18” whip is for what frequencies?

petbytes · #1 (**permalink**) 01-18-2013, 12:53 PM

I have a discontinued Radio Shack scanner/ham mobile magnetic-mount antenna. When I bought it, it came with 3 whips in different sizes. I can only find the 18” whip (exact size is 18 1/8”).

Can you tell me what frequency range it’s for?

Thanks much!

ridgescan · #2 (**permalink**) 01-18-2013, 1:01 PM

Half-wave @ about 300mHz

Here's a fun tool
http://www.csgnetwork.com/antennagen...eqlencalc.html

Don't forget, you can cut that whip to tune it up to UHF and above. Just key mHz desired in and that calculator will tell you how many inches.

902 · #3 (**permalink**) 01-18-2013, 1:07 PM

That length will work best for VHF high band. That's what's called a "quarter wave" antenna.

monitor142 · #4 (**permalink**) 01-18-2013, 1:07 PM

An 18" whip is a VHF quarter wave at or around 150MHz.

If I'm thinking of the right antenna you speak of, it's got the 18" top whip, a 6" to 7" UHF whip, and a rigid 3"-4" 800MHz whip at the base all connected via coils.

VHF-Hi, UHF, and 800 and combined maybe some form of VHF-Lo coverage.

-M142

petbytes · #5 (**permalink**) 01-18-2013, 1:25 PM

Thanks much!

awattam · #6 (**permalink**) 01-18-2013, 2:36 PM

So let me ask this then. I think I have the same antenna so would it work better at the 800mhz range if one were to leave the top two sections off. Is this what is meant by cutting the antenna? Also, would it be a good idea to get a whole bunch of steel rod and make custom length antennas for different frequencies to screw into the base?

I know these may sound like silly questions but right now I am trying to experiment with my antennas rather than go out an buy new ones.

ridgescan · #7 (**permalink**) 01-18-2013, 2:53 PM

Why is it silly? Experimenting is where the fun and learning's at.

monitor142 · #8 (**permalink**) 01-18-2013, 3:17 PM

Quote:

Originally Posted by ridgescan

Why is it silly? Experimenting is where the fun and learning's at.

I couldn't have said it better!

LtDoc · #9 (**permalink**) 01-18-2013, 4:19 PM

"Cutting an antenna."
That typically means 'fine tuning' an antenna, making small adjustments in length. You start with an antenna who'z length is longer than needed but in the 'ball-park'. Then you make adjustments in that length to get it more 'right'. The biggy with that is that if you go too far, it's awful hard to put back, you know? The closer you get to 'right' the smaller the 'bites' you cut off of the thing. Make sense?
- 'Doc

(The 'catch' to tuning an antenna is that if you change that antenna's environment, mounting location, etc, you probably will need to re-tune it. That means that in some cases it may be too short, and that isn't very easy to get around sometimes. Not something you have to spend a -lot- of worry over, but keep it in mind.)

902 · #10 (**permalink**) 01-22-2013, 9:58 AM

For the sake of learning, here's the formula for deriving the length of a quarter wavelength antenna:

[(468/frequency)/2]*12 = length of your quarter wave antenna in inches,
or
Frequency = 1/[((length in inches/12)*2)/468]

Using that formula (which can be streamlined, btw), your 18-1/8" antenna is "perfectly" resonant at 154.9125. Adding an 1/8" to it would make it "perfectly" resonant at 153.865. Making it 18" on the dot would make it resonant at 156 MHz. You can see how easily things can get lost in minutia. That 1/8" isn't going to matter if you're only receiving.

prcguy · #11 (**permalink**) 01-22-2013, 12:03 PM

For calculating 1/4 wavelengths I find it easier to use 234 as the constant which saves keystrokes over using 468 (for figuring half wavelengths) then dividing by 2.

So, for a 1/4 wavelength use 234/frequency*12 for length in inches.
prcguy

Quote:

Originally Posted by 902

For the sake of learning, here's the formula for deriving the length of a quarter wavelength antenna:

[(468/frequency)/2]*12 = length of your quarter wave antenna in inches,
or
Frequency = 1/[((length in inches/12)*2)/468]

Using that formula (which can be streamlined, btw), your 18-1/8" antenna is "perfectly" resonant at 154.9125. Adding an 1/8" to it would make it "perfectly" resonant at 153.865. Making it 18" on the dot would make it resonant at 156 MHz. You can see how easily things can get lost in minutia. That 1/8" isn't going to matter if you're only receiving.

jhooten · #12 (**permalink**) 01-22-2013, 12:10 PM

Just use 2808/Freq to get the 1/4 wave length in inches or 2808/length in inches to get the frequency. (468X12/2=2808)

No need making it harder than it has to be.

902 · #13 (**permalink**) 01-22-2013, 10:23 PM

Quote:

Originally Posted by jhooten

No need making it harder than it has to be.

You're right. This is just how I learned it. I end up plugging the formulas into Excel and let it do all the heavy lifting.

W2NJS · #14 (**permalink**) 01-23-2013, 10:17 AM

I learn something new every day, and I've been using 468 and 234 for sixty years, so now I can skip that 12X multiplier to get inches. Thanks, guys.

mformby · #15 (**permalink**) 01-27-2013, 5:40 PM

18" antenna "stinger" is for the VHF range, 134-174 MHz.

Quote:

Originally Posted by petbytes

I have a discontinued Radio Shack scanner/ham mobile magnetic-mount antenna. When I bought it, it came with 3 whips in different sizes. I can only find the 18” whip (exact size is 18 1/8”).

Can you tell me what frequency range it’s for?

Thanks much!