Friday, October 20, 2006

In need for some answers

So, here I am again with some problems. As I was saying, I contacted a highschool teacher and we gathered a small group of boys and girls interested in this project. The first problem, building the antena, of course. It is very difficult to find 120 m of wire. So the first question. Can it be made of two or more parts? If yes, how can we make one piece from them? Solder?
Second question. Can we position the antena horizontaly?
Third question. Can it be stored under a roof or must it stay outside in the open?

Thank you

4 Comments:

Blogger Ray Mitchell said...

To answer your 3 questions:
1) Although 1 length of wire is recommended, if you make it from 2 pieces then splice, solder is a good idea and insulate it to protect it from the weather.

2) Antenna cannot be horizontal

3) It can be stored under a roof, inside, etc. Interference is the problem and that is why we find outside a better place. About 10 feet (3+ Meters) away from the computer CRT monitor is usually enough

4:29 PM  
Blogger Doru Dragan said...

Thank you for your answer! Thank you for your metric translation ;-) Our setup will be on the last floor of a 4 etages school and we plan to store the antena up under the roof of the building. We made our first antena 1 m wide and we plan to extend it to 2 m if the signal is too weak. The problem is that we have a very big distance between us and the emiter but we will see.

All the best,

DD

12:44 AM  
Blogger Lorne Davis said...

Antenna size may not be a very big factor in signal-to-noise.

The Stanford receiver has very nice low noise characteristics and seems to have an unusually large amount of spare gain. For my system the signal to noise is almost constant as I increase gain. Perhaps this is because the large amount of external noise for a normal application like an commercial radio receiver is the actual signal to us. We want to see the variability in the transmission environment.

As to antenna size, I considered the calculation of magnetic field strength for a loop antenna. The induced signal in a square antenna is roughly proportional to the area of the loop, L^2 - with L being the loop side, times the number of turns squared, N^2.

So the signal is (N*L)^2. Since the length of the wire in your antenna is 4L*N, the signal to your amplifier is proportional to (length of wire) squared. What really counts is the amount of wire you have, not how you use it.

Of course there must be some limits to this. For multiple layers of turns the mutual interaction of the wires will begin to reduce the sensitivity. I doubt that any of us will need to worry about that. But small antennas with closely spaced turns, particularly multiple layers, may do slightly worse.

I thought about stray electric field noise. That probably is proportional to L*N. So again the effect depends on the amount of wire. Since the magnetic field pickup is proportional to (L*N)^2, more wire would always be better. (At least until the inductance gets large enough to make the antenna-coax cable system self-resonant.)

But antennas are strange beasts, and your local environment may also influence what you find best.

Good luck and best wishes!

6:02 AM  
Blogger Doru Dragan said...

Thank you for your answeer. I will have to read it carefuly as it contains lots of technical stuff. As a quik comment: it's easy for you to say that "antenna size may not be a very big factor in signal-to-noise" because you are (as I see) in Texas :) but we are some thousands of km (miles, whatever) away :( (This is some kind of a joke!)

1:39 AM  

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