Radio Direction Finder

yes foxhunting they call it. I have done it but I have not bothered for years.

this will load you with data

http://www.google.ca/search?hl=en&safe=off&q=%22amateur+radio%22+%2B%22fox+hunting%22&btnG=Search

All of the techniques on the sites seem to using directional antennas but there is another technique that can be use. If you are trying to find a transmitter that is transmitting an AM signal then you can use a series of antennas arranged in a circle. The way it works is that you rapidly switch through the antennas in a rotary manner. Look at the diagram below.

What we have his a circular antenna array with a switch S that connects each antenna one at a time rotating through in the direction of the arrow. The result that the receiver sees is an increase in the frequency as you switch from c through d to a and a decrease from a through b to c. You can then use this information to calculate the direction of the transmitter in relation to the antenna.

The advantage of a system like this is that there are no moving parts, and it doesn't require the bulky directional antennas that many systems use.

You would have a hard time doing that at 2 meters if each of your antennas was a 2 meter yagi with 6 elements. Such a circular array as you picture would be huge.

In fact what they use are rotating yagis, or even folded or open loops with no reflector and use geography to determine which direction the signal arrives from by drving from point a to point b. This is similar t the way birds with a single eye on each side of their head get effective binocular vision with rapid head movements.

Yagis better, more directional, more sensitive, better front to back ratio (loops are synmetrical)

"You would have a hard time doing that at 2 meters if each of your antennas was a 2 meter yagi with 6 elements. Such a circular array as you picture would be huge."

The system I described is called a switched or phased_array radio detection finder and doesn't require fancy directional antennas like a yagi. In fact they won't work with directional antennas as it needs multi non directional antennas.

The radio direction finders in aircraft have worked on this principal for quiet some time so it's nothing new and it dose work. The link describes a unit that is specifically designed for the use Saurabh describes in his initial post and whilst the system he describes is quiet large you could always try and get hold of an ADF from an aircraft.

There is a lot of difference between a switched system and a phased system.

A switched system selects different antennas. A phased array is hard wired to radiating elements and a variable delay is inserted into the lines that change the transmitted pattern to perform the equvalent of mechanical scanning. It is far faster but has some problems with aliasing. Usually used to transmit a movable beam that is pulsed. Pulse...move...pulse...move. You will also listen between pulses or you may use a separate listen antenna or more than one.

Phased arrays are usually high frequency, radar, but you can create a vertical beam antenna on commercial FM that uses phased feeds. A station on the edhe of the ocean would do this so instead of a circular pattern that wasted 50% to seaward they would make a cardiod that sent little to the sae byt gave a higher ERP to landward. FCC may also dictate a patter to reduce adjacent channel interference with another transmitter 100 miles away

"There is a lot of difference between a switched system and a phased system."

You are of course quiet correct. I should have been more explicit. I was using the names that people have quiet erroneously called this system of RDF on the internet so I apologize for my poor choice of terminology.

By the way though the direction finders that are in aircraft work like this not just the ground based one that Wangito is referring to in post #8. In fact a ground based unit working on the aircraft band would be next to useless due to the fact that all the aircraft in a particular region would be transmitting on the same frequency and their transmissions are normally only of a short duration. You would more than likely end up with the indicator being a resonant electric motor rather than a direction finder. I could however prove to be very useful on the emergency frequency of 121.5 MHz as normally you would only expect a single transmitter on ths frequency.

Just a small operational correction:

The VHF RDF in aviation works on the entire COM range. The bearing is taking during the pilot's regular COM transmission, displayed and HELD on the display. The controller knows in an instant the bearing for this aircraft. The next transmission will erase this info, or not at the controller's choice. With the pilot indicating his compass bearing the controller will know whether this aircraft is in or out bound. that is all the information this system could provide. It will of course work on the emergency frequency of 121.5, problem is, that when this frequency is in use, the airplane is most probably not flying any more, and the D-Fing is done by the search and rescue team on the ground.

An interesting comment: Time ago, We started to work on a IFF transponder system that will give bearing information to ground controlling unit, using the onboard IFF transmitter (transponder) and that, without the use of ground radar, or where radar service is not available. The idea behind this was to be able to use the transponder out of radar range. The idea was later dumped as activating the IDENT mode, would make the airplane a sitting duck for enemy fire. and also the lack of sufficient digital technology at the time to achieve all we wanted from this system.

Wangito.

i m doing my work on ADF.i need guidance

i have two questions to ask

Q#1:-

i have to simulate the process of direction finding of an air craft with respect to the NDB on matlab.can i have any simulink model for that?

Q#2:-

flight simulators give us the cabinet of ADF in the aircraft along with other pannels.i just want to have the ADF simulated pannel where the indicator is showing the bearing of Aircraft around the transmitting station.

This system was widely used in control towers of rural or small airports to locate inbound/outbound aircraft prior GPS days. and it did work pretty well. They didn't use Yagis, but rather 4 omni directional pahsed vertical antennae. usually in the VHF range. CW or AM signals.

locating the transmitter (the on-board communications radio,VHF AM 118-136Mhz.) was done by time domain, calculating signal time differences between the 4 antennae. and was displayed usually, on a wind rose. Simple and effective.

Wangito.

Yes, with planes high and antenna high you would be in direct path so phase timing would work well.

I wonder what frequency they use...?

I know for my ferret finder a frequency of 70kHz is best as its underground... also as the ferret can't carry a heavy transmitter it only transmits a burst every ½ second.

The reciever aerial is a tuned ferrite rod so location is good and depth can be estimated...

John.

for small emitting buried objects, 70khz is much like the metal detectors that emit something and listen. your thing emits 70Khz on it's own = deeper. The ham fox hunt works on a 10 miles square and the fox moves under rules.

Different fox hunts use different areas

On 146 MHz, I use a Yagi I designed which is optimized for front to back ratio, and an active attenuator designed by Dale WA6BYU.

Some guys really get into it. There are some complex fox hunting rigs out there. Sadly the whole sport is dying out