[Fwd: LF: Elevated radials]
Wed, 25 Feb 1998 08:25:27 -0500
Peter Martinez wrote:
> >From Peter Martinez G3PLX Cumbria.
> Many thanks to John KD4IDY for his comments on this subject. It's
> good to know that others have similar ideas.
> In the last few days I completed the tuning coil to resonate the 12m
> vertical, fed against the radials laid on the ground, and match it to 50
> ohms. It seemed to me that rather than find the precise zero-voltage
> point on the coil and ground that, since there is no current flowing
> through this ground wire, it was not needed. With the whole thing
> floating, it would self-balance itself to the minimum-loss condition,
> with the current to ground from the radials equalling that part of
> the dispacement current from the vertical itself that flowed to ground
> outside the radial mat.
> I therefore just connected the tuning coil (12mH) between the base
> of the antenna and the centre of the radial system, and connected
> the feeder to an insulated link winding near the bottom of the coil.
> The coil was about 250 turns on a 175mm diameter glass bottle (a
> "home brew" tuning coil in fact), and the link winding was 5 turns to
> match 50 ohms. There is no connection between the feeder braid
> and the antenna system. As I found previously, grounding the centre
> of the radial system, or connecting the feeder braid to the radials,
> made things worse, not better, but there was another benefit to this
> "floating" system, in that the noise level on receive was much lower
> than when joining the centre of the radial system to the feeder, and
> much lower than on a longwire antenna which was also fed against
> the house earth. This is consistent with the possibility that
> mains-borne noise is present on the house earth, and gets fed out
> along the coax braid and ends up in series with the antenna (i.e. the
> house has noise on it relative to the antenna, which has the same
> effect as noise on the antenna relative to the house). This would also
> be consistent with what other people have found about the lower
> noise levels on loop antennas compared with transmitting antennas.
> There would therefore seem to be a good case for keeping any
> ground system, whether radials or rods, right at the base of the
> antenna and decoupled from the feeder.
> I have not yet tried raising the radials off the ground, but this will
> have to wait. The next step is to finish the PA and get some QSOs.