[Lf] [Fwd: LF: RE: VS: Feeding an inverted L-antenna]

[Andre Kesteloot]

Andy talbot wrote:

> Beacause the optimum lowest loss matching network is always the one with the
> lowest possible Q.  And the lowest possible Q is given by an L match.  Here the
> L match is formed by the series inductance which has the dual job of cancelling
> out most of the capacitive reactance.  One this is done whatever real part of
> the impedance has to be matched, which can be done with a little bit more L
> followed by a shunt C to give a required impedance - or more aften as is done
> on these bands the transmitter is then matched into whatever residual impedance
> is presented.  Due to the dominance of the ground resistance, this often turns
> out to be some useable value in the 20 - 100 ohm range.
>
> Any other components added over those required to make a basic match are likely
> to add extra loss.  The one exception to this general rule is where extra
> components are added in order to deliberately broadband match a load impedance.
>   The mathemateically correct procedure for this is probably to start off with
> something like a chebshev network (as in the filters) and go from there - but
> these days designers just plug the requirements into a circuit optimiser and
> press the go button - then trust the results.  Broadband matching something
> that is inherently very narrow band such as an LF antenna is not possible, or
> at least not without incurring considerable losses away from resonance.
>
> Andy  G4JNT
>
> -----Original Message-----
> From:   jan-martin.noding at telenor.com [SMTP:jan-martin.noding at telenor.com]
> Sent:   2002/11/04 12:50
> To:     rsgb_lf_group at blacksheep.org
> Subject:        LF: VS: Feeding an inverted L-antenna
>
> Wonder why seems to be a habit use a series coil to tune out the capacitance of
> the 136kHz aerial, instead of more normal sort of matching - using T- or
> L-network
>
> Below is shown some impedance for a thought antenna (had to enter some radial
> length for the program to work) using ENDFEED.exe provided by G4FGQ
>
>  H.  Height, metres ..   10.0       N. Number of buried radial wires .      2
>  L.  Length, metres ..  150.0       R. Length of radials, metres .....   50.0
>  W.  Wire dia, mm ....   2.00       D. Diameter of radial wires, mm ..    1.0
>  F.  Frequency Kilo-Hz    136       S. Strap length to radials, metres    3.0
>  G.  Ground characteristics:-   Average soil:    R = 100, K = 12
>  H+L+S =  0.074 wavelengths.
>                                    R ohms      jX ohms     Tune out jX with
>  FEEDPOINT INPUT IMPEDANCE:          18.9      -1096.1     1282.7 micro-Henrys
> -------------------------------------------------------------------------------
>                              <-- Transcvr      Shunt L      Antenna -->
>  T-NETWORK L & C values:           142 pF    3666.6 uH       295 pF
> -------------------------------------------------------------------------------
>                              <-- Transcvr     Series X      Antenna -->
>  L-NETWORK L & C values:                     2086.4 uH    3328.4 uH
>  Alternative L & C values:                      656 pF     794.7 uH
> -------------------------------------------------------------------------------
>  LOSS in T-match coil   25.4  % of Tx Pwr      GROUND ELECTRODE SYSTEM, INPUT Z
>  in antenna conductor    3.3  % .. .. ..             DC Resistance    2.9  ohms
>   in soil in vicinity   42.6  % .. .. ..             RF Resistance    7.2   ..
>  in system of radials   28.6  % .. .. ..             RF Reactance     7.0   ..
> -------------------------------------------------------------------------------
>  Overall power radiating efficiency ..   0.05  percent
>  Overall loss relative to ideal system   33.1  decibels or 5.5 'S-points'
>
>  Select H,L,W,F,N,R,D,S,G to change data,  B(egin again) or Q(uit program)
>
> you find the program on the link below
> >========================
> >Free Radio Design Software
> >Go to  http://www.g4fgq.com
> >========================
> 73,
> Jan_Martin
> LA8AK (in JO38XC
> http://www.laud.no/la6nca/la8ak/
> NOTE: Because of heavy spamming now applying eremove.exe, please avoid odd
> titles!
>  << File: ATT00000.htm >>