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Low-Band Receive Antennas How to hear that great DX that you’re missing on 40, 80 and 160!

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Tonight’s Topics… Introduction Receiving Basics RX Loops Elongated Terminated Loops EWE Antenna Flag Antenna Pennant Antenna K9AY Loop Beverages

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Why do we need separate TX and RX antennas? Because, they have different requirements: TX antennas need to deliver strongest possible signal into target area compared to other antennas. Efficiency and gain are most important factors. RX antennas need to have best Signal to Noise Ratio (SNR) – gain and efficiency are not necessary.

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Gain single Beverage: -11.2 dBi Gain two Beverages (70-ft sp): -8.2 dBi So, a pair of Beverages (with 70-ft spacing) has 3 dB gain over a single Beverage. But, has anything actually been gained in terms of Signal/Noise ratio?

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NO – nothing has been gained! The pattern is still practically identical Front/Back is the same Front/Side is within 0.47dB Unwanted noise is external to the antenna. Because the directivity of the two antenna systems is the same, the Signal/Noise ratio is exactly the same for both. We must use Directivity when comparing RX Antennas, not gain.

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How much Negative Gain can we tolerate with RX antennas? Modern receivers are very sensitive. If you can easily hear an increase in background noise when switching from a dummy load to an RX antenna under quietest conditions, then gain is sufficient. Minus10 to minus 20 dBi Gain is generally fine for most occasions.

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Noise The sum of all unidentified signals (thunderstorms, man-made, cosmic etc.). Requires its own presentation! RX antennas reduce noise through: Directivity Null placement Noise canceling devices Height

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Receive Loop Antennas

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Receive Loops Summary Pros Small, lightweight Easy to build Sharp null in 2 directions

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Elongated Terminated Loops Include Ewe, Flag, Pennant and K9AY Terminated loop produces a cardioid pattern Depth and angle of null depend on loop shape

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Theory of Operation Despite the shape, actually a pair of verticals Feedline on top and bottom gives crossfire phasing towards feedpoint when elements closer than ¼ Lambda Terminating resistor is equal to feedpoint impedance, and ensures equal current throughout Thus, vertical elements have phase difference of 180 deg plus electrical length of connecting wires (slightly more than element spacing) This gives the cardioid pattern

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Feeding Elongated Loops Impedances range from 500 Ohms in K9AY, to 950 Ohms in Deltas and Flags. Important characteristics: Lowest possible capacitive coupling between primary and secondary windings. Low loss, as signals are weak Good SWR if you want to phase loops into an array of loops

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Elongated Loop Summary Pros Small footprint Simplicity Can be phased to improve performance Much better than listening to a vertical!

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The Beverage Antenna!

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Influence of Length Following slide shows EZNEC results for a Beverage with following characteristics: 2 meters high Over good ground 600 Ohm termination 0.55 to 4.4 wavelength 160 M band

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How High? Not as critical as many think General rule: Higher Beverages produce higher output Higher Beverages have larger side-lobes Higher Beverages have a higher elevation angle Higher Beverages have a wider 3-dB forward lobe Laying on ground to 6 meters high is acceptable 1.5 x Antler Height is good idea! 2.5 meters is a good compromise

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Ground Quality The better the ground, the lower the output Ground quality has little impact on radiation angle The poorer the ground, the less pronounced the nulls between the different lobes Directivity remains almost constant Beverage does not work well over salt water

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Wire Inefficient antenna anyway, so size not critical as long as it is physically strong enough Insulated, not insulated – doesn’t matter Pre-stretch soft-drawn copper wire Copper-clad and aluminum wire also okay

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Theoretical Surge Impedance

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Termination Resistor Should be non-inductive Antenna will pick up TX power and lightning surges, so use 2 watt resistor Metal Film and Carbon Film cannot handle surges Use Carbon Composition Use a Spark Gap

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Supports Metal, non-metallic – doesn’t matter as long as antenna is insulated Poles, fence posts, trees, sheds, misbehaving children – whatever is available Do not wrap wire around an insulator Try to keep it straight and level, but minor variations are okay

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Parallel and Crossing Beverages Separate parallel Beverages by distance equal to their height above ground Separate by at least 10 cm when crossing Do not run close to parallel conductors (fences, telephone poles etc.)

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Matching the Beverage Antenna Several different core material/turns combinations available Separate primary/secondary windings advisable I prefer Type 73 Binocular Cores as recommended by W8JI

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Coax Can use 50 or 75 Ohm cable I prefer 75 Ohm cable Works very well (ensure it is good quality cable) Cheap! Easy to attach connectors in the field Easily identifiable as part of RX system – will not accidentally transmit into it Did I mention that it is cheap?

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Grounds One 8-foot ground rod may suffice Will probably need two or more to stabilize the ground system Can supplement it with a number of short radials to form capacitance hat to earth On coax end of antenna, do not ground the coax braid Ensure the coax braid ground is no closer than 5 meters to the ground attached to the transformer

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Phasing Beverage Antennas To improve directivity without using long antennas, can phase individual Beverages Two methods: Broadside End-Fire (or Staggered) Each has its own advantages

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Broadside Phasing Narrows frontal lobe Front/Back remains the same Fed in phase Multiband Require wide spacing 0.5 wl spacing good 0.67 wl excellent!

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End-Fire Phasing Greatly improves Front/Back directivity Front lobe remains much the same Spacing 5 meters Stagger NMT 0.5 wl 20 m for 40 – 160m ant 30 m if only 80 – 160m

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Broadside Phasing

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Crossfire Phasing Simple end-fire feed system developed by W8JI Usable over several octaves Termination value = twice that of single Bev 16:1 matching transformer used (900 Ohms) (X – S)/2 = Y2 Y1 = X – Y2

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Beverage Antennas at VO1NO/VE3 5 acres near Merrickville Dimensions ~ 650 x 320 feet 8 directions using end-fire phased Beverages Control Box in shack, with 3 switchboxes in field

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Property too small? Try a BOG (Beverage On Ground) Termination ~ 200 to 300 Ohms Need a 4:1 matching transformer Use ferrite beads to decouple feedline May require a preamp Beverage’s first antennas were laid on the ground

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For more Information… The “Bible”!! Also check the website of Tom Rauch, W8JI: http://www.w8ji.com Try the Topband Reflector as well: http://lists.contesting.com/_topband/ Joseph Carr’s book also has lots of good stuff.