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My receiver is about 5 years old and I cut the antenna short enough to fit the internal connection in the 3" WTC that came with my Thor Permit. I plan to use the receiver in an oil tanker (target) I'm building from scratch. Problem is I'm not sure the 3" stub of antenna on the receiver is going to work. I can add a length of antenna that matches what came with the WTC but is that necessary for a surface ship?
Thanks, the 3" stub worked fine on the bench. Then I spliced on another 18" and that worked fine also. Not sure what the difference would be once I get her in the water and farther away from me.
Antenna theory is way past my pay-grade, but it's enough to know that the optimum antenna length is some whole-fraction of the radio wave the receivers detection circuit is designed to see. A resonate circuit (which is what the detector is) is a circuit that will oscillate in response to the specific frequency, and pass that signal on into the de-modulation portion of the receiver. Again, the antenna is sized to intercept either the whole or some whole fraction (harmonic) of the desired signal frequency.
The antenna, one leg of the inductive-capacitive detector circuit, is optimized for a specific band of frequencies. If the antenna is a bit too long or short, the detector is a bit out of tune, but still able to resonate to the assigned frequency and it's attached intelligence (modulated coding of transmitter commands), but not out to a range were a better tuned 'front end' would.
75 mhz is approximately a 4.5 meter wavelength. From my HAM radio studies a while ago I seem to remember that to optimize (transmission) an antenna was often 1/2 wavelength. Sometimes this could be 1/4 wavelength. Dan Roe might have a better skinny on this than me, but I used to build antennas for the 2 meter band, and I'd usually start around 1/2 wavelength, but I know one could get away with 1/4 wavelength. 1/4 wavelength is about 1 meter for us on 75mhz....basically 3 feet.
There was some interesting discussion on aerials and underwater transmission on another forum. it seems that all theory related to antenna length is null and void once we submerge our boats.
What transpired is that the biggest attenuation takes place during the transition from air to water- if the TX aerial was dunked into the water, range would be much greater than if we stand with it in the air. For that to be practical would require a separate tuner.
I read an interesting research article that concluded that best shape of aerial for underwater use appears to be a bow-tie bent into the shape of a hoop.
OK, I'll wade in here (no pun intended) with my two pfennigs worth. I was involved with the discussion on the 'other forum' on antenna length. I'm an amateur radio operator with over 50 years of experience designing antennas for various purposes so I have a little background here.
The problem with all the theoretical calculations for antenna length are that they are valid ONLY if the antenna is in a vacuum - but dry air is close enough for our purposes.
When we submerge the antenna in water then something called 'velocity factor' kicks in to modify any calculated antenna length, i.e. makes the required length shorter. How much shorter depends on the conductivity of the water.
Also the theoretical calculations are for a straight antenna with no appreciable conductors in the immediate area.
When we 'jam' an antenna into a WTC all curled up or wound in a coil (inductance) or back on itself (capacitance) or near any other conductors (power wires, whatever - inductance/capacitance) all our best laid plans calculations are out the window.
Best idea: don't worry about it. Use what you have - if it doesn't work increase the antenna length or run the antenna outside in the wet as straight as possible. The commonly held idea that antennas can't be run in the wet is nonsense. Seal off the end of the antenna if you're using insulated wire so water can't wick up the inside between the insulator and the wire.
Don't worry about the frequency you're using. The theoretical differences in received signal strength between 35, 40 , 50, 72, or 75 MHz are just that, theoretical. There's a reason the 12 inches to the foot boys use the ELF frequencies; they penetrate water very well. Our commonly used frequencies work well enough for our purposes.
Dan, K8XW
Born in Detroit - where the weak are killed and eaten.
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