Why Two Navtex Antennae?

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Babylon

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Boat came with two NASA antennae for the NASA navtex, with a toggle-switch to choose between the two. I have no idea why this is the case - are different frequencies used in different places or for different types of transmission, or could the second one simply be an identical 'backup' in case the first one fails?

BTW, when checking the signal for each antenna, one gives the correct sort of 'bar-graph' signal-reception display (according to the handbook) and recieves data fine, the other has a skewed signal-reception display and receives no data.
 
There is no reason to have two navtex antennae. Must have been for backup, but if one doesnt work, not much point in having it.
 
Navtex is on two frequencies international and local, when the local started a lot of manufacturers inc. NASA brought out another a/e until they could incorperate both in same case.
 
Eh? How would the antenna pick up one frequency and not the other as it all goes into the same box, the reciever is in the box, not in the antenna, I think.
 
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Eh? How would the antenna pick up one frequency and not the other as it all goes into the same box, the reciever is in the box, not in the antenna, I think.

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No, for the NASA gear, the 'active antenna' contains a large chunk of the receiver.
 
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Eh? How would the antenna pick up one frequency and not the other as it all goes into the same box, the reciever is in the box, not in the antenna, I think.

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Most navtex and GPS a/e are active so that only the i.f. frequency has to be sent down the cable to the box thus less loss of signal in the cable
 
But I have an active antenna for my NASA ssb, and it is "active" for all frequencies in the ssb range. I always thought the antenna was an IF amplifier, not part of the receiver, my NASA navtex works with a non active antenna and with the ssb active antenna. So I'm a bit confused now.
 
Slightly different things

The "Active" antenna for the SSB receiver is basically a wideband amplifier, with a bit of impedance matching thrown in.

The "Active" antenna on the Navtex is the front end of a receiver. It converts the received frequency you want to a different frequecy (IF) to feed down the coax to the receiver.

One antenna will change 518 to the IF - the other will convert 4** (I forget the frequency) to the IF

IF is probably a misnomer as it will almost certainly be higher than the received frequency.
 
The reason why you have to antenna is as follows.
Early nasa navtex receivers were single frequency. (518Khz.) When the second frequency became available (490Khz.) the guys at nasa produced a converter antenna to update receivers in service. The converter antenna receives at 490Khz and converts the signal to 518Khz which the navtex can cope with. Simply switching between the two antenna gives you both frequencies on a single frequency receiver.
 
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