antenna-theory.com

[franzellin]

0) here in europe we have now the DBT (digital television), and you only need a receveir and an antenna and you have a wonderful TV quality

1) I have at home 3 types of antennas , the amplified little dish antenna with a front closed dipole in form of a "8" that can be rotated about 90 degrees
2) the one pole little antenna, 11 cm with a magnetic basis for the ground
3) 1 selfmade little dipole, 2 x 11 cm of naked copper wire 180°

ad1) theoretically the first one should be the best with variable amplification and in another location is effectively the best one
ad2) I tried to connect the little stab antenna to 1 receiver and I had no image till a wounded the tiny cable (only the thin plastic covered antenna cable) to have the form of a "8" and at once the receiver works
ad3) on another receiver, on the same location (1 meter distance), I plugged the nr.1, but nothing either amplified or not, I plugged the second, nothing till I plugged the self made one and this last one works wonderfully

now my questions for the experts:

1?) why a simple dipole selfmade can have a better result than an amplified dish antenna

2?) when we speak about Dipoles I read and looked at all infos in "antenna theory" and I understand the 180 dipole, but I do not understand the classical stab (one pole) 11 cm antenna, where can I find more theory about it and why the modification of the cable layout makes a change in the result?

3?) in the dipole antennas somebody explained that we could build one, a simple vertical one, cutting a coax cable 11 cm naked and leaving beneath other 11 cm without the plastic cover only, it means it is a 22 cm coax cable totally naked the first half and without plastic cover the second half and the rest is coax cable to receiver
I do not understand how it works, compared with the classical 180° dipole, where in the middle there is the connection with the coax cable, please tell me where can I find some theory about it.

thanks for all possible infos - renato

[bigSteve]

For the first question - on amplification - note that your TV already has an amplifier in it. The quality of your signal will be a function of the Signal to Noise Ratio (SNR). When you add an amplifier to the antenna, the amplifier will amplify both the signal and the noise, and in addition will add some of its own noise too. This means your signal to noise ratio is actually degraded after your amplifier. This is commonly referred to as Noise Figure for an amplifier - it always adds noise.

As to moving the cable around, note that the cable is part of your antenna - the currents on your antenna don't know what is antenna and what is cable, so by moving that around you are changing the antenna as well. It may have happened that when you moved it around it just so happened to make a good antenna for the configuration you had.

Don't really follow the 3rd question

[franzellin]

thanks for the fast reply:

ad2?) "the cable is a part of the antenna"
I suppose that is only for thin cables and not for the classical coax cables with
5 elements or not?

about the one pole antenna (11cm) , do you have some references?

thanks

[bigSteve]

For all dipoles, the antenna needs to be fed somehow with a cable. The cable becomes part of the antenna unless it is "choked" somehow with a balun:

http://www.antenna-theory.com/definitions/balun.php

This definitely applies to coaxial cables as well. Do you have a picture of your "one pole antenna"? That might make things more clear.

[franzellin]

1) Thanks, I understood now the problem of Ic, that should be zero

2?) In my selfmade dipole antenna I used a simple double screwterminal to connect the first leg to the center conductor and the second leg to the rolled outerconductor composed of a thin alu foil and net alu filaments.

Now how can the current decide where to flow (inside or outside) if I rolled the outer conductor in the screwterminal?


3) tomorrow I send the photos of the antennas

[franzellin]

here the 3 antenna types that I use
as You see the little one pole Antenna is ca.11 cm long

as written in the website, the antenna should have the ground as a pole, and for this reason they have often a magnetic foot to fix on the metal body of a car for instance
If the foot of the antenna is on a wood surface or fixed with an adhesive, the contact with the ground is lost , at that moment how can the antenna work?

I confirm that the best one is the self made and I am surprised...

thanks