On 08/04/2010 07:01 PM, Mike Parkin wrote:
> <mikenjulie.parkin@btinternet.com>
>
> Frans,
>
>>> It is recommended to use a splitter and a separate antenna for
>>> reception
> of
>>> the glidescope info.
>
> This statement is incorrect!!! The installation instructions for the
> SL30 recommends a single antenna installation.
I was not parotting a product manual, but explaining simple physics. The
glidescope signal is transmitted somewhere around 330 MHz, while VOR/LOC
is transmitted around 110 MHz. A quarter wavelength antenna for 110 MHz
is simply far too long for 330 MHz, it is then acting as a 3/4 wave
antenna, which has no impedance of 50 ohms, the impedance the coax cable
is designed for. Losses are huge if you try to implement it this way,
and no product manual can change this simple fact of physics. IF you go
this route, a lot depends on the actual antenna configuration and lentgh
of the cable, cable routing, and so forth. (With a 50 ohms impedance the
length and routing of the cable doesn't matter, but with a huge mismatch
cutting a centimeter of a cable can make a significant difference) For
the average homebuilder without measuring equipment it is a matter of
simple luck and trial and error... or one can go the safe route and
install a splitter and an antenna which is optimized for 330 MHz and
outputs a nice 50 ohms impedance at this frequency. Pick your choice.
Complaints about the SL-30 manual should be directed to Garmin, not to
me. ;-)
Said this, I revert to my original statement that a simple dipole most
likely offers more signal at this mismatched 330 MHz frequency than some
fancy commercial antenna specifically built for 110 MHz.
In my setup this works well, but again, it IS a mismatch and the result
is probably hard do duplicate because it is dependant on cable length
and other hard to determine properties. Relocating the antenna can make
a difference, if not just only for a different cable routing and length.
If I had to fly IFR and my life would be depending on proper signal
reception, I would for sure use a dedicated 330 MHz antenna and a splitter.
Frans
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