My wife and I are travelling around the world in our yacht. We are
currently in Thailand carrying out some repairs on the boat.
We have an Iridium Sat Phone that allows us to communicate by voice
and email via the uuplus.com service.
The fixed masthead antenna seems to have developed a fault and we can
find no info in Thailand or on the net about its circuit or test
parameters.
The antenna is a motorola model that has a spiral antenna some 4
inches tall with a small number of surface mount resistors all encased
in a conical plastic housing. The resistance measured by a multimeter
if 55 Ohms on the connector. Is the antenna active or passive.
Thanking you in advance.
Tony Rowlands
S/V Ambrosia

Hi Tony,

I'm 99% sure the antenna is passive, not active. Check the solder
joints where the four helical wire antennae join onto the board at the
bottom. I've had one experience with faulty connections (came loose
due to excessive vibration) here.

If a visual inspection of the antenna otherwise looks OK, check the
coaxial cable, in particular the connector at the antenna end. Has
any water got in here?

Iridium antennas don't have much gain, and any additional losses in
the coax cable and connectors really reduce the receive signal.

Regards
Vaughan Henderson

(Vaughan Henderson) wrote in message . com...
Hi Tony,

I'm 99% sure the antenna is passive, not active. Check the solder
joints where the four helical wire antennae join onto the board at the
bottom. I've had one experience with faulty connections (came loose
due to excessive vibration) here.

If a visual inspection of the antenna otherwise looks OK, check the
coaxial cable, in particular the connector at the antenna end. Has
any water got in here?

Iridium antennas don't have much gain, and any additional losses in
the coax cable and connectors really reduce the receive signal.

Regards
Vaughan Henderson

Vaughan
Thanks for the reply.
I have checked the antenna (at the antenna) with a multimeter and have
55 Ohms resistance form the centre connector to shield. Have also
replaced the coax. There are a number of SMD (resistors i think) with
221 stamped onto them. The 5th component is connected on the ground
plane of the antenna to the centre tap. No markings so don't know what
it is. Whe I first opened the antenna it had a fair amount of water
inside it. Dried it out and lacquered the circuit.
Should the antenna have any connection between the centre tap and
shield? All help is appreciated.
Tony
S/V Ambrosia

In article ,
(Tony Rowlands) wrote:

(Vaughan Henderson) wrote in message
. com...
Hi Tony,

I'm 99% sure the antenna is passive, not active. Check the solder
joints where the four helical wire antennae join onto the board at the
bottom. I've had one experience with faulty connections (came loose
due to excessive vibration) here.

If a visual inspection of the antenna otherwise looks OK, check the
coaxial cable, in particular the connector at the antenna end. Has
any water got in here?

Iridium antennas don't have much gain, and any additional losses in
the coax cable and connectors really reduce the receive signal.

Regards
Vaughan Henderson

Vaughan
Thanks for the reply.
I have checked the antenna (at the antenna) with a multimeter and have
55 Ohms resistance form the centre connector to shield. Have also
replaced the coax. There are a number of SMD (resistors i think) with
221 stamped onto them. The 5th component is connected on the ground
plane of the antenna to the centre tap. No markings so don't know what
it is. Whe I first opened the antenna it had a fair amount of water
inside it. Dried it out and lacquered the circuit.
Should the antenna have any connection between the centre tap and
shield? All help is appreciated.
Tony
S/V Ambrosia

The water is really a dead givaway, that your antenna needed some
maintainance, but the addition of a lacquere to the top of the dried
PCBoard inside the antenna, may well have caused more problems then it
solved. Most UHF and above RF Frequency circuits are designed using
Stripline tuned systems, and by the addition of a lacquere over these
components can cause them to detune from the intended design perameters.
The lacquere adds coapactive coupling and on Stripline tuned circuits
this detunes the stripline components. Most are designed with the
conformal coatings that cover them included, in the design parameters,
so adding more stuff over the top causes problems. If they had no
coating in the original design, then that was not intended to be used in
the final product and addition of any coatings was never intended, and
will cause bad stuff to happen, that was never intended. Now that it is
done, there is no way to undo it and if the lacquere has detuned the
system, it is destroyed and useless, from this point on. Better start
lookking for a replacment.


Me who knows this, by many years of field experience.........

Me
Thanks for the advice. Looks like I will get a new one.
I guess it is a good time seeing that the US$ has gone down against the Aust$.
Do you have any recommendations as to an antenna supplier.
Cruising is great but there is no time for play.
Tony
S/V Ambrosia

(Tony Rowlands) wrote in message . com...
Me
Thanks for the advice. Looks like I will get a new one.
I guess it is a good time seeing that the US$ has gone down against the Aust$.
Do you have any recommendations as to an antenna supplier.
Cruising is great but there is no time for play.
Tony
S/V Ambrosia

Before you spend real money Tony, I'd suspect the fifth component.
The smd devices marked 221 should be resistors, each with a value of
220 ohms. Four of those, effiectively connected in parallel, would
give you the 55 ohms you are measuring on the multimeter. The fifth
component (unmarked) is most likely a capacitor and would be more
liable to be permanently damaged by the water. Difficult to test with
a simple multimeter unless it also has a capacitance range(s). If you
measure across it, it should read some very high value of resistance
(greater than 10 M-ohm). I don't have any idea what it's value would
be, but could be around 100pF (picofarads). If you are up to
replacing smd components and can get one, (should cost less than
50cents) you could try replacing that.

"Me" may well be right that the lacquer coating you applied has
totally detuned the antenna, so all the above may be a waste of time.

Regards
Vaughan Henderson