[TowerTalk] antenna for data collection out at sea

[Mark .]

Hello Henry,
Are you asking for a dual-band antenna for 148 MHz and 1.2 Ghz?
It sounds like your application uses two transmitters on different bands.
Horizontal dipoles on a shared mast (one dipole for each band )could do 
this. This should produce better overhead sky wave covereage than a 
vertical, j-pole radiator (essentially a vertical, end-fed dipole).

Use separate feedlines if you have two transmitters. The two dipoles can 
also be connected in parallel on the same feedline if your device's 
transmitter works that way.
Wind a tiny choke balun in the coax at the feed point. Try 3-4 turns at 3 
inch diameter. You can also slip on several ferrite beads and secure them 
with heatshrink tubing for a lower-profile balun solution.

As the device bobs in the waterline, the antennas will sees varying heights 
above the seawater, but shouldn't detune enough to make an unacceptable 
match for the transmitter. You mentioned 1800m. Does that mean your device 
also submerges?

An open sleeve matching stup in a traditional 'copper cactus' j-pole can be 
quite subject to detuning from flexing and nearby objects, depending on 
construction.

Hope this helps.

Tell us more about your application.

--...MARK_N1LO...--
  Gloucester, VA

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