Changing ferrite antenna for external omnidirectional

[ChiPete]

Hi all,

I've got a Morer Navtax unit which I'm very please with it given it's size and portability. The one issue I have with it is the reception, or rather, lack of. So! Now it's out of warranty, I plan to make a mod and replace the internal ferrite rod with an external omnidirectional antenna.

I've sourced the tuned 490-518Mhz passive antenna from ICS Electronics - very fair price including a pushpit mount and just around the corner from me. I plan to fit a BNC connector to the unit to connect the antenna and ground the co-axial cable shielding to an anode.

There are, of course, two ends to the ferrite coil, let's call them ''A'' and ''B''. Each end is soldered to the PCB at different points - makes sense. When I fit the new antenna, there'll only be one conductor - can anyone (perhaps any radio hams out there who got past the ''Propagation & Antennas'' chapter) give me any pointers as to which end of the coil I should connect the one conductor - ''A'', ''B'', both...?

Cheers,
Pete

 

[lenseman]

. . . . . . . There are, of course, two ends to the ferrite coil, let's call them ''A'' and ''B''. Each end is soldered to the PCB at different points - makes sense. When I fit the new antenna, there'll only be one conductor - can anyone (perhaps any radio hams out there who got past the ''Propagation & Antennas'' chapter) give me any pointers as to which end of the coil I should connect the one conductor - ''A'', ''B'', both...?

Cheers,
Pete

I suspect that the coil that is wound on the ferrite rod is grounded at one end and the other coil end goes to the first stage of RF amplification (TRF, Superhet?).

Your new antenna would need to be connected in place of this coil with then centre connector of your new antenna coax going to the first stage of RF amplification and the outer of the new antenna coax connected to the other end (earthy end) of the ferrite rod coil. As it is a new passive antenna, there will be little chance of letting the box of smoke escape.

Without seeing the exact circuit layout it can be difficult to suggest further?



.

 

[Nigel-in-Oxford]

The coil on the ferrite will be part of the tuned circuit used for tuning the receiver into the signal, leave well alone. However you can couple the ferrite antenna to the external one by wrapping a few turns of copper wire (the sort with varnish on that is insulated) around the ferrite and connecting the ends to the core and screen of the coax appropriately. Best wound on the end of the ferrite away from the main coil if there is room. You will have to experiment to get the number of turns right.

 

[maby]

Hi all,

I've got a Morer Navtax unit which I'm very please with it given it's size and portability. The one issue I have with it is the reception, or rather, lack of. So! Now it's out of warranty, I plan to make a mod and replace the internal ferrite rod with an external omnidirectional antenna.

I've sourced the tuned 490-518Mhz passive antenna from ICS Electronics - very fair price including a pushpit mount and just around the corner from me. ...

You do mean kHz, don't you? Navtex runs at medium wave frequencies, not UHF.

 

[Bilgediver]

Hi all,

I've got a Morer Navtax unit which I'm very please with it given it's size and portability. The one issue I have with it is the reception, or rather, lack of. So! Now it's out of warranty, I plan to make a mod and replace the internal ferrite rod with an external omnidirectional antenna.

I've sourced the tuned 490-518Mhz passive antenna from ICS Electronics - very fair price including a pushpit mount and just around the corner from me. I plan to fit a BNC connector to the unit to connect the antenna and ground the co-axial cable shielding to an anode.

There are, of course, two ends to the ferrite coil, let's call them ''A'' and ''B''. Each end is soldered to the PCB at different points - makes sense. When I fit the new antenna, there'll only be one conductor - can anyone (perhaps any radio hams out there who got past the ''Propagation & Antennas'' chapter) give me any pointers as to which end of the coil I should connect the one conductor - ''A'', ''B'', both...?

Cheers,
Pete

If this is the antenna you have your eyes on then be aware it probably needs a 5 volt supply up the coax to power it. A basic circuit will be needed to feed the DC supply and pass the received signal to your navtex unit. I would speak to the ICS technical team and who knows they may sketch out a power supply etc if needed. If it was me I d just connect a long wire to your Morer unit via a wee capacitor to where you were going to connect the coax or via mutual inductance by winding a wee coil on the ferrite rod and connecting this coil to the long wire and ground.

http://www.icselectronics.co.uk/leisure/antennas/passive

If you are looking at the older passive antenna which was a wee dipole on a mushroom then this is not for you as ICS would confirm.

 

[nigelmercier]

If this is the antenna you have your eyes on then be aware it probably needs a 5 volt supply up the coax to power it...
http://www.icselectronics.co.uk/leisure/antennas/passive...

Don't think so, this is the passive version.

Got a link for the purchase of the ICS antenna Pete?

 

[William_H]

The coil on the ferrite will be part of the tuned circuit used for tuning the receiver into the signal, leave well alone. However you can couple the ferrite antenna to the external one by wrapping a few turns of copper wire (the sort with varnish on that is insulated) around the ferrite and connecting the ends to the core and screen of the coax appropriately. Best wound on the end of the ferrite away from the main coil if there is room. You will have to experiment to get the number of turns right.

+1 is my guess too. olewill

 

[ChiPete]

All, thanks for all your responses, greatly appreciated.

You do mean kHz, don't you? Navtex runs at medium wave frequencies, not UHF.

Doh! kHz not mHz, just checking you're paying attention!

BD and Nigel, this is the antenna: http://www.icselectronics.co.uk/leisure/antennas/passive.

I do believe it is indeed passive; no mention of power supply is made.

Here's the unit PCB:



As you can see, there's not a great deal of room on the ferrite rod to wind any further turns on. One end of the coil terminates and links to the brown-cased potentiometer which I'm guessing is the circuit tuner. The other end (the earth?) terminates to the PCB and disappears into the layers.

Is there any reason I MUST make the link between the ferrite rod and any external antenna through turns onto the rod? It does seem as though there is going to be some experimentation required but given the physical size constraints of the unit, if at all possible, I'd prefer to go with the connect the external antenna/long wire to the board solution.

Cheers,
Pete

 

[Bilgediver]

All, thanks for all your responses, greatly appreciated.



Doh! kHz not mHz, just checking you're paying attention!

BD and Nigel, this is the antenna: http://www.icselectronics.co.uk/leisure/antennas/passive.

I do believe it is indeed passive; no mention of power supply is made.

Here's the unit PCB:

View attachment 35711

As you can see, there's not a great deal of room on the ferrite rod to wind any further turns on. One end of the coil terminates and links to the brown-cased potentiometer which I'm guessing is the circuit tuner. The other end (the earth?) terminates to the PCB and disappears into the layers.

Is there any reason I MUST make the link between the ferrite rod and any external antenna through turns onto the rod? It does seem as though there is going to be some experimentation required but given the physical size constraints of the unit, if at all possible, I'd prefer to go with the connect the external antenna/long wire to the board solution.

Cheers,
Pete

I would be wary about direct connection as a long wire can do damage if in the presence of strong nearby signals. There is room for a coil at either end between the long coil and brackets. We are only talking about 0 turns or so of fine wire. Just try it and see.

Sometimes holding something like this unit close to a stay is enough to get mutual inductance. On one ship I sailed on I used to clip a wee portable am radio
to the for mast stay. This allowed me to pick up AM stations up to 1500 miles away which were inaudible when away from the stay. Maybe just running a long wire across the end of your unit will be enough.

Yep should have picked up it was a passive

 

[ChiPete]

I would be wary about direct connection as a long wire can do damage if in the presence of strong nearby signals. There is room for a coil at either end between the long coil and brackets. We are only talking about 0 turns or so of fine wire. Just try it and see.

Sometimes holding something like this unit close to a stay is enough to get mutual inductance. On one ship I sailed on I used to clip a wee portable am radio
to the for mast stay. This allowed me to pick up AM stations up to 1500 miles away which were inaudible when away from the stay. Maybe just running a long wire across the end of your unit will be enough.

Yep should have picked up it was a passive

Many thanks for clarifying why the direct connect is not the best solution BD, that does of course make sense.

One query - you state ''0'' turns - is this a typo perhaps??! Did you mean 10/20/etc.?

Cheers,
Pete

 

[William_H]

Yes I am sure he meant 10 turns. The firrite rod coil with "brown cased potentiometer" connected to one end (it will be a vaiable capacitor) make the tuned circuit for the input of the radio. One end will be earth and the other end will be a very high impedance. Hence connection of any coax cable which is low impedance will tend to ruin the tuned circuit performance. Further the capacitance of the cable being high will detune the circuit anyway.
If you connect a passive antenna through a small coil this will be low impedance and so not affect the tuned circuit. Further the coax cable may not affect the carriage of the signal so much. The small coil to large coil will act as a transformer to transform the input signal to a higher voltage. (still microvolts of course)
However you could try a simple wire connected to the non earth end of the coil. The wire will be very short compared to the wavelength of the signal so will appear as a high impedance. So hopefully will not upset the tuned circuit. If there is any way to indicate signal strength then you could try adjusting the variable capacitor with the wire attached for max signal. good luck olewill

 

[Bilgediver]

Many thanks for clarifying why the direct connect is not the best solution BD, that does of course make sense.

One query - you state ''0'' turns - is this a typo perhaps??! Did you mean 10/20/etc.?

Cheers,
Pete

Yes just about 10/20 of fine enamelled wire. This is available from old radios or Maplins.

 

[ChiPete]

William and BD, many thanks for the responses and clarification. I'll head off to Maplins and warm up the soldering iron!

Cheers,
Pete