RDF

[CapnK]

This is interesting...

http://www.handi-finder.com/

A very basic RDF that you finish yourself, with a homemade antenna, if you want. AM/FM freqs. At time of this writing, US$34 includes S&H. There are even hardware hacks to make it more directional. Pretty kewl little gadget. :)

[Captain Smollett]

Interesting.

A couple of years ago, I built my own tunable AM receiver - cost? About $3.  The DF part of RDF comes from the antenna, so if you REALLY wanted to save $$, build your own completely from scratch.

For $34, you getting a bare board?  If so, that explains the 'low' cost.  Much of the cost of a marine RDF (or any other electronic gadget) comes from marinizing the ENCLOSURE (and its associated feed-thrus).  Uh, and you STILL have to have an FM receiver to actually detect the signal.

Looking at the schematic, this thing would be dirt simple to build and for much cheaper than $34.  And I'm wondering why bother.  A properly constructed antenna on the front-end of the radio receiver you have to use to hear the tone anyway would serve (at least for AM that is fairly easy, a loop is directional).

Sorry, I don't mean to poo-poo the gadget....just cannot see that it really offers that much.

Quick note/question: Is the plan to use commercial FM broadcast stations for RDF?  I've always read (like on charts, iirc) that those are not reliable.  From what I understand, the only 'reliable' RDF signals are those intended for the purpose, and aren't those AM?

Just thoughts to spark discussion...   ;)

[Captain Smollett]

Sorry, to double post...but I took another look at that schematic.  Here's what I think the board is doing.

The CD4047 is an oscillator chip, similar in function to the venerable 555 Timer (though lower power).  It generates a tone whose volume is related to the signal difference between the two antenna inputs.  This tone is then sent to the AM/FM reciever for actual detection.

Basically, you tune your AM/FM receiver to a station.  When you turn this unit "on," you get a tone superimposed on your station.  You rotate the unit until the tone disappears.

What I am now wondering is what if there is more than one station nearby, but in different directions (think about the number of radio towers visible from offshore of Charleston, for example).  I doubt seriously you will get any meaningful position information in this situation.  From far enough offshore, I guess it could show you the general direction of Charleston, but you'd still get an artifact from other transmitter locations.

A better RDF would have a tuner in the circuit, so you can pick what station you are fixing. 

This is a kit project designed as a teaching tool, not really a field grade navigation aid.  Sorry.

[AdriftAtSea]

All the radio stations I've ever seen listed on a chart for navigation purposes have been AM ones, not FM.  Not sure why that is but there must be a reason for it. :) Most cheap transistor radios, especially older ones have fairly directional antenna in them.  Not quite as nice as a purpose built one, and would take a bit of testing to figure out how to "read" it, but in a pinch they'd be better than nothing—once you've done the testing.

[CapnK]

For folks like me, who don't know a resistor from a capacitor on that confusing looking crazy squiggle of a drawing, $34 is cheap. Cheaper than the time it would take for me to decipher the thing, and learn how to get the parts and where and then solder them all onto a PCB.

If, like you, it is something you can make as easily as you would toss together a PBJ - have at it, the drawing is right there.

Commercial FM stations here have been reliable and on, even through hurricanes like Hugo, for the 40 years I have been alive. Might not be that way in 3rd world countries, but its another tool in the navigational arsenal, for not much money, even if you only were to use it for training purposes. I don't think RDF for small boat sailors has ever been something you'd use to find your way into that strange narrow shipping channel at night. But to aid in dead reckoning as you ran down a strange coastline, that's a different thing...

Better than spending the $2K I've seen on some. And - it's hackable.

Charts also show skinny water where I sail regular, or nav markers that have since disappeared/moved. Like the net, just cause its on a chart, don't necessarily make it so... Disclaimers abound in this society... :)

You could marinize this board with a couple-dollar glop of epoxy, looks like to me.

[Captain Smollett]

For folks like me, who don't know a resistor from a capacitor on that confusing looking crazy squiggle of a drawing, $34 is cheap. Cheaper than the time it would take for me to decipher the thing, and learn how to get the parts and where and then solder them all onto a PCB.

In the spirit of discussion:

Part of being self reliant underway will include being able to repair any electronic gadgetry yourself.  Certainly troubleshooting electrical problems on the engine falls into this category as well.  Thus, it might be worth considering using this as an opportunity to expand one's skillset.  Schematics and simple circuits like these are NOT difficult; it's like anything else - use makes master.

So, I'd take the position that building this kit from component parts for a fraction of the cost of buying it would be worth the exercise.  Of course, this is an individual opinion and as always, YMMV.

Commercial FM stations here have been reliable and on, even through hurricanes like Hugo, for the 40 years I have been alive.

Fair enough, but I ask (for my own edification) why in ALL the navigation documentation that I have ever seen, including I think some US charts, it states clearly to NOT rely on them (the signals) for position fixing? 

I freely admit that you use whatever tools you have, and absolutely WOULD get a fix off a commercial signal if that's all I had to go on (if I understood the limitations of doing so).  I'm just curious why they are not considered "reliable" because frankly, The only reason I can fathom for why this might be is that I know some commercial stations have to change their broadcast characteristics at different times of the day (day vs night, for example), so that the signals you recieve can be VERY different.  For example, WBT 1110 kHz out of Charlotte has to 'detune' their antenna at night and are limited to broadcast only in certain directions (ie, a general N-S line, rather than a spherical coverage).

And I still don't know how in the heck you know WHICH tower you are fixing on.

You could marinize this board with a couple-dollar glop of epoxy, looks like to me.

There I beg to differ.  I'm talking about more than just sealing over the board itself.  Using a bare board in any field environment is inviting failure.  You really need an enclosure of some sort.  The box itself might be easy to marinize (an o-ring or perhaps 5200)...the tricky part is weather-proof feedthrus for the antenna and output lead as well as the switch.

True, it's not a BIG deal, but it will run up the cost a bit.  There's a bit of a price difference between a marinized three-way switch and a 'regular' one, ditto the lead feed-thrus.

Bottom line, as I said before, this is not a field grade instrument, but a learning "project" tool.  That's not to say it is not worth playing around with.  If you get it and try it out, I'd be very interested to learn how well it works in the environment of multiple signals that I have mentioned. 

As always, I'll be more than happy to admit I was wrong if experiment proves me so.

[Captain Smollett]

I thought of another way to go at this. This "Device" is NOT an RDF.  It is a tone generator, nothing more.  The IC in the schematic is used only to generate the tone, adn the various resistors and capacitors are only for "signal conditioning" the inputs and outputs (except for the ones on the IC itself, they set the frequency of the tone).  So, you would be paying $34 for a tone generator; you still have to supply the antenna (the KEY to RDF) and the actual receiving unit (a radio).  It is therefore not that cost beneficial for practical use.

The direction finding capability of this unit comes solely from the antenna; the tone generated by the device is only to detect the signal DIFFERENCE from the two antenna poles.  Typical RDF units do the "reporting" of the signal difference with a meter or some such.

Any AM receiver can be used as an RDF.  You have two options.  First is an AM radio with internal antenna.  These are ferrite rods and are directional.  The other option is an external antenna for AM, like my home stereo receiver has.  It is a "loop" style antenna and is very directional also.  I can completely lose a given AM signal by turning the loop about 45 degrees, and there is measurable difference in signal strength even over a couple of degrees.   That's pretty precise instrumentation for something not even designed for the purpose.

Typical FM antenna's are directional as well.

Here's a cheaper way to go for you, and a cool "hack" so I know you'll like it.   ;D ;D  Build a Loop antenna and splice that into your existing AM receiver. Cost: 1 wire coathanger, a few feet of coax and the willingness to hack your receiver.   ;D ;D   ;)

[Auspicious]

I have done a good bit of radio fox-hunting. This type of unit doesn't do well. In my opinion you are better off keeping track of where the sun rises and sets and work from there (okay, slight exaggeration). Really, the beam pattern is too broad to be very useful for navigation.

Watch eBay for surplus RDF rigs. You'll be happier.

[AdriftAtSea]

Could it be due to the distance at which the stations can be detected.  I know, at least when I lived out in the mid-West, that under the right conditions, you could AM stations that would other wise be way out of range.

[Captain Smollett]

Could it be due to the distance at which the stations can be detected.  I know, at least when I lived out in the mid-West, that under the right conditions, you could AM stations that would other wise be way out of range.

There's no such thing as "out of range" for AM, especially at night.  I once talked to a guy in Venezuela with a 4 watt CB radio - with a whopping ground plane antenna - while I was in NC.  HAM's DX around the world with no trouble at all.  It's all about "skip" of the skywave off the ionosphere.

Commercial broadcast AM travels similarly.  At night, I routinely listen to AM stations from all over the country.  One of my favorite anecdotes is the time I was in Syracuse, NY listening to WBT out of Charlotte - my then-favorite talk host - just about as clear as if I were on Independence Blvd.  As I said, they are now regulated by the feds to actually detune at night so they don't walk over a competitor on the same frequency in Nebraska, iirc.

That's part of the reason I am wondering about real world radio fixing with any unit that does not include a tuner to select the station of interest.  Especially at night, you are going to pick up crud from all over the world.  All a unit like the above is doing is measuring the difference between two signals on the antenna poles - that's total signal on the wire without regard to carrier freq or station location.