MW Signal Coupling

[amdx]

I have an AM radio on an aluminum boat. The radio is shielded on
six sides by aluminum. I want to mount an antenna on the boat and
run coax inside and couple the signal to the ferrite rod in the radio.
I'm thinking about an active whip antenna with a 50 ohm output. If I
did this, I would have a coax with 50 source impedance at the radio!
How do I create a 50 ohm load at the radio over the AM band? The radio
has a 2-3/8" x 3/8" ferrite rod.

I would appreciate any other solutions to get a signal to my radio.

Mike

 

[John Crighton]

On Wed, 28 Apr 2004 11:58:28 GMT, "amdx" <amdx@att.net> wrote:

I have an AM radio on an aluminum boat. The radio is shielded on
six sides by aluminum. I want to mount an antenna on the boat and
run coax inside and couple the signal to the ferrite rod in the radio.
I'm thinking about an active whip antenna with a 50 ohm output. If I
did this, I would have a coax with 50 source impedance at the radio!
How do I create a 50 ohm load at the radio over the AM band? The radio
has a 2-3/8" x 3/8" ferrite rod.

I would appreciate any other solutions to get a signal to my radio.

Mike

Hello Mike,
is this just a regular old cheap transistor radio that you want to
listen to while painting the decks or something special? The
reason I ask is something like an active whip sounds expensive.
How much will that cost?
What sort of vessel, how big, what purpose?

If you have say a common 100 W output, HF, SSB transceiver
on your boat, already using a whip, then this active whip situated
close by might suffer from the strong RF and you would have
wasted your money.

I have used a special aerial coupling transformer for ships
commercial radio receivers. It was a box about the size of
your fist with a porcelain insulator, earth bolt and coax connector.
Just contained a ferrite torroid and a few components. Some
diodes, back to back to act as a voltage limiter to protect
the receiver. Passive only.
Not as good as a proper tuning unit giving 50 ohms out for the
coax lead-in but still good enough. Debeg and Marconi
Marine were the brands I saw. Contact marine radio service
companies in your area and speak to the radio technicians,
and they might give you something for nothing from their
junk. Don't talk to the female dragon or the boss, they will
want money. Grab hold of a technician.

Me being a cheapskate, I would run the coax to a small box
containing a crude homemade tuner of sorts, just a tapped
coil for starters and if I was keen maybe a variable capacitor.
Crystal set type tuning arrangement. Selection of taps for
the aerial connection and coax inner connection.
Have a look in the ARRL handbook for ideas.
You might even get away with just the end of the coax
braid to the metal hull/structure and inner to your whip.
Try it and see before getting too techo.

At the radio end, I have seen crude modifications work
quite well, BNC connector fitted to the case, and just
several turns of hook-up wire loosely coupled to one
end of the ferrite rod. There might have been a series
resistor of 50 ohms or less, can't remember, have a play
and see how you go. On the bench with a signal generator
would be ideal for experimenting.

Have Fun
John Crighton
Sydney

 

[amdx]

"John Crighton" <john_c@tpg.com.au> wrote in message
news:408fb055.10556452@News.individual.net...

On Wed, 28 Apr 2004 11:58:28 GMT, "amdx" <amdx@att.net> wrote:

I have an AM radio on an aluminum boat. The radio is shielded on
six sides by aluminum. I want to mount an antenna on the boat and
run coax inside and couple the signal to the ferrite rod in the radio.
I'm thinking about an active whip antenna with a 50 ohm output. If I
did this, I would have a coax with 50 source impedance at the radio!
How do I create a 50 ohm load at the radio over the AM band? The radio
has a 2-3/8" x 3/8" ferrite rod.

I would appreciate any other solutions to get a signal to my radio.

Mike

The reason I ask is something like an active whip sounds expensive.

The reason I started with an active whip-- I saw an article in the Sept
2001
QST and thought I would build it and have an high end active whip for future
use.

If you have say a common 100 W output, HF, SSB transceiver
on your boat, already using a whip, then this active whip situated
close by might suffer from the strong RF and you would have
wasted your money.

I don't have any other antennas on the boat.

I have used a special aerial coupling transformer for ships
commercial radio receivers. It was a box about the size of
your fist with a porcelain insulator, earth bolt and coax connector.
Just contained a ferrite torroid and a few components. Some
diodes, back to back to act as a voltage limiter to protect
the receiver. Passive only.
Not as good as a proper tuning unit giving 50 ohms out for the
coax lead-in but still good enough. Debeg and Marconi
Marine were the brands I saw. Contact marine radio service
companies in your area and speak to the radio technicians,
and they might give you something for nothing from their
junk. Don't talk to the female dragon or the boss, they will
want money. Grab hold of a technician.

Me being a cheapskate, I would run the coax to a small box
containing a crude homemade tuner of sorts, just a tapped
coil for starters and if I was keen maybe a variable capacitor.
Crystal set type tuning arrangement. Selection of taps for
the aerial connection and coax inner connection.
Have a look in the ARRL handbook for ideas.
You might even get away with just the end of the coax
braid to the metal hull/structure and inner to your whip.
Try it and see before getting too techo.

At the radio end, I have seen crude modifications work
quite well, BNC connector fitted to the case, and just
several turns of hook-up wire loosely coupled to one
end of the ferrite rod. There might have been a series
resistor of 50 ohms or less, can't remember, have a play
and see how you go. On the bench with a signal generator
would be ideal for experimenting.

I"ll probably put a few turns of wire on the ferrite rod in the

radio and install a BNC connector on the radio case. Then
start experimenting with antennas outside the boat.

Thanks, Mike

 

[GPG]

Wrap acoil of about 4 turns on rod at end away from tuning coil.
Adjust distance for coupling.