Josh is correct....the antenna will work fine without being cut if you aren't going to transmit on it.
Josh, I thought any of the forums were for inquisitvie folks.....as we all listen to scanners....nosy by nature aren't we. I wouldn't be making my living in the radio field had I not asked a lot of questions and tried and erred many times over. I wish I had the internet, and a forum like this, to ask all of my questions 20 years ago when I was learning about antennas and radios.
I see a lot of the same questions over and over...asked a different way. I had to remind myself the other day when I saw one of these questions...it's okay! If I don't want to hear the question again or don't have anything to add, I just won't log on.
Ok, here's a quick comparison for you, I just did this yesterday...
Ford F150 pickup, distant 155 MHz signal, AOR8200 scanner
RadioShack glass mount scanner antenna on rear glass : 1bar
1/4 wave 2 meter well used beat up magmount on the roof: 4 bars
I use the 1/4 wave and my FT100 ham rig as my main RR monitor, it hears about as well as anything would on a mobile although I keep itchin to try a 5/8 wave for those signals not quite out of the noise.
the difference is about 2' of additional antenna length for what ends up being a giant CB-Whip look-alike.
The problem I experience with the 5/8 wave is that performance is severely degraded while in a moving vehicle at speeds above around 50mph. The whip begins to blow all around in the wind and also bend down. A basic quarter wave will stand up and fare a lot better.
3/8? (;->) OK, your answer is 43 3/4" assuming 5/8 which is 0.625 BUT like the man said it will receive OK just the same without cutting it down. Likely the actual cut is 0.64 which doesn't come out to any reasonable fraction and for that it's 45".
Flip a coin or tune it a bit at a time with an SWR analyzer until it resonates where you want it (161 MHz). Manufacturer's published specs are confusing and frankly I have never used a ham antenna mobile, commercial antennas are my choice. They come with cutting charts that usually are so close no fine tuning is needed, the SWR comes out right on the money first try.
"RadioShack glass mount scanner antenna on rear glass..."
"...1bar, 1/4 wave 2 meter well used beat up magmount on the roof: 4 bars."
Uh huh, does the phrase "I told you so." keep ringing in your ears? (;->)
"I keep itchin to try a 5/8 wave for those signals not quite out of the noise."
Well, scratch it. The results will be less than astronomical but the couple of dB you get out of it'll surely help.
OK Natty, now that you've read the confusion unless you have access to an MFJ, Autec or other analyzer maybe you've decided to leave well enough alone. The little bit you'd gain retuning the antenna probably isn't worth it unless like me you're a perfectionist.
NJ Rail 4192 Brielle, wye 5 at Long Branch, clear of the Bay Head running track.
Josh, I never had that problem with commercial antennas, specifically the Antenna Specialists VHF Hi Band antenna I used on 2M. At highway speed it tended to howl a bit but a few MPH either side it shut up. Right about the CB look-alike but it was hardly gigantic, little difference between the ASP HF used for CB years ago, the VHV Lo used for 6M or the VHF Hi used for 2M. It's all in the "base load" coil but unless you look at the marked frequency range it's hard to tell the difference.