Antenna Q n A

[zachtheterrible]

I got two questions about antennas.

1. I know that an antenna's length has to correspond w/ the wavelength, it can't be too short, but can an antenna be too long?

2. An antenna can be made shorter by putting a coil in series w/ it. What is the math for this kind of thing? Or can anyone tell me what this is called so i can google it?

Thanx

 

[Nigel Goodwin]

I got two questions about antennas.

1. I know that an antenna's length has to correspond w/ the wavelength, it can't be too short, but can an antenna be too long?

It needs to be tuned to the frequency in question, and to match it's connecting cable, which in turn must match the output of the transmitter. So too long is just as bad as too short - and may be far worse.

Aerial impedance is usually 50 ohms using screened cable, or 300 ohms using balanced feeder.

2. An antenna can be made shorter by putting a coil in series w/ it. What is the math for this kind of thing? Or can anyone tell me what this is called so i can google it?

I've no idea what the formula is, but this type of VHF aerial is generally called a 5/8 whip, a loading coil is used to match the aerial to 50 ohms. This generally isn't 'shorter' though (more than twice as long), but it gives better performance than a standard 1/4 wave whip. Similar techniques are used for CB aerials at 27MHz, where the aerial is far too short for 1/4 wave.

A good source of information are the Radcom books in the UK, or ARRL books in the USA.

 

[zachtheterrible]

Thx Nigel

 

[stevez]

Zach - to expand a little on Nigel's responses.

For a given frequency, changes in the length of an antenna do affect the feedpoint characteristics. For simplicity we often think in terms of 1/4 wavelength, 1/2 wavelength, 5/8 wavelength. A half-wave dipole for 21 mHz would be about 22 ft long and is assumed to have a feedpoint impedance of 50 ohms (it doesn't but to keep it simple we'll stick w/50 ohms). If you made a dipole somewhat longer - say 30 ft - it might not work at 21 mHz. Let's keep going.

Multiples of these can be considered if space allows or your needs dictate. A good example of this - it's common for amateur radio operators to cut a dipole antenna for 1/2 wavelength at 7 mHz (40 meter band) and use it for both 7 mHz and 21 mHz - at both 7 mHz and 21 mHz the antenna can look like about 50 ohms at the feedpoint. At first the 67 ft long antenna might look like it's way too long for 21 mHz but it's not.

A loading coil can help to make a short antenna look longer. Coil characteristics depend on placement of the coil. While too complicated for me to explain here, there are numerous methods worked out in antenna and amateur handbooks or publications. Construction is fairly straightforward and good results can be achieved by most hobbyists. Take a look on the ARRL website or similar radio amateur websites.

For what it's worth a capacitor can make an antenna that is slightly long "look" a little shorter electrically. Heath used to make a motorized variable capacitor that could be placed across the feedpoint of a slightly long dipole. It would be adjusted to tune the antenna more precisely.

Good luck.

 

[zachtheterrible]

Thanx Steve, that was very helpful!!