Antenna length

[♠SARKID♠]

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?

[floridacyclist]

Yes, antenna length does matter on receive. The closer to resonant, the better the performance.

[Major Lord]

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

[♠SARKID♠]

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?

[floridacyclist]

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.

[lordmonar]

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.

[Hartley]

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

[fyrpilot2202]

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).