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Small Antenna Types
Isotron Antennas:


  • There are two distinct variations and several configurations of the very small Isotron antennas .

  • Both versions of the Isotron consist of a coil and some adjustable capacitance that allows you to resonate the antenna at the target frequency.

  • These antennas work reasonably well as receiving antennas. Transmitting gets a little tricky!

  • Effective transmitting requires a sizeable grounded metal pole, a long heavy ground wire and/or a long length of coax feedline.

  • An graphic showing a typical example from the Isotron web site illustrates this clearly.



  • These can be really difficult to tune. The isotron input impedance and SWR are dependent on the length and routing of both the ground cable and the feed line.

  • It gets worse. If you actually try to use the Isotron as a "stand alone antenna" without the long metal pole or the "grounding cable" connected to the antenna and you install a choke to stop the feedline from radiating back into the shack, the Isotron stops working as a transmitting antenna.

  • Unless an antenna is specifically designed to use the vertical as part of the antenna (such as in an inverted "L") and you cannot install a choke at the antenna feedpoint, you don't really have an antenna. A dummy load will function in a similar manner with the same "accessories" connected.

  • No magic here - these "antennas" are actually resonators requiring large "additional radiators" to transmit effectively.

  • I build several of these. Reception was fair. Transmitting was questionable.



  • Conclusion: The long metal pole or the non-metal tower with the long "ground wire" required for transmitting, does not make this a good choice for a small backyard antenna.




Small Magnetic Loop Antennas:


  • Small magnetic loop antennas are Electrically Short Antennas (ESAs) when they are shorter than 1/4 wavelength.

  • As ESAs, small magnetic loops have the same characteristics as other ESAs: low radiation resistance, high Q, narrow bandwidth and low efficiencies.

  • Optimum size for a "small" magnetic loop transmitting antenna is 1/4 wavelength in circumference.

  • At 1/4 wavelength and with proper construction, efficiencies over 90% are attainable.

  • Even at 1/10 wavelength, a small magnetic loop antenna can function as a good recieving antenna.

  • The high Q, narrow bandwidth and deep side nulls reduces noise.



  • A loop tuning capacitor functions similarly to vertical antenna capacitive top hat: it makes the loop "appear" to be electrically longer, allowing resonance at a frequency lower than the self resonance of the loop.


  • Example of a "small" loop antenna for 160 meters (this 40 ft. circumfrence loop is too small for efficient transmitting - 22% eff.).




  • An optimum 1/4 wavelength 80 meter loop antenna would be about 20 feet in diameter, presenting difficult mounting and stability problems.

  • Attempt to make a compact 80 meter loop antenna proved to be too small in "amp area" to be an effective transmitter for 80 meters. The area inside the circle defined by the loop is the critical dimension here.



  • Conclusion: Although these actually work, an efficient 80 meter "small" loop antenna for a small yard is not very practical!






Electrical Magnetic (EH) Antennas:



  • Typically 1.8% to 4.0% wavelength in size.

  • Uses large cylinders instead of thin wires for the antenna elements.


  • Russian inventor Vladimir Korobejnikov claims that the EH antenna uses a "new concept in radiation".


  • A "unique configuration" of the large cylinders and tuning coils is reported to be responsile for very high efficiencies and wide bandwidth.


  • The claimed high efficiencies are the result of Vladimir making the following change to the Radiation Efficiency formula:

    From the conventional efficiency formula:


    To this very creative but incorrect formula:


  • The term "Rm" (measured resistance) actually includes a small radiation resistance, coil resistance and coax outer shield feed-line impedance.


  • Only the coil resistive losses are included in "Rloss". This makes the efficiency look really great!


  • Vladimir has turned the efficiency equation on it's head, making measured losses an asset! The higher the measured resistance, which includes losses, the higher the efficiency will appear to be.


  • Using this formula Vladimir can claim that these very small EH antennas have efficiencies in the neighborhood of 95%.


  • The very low radiation resistance of an antenna as short as these EH antennas means that even insignificant resistive losses would make the efficiency very low. By ignoring the significance of radiation resistance in determining an antenna's efficiency, Vladimir can make his high efficiency claims.


  • The justification for this change, according to Vladimir, is that,

    "in reality, the radiation resistance is only a mathematical convenience."


  • If we were to use this formula for a dummy load, we would get an efficiency of close to 100%!


  • The wide bandwidth of a small EH antenna is an indication that something is wrong: ground and coil resistive losses are usually to blame. In this case it appears that the inclusion of coax shield impedance is also a factor.


  • Using the conventional formula for efficiency, we can see that this antenna will not be 95% efficient, but probably much much less. A good portion of the power would be dissipated as heat due to the resistive losses in the large coils.


  • A lot of energy is being radiated from the feed line - the real antenna.


  • See W8JI's (Charles Rauch) debunking of the EH antenna. EH Antenna Examined Rauch indicates that the EH antenna is "actually an off-center fed dipole with the shield being the long leg (measured high radiation resistance) and the antenna the short leg."


  • See also what your fellow hams operators think about these antennas: EH Antenna Reviews It's not good!


  • Conclusion: If you go to one of thes EH antenna web sites and see a crazed looking Russian like this:


    and he tries to sell you construction plans or an antenna for 21 rubles, RUN LIKE HELL!





The Spiral Dipole Antenna:


  • The spiral dipole ESA antenna consists of a central radiating element and two end "assemblies".

  • Each end "assembly" consists of a 1/4 wavelength of wire or tubing.

  • Small magnetic loop antennas (and the ones that work quite well) also consist of a 1/4 wavelength of wire or tubing - this sounded promising.

  • I was convinced that I had found the HOLY GRAIL OF THE SMALL DIPOLE ANTENNA?

  • Research shows little technical information available on this antenna - red flag!

  • The Tak-tenna antenna project web site on this by N6PET (Pet Natividad) indicates that he had 100 feet of feedline hooked up even though he was only 15 feet away and that he was getting RF back into the shack. He also likes listening to Elvis - another red flag! For more see: http://www.N6PET.com/Taktenna-antenna-project/

  • Pet also states that, "the combined length of antenna and hookup wire has a significant effect on the antenna’s resonant frequency. The longer the combination, the lower the resonant frequency."

  • Sounds like the coax is the antenna: another feed line radiator!!!

  • The spiral diople is actually a very short dipole with capacitive end hats.

  • Here is what your fellow ham operators think: "lousy antenna"; "bad news"; "the performance is not great"; "horrible antenna", etc.
    See Eham Spiral Dipole Reviews

  • Conclusion: I decided not to build this one. I did, however, try to build a longer end and coil loaded dipole for 80 meters. I could not support the elements with the capacitive loading at the ends, so I improvised: the result was not pretty! I never got it working!






The Super Compact "HD Antenna":



  • I would like to announce the discovery of a new type of antenna: I call it the HD antenna!

  • This small antenna is resonant at 80 meters and fits in the palm of your hand.

  • I haven't tested transmitting on it yet, but I am sure that it will work just as well as the isotron and EH antenna with enough feedline hooked up to it.

  • I am currently taking orders: only $79.99 or 21 rubles.

    Click link to see the HD Antenna Revealled: HD Antenna