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CAP Talk  |  Operations  |  Tools of the trade  |  Topic: Antenna length
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♠SARKID♠
Salty & Seasoned Contributor

Posts: 1,835
Unit: GLR-WI-002

Timmerman Composite Squadron - WIWG - CAP
« on: November 06, 2007, 06:12:32 PM »

I was toying around with my scanner today and wondered, "can you have an antenna on a receiver thats too long?"

I know that you have to have a proper SWR for transmitting.  But is there a point where an antenna is too long for scanning purposes?
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           Capt. Dan Turkal
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                WI-002/CC
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floridacyclist
Salty & Seasoned Contributor

Posts: 926

Tallahassee Composite Squadron
« Reply #1 on: November 06, 2007, 10:41:59 PM »

Yes, antenna length does matter on receive. The closer to resonant, the better the performance.
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Gene Floyd, Capt CAP
Wearer of many hats, master of none (but senior-rated in two)
www.tallahasseecap.org
www.rideforfatherhood.org
Major Lord
Suspended

Posts: 1,817

« Reply #2 on: November 06, 2007, 11:22:42 PM »

Floridacyclist is right that the antenna length should optimally be a function of frequency for optimum perfprmance, generally scanners will use a telescoping whip that approximates 1/4 or 1/2 wave antenna vover a fairly wide bandwidth. Since public service radios generally use quite a bit of power (relatively), having an optimal antenna length is not super-critical.

Your original question though is more technically interesting: Can an antenna be too long? The answer is yes. A long wire antenna can be a very effective antenna for low frequency signals, which can tend to overwhelm the (generally) higher frequencies of interest. A poorly matched random length antenna can "drag" the performance of your receiver down. For monitoring CAP VHF frequencies for instance, a whip longer than 1/4 wave ( about half a meter) will probably not buy any addtional performance, until the antenna hits 1/2 wave length ( one meter) and shows a increase in performance again. Longer than that just makes you a good target for lightning and tripping family members.

Major Lord
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"The path of the righteous man is beset on all sides by the iniquities of the selfish and the tyranny of evil men. Blessed is he, who in the name of charity and good will, shepherds the weak through the valley of darkness, for he is truly his brother's keeper and the finder of lost children. And I will strike down upon thee with great vengeance and furious anger those who would attempt to poison and destroy my brothers. And you will know my name is the Lord when I lay my vengeance upon thee."
♠SARKID♠
Salty & Seasoned Contributor

Posts: 1,835
Unit: GLR-WI-002

Timmerman Composite Squadron - WIWG - CAP
« Reply #3 on: November 07, 2007, 12:04:12 AM »

Okay, thats what I had thought.  Now my next question (and this is my newbie Ham curiosity poking out here): will you get better reception by having an antenna tuned to double the wavelength as opposed to just a single wavelength?  i.e - if the wavelength is exactly 1 meter, will you get better reception out of an antenna 2 meters long as opposed to just 1meter long, or will it not make a difference?
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           Capt. Dan Turkal
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                WI-002/CC
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floridacyclist
Salty & Seasoned Contributor

Posts: 926

Tallahassee Composite Squadron
« Reply #4 on: November 07, 2007, 12:18:23 AM »

Unless you have a fancy matching network, you simply want an odd multiple of 1/4 wave, either 1/4 or 3/4 as those have 50ohms of impedance at the resonant frequency. The 5/8th wave and 1/2 wave antennas always have some sort of gadget or network to match them to 50 ohms and are therefore not as simple or cheap.
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Gene Floyd, Capt CAP
Wearer of many hats, master of none (but senior-rated in two)
www.tallahasseecap.org
www.rideforfatherhood.org
lordmonar
Too Much Free Time Award

Posts: 10,629

« Reply #5 on: November 07, 2007, 02:12:07 AM »

Okay, thats what I had thought.  Now my next question (and this is my newbie Ham curiosity poking out here): will you get better reception by having an antenna tuned to double the wavelength as opposed to just a single wavelength?  i.e - if the wavelength is exactly 1 meter, will you get better reception out of an antenna 2 meters long as opposed to just 1meter long, or will it not make a difference?

When you start going to multiple wave Lents you have to start worrying about multi-path interference, odd phase variance and other "FM" that happens.

In theory...you should get better performance a multiple wave lengths but in practice you don't get enough an improvement to justify the costs.
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PATRICK M. HARRIS, SMSgt, CAP
Pacific Region
Hartley
Recruit

Posts: 36

« Reply #6 on: November 07, 2007, 12:58:25 PM »

Hi Guys,

  Actually, as the antenna becomes longer, it starts to exhibit gain (usually good) and directivity (maybe bad).  For a single element, a half-wave has a null off the ends, and the best performance broadside (a "quarter-wave antenna" is actually a center-fed halfwave - the ground plane is the other half).  As you make the antenna longer, the pattern becomes more & more aligned with the element, with increasingly poor performance broadside. (definitely a bad thing if the antenna is vertical!)
  The only way to increase broadside performance is to break the antenna up into separate elements that are fed in proper phase (a "collinear") - this is what is going on inside those long repeater antennas you've seen.

73 DE Hartley
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fyrpilot2202
Newbie

Posts: 4

Chief Miller at work
« Reply #7 on: November 18, 2007, 02:46:37 AM »

I have found the best scanner antenna to be a VHF 5/8 wave, base loaded [larsen or maxrad] cut the rod for 150 mhz.  Works at 150 mhz, 450 mhz (harmonic), and 6 meters (52 mhz).
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Senior Member - Doug Miller
Pangborn Composite Sqdrn
Washington State
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CAP Talk  |  Operations  |  Tools of the trade  |  Topic: Antenna length
 


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