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?

Thank you in advance for any help.
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