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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 10-09-2011, 11:03 PM
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Default Magnetic mount CB antenna to scanner antenna mod?

Ok folks. This might be a deep pile of stuff im stepping in here, but i have an old magnetic mount cb antenna that i want to use for my mobile scanner. It has a coil in the middle of it. Ive tried to read up on antennas but I dont understand exactly what they are talking about.

My way of thinking is that I want it to be tuned to 460.475 Mhz, since that is about middle of the road between 800Mhz channels and low band and also that is the sheriffs dept dispatch here.

From what I understand, is that 5/8 is as long as you want to go as it looses gain after that. Is that correct? Originally i was planning a full wave until I read that.

According to this page ( Amateur Vertical Antenna Calculator by dxzone.com ), 460.475 at 5/8 wave is 1' 3 1/4". Now is this measured from the bottom of the base to the top of the antenna with a measuring tape or is the length of wire in the coil factored in with the measurement? There could be 2 to 3 feet of wire wound up in that coil and the coil is only about 3 inches long.
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Old 10-09-2011, 11:47 PM
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Quote:
Originally Posted by jfhtm350 View Post
Ok folks. This might be a deep pile of stuff im stepping in here, but i have an old magnetic mount cb antenna that i want to use for my mobile scanner. It has a coil in the middle of it. Ive tried to read up on antennas but I dont understand exactly what they are talking about.

My way of thinking is that I want it to be tuned to 460.475 Mhz, since that is about middle of the road between 800Mhz channels and low band and also that is the sheriffs dept dispatch here.

From what I understand, is that 5/8 is as long as you want to go as it looses gain after that. Is that correct? Originally i was planning a full wave until I read that.

According to this page ( Amateur Vertical Antenna Calculator by dxzone.com ), 460.475 at 5/8 wave is 1' 3 1/4". Now is this measured from the bottom of the base to the top of the antenna with a measuring tape or is the length of wire in the coil factored in with the measurement? There could be 2 to 3 feet of wire wound up in that coil and the coil is only about 3 inches long.
The CB antenna would be fine for 25-54 MHz reception, but for higher frequencies you need an antenna tuned to that band for best reception. Your best bet would be to buy a ham 144/440 MHz 5/8 antenna and your reception will be fine even for 800 MHz . I use a 144/440 MHz antenna with my BCD996XT scanner and it works great for 150/460/850 MHz.
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Old 10-09-2011, 11:59 PM
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Yes I see.
I dont really want to buy one although its like you say that I would have a better antenna if I buy one that is tuned for the correct frequencies already. I am just trying to fool around with this antenna and see what I can get out of it since its not gonna cost me anything.

I am on a webpage now that explains it pretty well: The Ultimate Guide to 11 Meter CB Antennas

The antenna I have comes apart in 3 sections so by the time im done putting it all together it will be a bottom base loaded antenna which is not that efficient. So Im probably wasting my time with this thing but thats alright I like tinkering with stuff.
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Old 10-10-2011, 12:12 AM
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6" for 460 mhz.....
18" for 156 mhz
3.25" for 855 mhz
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Old 10-10-2011, 7:48 AM
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Mobile CB antenna's are ok for Low band reception but that's about it.
Go get one of these:
Magnet-mount mobile scanner antenna - RadioShack.com

You will be pleased by how well it does around here.
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Old 10-10-2011, 7:58 AM
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Like everyone has been saying,CB radio antennas are made for the CB radio band.It might do alright on low band but on anything higher it won't do much of anything.It is always fun to tinker around with antennas.
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Last edited by Rt169Radio; 10-10-2011 at 8:00 AM..
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Old 10-10-2011, 8:49 AM
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It just depends on how much 'compromise' you think you can put up with. Resonant, or antennas 'cut' for the band of use/interest tend to 'hear' better than non-resonant antennas. How big a difference? No idea of how to quantify that, so "some" is a good general answer. If you have one particular band of interest, then make the length of that antenna for that band. If you want to listen to several bands and still keep things simple, use an antenna that's 'made' for the lowest band of interest (too long is usually better than too short).
Since you already have an antenna, try using it and see how it performs. No, it won't perform -well-, but is it adequate? If not, then try 'adjusting' it, as in chop the thing off just below that center coil. Or just above it for that matter, see what happens. (If it's possible to disassemble it with out destroying it, that's even better!)
[Off topic a little, but the amount of wire used to make a coil doesn't contribute/equate directly to how 'long' an antenna is. That coil has a property called inductance which is one 'part' of making an antenna resonant (the other 'part' is capacitance). A straight section of wire/conductor also has those properties. The right amount of inductance and capacitance added together properly make for resonance. Which basically means that resonant antenna really 'likes' a particular frequency. No, there's nothing scientific about that explanation! It does get the point across though.]
There's lots more to a 'good' antenna than just resonance, but it's also not something that's easy to digest without a lot of effort to understand. A 'forum' isn't the best way to get that knowledge, but it can help.
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Old 10-10-2011, 4:07 PM
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Quote:
Originally Posted by LtDoc View Post
Off topic a little, but the amount of wire used to make a coil doesn't contribute/equate directly to how 'long' an antenna is. That coil has a property called inductance which is one 'part' of making an antenna resonant (the other 'part' is capacitance). A straight section of wire/conductor also has those properties. The right amount of inductance and capacitance added together properly make for resonance. Which basically means that resonant antenna really 'likes' a particular frequency. No, there's nothing scientific about that explanation! It does get the point across though.
So if I understand you right, the coil on the antenna is coiled up to match the cb low bands and is not gonna recieve pretty much anything out of that spectrum without me unwinding the coil and winding back several coils, a couple coils or straight wire. Yes the antenna is 3 sections, the top, the coil and the bottom and the coil comes apart to expose the windings in it.

What im figuring is removing the bottom section and screwing the coil down onto the mag base so that i will have the coil and the top section which eliminates most of the lenght.

I think i understand your coil description, but how does a man measure the coil section for the length of the antenna to get a 5/8 wave length? If it was just a straight wire antenna you would just measure and cut.
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