Quick antenna question

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scan_madison

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I primarily use rubber ducky with the scanner. Couple of days ago I experimented​ with a solid copper wire pulled from AC power cable. The length was about 3 inches and I simply inserted on the bnc port. I was expecting the reception to be poorer than the rubber ducky on 450 MHz band. However I was able to get much better result.

I know a bit about how antenna plays important role in radios. But I don't understand how antenna that is of certain length makes the reception better. Does it have to do with voltage gain or something of that particular frequency signal? If you can explain in a simple way, that would be greatly appreciated. Thanks.
 

br0adband

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Radio waves have a specific wavelength given the frequency, and the closer you can match the length of the antenna to that wavelength the better. Because lower frequencies like longwave and shortwave have incredibly long wavelengths that is the primary reason - if you do research - you'll find such antennas being dozens of feet long when they're done at full size. Thanks to how radio waves propagate a lot of the time you can get excellent results with antennas that are 1/4 of the full wavelength aka 1/4 wave antennas. There's also 1/2 wave, 5/8 wave, and some other odd but workable fractions of the full wavelength.

Ever see someone driving a car or truck that has one of those super long CB antennas mounted on the bumper and it just sways back and forth when the vehicle is moving almost like it's in slow motion or something? They're typically ~8 feet long which happens to be just about a perfect 1/4 wave antenna for the 27 MHz spectrum where most of the CB radio frequencies happen to be from 26.9650 MHz to 27.4050 MHz - the middle of that range is about where channel 20 is (CB radios have 40 channels since the late 1970s, prior to that it was just 23) at 27.205 MHz. If you use an online antenna wavelength calculator you can find that at that frequency a full wavelength is 36.2 feet so those ~8 feet long whip antennas at 8 feet make for the 1/4 wavelength.

Another example would be an FM radio antenna on a car - a lot of cars use antenna wires inside the front windshield nowadays but some do still have actual antennas poking from the front or rear fenders depending on the model/manufacturer. In the US those almost always end up being about 31 inches long which is a good match for a 1/4 wave antenna at ~98 MHz which is the center of the FM band (88 to 108 MHz).

The principle holds at any frequency - at 460 MHz (the middle of the 450-470 MHz land mobile radio service band the 1/4 wavelength works out to about ~6.4 inches so that ~3 inch wire you're using becomes a somewhat useful 1/8 wave antenna, more or less. Not perfect by any means but if it's getting you a better signal than the stock antenna does, hey, bonus and it cost you basically nothing at all to do it. ;)

I'm not going to get into the potential gain aspects because that's a can of worms I'm not interested in opening and other people are vastly more proficient with antenna technology than I'll probably ever be, but the basic gist I would say is what I said above: when you can match the length of the antenna to the wavelength you'll get better signal reception but at some frequencies having very long antennas is very impractical hence using fractional lengths that fall upon mathematical points (1/2 wave, 1/4 wave, 1/8 wave, and others).

Paperclips and just straight pieces of wire can sometimes work very well in some situations as you've just discovered. Experimenting is fun to do and learning about antennas and how to make your own simple ones can be pretty awesome at times. I regularly construct odd things from spare wiring or coat hangers like 1/4 wave ground plane antennas and they work remarkably well more often than not.

Also, if you're serious about the hobby of monitoring I highly recommend getting at least two antennas as part of your "tool kit" for monitoring: something like a Diamond RH77CA antenna which offers great general purpose "wideband" reception capability for anything from about 50 MHz up to 950 MHz (it's designed for 144-148 and 420-450 MHz but does well across other bands as well above and below those) and also an actual telescopic whip which can be adjusted to a very wide range of frequencies given it's fully collapsed state (the highest frequency it would be "tuned" for) and also the maximum extended length (the lowest frequency it could be tuned for). Having both would help in most any situation and be a rather dramatic improvement over any stock scanner antenna.

Piece of advice: be careful with using wiring stuck in the BNC jack "pin hole" - it can work as you've just proven yourself, sure, but just be aware and be careful when doing it even so. While creating a short between the center main conductor and the ground of the BNC port shouldn't damage a modern scanner (since it's not transmitting which would destroy it if that situation happened during an attempt to transmit) there's always the potential of a static discharge back into the radio circuitry which could do severe damage and render it inoperable so again, be careful and ground yourself whenever possible.

As winter is upon us now (in some areas much worse than others) the chance for static electricity is actually a bit higher considering a lot of people wear more clothing during such colder months. Your info says you're in Madison, WI (been there, done that, Racine for a while too) the winters aren't necessarily kind so keep warm too. :)

Hope this helps...
 
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scan_madison

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Thanks a lot for great explanation br0adband! Now I feel I definitely know more about the antenna. One thing I still feel confused is the way radio processes the particular frequency signal. Is it similar to using a funnel with certain diameter hole letting only certain sized items while blocking other? Or am I thinking too hard? Oh, I'm not currently in Madison. But I know what you mean. Thanks.
 

teufler

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Design a Dipole 468 divided by the frequency in mhz is a dipole, or 234 divided by the frequency in mhz would be a quarter wave. a dipole is basically 2 quarter waves. your 3 in antenna is half what to what a quarter wave should be. So 234 divided by 450 is .52 feet. . Now if the signal arriving at your radio is strong, about any wire length will work. If you are going for an outside antenna, this is the loss you would have for the different brands of coax. Coax Loss Calculator
Just accept that the antenna makers have all done the calculation. You see some police units that are using a quarter wave for vhf as a scanner antenna. 18 inches covers the vhf spectrum and 18 inches is a gain antenna for uhf. The quarter wave of .52 feet, close to a multiple of,52 feet. A story that bigger is better.
 
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