Quote:
Originally Posted by Auburntsts
The Garmin install manual for their GTX 23 doesn't specify a minimum cable length. Instead it talks in terms of maximum dB loss and the maximum cable length by cable type that will not exceed the dB limit (which for the 23 is 1.5dB)--this is all depicted in a table. For example the maximum length for RG400 (M17/128-RG400) is listed at 6' 1.3".
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Interesting... I had to spend a minute to go find what I was trying to remember. I was looking in the Garmin GTX35R/45R manual, rev 2:
- Section 3.13 Antenna Considerations, final bullet: "The antenna should be attached a minimum of three feet from the GTX35R/45R to prevent RF interference"
- Section 4.5 Note: "Make sure the length is set for the necessary cable loss. Some antennas have minimum cable loss specifications, that may need a cable longer than the physical run in the aircraft".
Also:
- Section 1.9 Transmitter power 125W min at antenna, with max 2dB cable loss
So it seems there is *at least* a 3' minimum length (I'm interpreting it as a physical separation, so the cable likely needs to be a bit longer). The documentation for my CI-105 antenna does not seem to say anything about minimum loss per what was mentioned in the Garmin manual, so perhaps it is safe to assume I don't need to go any *more* than 3'.
What it seems I missed is that there is also a maximum length so as to not exceed the 2dB loss. However, I seem to have gotten a slightly different number for this, using the calculator
here it seems I hit 2dB loss at around 12' 6". So, if I'm understanding this correctly, I need to be between ~3ft (physical sep) and 12'6" (cable length)... yes?
EDIT: Sounds like at least some of the difference between your numbers and mine is the 1.5dB and 2dB max loss... 9'6" and 12'6" respectively according to the calculator linked above.
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