Zener_Diode

Hey
as much as the freq is low the antena should be longer. l=c/f (c=3*10^8)
if I take a freq of 2Mhz the antena is about 150 meters.
I hear about the option to connect a serial inductor to the antena and then you can choose the antena long you want.
how can I calculate this inductor????

thanks ahead....

Nigel Goodwin

I suggest you consult some Amateur Radio books, by either the ARRL from the USA or the RSGB from the UK - both publish excellent books which will tell you all you need to know.

I presume you are aware that using a shorter aerial will drastically reduce performance?, essentially all it's doing is matching the impedance of the radio (usually 50 ohms) to the impedance of the aerial.

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Zener_Diode

nope, the antena have 2 length. one is a lenght in meters, cm and so on.
and another is electronic length, L(electronic)=l(meter)/0.95.
as I know, you can add a serial inductor to antena that simulate a longer electronic lenght.
and it doesn't metter what is antenas impedance.

Nigel Goodwin

It's the impedance which is critical, that's what the series inductor (or other components) are for - simply to match the aerial to the receiver. It's also basically what ATU's (aerial tuning units) do, one of those would probably be most useful for you.

Buy the books and try reading about aerials!.

__________________
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Zener_Diode

In the AM radio 530Khz - 1600Khz. the antena lenght is about(566 - 187.5 meters) did u ever see tuner with antena more then 20cm????
if u ever open the tuner, u saw an inducor coiled on a black material. it's material is s special unmagnetic material that can change its poles very fast. This part in the radio replace the lenght of the antena.

Nigel Goodwin

It's called a ferrite rod aerial, and only works at fairly low frequencies, they have two windings for each band, the main tuning winding, and a secondary coupling winding - which matches the input impedance of the receiver. However, in this type of receiver the input impedance is almost certainly NOT a standard 50 or 75 ohms, there's no need for it to be.

Another type of small medium wave aerial is a loop aerial, again it's tuned to the incoming frequency as part of the radio tuning, and coupling to the receiver is usually via an impedance matching transformer winding (like on a ferrite rod) - although I have seen instances where high impedance MOSFET's are used to provide the matching as well.

Older radios, or often communication receivers, will provide an external connection for a large external aerial, to give better performance - but the popularity of small portable radios has resulted in the universal acceptance of ferrite rod aerials for medium and long wave radios.

Perhaps you might like to tell us EXACTLY what you are trying to do?.

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samcheetah

i think your question has been answered. could you explain your confusion a little bit more

Zener_Diode

the dipol anten should be half of the wave lenght. How does it take in figure?

JimB

OK lets try and approach the question from a different angle.

The basic wire antenna is a half wave dipole, at 1Mhz it would be about 150 metres long.

If you have a sensitive receiver, you do not need such a long antenna, something much shorter will work OK.

Radio waves have an electric component and a magnetic component, a wire antenna receives the electric component.

A ferrite rod antenna, which is what Mr Zener appears to be describing, receives the magnetic component of the radio wave.

By using a rod of ferrite wound with a coil which is resonated by a capacitor, a very small efficient receiving antenna is the result. This is what is generally used in long and medium wave recievers where the signal strength from the transmitter is usually quite strong.

JimB

Zener_Diode

ok....
if I understand you right, I can use or normal dipol aerial that can be very long, OR a ferrite rod antenna instead of dipol aerial. Did I understand you right????

and if it is so... how can I culculate the numbers of coils that I need to wind on the ferrite rod antenna to get an excellent receive??

zachtheterrible

ya rip 1 off a radio :lol:

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I'm no electronics god, i just talk too much.

JimB

Yes you understand correctly.
However, the dipole will always give a bigger signal than a ferrite rod, but for a practical receiver for use at home, I think the ferrite rod is better!

If you want to calculate how many turns to use on a ferrite rod antenna, consider this:

Assuming that we want the antenna to be resonant at 1 Mhz with a 150pF capacitor, we can calculate the required inductance from:

L= 1/(4 x PixPi x FxF x C)

Where L is inductance
F is frequency in Hz
C is capacitance in Farads

And for 1Mhz with 150pF, I make it 169uH.

To find how many turns to put on our coil, first consider that putting the coil on the ferrite rod will double the inductance.
So we need an air cored coil of 85uH (half of 169, near enough).

To find the inductance of an air cored coil, we can use:

N = (SQRT(L x (9 x A + 10 x B))/A

Where N is number of turns
A is the radius of the coil in inches
B is the length of the coil in inches

And for 85 uH, radius 0.1875 inch, length 2 inch, I make it 68 turns


JimB

Zener_Diode

how did u get 68 turns???? if u put all numbers into the for formula you will get number around 0.229, and not 68.

JimB

OOPS,

we are both wrong, and its all my fault.

I should have said that L is in uH, I think you have used Henries.

Also, I used a spreadsheet to do my calculation beacause it was more convenient than a hand calculation, and I missed out a pair of brackets ( ).

The number of turns should be 228.

A valuable lesson here.....
Always do a manual check on a spreadsheet calculation.

JimB

Zener_Diode

OK.
thank you very much.
And to all people that tried to help me.