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Navtex 490 kHz LF-antenna signal amplifier

earckens

Member
Below is the schematic for an antenna amplifier circuit for long wave radio signals. Where can I find the tuning transformers?

schema.jpg
 

Les Jones

Well-Known Member
Most Helpful Member
465 khz was used as an IF frequency in old AM radio receivers. I think one of these could be tuned to 490 Khz. (You may have to change the built in tuning capacitors.) They seem to be available on ebay.

Les.
 

JimB

Super Moderator
Most Helpful Member
Where can I find the tuning transformers?
Try these:

There is a mistake in the pin numbering of the SA612, there are two "pin 6" connections.
The connection to the 1N914 diode should be pin 5.

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
465 khz was used as an IF frequency in old AM radio receivers. I think one of these could be tuned to 490 Khz. (You may have to change the built in tuning capacitors.) They seem to be available on ebay.

Les.
A good many years ago (40?) I called in a small 'junk' shop one day (not there any more I'm afraid) while we out in the van at work, and we'd parked just a couple of houses away from it on a job. The only item that's stuck in my mind was a HUGE box (think 45 gallon drum) full of various small IF transformers.

I didn't have any money on me, as I was at work, and I always meant to call back - but never did :(
 

earckens

Member
A good many years ago (40?) I called in a small 'junk' shop one day (not there any more I'm afraid) while we out in the van at work, and we'd parked just a couple of houses away from it on a job. The only item that's stuck in my mind was a HUGE box (think 45 gallon drum) full of various small IF transformers.

I didn't have any money on me, as I was at work, and I always meant to call back - but never did :(
You would have made a fortune these days on that 45 gallon drum! Retired in style ;). Or maybe that shopkeeper did.
 

earckens

Member
How should an ferrite rod antenna be connected to the diagram in my initlal post?

The input signal on the schematic is for an unbalanced input, a ferrite rod has two (balanced) connections. How to proceed best?

Would this (attached) do it (center tap to RF filter, one end to GND, tuning cap end-to-end (what value for the cap?)..
 

Attachments

Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
That would work, but usually they simply wind an extra winding on the rod to feed the transistor, rather than using a tap, as in this example:


You could simply use a standard MW ferrite aerial, and add extra capacitance across it to tune it down a bit.

You can't use the entire superhet radio because of the clashing IF frequency.
 

earckens

Member
That would work, but usually they simply wind an extra winding on the rod to feed the transistor, rather than using a tap, as in this example:


(....)
Thanks Nigel. Would you know the values of C1, C2, C4, R1 and the amount of windings for both coils; approximatively (490kHz)?
 

earckens

Member
Also, I think ferrite antenna's are directional; but the application is on a boat. Therefor should I use two ferrites in a horizontel plane at 90° angle?
How to connect both?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks Nigel. Would you know the values of C1, C2, C4, R1 and the amount of windings for both coils; approximatively (490kHz)?
No, but R1 simply biases the transistor, C4 is a decoupler (0.01uF should be fine, not critical), and C1/C2 are just the tuning capacitor and trimmer - tuning capacitors for AM are usually up to about 200-300pF maximum.
 

JimB

Super Moderator
Most Helpful Member
My interest has been piqued by this thread.

NAVTEX, is transmitted on two frequencies 490kHz and 518kHz.
The signal is basically a 100bps teleprinter type signal. (no it does not use the old 5bit CCITT no2 code).

As I sit typing this, the receiver behind me is receiving data from Rogaland Radio near Stavanger in Norway.
The data is being displayed on a laptop computer running a bit of software called Frisnit.

Getting back to the receiver in the first post of this thread, I was confused as to how it worked.
It is basically a direct conversion receiver, basically a superhet where the intermediate frequency is zero, ie the conversion is straight from RF to AF.

The incoming 518kHz NAVTEX signal is is mixed with the local oscillator at 516.5kHz, to give an audio signal at 518 - 516.5 = 1.5kHz.
This audio signal is filtered by the op-amps and fed to the decoder.
( The little sketch of the filter passband shows the centre frequency to be 1.4kHz. )

Coming back to:
How should an ferrite rod antenna be connected to the diagram in my initlal post?
I would suggest like this:
NAVTEX with Ferrite Rod.png
The RF amplifier is a cascode circuit using FETs, so it is safe to assume the the input impedance will be high, so connecting the tuned circuit of the ferrite rod antenna directly to the gate of the FET will not result in any loading of the tuned circuit.

However, be aware that using the ferrite rod antenna on the bench with all sorts of other equipment could result in a lot of noise which swamps the wanted NAVTEX signal.

JimB
 

earckens

Member
Hi JimB,

thank you for your reply!

My intent is to initially build an active antenna for an existing Navtex receiver: the commercial grade antenna sells for €100 approx.

For that I thought of using the RF filter and cascode amplifier of the schematic in my first post. Since long or medium wave signals are better captured with a ferrite antenna I wanted to add this to the initial RF filter stage (hence my question in post #9 on converting a balanced ferrite rod output to the unbalanced input of my first post).

May I presume from your post that this RF filter effect is obtained from the ferrite rod with trimming cap?

Does the idea of combining a resonator (ferrite coil + trimming cap) with an RF 465kHz IF transformer (such as the input RF filter stage in the schematic of my post #1) result in unnecessary complication?
 

JimB

Super Moderator
Most Helpful Member
OK, if you want to build an active antenna, consider something like this:

MW Amplifier.JPG

A rummage around in my dead projects archive revealed this, shown above.
It is a medium wave active antenna.

In the 1980s when I lived in Shetland, medium wave radio stations were a bit weak during the day, so I built this amplifier.
There is a ferrite rod, a tuning capacitor and an amplifier.
The orange wire is an external wire antenna about a metre long.
The cable with the spiral wrap brings power from the main receiver, and takes the amplified signals back to the main receiver.

The schematic is as shown here:
Amplifier.png

The circuit originated in some magazine, Practical Wireless probably.
I think that the magazine is long gone, so I spent a happy hour or so reverse engineering the schematic.

JimB
 

earckens

Member
Fantastic JimB!! Thank you so much for that effort.

What is the TR1, meaning: its physical description? Any idea with what descriptors to find that transformer?

Also: here a balanced amplifier is used, what would be the advantage/disadvantage over a cascode amplifier using one end of a ferrite rod antenna (the other end being earthed)?

Grts, Erik

Edit: I can see a minuscule toroid transformer (TR1); self made? Would it matter what size that toroid is?
 

JimB

Super Moderator
Most Helpful Member
I can see a minuscule toroid transformer (TR1); self made? Would it matter what size that toroid is?
Yes, it is something which I wound myself.
There appears to be 8 turns on the primary and 6 turns on the secondary.
The core is 0.23 inch outside diameter, and the core material is probably a 61 or 67 mix.

I don't think that the thing is critical, if you can find an FT23-61, FT23-67, FT37-61 or FT37-67, any one of those will be OK.
Obviously FT37 cores will be bigger (0.37 inch) and can accommodate a few more turns on the windings.

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The circuit originated in some magazine, Practical Wireless probably.
Looks more like an American magazine project?, as they were always very keen on using little toroids, while the UK wasn't.

There's a few things I don't like about it as well.

The entire balanced nature of the front end seems pretty pointless, and doesn't work well with the tuning capacitor, one side of which is designed to be grounded, so won't help the balancing - and then they even stick an external aerial directly on the end of the high impedance 'balanced' tuning coil, so heavily damping the coil, and completely unbalancing the front end.

The conventional way of adding an external aerial is to have an extra small winding on the ferrite rod, to provide a more accurate impedance match and not dampen the tuned circuit. An added bonus of course, in this case, is it wouldn't upset the balancing.

But an intersting little circuit, and nice to see it still exists after all these years.
 

earckens

Member
(....)

NAVTEX, is transmitted on two frequencies 490kHz and 518kHz.
The signal is basically a 100bps teleprinter type signal. (no it does not use the old 5bit CCITT no2 code).

As I sit typing this, the receiver behind me is receiving data from Rogaland Radio near Stavanger in Norway.
The data is being displayed on a laptop computer running a bit of software called Frisnit.

(....)
The RF amplifier is a cascode circuit using FETs, so it is safe to assume the the input impedance will be high, so connecting the tuned circuit of the ferrite rod antenna directly to the gate of the FET will not result in any loading of the tuned circuit.

However, be aware that using the ferrite rod antenna on the bench with all sorts of other equipment could result in a lot of noise which swamps the wanted NAVTEX signal.

JimB
Where can I find the Navtex transmission protocol? I intend to eventually build my own RF receiver (with a cascode setup in the input stage) as well as message decoder using a controller.

The antenna would be 3 ferrite rods electrically connected in parallel and physically at 60° angles in horizontal plane.

Frisnit: impressive site; do you use the Java app or the windows download?
 

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