RDF - Need a smart feller's help

[Gene :^)]

I was wondering if there were anyone out there that could help me learn to take apart and modify an RDF to use with my existing antenna.

In the late 1950's an Apelco Ship to Shore RadioTelephone and Radio Direction Finder (RDF says "Applied Electronics") were installed in my boat.

Well, here we are 50 years later and I am sure you can guess... it's gone and probably quit working long ago. However, the RDF antenna is still in place.

To keep the authentic look and coolness of this antenna, I was wondering if a more modern RDF could be taken apart and modified to work with my existing antenna?

Even if not accurate, it would just be for show and tell. I would hide the modern bits of equipment.

My antenna has the "LOOP" on the cabin top and inside the pilot house has a wheel to direct the antenna. There is a fixed compass card and a needle indicating the direction in which the antenna is facing. There is also a second needle that is 90 deg off. Above the compass card is a dB guage that swings both directions.

Any help would be greatly appreciated.

[GordMay]

Connect a conventional AM/FM radio (with external antenna connections) to it.

[btrayfors]

Gene:

There are lots of old RDFs available on eBay, most at very low prices. Most any of these could likely be easily adapted to work OK with your antenna.

My favorites are:

1. the Homer/Heron units by Brooks and Gatehouse;
2. the Japanese-built metal case units variously marketed as "NewMar" or other name;
3. the Sailor units like the 66-T; and
4. some of the Ray Marine units, like the 670 with digital readout.

Bill

[Gene :^)]

Thanks guys. It's a start.

I guess the tricky part will be making the gage read something real.

Gord, will the AM/FM pick up RDF signals? What station do I tune to?

As you guys can probably guess, I have never used RDF.

[David M]

Cool, I have not seen an RDF since I was a teenager in the 70's. Have fun with it. Modern navigation electronics are pretty boring when you think about it...they are too easy! What challenge is it to get a fix with a GPS?...like none! It would be kind of fun to have an Omega, Loran and maybe even an old Sat Nav unit...and of course don't forget the sextant.

[Gene :^)]

Got the Loran C as well. That still works great.

You should see my old Bendix Model 155 auto pilot with hand held remote. Not working, but it is on my list of things to fix and keep old school.

Still hoping someone can help me figure out how to actually connect my dB gage so it works and I can use the RDF. Even if only for fun and not accurate. Though accurate would be nice, I would use it.

[dacust]

Originally Posted by Gene :^)

What station do I tune to?

You tune to any station you can pick up. Listen to it and figure out where it is located. Then you know what direction the signal is coming from.

If you are close to the station, remember, the station may be miles from the transmit tower.

Also the signal can bounce and you'll get an erroneous reading.

For those reasons, it's best to get fixes on as many different stations as possible, not only to be able to triangulate, but so errors will become obvious. If you get 4 fixes and 3 of them cross at about the same point, but one is off, you discard that reading or check it again. 4 is the minimum number of fixes you can use for error detection. With three, you can't tell which one is wrong.

-dan

ADDED:
If you have 3 fixes, and they all cross at the same point you can feel pretty good about the position. If they cross at two different points, you can feel pretty sure you are somewhere around one of those points or somewhere between them. If the two cross points are too far apart, then another fix is needed to narrow it down.

[Gene :^)]

Ok, now I understand the disconnect between what you guys are telling me and what I was thinking. I didn't realize that the Beacons (navigational) are gone. So I will only be able to home in on actual radio stations which I would have to figure out the city they are coming from. Then the transmitter could be miles away outside of town. Thus, not very accurate and need multiple bearings for a fix.

That explains why Gord said to just hook up an AM/FM radio. I was thinking I needed Beacon frequencies and a special receiver.

I won my eBay bid today and a used Ray Jefferson RDF is on the way. It picks up Am .54-1.6 mhz, Fm 88-108 mhz, Cb 26.8-27.8 mhz, Vhf 145-174 mhz, Lw .15-.40 mhz

This way I can see what bearings I get from it's antenna and compare to what I get from my antenna. (it has an external antenna plug in).

The VHF homing should be fun to play with.

Any idea what the Lw .15-.40 mhz band is for?

[sluissa]

It looks like, via wikipedia Longwave - Wikipedia, the free encyclopedia

That there are still navigational beacons being transmitted on Lw frequencies... However I'd guess these are rare, given the giant arrays needed to transmit on these frequencies. It looks as if there are a few stations in Europe, North Africa, and North Asia that still transmit various things on these frequencies, from radio programs to time beacons.

According to Wikipedia also, Radio direction finder - Wikipedia, the free encyclopedia

You use the side of the antenna(the reason for the 90 degree angle on the dial) to find where the signal stops. Doing it that way appears more reliable than actually trying to find the direction where the signal is strongest. Apparently one side is "null" so you know which direction the antenna is actually receiving from.

I don't pretend to understand everything said, but from what I can figure, it makes sense.

Just figure out which one of those needles corresponds to the front of the antenna (the front looking like a circle) and which one the side of the antenna (looking like a straight bar)

[bill good]

If you have the old unit it maybe able to get it going then the field strength meter would work as well. Being of the age it was built it will not have many components that could not be found or remade. The instructions already supplied is correct the null is offset 90 degs & that is used for one dirrection. A sense antenna is used to resolve the correct dirrection in some units otherwise some known bearing is used or the additional stations to pinpoint the dirrection. Also daytime bearings (groundwave signals) are used. If you are trying the loop on another radio like yor HF then one end of the loop will go the the earth & the other to the normal aerial input.

Regards Bill Goodward

[Gene :^)]

Thanks everyone!

I am getting closer to understanding all of this. In simple terms, my loop antenna has a front and back (probably why my dB meter reads +/-). This way I could tune the signal to the plus side so I know what direction approximately it is. Then I turn 90 deg away (in the direction of the second arrow) until I find the "null" or dead spot. Then I read my bearing from the second arrow.

Here are the frequencies of the unit I am getting:

Beacon - Long Wave .15 - .40 mHz or 150 - 400 kHz
Broadcast - Am .54 - 1.6 mHz or 540 - 1600 kHz
Marine - Medium Wave 1.6 - 4.6 mHz
Citizens Band - CB 26.8 - 27.8 mHz
Marine - VHF 145 - 174 mHz
Broadcast - FM 88 - 108 mHz

Sluissa,

Thank you for the links. From what I read, nearly all of the marine NDB (non-directional beacons) are out of service. These would have used the "Beacon - Long Wave 150-400kHz". Anybody know of any still operating? Either east or west coast US.

Bill,

Unfortunately I do not have the "brains" of the old unit. All I have are the antenna and meter. It was Apelco.

I was hoping to connect the brains of a newer unit. The unit I purchased on eBay is a portable "self contained" unit but does have an external antenna connection. I will experiment with this and see where things go.

[bill good]

The meter would have been wired into the main box & will not function unless the unit you are getting can be modified. The idea of the meter was to allow the operator to tune for max signal & to ensure the signal was constant , correctly ID & good strength before looking for the null. These units only had manual tuning knobs & the dial calabrations may have not been exact. Maybe the unit you are getting will have a "s" meter" which will do the same function. By enlarge the stations have been pulled out of service. You can use the AM broadcast band. You might even find the lat & long from some aviation publications relating o frequencies. The aero services in the States pulled all the NDB stations out & now you will have to go to a museum to see any reference to this service!!

regards Bill

[sluissa]

That 145-174mhz band looks like it might cover some HAM 2 meter frequencies as well. Including some repeaters. Not the best things to take a bearing from though, since unless you can find one that's being used, they generally only transmit between once every 10 minutes to once an hour. Still, if you've got your ham license, and a 2 meter radio within range of it, you could trigger it to broadcast yourself. All you would need to know is the exact location of the repeater, which you MIGHT be able to find out from talking to someone through the repeater, although most people probably wouldn't know the exact location except maybe the person who maintains it at the tower.

Just an idea.

[Wotname]

FWISW Australia still has quite a few aero beacons (Non directional Beacons - NBD's). They are in the 200 to 400 KHz band. I believe South Africa still has a few, maybe South America. Oil rigs often have them. Remember you can get over 20 degrees of bend as the radio waves cross the coast if the transmitter is located inland (even if only a few miles inland).

[btrayfors]

Gene,

Aero beacons are a good source for signals your RDF can pick up.

Here's a list of all non-directional beacons (NDB) in the U.S. :
Navaid Info

Note that most of these are VHF, and won't be very useful. However, there are some LF beacons still in operation, and these are great for RDF-ing.

For example, if you choose Washington, DC, there are three NDBs listed. The one for OXON is a LF beacon.

Bill