coiled and folded antennas

[Jamie Morken]

Hi,

For coiled antennas, ie. for a portable AM radio, is the coiled wires
length the critical variable for reception gain? Is the wire length of
a coiled antenna the same as that of a straight wire for the same RF
band? If you take a straight wire antenna, and fold it twice so that
the wire is half as high, and then fold it again so that the wire is 1/4
as high, but the overall length is the same, will the drop in gain be
proportional to the number of folds?

I think if an antenna is folded it might require multiple beats of the
RF signal to pass over it to propagate a signal back and forth up each
of the folds to the receiver, and this results in a drop in gain?

cheers,
Jamie

 

[Tim Wescott]

Hi,

For coiled antennas, ie. for a portable AM radio, is the coiled wires
length the critical variable for reception gain? Is the wire length of
a coiled antenna the same as that of a straight wire for the same RF
band? If you take a straight wire antenna, and fold it twice so that
the wire is half as high, and then fold it again so that the wire is 1/4
as high, but the overall length is the same, will the drop in gain be
proportional to the number of folds?

I think if an antenna is folded it might require multiple beats of the
RF signal to pass over it to propagate a signal back and forth up each
of the folds to the receiver, and this results in a drop in gain?

It's more simple than that: the coiled antenna is an inductor that's
coupled to free space; the important parameters are it's inductance,
radiation resistance, and it's actual resistance. The radiation
resistance is a measure of how well it's coupled to free space, and it
isn't very high -- in fact, it'll be much less than the actual
resistance of the wire, so losses will be high.

Winding the coil with silver wire would help, or using a
high-temperature superconductor (and a non conducting vessel for the
LN2). Best would be a bigger antenna...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Don Lancaster]

Hi,

For coiled antennas, ie. for a portable AM radio, is the coiled wires
length the critical variable for reception gain? Is the wire length of
a coiled antenna the same as that of a straight wire for the same RF
band? If you take a straight wire antenna, and fold it twice so that
the wire is half as high, and then fold it again so that the wire is 1/4
as high, but the overall length is the same, will the drop in gain be
proportional to the number of folds?

I think if an antenna is folded it might require multiple beats of the
RF signal to pass over it to propagate a signal back and forth up each
of the folds to the receiver, and this results in a drop in gain?

cheers,
Jamie

Antennas that are "small" compared to the wavelength are pretty much
dominated by their effective area.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

It's more simple than that: the coiled antenna is an inductor that's
coupled to free space; the important parameters are it's inductance,
radiation resistance, and it's actual resistance. The radiation
resistance is a measure of how well it's coupled to free space, and it
isn't very high -- in fact, it'll be much less than the actual
resistance of the wire, so losses will be high.

Winding the coil with silver wire would help, or using a
high-temperature superconductor (and a non conducting vessel for the
LN2). Best would be a bigger antenna...

But there is the old AM resonance ploy.

Resonate a fairly large loop antenna nearby, and the AM reception
sensitivity will sharply increase.

Sort of a Q multiplier effect.



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[John Popelish]

Hi,

For coiled antennas, ie. for a portable AM radio, is the coiled wires
length the critical variable for reception gain?

No. The area surrounded by the coil is a lot more important.

Is the wire length of
a coiled antenna the same as that of a straight wire for the same RF
band?

No. The coiled length is usually a lot less than the long
wire, uncoiled length.

If you take a straight wire antenna, and fold it twice so that
the wire is half as high, and then fold it again so that the wire is 1/4
as high, but the overall length is the same, will the drop in gain be
proportional to the number of folds?

An antenna is a transducer than connects to the energy
passing through some volume of space. The length of the
conductor is not so important as how the energy is coupled
to it.

 

[Tim Wescott]

But there is the old AM resonance ploy.

Resonate a fairly large loop antenna nearby, and the AM reception
sensitivity will sharply increase.

Sort of a Q multiplier effect.

More like inductively coupling a larger antenna to the loop in the radio.

At 300kHz my understanding is that the best antenna for a receiver is a
one meter probe feeding the gate of a FET. It'll certainly pick up
enough atmospheric noise to overwhelm the thermal noise in a modest
quality receiver.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Bill Beaty]

For coiled antennas, ie. for a portable AM radio, is the coiled wires
length the critical variable for reception gain?

Nope. The critical thing is resonant frequency. In other words,
there
are parasitic capacitors between the close-spaced wires. EM energy
can jump across.

In older AM radios, the ferrite coil antenna is tuned by one section
of the main tuning capacitor. When in resonance with the desired
AM station, the antenna receives lots more RF power than with
a simple untuned coil of the same size.

Is the wire length of
a coiled antenna the same as that of a straight wire for the same RF
band?

No, but it's not that far different. It depends on the spacing of
the turns, and the inductance of the loops formed.

If you take a straight wire antenna, and fold it twice so that
the wire is half as high, and then fold it again so that the wire is 1/4
as high, but the overall length is the same, will the drop in gain be
proportional to the number of folds?

Maybe. Or perhaps it falls as the number of turns squared. You
reduce the gain by making the antenna physically smaller, but you
increase the gain because the Q is higher.

I think if an antenna is folded it might require multiple beats of the
RF signal to pass over it to propagate a signal back and forth up each
of the folds to the receiver, and this results in a drop in gain?

The gain rises. If multiple waves must pass, that means the
peak at resonance is sharper. Higher Q gives better coupling and
larger received power. But a resonant antenna must be tuned to
the desired frequency, not like an untuned quarter-wave antenna.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty Research Engineer
beaty chem.washington.edu UW Chem Dept, Bagley Hall RM74
billb eskimo.com Box 351700, Seattle, WA 98195-1700
ph425-222-5066 http://staff.washington.edu/wbeaty/