In article ,
hunt4mac wrote:

I have an Garmin Astro 220 GPS that uses the MURS frequencies, 151.82 - 154.60 Mhz. Using an online calculator I learn that the antenna should be 18.5
inches for 1/4 wavelength and 9.25 in. for 1/8 wavelength (based on the 151.82 Mhz frequency)

.I was considering buying an antenna on eBay but it is only 8 inches. (eBay item# one, eight, 1, 3, zero, two, 327, 578. Sorry for this workaround, the
admins think I'm posting a phone number).

How much difference would this mismatch cause? Or is it a mismatch?

A lot of the "rubber duck" antennas on the market are not simply
quarter-wave whips. They include inductive loading, which allows the
antenna to be resonant (or close to it) even when it's physically
shorter than a quarter-wave.

The thin-whip "spaghetti-noodle" style generally uses a flexible-
wire whip, and a loading coil at the base (moulded into the BNC or SMA
connector).

Fatter "rubber ducks" often build the inductive loading into the
radiator itself... the radiator is a flexible wire spiral, wound
around a core of some sort (sometimes air, sometimes a supportive
structure such as rubber or plastic).

These "loaded" antennas are generally a good enough match to keep the
transmitter happy (if you have a transmitter... the Astro 220 appears
to be a receive-only device so this isn't an issue).

They tend to be lossy, though. The "short radiator which is
capacitive, resonated by a lumped or distributed inductance" tends to
result in higher current in some parts of the antenna than would be
the case for a true quarter-wave antenna with a decent ground, and
this leads to higher I^2*R losses.

Some short rubber duck antennas are very lossy... as much as 10 dB or
so worse than a full-length whip with a decent counterpoise. This
costs you transmit range and receive sensitivity... the price for
the physical convenience of having a short antenna that doesn't poke
people in the eye :-)

If you're going to need more dog-tracking receiver sensitivity than a
"duck" antenna provides, you'd probably want a full-sized directional
antenna... maybe a 3-element Yagi (search for "measuring tape Yagi"
for DIY plans). These aren't terribly field-friendly, of course.