Radio Orienteering

Hi, I've been given a task to build a receiver to detect some low-power transmitter. The following are the given details about the transmitters:

1. 1. It will operate in the 3.5~3.6MHz band
2. 2. It will generate an output power complying with British Standard EN55011:1998(2.194~3.950MHz, E=85dB:mu: V/m)
3. 3. There will be more than 5 transmitters activated at the same time with different identification code (Morse code).

Here are my questions...
I'm confused about the frequency it will operate in and the frequency for it's output, is the frequency in the output a hint for decoding the Morse code?

Is there some special name for the antenna, filters, amplifiers, etc. used for building a receiver of this type? As when I googled "receiver antenna", I sort of get some random information, and I'm not sure if it's relevant to what I'm looking for....

Could someone please point me in the right direction as a start for this task?

Thanks in advance

try "fox hunt receiver"...

Thx ^^, this shld help me get started nicely

Yagi's (antenna) are often used because they're highly directional.

Sceadwian said:

Yagi's (antenna) are often used because they're highly directional.

Click to expand...

But at 3.5 Mhz?

I dont think so, at least not for this application!

JimB

Must the Morse code be identified through sound?
Because I came through some other transmitter hunting competition and they all used sound to identify the Morse code...

Another question: What is the relation between the 80m and 3.5MHz, what does 80m actually means?

No your right Jim. S'what I get for quick responces =)
I did a little more googling, and found this link.

**broken link removed**

forever99482 said:

Must the Morse code be identified through sound?
Because I came through some other transmitter hunting competition and they all used sound to identify the Morse code...

Another question: What is the relation between the 80m and 3.5MHz, what does 80m actually means?

Click to expand...

speed of light (c) = frequency(f) times wavelength (lambda)

(300000 km/s) = (3.5 MHz) times (80 m)

80 meters is the wavelength
sam

Thank You

80 meters is an international amateur radio band and "fox hunting" is one of many amateur radio activities.

A loop antenna might prove to be useful as they tend to be quite directional. There are two parts to directional here - orientation of the antenna for maximum signal and orientation for minimum or null. My experience is that the null is quite sharp. The loop can be a ferrite loop or simply a loop of wire or coax. Tuned loops perform well. A tuned loop for the AM broadcast band in the US might be 24 inches square with about 20 turns of wire and a variable capacitor to resonate the loop. For higher frequencies you might short some of the turns.

Here's a site all about this. It is called Radio Direction Finding (or Fox Hunting).

**broken link removed**

There seems to be a lot of information about this activity on this site, including links to reciever projects.

Here's another google hit with project info:

**broken link removed**


A receiver for 80 meter hunting is probably just a conventional analog AM (voice) shortwave receiver with an added beat frequency oscillator to allow you to hear morse code. The morse code comes out of a speaker as simple beeping sounds, with an on-off pattern. So, for your receiver project, you should probably try to find a very simple receiver schematic for a basic SSB/CW receiver for 80 meters. There are many of such things on amateur radio sites and in fact the simplest designs are often those specifically for what the hams call QRP operation. QRP simply means low power operation and is the kind of equipment that hikers take along on their travels.

Obviously you are new to receiver design and construction, so I would recommend this: if your task is to build a receiver, then buy a kit. If your task is to design a receiver as a learning exercise, then start with a receiver IC data sheet, say for example with the
https://www.electro-tech-online.com/custompdfs/2007/10/TEA5551T.pdf

There are many other receiver chips out there that can form the basis for a one-chip radio for your purposes.

I don't have a kit to recommend, but perhaps you can find one with google or the above enthusiast sites.

Using an IC seems very over complicated?, an 80m CW receiver is a VERY simple and easy device, try googling for "direct conversion receiver". They are easy to build, simple, yet work incredibly well.

Thanks, will look up on it now

Actually, I was wondering if it is possible to build an antenna which the shape look like the antenna of the old type mobile phone (those that you'd need to pull out the antenna type)? because the loop type antenna is sort of bulky and not really user friendly. (In terms of having to point in the right direction to get a maximum signal)

Actually, this is my 3rd year project, trying to beat other teams XD

It seems there are at least two ways to look at this.

Both suggest a receiver that will indicate signal strength - or at least it is helpful to have the meter.

In one approach you attach a directional antenna and plot lines on a map that might suggest direction. You change locations somewhat and plot more lines - the thought being that where the lines converge is the source. This is probably best where distances are greater.

In another approach you have a receiver and non-directional antenna. You move from place to place checking field strength. You might plot info on a map to help point the way. This approach might be most useful for low power stuff that's relatively close.

I mention this because you ask about a telescoping antenna - which won't have much directional capability. It may be that for your use a directional antenna might not be of much help. A metered receiver and possibly an attenuator on the antenna input might be helpful. The attenuator would serve to reduce the signal to the receiver as you got really close in.

Another question just came to mind when you mentioned plotting lines on a map to find the transmitters. As usually these type of competition would be in an open field with trees, but in my case, it is an open field with buildings.

Is there a possibility that the radio waves might get reflected by the buildings(mostly bricks and windows)?

Yes, reflection is possible though at 80 meters I'd think it less of an issue up close.

If I wanted to install a PIC/AVR into the circuit, at which stage should I actually do it? (as in after bandpass filtering? or put a differential amp after the antenna then feed it to the PIC?)

You don't use PIC's in RF circuits!.

forever99482 said:

Thanks, will look up on it now

Actually, I was wondering if it is possible to build an antenna which the shape look like the antenna of the old type mobile phone (those that you'd need to pull out the antenna type)? because the loop type antenna is sort of bulky and not really user friendly. (In terms of having to point in the right direction to get a maximum signal)

Actually, this is my 3rd year project, trying to beat other teams XD

Click to expand...

No, if you make an antenna that is a straight wire it will not be directive at all. This is mainly because your frequency, around 3.5 MHz, is quite low and the wavelength at this frequency is approximately 80 meters long. Any straight conductor antenna that is much shorter than a quarter wavelength will have no directivity at all. The only way to get some directivity out of something like this is to build an array of them and use some sort of phase indicating receiver to show direction of arrival for the wavefront. This is complicated. The loop is a lot simpler, and will actually work. A straight wire such as you ask about will not.

forever99482 said:

If I wanted to install a PIC/AVR into the circuit, at which stage should I actually do it? (as in after bandpass filtering? or put a differential amp after the antenna then feed it to the PIC?)

Click to expand...

Why would you want to stuff a processor into something that doesn't need it? Sounds like you are trying to create a "software defined receiver" by replacing the receiver analog circuits with a PIC/AVR, which is a needless overcomplication.