Dimensions for commercial antennas come from a variety of sources. Some the manufacturer provided. Some I found without provenance on websites. Others individuals measured and reported in detail. Still other antennas I measured myself, rounding values to the nearest 1⁄16″. I resolve inconsistent dimensions from multiple sources by picking the value that seems most likely to be encountered in practice.
The designer rounds dimensions for noncommerical antennas or lists them as decimal numbers for the builder to round. Antenna files list element half-lengths since the AO Antenna Optimizer takes advantage of antenna symmetry. For best accuracy, double the values, round to the desired precision, and measure whole lengths. Multiply cubical quad parasitic half-sides by eight, round, allow for splice overlap, and measure total wire length.
To avoid error accumulation, measure element positions instead of spacings. Distance between elements or other conductors is from axis to axis unless otherwise noted.
I use a generalized notion of front-to-back ratio to characterize pattern quality. Conventional F/B is the ratio of forward power to that in the opposite direction. I use the ratio of forward power to the peak power within a specified region to the rear of the antenna, normally the rear half-plane (90°–270° azimuth). When the rear region is 180°–180°, the measure is F/B. For any other region, I call it F/R, which stands for front-to-rear ratio. When not 90°–270°, I note the region limits. To be explicit, I entitle F/R graphs Front-to-Worst-Backlobe Ratio.
I model linearly polarized antennas in free space to maximize generality and to facilitate comparsion among designs. Since ground may affect the azimuth pattern of highly directive designs, sometimes I show F/R curves for various antenna heights. I always model circularly polarized designs over ground since their azimuth patterns depend on antenna height and ground constants. Models over ground are for flat earth with no obstructions.
These generic ground constants are suitable for 98 MHz:
Ground Perm Cond Open ocean 75 4500 mS/m Freshwater lake 80 15 Marsh, unflooded rice paddy 16 191 Rich agricultural land 10 92 Pastoral land, medium hills, forest 9.1 20 Permafrost, winter 8.4 6.8 Mountains; rocky, steep hills 5.9 2.7 Flat desert, cities 2.6 .51
Land values are extrapolated from the Hagn generic curves at 30 MHz.
When the antenna feedpoint coincides with an element, I model the conductor as continuous and place a voltage source at the feedpoint. This procedure sidesteps unknown feedpoint gap detail. For noncoincident feedpoint terminals, I add a jumper wire and place a source at its center. To isolate the model from balun effects, I do not model 300Ω balun leads. They form a transmission line whose impedance is higher than that of the antenna. This adds inductive reactance that alters SWR and mismatch loss. Avoiding this effect makes it easier to predict performance when replacing a long-lead ferrite balun with a halfwave coaxial or L-network balun to reduce loss.
The noncommercial antenna models assume isolated elements that do not pass through a conductive boom or use conductive mounting brackets. Use polypropylene tubing clamps to support elements. Metric clamps are available for 10 mm tubing or as 9.5 mm for ⅜″ tubing. Use clamps with inner ribs to stabilize the element.
If I know their dimensions, I correct for conductive mounting brackets on commercial antennas. I use the YO Yagi Optimizer to derive either a short conductor section whose diameter is equivalent to that of the element plus bracket, or an entire element whose length and diameter are equivalent.
A metallic boom will not detune elements as long as they are spaced a small distance from it by nonconductive mounts. When the boom bisects an element, signal currents induced into each half-element cancel. But the boom can induce current in a nearby phasing line when the conductors are not equidistant from it. This can happen at line crossover and may degrade the azimuth pattern.
A nonconductive boom eliminates induced current and can simplify element mounting. Fiberglass tubing is strong but may be expensive or hard to obtain. PVC or ABS pipe is cheap and readily available. Pipe with thick walls can support small antennas with little sag. Use boom guys for longer antennas. Dry wood also works fine. The residual conductivity of green wood may detune elements that pass through it. Wood may add dielectric loss at high-voltage points if used for loop spreaders. Always weather-treat wood for long-term outdoor use.
Boom length means electrical boom length, the distance between first and last elements.
An orthogonal conductor electrically vanishes only when it is in the antenna plane of symmetry. An offset mast and clamp may couple to a nearby element. Locate the mast midway between elements whenever possible to minimize coupling. This will position a log-Yagi mast next to a phasing line crossover, which will help cancel any line interaction. A centered mast attached below the boom may benefit designs with very low backlobes.
For long-boom designs, locate the mast near the point where torque is zero in steady wind. This is roughly the center of the boom. If this unbalances the antenna, add weight inside the light end of the boom to rebalance it.
Vertically polarized and circularly polarized antennas work best rear-mounted. When that's impractical, use a nonconductive mast section in the vicinity of the antenna to avoid degrading performance.
When the antenna and feedline impedances differ, signal power reflects at the feedpoint, reradiates into space, and is lost. Mismatch also may result in a nonoptimal source impedance that degrades the receiver noise figure.
Conductor loss due to material resistivity normally is insignificant. But it may rise at a band edge where antenna currents increase for many designs.
Forward gain includes mismatch and conductor losses. Modeling results list SWR, mismatch loss, and conductor loss so you can spot unusual values. Listed results include no other losses.
Loss for 300Ω ferrite baluns is about 0.5 dB for small cylindrical baluns with twinlead terminated in spade lugs and intended for indoor use. It is about 0.75 dB for larger outdoor baluns with unterminated copper leads and for indoor push-on baluns with screw terminals. Loss for a 300Ω RG-6 halfwave coaxial balun is < 0.15 dB and for an L-network balun, < 0.2 dB. Loss for a 75Ω RG-6 coiled-coax choke is 0.05 dB.
I have seen up to 0.5 dB loss for a ferrite power combiner, but typically it is 0.3–0.35 dB. Loss for a junction splitter followed by 22″ of 54Ω Belden 8219 RG-58A/U to transform 37.5Ω to 75Ω is about 0.1 dB.
Plastic end caps may cause dielectric loss due to the high electric field at element tips. This is a better idea.
Loss for RG-6 feedline is about 2 dB/100′. Check the manufacturer's data for your cable for a more exact figure.
A short wire can have significant reactance at VHF. Use the shortest possible leads for coax connections and feedpoint capacitors. Bundle split coax leads closely together to minimize the impedance bump. Insulation or waterproofing material may add shunt capacitance. Stray feedpoint reactance can degrade SWR and increase mismatch loss. Plastic end caps may detune elements by dielectrically loading them. Do this instead.
Use galvanized, cadmium-plated, or stainless steel hardware. Use an antioxidation compound, such as Noalox or Ox-Gard, at electrical junctions to prevent corrosion. Bimetallic washers with copper and aluminum surfaces can inhibit corrosion due to dissimilar electrode potential. Weatherproof joints, but avoid silicone sealants that smell like vinegar. Their acetic acid will corrode metal.
Due to their specialized element mounting brackets and phasing lines, attempts to duplicate commercial antenna designs using substitute parts may not be entirely successful.
Adhere closely to the dimensions and construction guidelines for noncommercial designs. You can substitute conductor material of equal diameter, but avoid bimetallic corrosion and high resistivity (chrome plating and stainless steel are rather lossy). ⅜″ and 10 mm tubing diameters differ by just 5% and are interchangeable for wideband designs.
You can substitute 6063-T5 aluminum angle from Home Depot for ⅜″ or 10 mm elements. The 0.5″ × 0.5″ right-angle shape is electrically equivalent to a 0.4″ round conductor. For accurate conductor spacing, mount all elements with the angle oriented the same way.
Use a vector network analyzer to measure antenna impedance and SWR. Use the 75Ω menu option to calibrate with a 50Ω load, or calibrate with a 75Ω load. Use a test cable with a common-mode choke and calibrate at the far end. Compare measured values with those from the antenna model to verify construction.
To avoid ground effects, point a directional antenna straight up with the rear element a few feet off ground. Remove any nearby conductors when making a measurement, including yourself.
The azimuth pattern of a vertically polarized antenna can be measured near ground, but that of a horizontally polarized antenna is best measured at installation height. Use signals from local stations.
Though mechanically challenging, stacking horizontally polarized antennas side by side works better than stacking one above the other. Vertical stacking causes high mutual coupling that detunes elements and creates large backlobes. Moreover, elevation patterns differ for antennas at different heights, which significantly reduces stacking gain from free-space values. Finally, horizontal stacking greatly narrows the forward lobe, which can reduce interference.
Horizontal stacking for maximum forward gain generates large secondary forward lobes. A reasonable trade-off is to stack so that these lobes are down 20 dB at 98 MHz. This yields 2.0 dB stacking gain for typical feeder and ferrite power combiner losses and approximately half the beamwidth. For this trade-off,
d = 76.3 + 0.415g²
where d is boom stacking distance in inches and g is forward gain in dBd for a single antenna at 98 MHz.
To avoid degrading performance, the crossboom should be nonconductive and the feeders should drop vertically for several feet. Beginning at the feedpoints, install current chokes at 30″ intervals to help decouple the feeders.
Unless its elements bisect and are orthogonal to the FM antenna elements, mount any antenna designed for a frequency between 50 and 150 MHz on a separate mast. Outside this range, mount the booms at least ten feet apart on a single mast. Closer spacing may degrade the pattern of designs with very low backlobes. Whenever possible, model the antenna combination instead of relying on these conservative guidelines.
Most of the noncommercial designs have little or no design margin at the upper end of their operating range. Rain may affect performance and ice can severely degrade it.
Sensitivity analysis evaluates the robustness of a design to construction errors. To each symbol the modeling program adds and subtracts a tolerance. Listed are the largest forward gain and F/R degradations over the stated frequencies. The numbers can suggest when a particular measurement needs extra care.
I have standardized on a tolerance of 1 mm (0.04″) to facilitate design comparison. It should be possible to construct an antenna to this accuracy using ordinary hand tools. Symbols that represent element half-length use half the standard tolerance since the antenna builder can and should measure the full length.
Sensitivity analysis varies one dimension at a time and does not address accumulated performance degradation. To maximize antenna performance, measure all dimensions as accurately as possible.
88–108 MHz