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: [email protected] [SMTP:[email protected]]
Sent: 2002/11/04 12:50
To: [email protected]
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 >>
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