Message-ID: <01C28407.92EC1C60.g4jnt@thersgb.net>
From: "Andy talbot" <g4jnt@thersgb.net>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 4 Nov 2002 13:39:17 -0000
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Subject: LF: RE: VS: Feeding an inverted L-antenna
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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@telenor.com [SMTP:jan-martin.noding@telenor.com]
Sent:	2002/11/04 12:50
To:	rsgb_lf_group@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!
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