Antenna for a specific frequency ?

[shuby]

Hi,

We're trying to build a simple antenna that's able to pick up a frequency of 457 KHz from a maximum distance of 20 metres.

I'm totally new to this stuff, so I looked up the yagi antenna and some formulas used to determine the length of the antenna, but the results I got were huge compared to all other low frequency antennas I know(AM radios for examples are not that long)

So I thought since our transmitter will be 20 metres far, maybe any sort of antenna should be able to pick up the signal from a small distance, maybe all we need is a good filter and an efficient amplifier ?


-What would be the best solution ? (Is it possible to use a toy's controller antenna for this ?)
-I know the formulas for band-pass RLC series filters look like this

f = 1/(2pi sqrt LC) Q = 2pi f L /R and Q = f/B
I read somewhere that we take Q= f/B if we have a range of frequencies or Q = 5
but how does it really work in practice ? and what's the best way to choose the others variables ?

-What kind of amplifiers should we use ?
-Any other advices ?

Thanks for your help!

 

[berkeman]

Hi,

We're trying to build a simple antenna that's able to pick up a frequency of 457 KHz from a maximum distance of 20 metres.

I'm totally new to this stuff, so I looked up the yagi antenna and some formulas used to determine the length of the antenna, but the results I got were huge compared to all other low frequency antennas I know(AM radios for examples are not that long)

So I thought since our transmitter will be 20 metres far, maybe any sort of antenna should be able to pick up the signal from a small distance, maybe all we need is a good filter and an efficient amplifier ?


-What would be the best solution ? (Is it possible to use a toy's controller antenna for this ?)
-I know the formulas for band-pass RLC series filters look like this

f = 1/(2pi sqrt LC) Q = 2pi f L /R and Q = f/B
I read somewhere that we take Q= f/B if we have a range of frequencies or Q = 5
but how does it really work in practice ? and what's the best way to choose the others variables ?

-What kind of amplifiers should we use ?
-Any other advices ?

Thanks for your help!

For that low of a frequency, you would use a ferrite rod antenna, like you see in AM radio receivers. The AM radio band is basically around 1MHz, so your signal is very similar.

What is the source of the signal? What power is it being transmitted with?

Basically you can start with an AM radio receiver circuit, and just re-tune it for 457kHz.

 

[berkeman]

Oh, it's for an avalanche rescue tranceiver:

http://en.wikipedia.org/wiki/Avalanche_transceiver

Right?

 

[shuby]

Exactly.

For the circuit the technician of the team was talking about that the other day I think(AM radio receiver circuit) I got lost with filters and stuff sorry..

http://www.circuitstoday.com/tag/radio-receiver-circuit
In this website for example, they state that a 100 cm metal wire should do it, we tried with a 50 cm and it didnt work quite well(we had to roll it around the transmitter to get a response...) that's why we're trying to make an antenna.

What else can you tell me about ferrite rod antennas ?

 

[berkeman]

Exactly.

For the circuit the technician of the team was talking about that the other day I think(AM radio receiver circuit) I got lost with filters and stuff sorry..

http://www.circuitstoday.com/tag/radio-receiver-circuit
In this website for example, they state that a 100 cm metal wire should do it, we tried with a 50 cm and it didnt work quite well(we had to roll it around the transmitter to get a response...) that's why we're trying to make an antenna.

What else can you tell me about ferrite rod antennas ?

I haven't used ferrite rod antennas much, but got good hits with a google search on ferrite rod antenna tutorial:

http://www.google.com/search?source...L_enUS301US302&q=ferrite+rod+antenna+tutorial

Basically it's a way to get an antenna to resonate at low frequencies, without having to make it huge. The L of the antenna is matched to resonate with an adjustable C, at the frequency that you want to receive. You use a couple stages of amplification and filtering to get the receive signal that you want.

 

[berkeman]

BTW, to get directionality from loop (and ferrite rod) antennas, you need to hold them sideways (rod parallel to the ground, going left-to-right in front of you), and their max receive directions will be in front of you and behind you.

At those low frequencies, I'm not sure how best to shield the signal from behind you (so you know if the transmitter is in front of you or behind you, as you rotate yourself around to get a bearing).

 

[shuby]

The reason I was asking is because I find it weird that nobody mentioned this ferrite rode loop antenna not even the graduate student, and we really need something small.

For directionality, I know that it's not easy to shield the signal at that frequency, so we're going to make sure that the transmitter will never be at more than +/- 90 degrees.

However, since the tests are to be made indoors, somebody mentioned that the signal might be re-bound by some materials and that we will detect like two signals, what are the chances of that happening ?

Thank you for clearing this up for me !

 

[skeptic2]

I have worked with ferrite rod antennas at frequencies between 200 & 400 kHz. Like a dipole the radiated pattern around a ferrite rod is a torus but unlike a dipole, the best reception isn't always when the the transmit and receive dipoles are parallel but when the length dimension of the receive ferrite rod is tangent to the torus. This means that in order to be sure of picking up the signal you must constantly change the orientation of the receive antenna. I don't know how sophisticated you want to get but to minimize the effect of dead spots due to antenna orientation we used crossed dipoles and switched between them looking for the strongest signal. To really do it right, there should be three rods, each perpendicular to the other two.

You should not couple the transmitter or receiver across the whole coil wound around the rod. Instead I recommend you use a high voltage variable capacitor in series with a fixed high voltage capacitor and connect your transmission line across the larger of the two caps. If you use high quality ferrite rods, you can develop fairly high Q resulting in a very narrow bandwidth and high voltages across the caps. You should experiment a little to determine the best ratio between the fixed and variable capacitors but I would start out with a ratio of about 10:1. When in use, any metal within a few inches of the ferrite rod will detune it.

Because the wavelength is so long you don't need to worry about reflections or multipath. As long as you aren't doing extremely precise measurements, you shouldn't have any problems indoors.

Our transmitters transmitted about 30 mW, the receivers had a sensitivity of 0.1 uV and we achieved a maximum range of about 150 ft.

 

[berkeman]

I have worked with ferrite rod antennas at frequencies between 200 & 400 kHz. Like a dipole the radiated pattern around a ferrite rod is a torus but unlike a dipole, the best reception isn't always when the the transmit and receive dipoles are parallel but when the length dimension of the receive ferrite rod is tangent to the torus.

Interesting. I'd assumed that the pattern was similar to a loop antenna, but it sounds like it's more complicated?

 

[shuby]

Hmm. looks like it's more complicated than I thought, I should consult a professor because I didn't get to these notions yet.

I'll take good note of all that.

Thanks.

 

[skeptic2]

Attached is a diagram of my observations. When the receive ferrite rod was offset from the transmit ferrite rod by 45 deg, it had to be rotated 90 deg to get the best signal. If the rod remained parallel, it hit a null at the 45 deg offset.

When I thought about it, it seemed to make perfect sense. Since the coupling was primarily magnetic, in order for the lines of flux to couple efficiently into the receive antenna they need to pass through the length of the ferrite rod. I wouldn't say it is more complicated only different, just as a magnetic field has a different shape than an electric field.

I never experimented with loops but I would be surprised if they didn't behave the same way. If you have a couple of loops, Berkeman or anyone else, maybe you could do a quick experiment and report back.

Question: If two dipoles are offset by 45 deg., would the best signal be obtained when they are parallel or when the receive dipole is rotated 45 deg to the transmit dipole so that it is aligned with the electric field?

 

[berkeman]

Interesting. I guess with these low frequencies and distance we are discussing in this thread, it's all near-field stuff? In the far field, you'd want your linear antenna lined up with the E-field vector, and a loop orthogonal to that orientation.

 

[sophiecentaur]

Electric monopoles or dipoles are not very sensitive at mf (they are a tiny fraction of a wavelength). The only reason they are used on motor cars is that the horizontal directivity patters is pretty omnidirectional. More or less all portable mf receivers use ferrite rod antennae because they are much more efficient, having an effectively huge receiving area (are actually 'tuned' to the specific receive frequency) and can be orientated to avoid the inevitable 'nulls' in the response. This is because all mf transmissions are Vertically Polarised because HP mf doesn't propagate well over ground - which demands the ferrite rod must be horizontal.
At this distance (near field), it would be important to know the form of the transmit antenna if you wanted to get the most out of your receiver antenna.
But, over just 20m, there should be loads of signal from even a tiddly transmitter. Is there any reason for not using an off the shelf MF radio receiver for this? 475kHz is well in-band and someone has done all the hard work for you. If you want to DF with it, you only need to know the orientation of the ferrite rod in the set (it would be horizontal, along the maximum dimension of the box, in most cases so you should get a 'null' with the receiver box 'pointing at' the rescuee. To find in which direction, you would need to find the direction from two separate places and that would resolve any ambiguity due to the rod having two nulls 180 degrees apart. To reduce any asymmetry effects, you could always calibrate your system against a signal from a known direction.

If you really need to know the direction just from one position, then you need a 'cardioid' receiving pattern. Two dipoles (as long as you can manage) can be spaced by a small amount (say 1m) and can be fed such that the phases of their signals from just one direction will cancel to produce a deep null. This is do-able at a spot frequency. There must be a number of google hits to Direction Finding Antennae. We've all seen the war films with spies and units trying to DF their transmitting location.

There may be a problem with the effects of nearby conducting structures, which can modify the fields - but it may not be so much of a problem in avalanche situations, out in the open.