There is no such thing as a "miracle" antenna or antenna-system (antenna + feed line + tuner). However, some small antennas perform remarkably well - relatively speaking: given the fact that for those of us who use such antennas, the alternative usually is "no antenna".

An antenna that I cannot afford, can't build myself, or that won't fit on my terrace, simply has extremely poor performance (i.e., useless to me), no matter what its gain or radiation pattern is.

A well constructed antenna radiates nearly all input energy. If this were not the case, many antennas would develop hot spots and melt. Note that this has little or nothing to do with antenna efficiency. For an interesting view on the efficiency of small antennas, I highly recommend reading this: "All sorts of small antennas – they are better than you think – heuristics shows why!", Mike Underhill, G3LHZ, February 2008.

Dimensions (sizes, number of coil windings, etc.) provided in antenna designs should be considered as "nominal" values. When building a design, the actual values will depend on many factors.

Your construction: your choice of materials (tubing, fasteners, antenna wire (hard/soft copper, bronze, litze, flexweave, etc.), type and thickness of insulation, type of feed line, associated velocity factors, coil cores, etc.) Note that "Harry's Law of Coils" applies. As Harry (SMØVPO) says:  1) You cannot wind coils like I, and I cannot wind coils like you.  2) Coil-winding data is a constant that varies from person to person.

Your environment: position with respect to "ground/earth" and to nearby conductive and/or capacitive objects in buildings, in & on the ground (electrical wiring, steel reinforcement bars (rebar) in concrete walls and floors, other antennas, masts, fences and railings, rain gutters and down spouts, water pipes, etc.), "living" vegetation (trees, bushes), etc.

Resonant antennas may exhibit (very) high voltages and field strength near the antenna. Keep animals and people (that you care about) away from such an antenna while transmitting.

For an elevated vertical antenna, a single elevated radial may be much more effective than an entire network of radials on (or in) the ground! The radiation pattern will still be omni-directional, but now favor the direction of the radial.

Antenna tuners do not tune the antenna! They are not there to shift the resonance frequency of a resonant antenna to the operating frequency of the transmitter! Its function is to (try to) provide the transmitter with a purely resistive load - typically 50 ohm. That is: cancel out the non-resistive (i.e., inductive, capacitive) part of the load impedance that the transmitter "sees". This enables maximum power transfer from the transmitter to the antenna system.

Did you tune the resonance frequency of your antenna to where you wanted it, but it shifted when you added a current-choke somewhere along your coax? Congratulations: your choke is working - and you needed it (unless, of course, you want the coax to radiate)! But when you tuned the antenna, the coax was part of the antenna. Adding the choke (for instance right at the feedpoint of the antenna), basically isolated the antenna from the coax. This not only changes the impedance of the antenna system, it also makes it electrically shorter: the resonance frequency of the antenna goes up. Unfortunately, you tuned the antenna, and now it is too short... Tune the antenna with the choke already at the antenna!

Keep antennas away from above-ground electrical power lines.

Antenna performance reports that are based on QSO reports, without mentioning time of day, transmitter & receiver locations, and date (sun spots number, geomagnetic storms, ....) are basically meaningless. During a good sunspot maximum, a wet broomstick may work FB.

Antenna analyzers only inject a very small amount of power into the antenna system - nowhere near enough to excite effects that normal operating power may, such as saturation in balun or unun transformers.

A fool with an antenna analyzer tool, is still a fool.

Bandwidth, SWR, and impedance of an antenna systems says nothing about the antenna's efficiency.

Be kind to your coax!

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