[Fwd: LF: LF antennas]

Andre' Kesteloot akestelo@bellatlantic.net
Mon, 07 Dec 1998 21:42:20 -0500



Peter Dodd wrote:

> LF ANTENNAS
> The most successful antenna used on 136kHz is the Marconi as a T or
> an inverted L fed against ground. The performance of this antenna,
> for a given size,  is largely dependent on the quality of the ground.
> The Marconi antenna was invented over three quarters of a century ago
> and is described in The Admiralty Handbook of Wireless Telegraphy
> 1925. (I am sure that there are many earlier publications describing
> this antenna). The only other antenna in use besides the Marconi is
> the single or multi-turn loop, which has been used with varying
> degrees of success.
> This reflector might provide a good forum for those interested in
> experimenting with other forms of antenna. To make a start, Roger
> Jennison, G2AJV, described on the last 80m LF net, his success with
> his Toroidal Antenna on 160m. Roger has described this antenna in [1].
>
> I was Technical Editor at the RSGB in 1994. I felt that such a
> radical antenna, described in [1], could only be published if it
> could be shown to work. I built one but had a few difficulties with
> it until the problems of feeding it were overcome. I then made two
> more and the results were described in [2].
> A double-toroid antenna used as a mobile antenna gave about 1S-point
> less than a conventional centre loaded mobile antenna. EZNEC2
> predicts a gain of a centre loaded mobile antenna of around 0.19dBi.
> If we could get a gain figure of around -7dBi for an LF antenna, then
> big PAs would be redundant!
>
> The toroid antenna is not new. It was first mentioned in Amateur
> Radio Literature by Pat Hawker in Radcom's TT in October 1983[3],
> which described a patent on this antenna by James Corum, K1AON[4].
> I had the good fortune to meet K1AON while visiting a friend Bert
> Weller,  WD8KBW. This antenna was apparently developed at Batelle
> (industrial research company), Columbus, Ohio, to reduce the cost of
> medium wave broadcast station antenna installations.
>
> In describing the toroid antenna K1AON states "by virtue of their
> construction possess a greater radiation resistance than known
> antennas of similar electrical size not having the slow-wave winding
> features possess greater radiation resistance and radiation
> efficiency than loop antennas of similar size".
>
> The patent gives some examples of practical antennas that have been
> claimed to have been built and tested. One of these is a contra-wound
> toroid for the medium wave band and is as follows:
>
> "MEDIUM FREQUENCY VERTICALLY POLARIZED TOROIDAL LOOP
> A 106 turn bifilar toroidal loop of the form of Figure 13 was
> constructed with the following parameters:
> a = 5.95 ft.; b. = .95 ft., S = 4 inches.  The turns were measured at
> the feed point AA' and the results are shown in Figure 22. [these are
> impedance measurements
>  The loop was constructed at a mean height of 3.5 ft. above soil with
> a measured conductivity of 2 millimhos/meter.  The graph shows two
> sets of curves.  One set of curves 2201 shows the feed point
> impedance vs. frequency for the situation where 40 twenty foot long
> conducting ground radials were symmetrically placed below the torus
> at ground level.  The second set of curves 2202 shows the same data
> for the case where the ground radials have been removed.  What is
> interesting is that the conducting ground plane has very little
> effect on the feed point impedance.  This is to be expected if the
> electric current tends to zero and the major fields are produced by
> the magnetic current, 1m'  However, the proximity effect of the
> ground has not been analyzed theoretically   It should be noted that
> the measured velocity factor was Vf = .094 while the theoretical
> value is Vf = .103. This corresponds to a difference of about 8.7%.
> This may be due to the ground or it may be due to mutual coupling
> effects on the bifilar windings.  The theory which was developed
> above was for an isolated single toroidal helix.  It would be
> applicable to multifilar helices if mutual effects are negligible."
>
> NOTE. The antenna letter dimensions are:
> b = the radius of the coil winding.
> a = the radius of the toroid to the centre of the coil winding
> S = the coil winding spacing
>
> No data is given for the radiation performance of this antenna
> compared with something more conventional.
> No description of how antennas are fed is given in the patent.
>
> To be make progress with this antenna we need to know how it works.
> The descriptions and discussions in [1] and [5] are a little bit
> hazy. I have a mathematical description by James Corum, (which is
> beyond me!) which I could e-mail if anyone wanted to investigate to
> toroid antenna.
>
> The Moment of Methods mathematical model, used by most amateur radio
> antenna analysis software does not seem suitable for modelling the
> toroid antenna - but perhaps someone out there has done it.
>
> Is there any possibility of making progress?, or was LF antenna
> design optimised a quarter of a century ago.
>
> REFERENCES
> [1] 'The G2AJV Toroidal Antenna', Roger Jennison, G2AJV, Radio
> Communication, April and May 1994.
> [2] 'Evaluation of the G2AJV Toroidal Antenna', Peter Dodd, G3LDO,
> Radio Communication, August 1994
> [3] 'Toroidal Helix Antennas', Pat Hawker, G3VA, Technical Topics,
> Radio Communication,October 1983.
> [4] The James Corum patent on the toroid antenna is available from
> The British Library, Science Reference and Information Service, 25
> Southampton Buildings, London WC2A 1AW; enclosing a cheque for 10
> and quoting Patent Application EP 0 043591. (check the price - this
> was correct in 1994!).
> [5] The last word, page 93,  Radio Communication, August 1994
>
> --
> Regards, Peter, G3LDO
>
> <g3ldo@zetnet.co.uk>