Building A Simple Wire Beam Antenna. Kurt Kochendarfer, KE7KUS Sacramento Mountains Radio Club

Transcription

1 Building A Simple Wire Beam Antenna by Kurt Kochendarfer, KE7KUS Sacramento Mountains Radio Club

2 OVERVIEW The Problem The Path Signal Math Antenna Candidates Modeling the Antenna Construction Techniques Conclusion

3 THE PROBLEM Need a high-availability HF connection between KE7KUS (DM72) and WD4FMO (FM07) for emergency communication Direct path = 070 / 1508 miles both stations / fixed antennas w/ tree support structure Need 24/7/365 availability for asynchronous operations using FSQ

4 THE PATH SIGNAL MATH Two major components to signal computations Angular computations based on signal hops IONOSPHERE F2 LAYER hmf2 Θ KE7KUS 1508 MILES WD4FMO Single hop path was mandatory Multi-hop paths generate ~10dB signal loss per hop This reduces a 5W QRP (37dBm) signal to the equivalent of 500mW (27dBm)...before any other losses!

5 THE PATH SIGNAL MATH The max F2 ionosphere height (hmf2) is determined by viewing the chart at:

6 THE PATH SIGNAL MATH Evening mid-path hmf2 is approximately 335km (208mi) Daytime mid-path hmf2 is approximately 279km (173mi) IONOSPHERE F2 LAYER 208 MILES Θ KE7KUS 1508 MILES Evening takeoff angle Θ = ATAN (208/754) = Daytime takeoff angle Θ = WD4FMO

7 THE PATH SIGNAL MATH CONCLUSION... We need an antenna with a low takeoff angle to achieve a single-hop F2 contact

8 ANTENNA CANDIDATES Wire Dipole PROS Good antenna for all around performance Easy to construct and install Add frequency-specific elements for multiple bands (fan) CONS Need feed at least 1/2λ high to get low takeoff angles Problematic for 60m/40m operations Partially directional Lobing at higher frequencies causes unwanted nulls Problematic operations above 20m BETTER OPTIONS ARE AVAILABLE

9 ANTENNA CANDIDATES Vertical Antenna PROS Good antenna for low takeoff angles Moderately easy to install Multi-band performance...with a matching system CONS Need to construct robust ground radial system Non-directional Significantly lower gain 1/4λ vertical w/ 32 radials = -7.5dB from dipole Definitely a factor for QRP operations BETTER OPTIONS ARE AVAILABLE

10 ANTENNA CANDIDATES Yagi-Uda Antenna PROS Good low takeoff angle performance w/ height Directivity increases SNR performance x 2 Gain antenna CONS Moderately complex construction & installation Boom/element construction requires specific mounting HOW CAN WE IMPROVE THIS DESIGN?

11 ANTENNA CANDIDATES Yagi-Uda Antenna Build it out of wire Low-cost/light weight construction method Low impact visual signature

12 ANTENNA CANDIDATES Yagi-Uda Antenna Settled on a 3-element 30m KE7KUS station Two element is easy to build, but not as good F/B & gain 30m band is generally good for afternoon/evening contacts regardless of time of year/solar cycle 30m antenna dimensions are manageable Built and installed a dual band 30/17m WD4FMO Two element 30m & two element 17m w/ fan dipole driven element (30m reflector & 17m director) QRP SSB w/ K5WAZ during summer of 2015 QRP CW w/ Portugal, Belgium, Argentina, and Iceland

13 MODELING THE ANTENNA Software Tools My tools are Linux-specific Can be found in most Linux-based Raspberry Pi distros Other tools are available Some cost $$$ but many open-source/free options exist NEC-based programs abound (EZNEC, 4NEC2, etc.) ARRL Antenna Handbook has CD full of software Modeling Software Suite Linux packages yagiuda xnec2c

14 MODELING THE ANTENNA Building a rough draft 'first' from yagiuda package will build a rough antenna file Optimized for VHF+ but works fine for HF We're just looking to get a rough hack at antenna dimensions

15 MODELING THE ANTENNA Building a rough draft Below is the 'first' output file Dimensions are in meters

16 MODELING THE ANTENNA Refining the draft Run the output file through 'optimise' Must read the man page for optimise...many choices! Produces.bes file note the differences from previous slide

17 MODELING THE ANTENNA Putting it into pictures Not so fast...time for xnec2c NEC=Numerical Electromagnetics Code/Method of Moments Built at Lawrence Livermore Labs for US Navy in 1981 Originally written in FORTRAN now ported to C Released to the public most modeling software uses NEC2 NEC3 adds a Sommerfield ground model NEC4 most current adds small (GSM, WiFi, etc.) antenna modeling NEC4 most current NEC4 is proprietary/license required from LL Labs/UC

18 MODELING THE ANTENNA Building the wire diagram The original FORTRAN variant used punch cards The.nec antenna model file reflects this

19 MODELING THE ANTENNA Building the wire diagram Cards define the individual segments of the antenna Also used to define Feed point location/voltage Transmission lines Ground Near field parameters Visit the ARRL website for an introduction on how to model:

20 MODELING THE ANTENNA Eventually you get a picture! FEED POINT

21 MODELING THE ANTENNA Once the NEC model is built, you can generate all kinds of interesting output CURRENT FLOW VOLTAGES

22 MODELING THE ANTENNA Radiation patterns can also be plotted VERTICAL RADIATION HORIZONTAL RADIATION

23 MODELING THE ANTENNA The model needs adjustment SWR 10.03MHz Peak 10.05MHz...but We're 2.5dB below the antennas peak gain at 15 takeoff Raising the antenna will decrease TO angle Gamma match the impedance mismatch

24 MODELING THE ANTENNA Raising the antenna to 20m above the ground significantly reduces the peak gain takeoff angle ORIGINAL NEW

25 MODELING THE ANTENNA 15 takeoff increases to 12.59dBi...a 3.5dB improvement! Antenna peak gain & direction peak gain are much closer Impedance also closer to 50Ω

26 CONSTRUCTION TECHNIQUES Two major construction methods: Triangle/2-point method Only two support lines/trees required 180 reversible

27 CONSTRUCTION TECHNIQUES Two major construction methods: 4-point corner method More stable Requires more trees Tougher to aim properly

28 CONCLUSIONS QRP forces you to engineer good antenna systems! The power delta between a 100W and a 5W radio is 13dB...over 2 S-units...good antennas beat the noise margin A spool of wire is usually cheaper than an amplifier Narrow filters significantly improve SNR for digital/cw Wire beams can be an easy, affordable, simple solution to point-topoint connectivity problems...bi-directional gain & FB ratio Height requirements similar to dipoles still apply for low takeoff Modeling can save a lot of frustration during construction However...it's only a model Lots of software options...pick your favorite and get to know it

29 QUESTIONS? Kurt Kochendarfer, KE7KUS Sacramento Mountains Radio Club