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VK3ABK > ANTENA 17.04.06 08:37l 78 Lines 3509 Bytes #999 (0) @ WW
BID : 26799_VK3HEG
Read: DL1BAH GUEST DO1RSR DC9ZP
Subj: Another Antenna Trick.
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<DB0RES<DK0WUE<GB7FCR<VK7AX<
VK3HEG
Sent: 060416/0744Z @:VK3HEG.#WEV.VIC.AUS.OC #:26799 [Ballarat] $:26799_VK3HEG
From: VK3ABK@VK3HEG.#WEV.VIC.AUS.OC
To : ANTENA@WW
Hello Antenna Students.
A recent bulletin (Short Vertical Balun Trick) got me thinking. When do we
disperse more RF in the feed system than we do in the antenna? An antenna
that is a bad match to the feed system will not readily absorb the RF
generated by the transmitter, no matter how much we try. Energy will be
wasted in the transmission system.
I like to think of a transmitter connected to an antenna being like a car
engine connecting energy to the road. We can load a transmitter into a load
via a transmission line, but our load (the antenna) should be able to send
a maximum of energy into space (our road). It should be 'tuned' or have the
capacity to absorb the RF energy.
The diagram below represents a car engine and transmission, and also, with
some imagination, a comparative Amateur transmitter, antenna matcher, and
antenna. The purpose of a car's transmission is to match the high speed/low
torque of the engine to the rear wheels which transfer energy to the road.
This is similar to our transmitter (energy wise) which needs matching to a
load (antenna) so that energy can be transfered to space. Impedance matching
instead of torque matching. So, we have two requirements; a source of energy
that can be converted in a gear/match box, and a transfer system to send the
energy to the road/space load.
In motor racing (especially) it is usual to tune the engine for maximum power
output. A gear box combination then provides a match to the rear axle wheels
and tyres so that they provide maximum coupling to a particular circuit track
conditions. Differential gear ratios, and tyres, are selected to gain maximum
grip on the track.
This is what we do when matching and then loading RF energy of a transmitter
to an antenna and free space. Two actions that are separate but must be
closely related. An antenna that will not 'load' is like car that is under
or over geared for the power available or the road conditions.
A CAR ENGINE AND TRANSMISSION IS LIKE A HAM TRANSMITTER AND ANTENNA
The automotive side The Amateur side
Wheels Antenna
ÚÄÄ¿ Diff Balun ÚÄÄ¿
³ ³ ÚÄÄÄ¿ ³ ³
Road load ³ ÆÍÍÍÍÍÍ͵ ÆÍÍÍÍÍÍÍ͵ ³ Space load
³ ³ ÀÄÒÄÙ ³ ³
ÀÄÄÙ º ÀÄÄÙ
º
º
Tail shaft º Transmission line
º
º
º
Úп
Gear box ³ ³ Match box
ÀÒÙ
ÚÄÄÄÐÄÄÄ¿
Engine ³ ³ Transmitter
³ ³
ÚÄÄ¿ ³ ³ ÚÄÄ¿
³ ³ ÀÄÄÄÄÄÄÄÙ ³ ³
³ ÆÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͵ ³
³ ³ ³ ³
ÀÄÄÙ ÀÄÄÙ
The two actions, tuning and matching, are sometimes interchanged by Amateurs
but they are two effects that must be combined to get the best results. This
is a visual lesson that can be appreciated by a beginner in cars or radio.
73. Dick. VK3ABK.
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