals and also across a set of banana jacks. Add a suitable knob to the capacitor
13 Testing & Calibrating the Matchbox If adjust your matchbox into a resistive load on the bench first and then adjust the antenna for a proper match then you should have a resistive match that will minimize current tthrough hthe coupler and through the "counterpoise Steve Yates AA5TB. Install a 4kOhm resistor across antenna terminals (for 9:1 ratio). Attach your antenna analyzer and adjust for the first band frequency Adjust knob until reactance goes to zero at that band s frequency. Mark knob position for that band Repeat for the additional bands.
14 Calibration Resistor for other Turn Ratios Za = antenna's resistive impedance then, Turns Ratio = (Za/50) 6:1 for Za = 1800 ohms 7:1 for Za = 2450 ohms 8:1 for Za = 3200 ohms 9:1 for Za = 4050 ohms 10:11 for Za = 5000 ohms
15 Final Trimming for 20m Band For 20m start with a wire 468/freq(MHz) long. ~33 feet on 20m. I am using #22 insulated speaker wire. Cut a counterpoise that is 33/10 = 3.3 ft Attach both to match box and orient antenna as desired (I setup vertically). Antenna wire on the red jack and the counterpoise on the black jack. Adjust knob to band setting previously determined for 20m Connect Antenna analyzer and measure at frequency of interest Trim length of wire until the antenna shows zero reactance at the frequency of interest SWR will drop as you trim resonance is more important than a 1:1 SWR!
16 20m SWR and Xs Chart
17 Antenna deployed ed as a Vertical on 20m Make a simple Common Mode choke by wrapping about six or seven Make a simple Common Mode choke by wrapping about six or seven turns of coax at the feed point.
18 Portable Deployment
19 Stations worked During Initial Testing Tested portable on FT-817ND with Amplifier running 45W SSB on 20m Made 16 contacts during Washington QSO party. Made two DX contacts to Brazil and Costa Rica No real difficulty making contacts Europe comes in clearly when propagation opens up. Was not able to make QSO as pile-ups were difficult to break on 45W Low background noise and good reception
20 Multiband Operation Use barrel connectors and insulators to form links. Engage links to make wire longer or shorter for band changes g g g g then adjust the variable capacitor to the appropriate band
21 Conclusions Basic design goal of an easily deployed, portable HF antenna was accomplished Easy to build and low cost resonant antenna Simplifies deployment Future work: Redo Matchbox to reduce stray capacitance and extend band range Tune additional bands using links
22 Thank Yo!!! Thank You!!! Questions?
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