WELCOME to my antenna pages! What you will find here, are illustrated descriptions of my never-ending antenna experiments. Basically my annotated "lab notes". I am not on a quest for the "holy grail of antennas" - just trying to find the best antenna system for my particular circumstances, and have some home-building fun & frustration.
Radio amateurs need an antenna - better yet: a shack with an antenna farm!
To get "on the air" you need an aerial or "antenna". It is much more fun (and less expensive) to build your own antenna, rather than buy a commercial one. And as a real radio amateur, you should! OK, I admit that I have sinned and use a commercial transceiver rather than build one myself - but I am redeeming myself by building all my antennas. In my book, if you need a 21-element logperiodic beam antenna and 1½ kilowatt of output power to do "amateur radio", you qualify for a visit to your preferred psychiatrist and possibly a straight-jacket and heavy medication.
Portable antennas, early 1900s...
(source: Radiobote, Vol. 5, Nr. 27, May-June 2010, p. 28)
"No vacation without antenna experimentation!" ©N4SPP, December 2016
"Jeder Hahn braucht seine Henne, jedes Radio 'ne Antenne!"
SOME GENERAL OBSERVATIONS
- The purpose of transmission antennas is to deliver a sufficiently strong signal at the site of the targeted receiver(s) - efficiency and gain are important. A good receiving antenna has a high signal-to-noise ratio - efficiency and high gain are not required, but directivity may be.
- 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! Depending on your situation, most energy may be radiated in the wrong direction, or dissipated in the environment as losses. For an interesting-but-controversial view on the efficiency of small antennas, I recommend reading this: "All sorts of small antennas – they are better than you think – heuristics shows why!", Mike Underhill, G3LHZ, February 2008.
- Efficiency of an antenna that is installed near earth/ground (in terms of operating wavelength) is almost always dominated by ground-losses. These are caused by induced ground currents due to capacitive coupling to the ground.
- 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, litz, flex-weave™, 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, even a single elevated radial may be 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! A "tuner" has two functions: loading and matching. The purpose is to provide the transmitter with a purely resistive load - typically 50 ohm. "Loading" means canceling out the reactive part of the load impedance. I.e., compensate the inductive or capacitive part ( = non-resistive). "Matching" means loading in such a way that the resulting resistive load is 50 ohm (or whatever the transmitter output needs to "see"). 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!
- Top-loading ("end hat") of an antenna can significantly increase the antenna's radiation resistance, and therefore system efficiency.
- Avoid using tinned wire in antennas or feedlines (e.g., cheap coax)
- 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 local conditions, date (sun spots number, geomagnetic storms, ....), etc., 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, or heating of transformer cores.
- Bandwidth, SWR, and impedance of an antenna systems says little or nothing about the antenna's efficiency.
- A fool with an antenna analyzer tool, is still a fool.
©1999-2020 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.