Hello,

Not an antenna expert, so thought I'd post my questions here, for all you
theorists.
Would appreciate any thoughts on the following, as it really confuses me.

Have a new handheld scanner; GRE 500 model.
Has a rubber duckie for its antenna, which is pretty typical for these kind
of scanners, of course.

Tried listening to an airport control tower (118.5 MHz AM), about 5 miles
from the house.

Here's what I found playing around.

When the unit is held normally, that is with the antenna vertical, the
airport comms from the tower were really poor.
Lots of static and noise.

But, much better when held horizontally.
a. Any idea why ?

I would have thought that their antennas would be polarized vertically
to communicate with planes.

b. any idea how their tower antennas are likely polarized (if at all) ?

c. and this one really confuses me.

The best reception, by far, is if I point the antenna vertically, but
DOWNWARD ! Big difference from vertically up.

Wouldn't vertically up, and vertically down be identical and symmetrically
the same re polarization, reception, etc. ?

Thanks,
Bob

On 14 ene, 14:47, "Robert11" wrote:
Hello,

Not an antenna expert, so thought I'd post my questions here, for all you
theorists.
Would appreciate any thoughts on the following, as it really confuses me.

Have a new handheld scanner; GRE 500 model.
Has a rubber duckie for its antenna, which is pretty typical for these kind
of scanners, of course.

Tried listening to an airport control tower (118.5 MHz AM), about 5 miles
from the house.

Here's what I found playing around.

When the unit is held normally, that is with the antenna vertical, the
airport comms from the tower were really poor.
Lots of static and noise.

But, much better when held horizontally.
a. Any idea why ?

I would have thought that their antennas would be polarized vertically
to communicate with planes.

b. any idea how their tower antennas are likely polarized (if at all) ?

c. and this one really confuses me.

The best reception, by far, is if I point the antenna vertically, but
DOWNWARD ! Big difference from vertically up.

Wouldn't vertically up, and vertically down be identical and symmetrically
the same re polarization, reception, etc. ?

Thanks,
Bob

Hello Rob,

Did you do the tests indoor, or very close to objects?

As far as I know, VHF AIR comms are vertically polarized (ground
station).

Due to reflection and refraction, polarization may change and the
field will no longer be uniform. So the strength and polarization
varies with position of the antenna. This is also valid for various
sources of interference.

Your rubber duckie will be part of the antenna; your hand, arm and
body will be the rest of it. Your hand serves as ground / opposite
pole for the antenna. Probably you will find more spots where you have
better reception (or even no reception).

I live about 16nm SE of EHAM (Schiphol Amsterdam, Netherlands), flat
terrain profile, very thick full wave dipole, 30 ft height above
average terrain. Signals from ground control are strong; signals from
airplanes at airport are reasonable. I use a 3 pole preselector to
attenuate broadcast signals. When you are going to use a larger
antenna, you may get intermodulation problems also.

Best regards,

Wim
PA3DJS
www.tetech.nl

Hi,

Thanks for reply.

Tests were done indoors.
Various locations; similar results.

Best regards,
Bob

------------------
"Wimpie" wrote in message
...
On 14 ene, 14:47, "Robert11" wrote:
Hello,

Not an antenna expert, so thought I'd post my questions here, for all you
theorists.
Would appreciate any thoughts on the following, as it really confuses me.

Have a new handheld scanner; GRE 500 model.
Has a rubber duckie for its antenna, which is pretty typical for these
kind
of scanners, of course.

Tried listening to an airport control tower (118.5 MHz AM), about 5
miles
from the house.

Here's what I found playing around.

When the unit is held normally, that is with the antenna vertical, the
airport comms from the tower were really poor.
Lots of static and noise.

But, much better when held horizontally.
a. Any idea why ?

I would have thought that their antennas would be polarized vertically
to communicate with planes.

b. any idea how their tower antennas are likely polarized (if at all) ?

c. and this one really confuses me.

The best reception, by far, is if I point the antenna vertically, but
DOWNWARD ! Big difference from vertically up.

Wouldn't vertically up, and vertically down be identical and
symmetrically
the same re polarization, reception, etc. ?

Thanks,
Bob

Hello Rob,

Did you do the tests indoor, or very close to objects?

As far as I know, VHF AIR comms are vertically polarized (ground
station).

Due to reflection and refraction, polarization may change and the
field will no longer be uniform. So the strength and polarization
varies with position of the antenna. This is also valid for various
sources of interference.

Your rubber duckie will be part of the antenna; your hand, arm and
body will be the rest of it. Your hand serves as ground / opposite
pole for the antenna. Probably you will find more spots where you have
better reception (or even no reception).

I live about 16nm SE of EHAM (Schiphol Amsterdam, Netherlands), flat
terrain profile, very thick full wave dipole, 30 ft height above
average terrain. Signals from ground control are strong; signals from
airplanes at airport are reasonable. I use a 3 pole preselector to
attenuate broadcast signals. When you are going to use a larger
antenna, you may get intermodulation problems also.

Best regards,

Wim
PA3DJS
www.tetech.nl

When the unit is held normally, that is with the antenna vertical, the
airport comms from the tower were really poor.
Lots of static and noise.

But, much better when held horizontally.
a. Any idea why ?

There are 10001 resons, but airfield communications ARE vertically
polarised. They tend to use discone antennas for VHF and the "biconical
monopole" for UHF. However, they are designed to broadcast to aircraft,
which do not sit so close to the ground and so far away. There is likely to
be an array of different antenna at the control tower and/or transmitter
site, all of which could slightly affect that which you are receiving.

The "rubber duck" antenna is NOT a 1/4-wave antenna at all. It is in fact a
3/4-wave antenna all coilled up, which gives a radiation pattern a little
close to the fictitious "isotropic" antenna. People playing with rubber duck
antennas on handheld gear rarely hold the things vertical, so the 3/4-wave
gives a more uniform radiation in all directions - especially slightly up
and down: more like a cylinder with a hole down the middle, than a donut.
Another point is that the impedance of the 3/4-wave is more constant when
your hands and head are close to the thing, but that is irrelevant for your
application.

Buildings tend to use metal structures, fittings, and wiring, all of which
affect the passage of radio signals through them, especially at VHF. I have
succesfully use existing mains conduit in the ceiling of a rented appartment
as an antenna, by mutual coupling of feed and resonant "current breaks".

Reflections from other buildings can also interract with the signal you
receive. You may think that you predominantly receive the main direct path,
but you would be surprised how often you are receiving reflections from
other sources. Most GSM and all later generation of cellular phone base
stations use polarisation diversity. That is one array of antennas
45-degrees to the left, and another array leaning to the right. All this is
necessary because reflections and movements change polarisation.

Q1: Are you sure that the antenna is vertically polarised?
To be vertically polarised then the end bit should be higher than the bottom
bit, and pointing at the sky. (Suoopid suggestion, I know, but you never
know :-)

Q2: Is the signal very strong? If weak then it could be arriving from any
source. If you building is shielding your then you may be relying upon
reflections from the bus park, aluminized billboard, power pylon, or the
neighbours TV antenna, for reception.

If you really want to receive these stations then use a simple antenna
outside, in a controlled environment, without any unknown influences. Such
an antenna need not be very complicated, in fact it could be built in a few
minutes using a bit of wire and a broom handle.

This link even does all the simple antenna calculations for you, for any
frequency you want.
http://web.telia.com/~u85920178/antennas/vpole0.htm

If you want to receive a specific band with decent sensitivity (weak signas)
then this link will give you some idea.
http://web.telia.com/~u85920178/antennas/6dbvhf0.htm

If you think that you have a weak signal, then see why at:
http://web.telia.com/~u85920178/data/pathlos.htm

Hi Harry,

Great explanations. Thank you.

I will try to build your suggested antenna first chance I get. Will let you
know how it works out.
BTW: is this what's (also) called a "J Pole" ?

Should it be pointed in any specific manner toward the source of what you
are trying to receive ?
e.g.: should a plane formed from the short & long lengths be pointed to the
source ?

Q1: I have no idea how it's constructed, but what I describe as vertical,
the "tube" of the antenna is true up-down vertical.

Q2: Signal is "fairly" strong; it's only the noise that makes it hard to
listen to when held vertically in the normal manner. Much better
horizontally, and vertically downward, like I mentioned.

All my listening is indoors; perhaps as you mentioned this accounts for the
S/N being better horizontally and vertically downward due to reflections,
etc. ?

Thanks again; appreciate your time and advice very much,

Bob

--------------------------------------
"Harry - SM0VPO" wrote in message
...
When the unit is held normally, that is with the antenna vertical, the
airport comms from the tower were really poor.
Lots of static and noise.

But, much better when held horizontally.
a. Any idea why ?

There are 10001 resons, but airfield communications ARE vertically
polarised. They tend to use discone antennas for VHF and the "biconical
monopole" for UHF. However, they are designed to broadcast to aircraft,
which do not sit so close to the ground and so far away. There is likely
to be an array of different antenna at the control tower and/or
transmitter site, all of which could slightly affect that which you are
receiving.

The "rubber duck" antenna is NOT a 1/4-wave antenna at all. It is in fact
a 3/4-wave antenna all coilled up, which gives a radiation pattern a
little close to the fictitious "isotropic" antenna. People playing with
rubber duck antennas on handheld gear rarely hold the things vertical, so
the 3/4-wave gives a more uniform radiation in all directions - especially
slightly up and down: more like a cylinder with a hole down the middle,
than a donut. Another point is that the impedance of the 3/4-wave is more
constant when your hands and head are close to the thing, but that is
irrelevant for your application.

Buildings tend to use metal structures, fittings, and wiring, all of which
affect the passage of radio signals through them, especially at VHF. I
have succesfully use existing mains conduit in the ceiling of a rented
appartment as an antenna, by mutual coupling of feed and resonant "current
breaks".

Reflections from other buildings can also interract with the signal you
receive. You may think that you predominantly receive the main direct
path, but you would be surprised how often you are receiving reflections
from other sources. Most GSM and all later generation of cellular phone
base stations use polarisation diversity. That is one array of antennas
45-degrees to the left, and another array leaning to the right. All this
is necessary because reflections and movements change polarisation.

Q1: Are you sure that the antenna is vertically polarised?
To be vertically polarised then the end bit should be higher than the
bottom bit, and pointing at the sky. (Suoopid suggestion, I know, but you
never know :-)

Q2: Is the signal very strong? If weak then it could be arriving from any
source. If you building is shielding your then you may be relying upon
reflections from the bus park, aluminized billboard, power pylon, or the
neighbours TV antenna, for reception.

If you really want to receive these stations then use a simple antenna
outside, in a controlled environment, without any unknown influences. Such
an antenna need not be very complicated, in fact it could be built in a
few minutes using a bit of wire and a broom handle.

This link even does all the simple antenna calculations for you, for any
frequency you want.
http://web.telia.com/~u85920178/antennas/vpole0.htm

If you want to receive a specific band with decent sensitivity (weak
signas) then this link will give you some idea.
http://web.telia.com/~u85920178/antennas/6dbvhf0.htm

If you think that you have a weak signal, then see why at:
http://web.telia.com/~u85920178/data/pathlos.htm

Hello Bob,

On 14 ene, 19:44, "Robert11" wrote:
Hi Harry,

Great explanations. Thank you.

I will try to build your suggested antenna first chance I get. Will let you
know how it works out.
BTW: is this what's (also) called a "J Pole" ?

Should it be pointed in any specific manner toward the source of what you
are trying to receive ?
e.g.: should a plane formed from the short & long lengths be pointed to the
source ?

Q1: I have no idea how it's constructed, but what I describe as vertical,
the "tube" of the antenna is true up-down vertical.

Q2: Signal is "fairly" strong; it's only the noise that makes it hard to
listen to when held vertically in the normal manner. Much better
horizontally, and vertically downward, like I mentioned.

Are you sure that it is noise, is there some speech or music in the
"noise"? It could be an intermodulation product or just overload to
your receiver from a local FM broadcast station. See what happens
when you listen outside. when it is interference from your own
domestic equipment, the level should drop when outside your house.
Many so called "wide band receivers" give problems when used with a
good outdoor antenna.

Wim
PA3DJS
www.tetech.nl

[rest deleted]

Wimpie wrote:
Hello Bob,

On 14 ene, 19:44, "Robert11" wrote:
Hi Harry,

Great explanations. Thank you.

I will try to build your suggested antenna first chance I get. Will let you
know how it works out.
BTW: is this what's (also) called a "J Pole" ?

Should it be pointed in any specific manner toward the source of what you
are trying to receive ?
e.g.: should a plane formed from the short & long lengths be pointed to the
source ?

Q1: I have no idea how it's constructed, but what I describe as vertical,
the "tube" of the antenna is true up-down vertical.

Q2: Signal is "fairly" strong; it's only the noise that makes it hard to
listen to when held vertically in the normal manner. Much better
horizontally, and vertically downward, like I mentioned.

Are you sure that it is noise, is there some speech or music in the
"noise"? It could be an intermodulation product or just overload to
your receiver from a local FM broadcast station. See what happens
when you listen outside. when it is interference from your own
domestic equipment, the level should drop when outside your house.
Many so called "wide band receivers" give problems when used with a
good outdoor antenna.

Wim
PA3DJS
www.tetech.nl

[rest deleted]

it is not unusal for the transmiters at an airport to produce a huge
amount of interrmod. This is because you may have a dozen or more
transmitters all keying at once and their antennas are closely spaced
often only 4 to 6 ft apart. This is not usally a problem for air
traffic control because the receivers are remotely located from the
transmitters often a mile or more separating the two. On the occasion
this is a problem circulators are placed on the transmitters to
eliminate the intermod'

If you are a good distance from the tx site and are still experiencing
an intermod probllem it is because the mixing action is taking place
in your rx in instead of the transmitters.

Jimmie

Jimmie