LF: 136 kHz antennas and radials

[Andre Kesteloot]

WarmSpgs@aol.com wrote:

> Greetings, Peter and the group.
>
> That is a worthwhile observation about the difference between earth rods and
> radial wires.  License-free 1750-meter beacon operators in the US generally
> follow the practice of mediumwave broadcasters, utilizing buried radial wires
> to minimize loss when displacement current returns through soil resistance.
> However, experiments with radials at or above the surface have shown promise
> for reducing losses further, especially where soil conductivity is poor.
>
> One of the complications in this scheme is interaction between surface or
> elevated radial wires and the ground itself, as reflected in your
> measurements.  Broadcasters have used counterpoise systems in the past, and
> are once again experimenting with elevated radials; thus, a few techniques for
> coping with this problem have been developed.
>
> One approach is to make the antenna and its radial system substantially self-
> contained.  The regulations which apply to our LowFER experimenters pretty
> much preclude any transmission line.  Therefore, when Todd, WD4NGG, tested an
> LF elevated-radial system some years ago, his entire transmitter had to be
> located at the antenna feed point.  He isolated the power and control lines
> with RF chokes, I believe, but also took the step of tuning the radial system
> with a variable inductor between the transmitter case and the tie point for
> the radials.  In this way, he was able to balance out any interaction.  After
> tuning the radial system, making or interrupting a connection with the
> physical earth had no effect on transmitter tuning or radiation efficiency.
>
> Another approach...and one which is more physically realizable in some
> circumstances...is to keep the earth, the counterpoise, and the radiating
> element each at opposite polarity to the others.  The objective is to balance
> the potential of the physical earth between that of the antenna and the
> counterpoise.  Hence, no displacement current needs to flow in the lossy soil.
> I apologize for not being very good with "ASCII art," but if you view the
> following with a monospaced font, perhaps it will make sense:
>
>                ___
>                \ /
>                 |
>                 |
>                  )
>   (variometer)   )
>                  )
>                 |
>                 |
>                  )
>                  )--- (RF feed)
>          ___(I)__)_______________
>          |       )
>          |       )
>        -----     )
> (earth) ---     |
>                 |
>         =================
> (elevated radials or counterpoise)
>
> Connection to the conventional earth system is made by a tap somewhere near
> the center of the balancing inductor.  The RF ammeter symbolized by (I) will
> indicate minimum current when the correct tap point is found.  If RF energy is
> fed to the antenna system by coaxial cable, the shield of the coax is tied to
> the same point as the earth connection.
>
> Perhaps these thoughts will help in planning your elevated system.  One thing
> that has been noted during computer modeling (and confirmed experimentally) is
> the desirability of elevating the radials enough to get away from interaction
> with the soil (i.e., coupling of magnetic fields arising from current in the
> radial wires).  At mediumwave frequencies, a height of greater than 3 to 5
> meters is deemed best.  At lower frequencies, still greater heights would be
> preferred.  One may not be entirely able to achieve the desired elevation at
> LF, but I believe you will probably obtain worthwhile results within practical
> constraints.
>
> 73,
> John Davis  KD4IDY
>
> (G3PLX wrote:) >I have been doing measurements on a 36ft vertical antenna,
>  >interesting result, namely that the losses are LOWER when I use a
>  >set of radial wires over the ground that when I use earth rods. If I
>  >connect the earth rods in parallel with the radials, the losses get
>  >WORSE and this is still the case if I bond the whole system to the
>  >house safety earth. I think this can be explained by the radials being
>  >a higher impedance than the ground rods but this impedance is a
>  >purer capacitance. (etc) >>