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