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Radio Direction Finding Forum - Discussions regarding direction finding and transmitter location

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Old 04-15-2017, 10:55 AM
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Lightbulb 2.4 GHz RDF assistance requested

I am Ron Milliman and working on a pretty specialized project with which I am seeking your help. I am an avid fisherman. However, I am also totally blind. Generally, when I am fishing, this is not a problem, except for one situation. When I am out in my boat with my fishing companions who are fully sighted, they sometimes toss out a buoy marker or float to mark our targeted fishing hole. This allows us to back off a ways from the targeted spot and cast our lines into our fishing hole. This is relatively easy for them since they can see the buoy marker, but it poses an obvious problem for me since I can’t see the marker.

Here is my proposed solution for which I am seeking your expertise and assistance. I have a duck decoy which is completely hollow inside. I have built an undercarrage on the bottom of the duck that allows me to lower and raise a weight that goes to the bottom of the lake securing the duck in place. I have cut the top out of it, making an opening giving me access to the inside of the duck. Of course, I made a water sealed lid from the cut out piece so I can close the opening in the duck’s back. I want to insert a RF transmitter inside the duck that operates in the 900 MHz or 2.4 GHz frequency range. This transmitter will be used to send a signal to be received by a receiver I have on the boat with a directional antenna. By moving the receiving antenna, I will be able to pick up the signal from the transmitter. In other words, I am simply using this technology in a very basic radio direction finding (RDF) application. Obviously, there is a little more involved, e.g. I want to convert the received signal into a tone that varies with the strength of the received signal. The distance between the transmitter and receiver would very rarely exceed 200 or 300 feet. Since it is over water with no obstacles, it would be “line of sight.”

I have invested considerable time researching this issue, and while I have found lots of information similar to what I want to accomplish designed for the 2m band (144 – 148 MHz), I have not found anything comparable that operates in the 900 – 2.4 GHz range. I want this range, preferably the 2.4 GHz band because the antenna is so much smaller, making it much easier to use out in the boat. Is there anyone here who can assist me with this project? I have some knowledge of electronics from my 60 years as a radio amateur (ham), and so, I am familiar with the basics. I just have no experience working with circuitry or transmitting/receiving devices above the 70cm band (430 – 480 MHz).
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Old 04-15-2017, 11:46 AM
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The 900MHz band would probably be best and you would only need 1mW (0dBm) or maybe less and a 3" long 1/4 wave whip for the range you need. If you could find or make a simple but stable 900MHz oscillator and AM modulate it with an audio tone, you could then use a small Yagi or loop antenna to DF the transmitter. A very basic hand held police scanner could be used as the receiver.

Its usually best and most accurate to go for a null rather than a peak in signal because the null on most antennas are much narrower than the main beam. A DF loop could be made and would be so small you could simply attach it to your fishing pole and point the pole for the weakest signal and cast.
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Old 04-15-2017, 6:32 PM
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I assume that distance to target is already known and you just need an indication that you're aimed at the target. A TDOA setup is easy to build and provides a solid "on target" indication. It will work with any FM receiver that you already own. I would think that you could use UHF if you wanted to, as the TX/RX antennas don't have to be particularly efficient, so wouldn't need to be full size.
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Old 04-15-2017, 8:53 PM
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Hello Ron, I like prcguy's idea of using the null of a loop antenna because its pattern will be rather sharp, narrow.

The output of the receiver could (with the correct interface) drive a voltage controlled oscillator. The on target indication could be either no tone or highest frequency tone or loudest tone.

If you would like it to be silent the receiver could drive a miniature pager motor vibrator, no vibration or maximum vibration when target.

Silicon Labs has a line of ISM transmitter and receiver chips from about 27 MHz to about 960 MHz but they would require some construction.

I am no sure how deep (no pun intended) you would like to go or what resources you have at your disposal.

Using off the shelf parts something like this could be built into a wearable device. Say fastened to your hat or cap and the correct direction would be where you head is pointed. You would push a button, the device would turn on for a few (adjustable) seconds, you would locate your target and the device would turn its self off so you could fish with out being distracted or if you wanted the device could be continually powered on.

Or it could be built into a small box. Push a button, find your target and then fish.

The antenna could be on the fishing pole as previously mentioned, but you would have to deal with the feed line unless you mounted the entire receiver on the fishing pole, something you may or may not want to do.

There are a number of options. What would work the best for you? How would you like it implemented so as not interfere with the fishing? Would you be happy using a scanner for the receiver and or dealing with the antenna mounted on the fishing pole along with the feed line?

Does the boat drift or turn at all or could you just locate you target once and be done? Such as using a scanner with a loop/directional antenna to find the target, then puting the scanner down and then fishing.

Again, a lot of options depending on how you would like to do it.
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Old 04-15-2017, 9:10 PM
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First, I think mounting the receiving unit and/or antenna to the fishing rod would make the rod difficult to handle. Second, as for my resources, if you mean financial resources, I can, within reason, invest whatever I need to achieve my objective. Third, the boat is not stable; while I have a GPS controlled trolling motor that does a good job of maintaining our position, the boat still swings and moves around with the wind and waves; so, I am constantly having to check my casting direction against the marker's location. Again, 2m devices are readily available, but the antenna is quite large and cumbersome to use in the boat. I am seeking assistance to make something work preferably in the 900 MHz or 2.4 GHz ranges simply because the antenna can be much, much smaller for the receiver and transmitter.
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Old 04-15-2017, 10:06 PM
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A DF loop for 900MHz would not be any larger than a line guide already on the pole. No reason it could not be mounted on the pole and fed with miniature coax to a hand held police scanner. Using an AM modulated tone on the transmitter and AM on the scanner and a transmitter with correct power level for your distance will allow you to easily null the signal by ear. Heck, I could probably work a setup like that with my eyes closed....
prcguy


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Originally Posted by k8hsy View Post
First, I think mounting the receiving unit and/or antenna to the fishing rod would make the rod difficult to handle. Second, as for my resources, if you mean financial resources, I can, within reason, invest whatever I need to achieve my objective. Third, the boat is not stable; while I have a GPS controlled trolling motor that does a good job of maintaining our position, the boat still swings and moves around with the wind and waves; so, I am constantly having to check my casting direction against the marker's location. Again, 2m devices are readily available, but the antenna is quite large and cumbersome to use in the boat. I am seeking assistance to make something work preferably in the 900 MHz or 2.4 GHz ranges simply because the antenna can be much, much smaller for the receiver and transmitter.
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