Why a capacitor for a shunt fed antenna (Mobile)
 

What is the reason a capacitor is added to a shunt fed antenna. Most
importantly why don't we just use an antenna tuner? Is there really a
big difference...Thank you
Mike

REF: Hamstick 10,20,40 meter

Capacitors are added to match the feeder to the antenna.

What you describe as an, "antenna tuner" actually matches the
TX to the feeder.

(Slightly more than that, in reality, because what is on the feeder
at the TX end depends on what is connected to the feeder at the
antenna end. However, one thing that the device know traditionally
as the antenna tuner doesn't do, is to tune the antenna!)

wrote in message
ups.com...
What is the reason a capacitor is added to a shunt fed antenna. Most
importantly why don't we just use an antenna tuner? Is there really a
big difference...Thank you
Mike

REF: Hamstick 10,20,40 meter

wrote:

What is the reason a capacitor is added to a shunt fed antenna. Most
importantly why don't we just use an antenna tuner? Is there really a
big difference...Thank you

Caps are cheaper than antenna tuners? One disadvantage
of shunt caps is that their reactance goes down as
frequency in increased thus requiring a separate cap
per band. I prefer a shunt coil whose reactance increases
with frequency so it can be left in place. It gives the
proper amount of parallel inductive reactance on 75m,
twice as much reactance on 40m, 4x on 20m, and 8x on 10m
thus having less effect as frequency is increased.

One other way to accomplish the above matching to 50 ohms
is to use a transformer, either a conventional one or a
transmission line transformer (UNUN).
--
73, Cecil http://www.qsl.net/w5dxp


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Airy R.Bean wrote:
However, one thing that the device know traditionally
as the antenna tuner doesn't do, is to tune the antenna!)

However, an antenna tuner at the source makes the
transmission line into a tuned feeder. It tunes the
antenna *system* to achieve a Z0-match that re-directs
all the reflected energy back toward the antenna.

Given a 1/2WL resonant dipole, 1/2WL of ladder-line
is a tuned feeder. 3/8WL of ladder-line plus 1/4WL
provided by the antenna tuner results in a tuned
feeder system.
--
73, Cecil http://www.qsl.net/w5dxp


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Except in the case where the feed point impedance of the
antenna is resistive and an exact match (rare, possibly
unlikely!) to the surge impedance of the feeder.

ie, for the concept of a tuned feeder, there has to be a
standing wave on it.

"Cecil Moore" wrote in message
...
Airy R.Bean wrote:
However, one thing that the device know traditionally
as the antenna tuner doesn't do, is to tune the antenna!)

However, an antenna tuner at the source makes the
transmission line into a tuned feeder.

Michael Mannung wrote:
"What is the reason a capacitor is added to a shunt fed antenna?"

Shunt feeding a radiatir has advantages. It eliminates the base
insulator. Shunt feed acts as a dingle-turn loop. Field of the loop
magnetically induces voltage into the radiator. Attachment point on the
radiator is picked to nearly match the resistive part of the radiator
impedance to the Zo of the transmission line at the connection point.
There is always an inductive reactance at this point which can be tuned
out by using a variable capacitor connected in series with the shunt
feed connection.

The capacitor is simple to adjust, cheap, and nearly lossless as
compared with a coil.

Best regards, Richard Harrison, KB5WZI

On Fri, 21 Jan 2005 16:04:06 -0000, "Airy R.Bean"
wrote:

|Except in the case where the feed point impedance of the
|antenna is resistive and an exact match (rare, possibly
|unlikely!) to the surge impedance of the feeder.

Mutually exclusive.

What you describe as an, "antenna tuner" actually matches the
TX to the feeder.

===============================

A misleading statement.

The TX design impedance is fixed, usually at 50 ohms.

So the antenna tuner matches the variable feeder input impedance to the TX
design impedance.

But no doubt you will manage to squirm your way out of it. ;o)
----
Reg

Airy, you'll be telling us next the SWR meter reads the SWR on the
eedline - whereas it does nothing of the kind.

The confusingly, so-called SWR meter is merely a useful TLI. which Indicates
whether or not the Transmitter is correctly Loaded with its design
resistance. Usually but not necessarily 50 ohms.

It is realised this means re-editing all the handbooks which have appeared
during the last 50 years or so.
----
Reg.

"variable feeder input impedance" = feeder

"TX design impedance" = TX

Why would I wish to "squirm my way" out of an
obvious agreement?

"Reg Edwards" wrote in message
...
What you describe as an, "antenna tuner" actually matches the
TX to the feeder.
A misleading statement.
The TX design impedance is fixed, usually at 50 ohms.
So the antenna tuner matches the variable feeder input impedance to the TX
design impedance.
But no doubt you will manage to squirm your way out of it. ;o)