1/4 wave vs 1/2 wave

[beerzkool]

Hi,

I want to cut an antenna rod for a mag mount antenna so that it will be tuned to 453.45 mhz.

I have found it needs to be 12.4 inches for 1/2 wave and 6.2 inches for 1/4 wave.

My question is what are the advantages of using one size over another?

(Pardon my ignorance and failure to find the answer through searches)

 

[jred184]

Where did you find the calculations for that? I would like to do the same thing, just for 2 meter.


JRed

 

[beerzkool]

http://www.qsl.net/kd4sai/antencal.html

 

[jred184]

What type of material are you making the whip out of?

 

[beerzkool]

Stainless steel

 

[jhooten]

Half wave does not need a ground plane but is not a direct impeadence match for 50 ohm cable and the transmitter it is connected to. Quarter wave must be installed on a good ground plane, has a little bit lower gain, but is a direct match to your cable/radio.

 

[beerzkool]

I forgot to add that I will only be receiving....not transmitting....if that makes a difference.

 

[DaveH]

The 1/4 wave will match 50 ohms but 1/2 wave will not. For most "gain" antennas, there is a base matching coil required. Otherwise, the potential gain from a larger antenna will be lost by the impedence mismatch.

You've probably seen "stacked" antennas which are two elements with a phasing coil in or around the center. Trying to duplicate the base coil or the phasing coil would be touchy at those frequencies. Best to just buy one, and cut according to instructions.

Last, there is usually some length correction factor due to antenna thickness. If you're just receiving, not critical.

Dave

 

[SCPD]

There's also something called take off angle. A quarter wave antenna has a higher take off angle compared to an antenna with higher gain. The quarter wave will transmit it's signal at a steeper angle off the horizon more than a gain antenna will. An antennas gain is measured by how much signal it will transmit, and receive, across the horizon. If you've ever been in a large city you might have noticed that the police cars, fire trucks, etc., have quarter wave antennas on them. Alot of repeater antennas are located on top of tall buildings, when you're driving amongst the buildings the quarter wave antenna will perform better than a gain antenna. The quarter wave is shooting it's signal higher into space, more so than the gain antenna. If you live in a rural area, like I do, the public safety vehicles use a 5/8 wave gain antenna. That way the signal will be transmitted more across the horizon rather than shot into space and wasted.

There are pros and cons to all types of antennas. Just choose one based on your particular application. If it's for monitoring, and you plan on staying within the geographical area of your monitoring, I would choose the quarter wave. They're less expensive and they're more "friendly" to getting bumped by trees and so forth. Shop Ebay, they have some good deals on commercial brand quarter wave antennas, coax, connectors, etc.

 

[kb2vxa]

Hi all,

Let's keep it simple, OK? The answer to the question is six inches, end of story.

 

[SCPD]

Read his original question. The question was, "What are the advantages of using one size over another?"

 

[Al42]

Read his original question. The question was, "What are the advantages of using one size over another?"

Quarter wave end-fed will work, half wave end-fed won't. As Warren said, simple.