Make my own UHF antenna...

[conve36]

Can someone tell me how I can make a transmitting antenna for GMRS using a mag-mount. What is the difference between a quarter-wave antenna and full-wave? I know its the length, but whats better?

 

[rfradioconsult]

Quarter wave. Antennas present an acceptable impedence at a quarter wave and odd multiples thereof. Half wave and even multiples thereof present a very high impedence without a matching network to transform the high impedence to a nominal 50 ohm impedence.

Don't be tempted to make a 3/4 wave antenna it will give you an acceptable match but the radiation pattern maximum will no longer be toward the horizon but will be elevated at about a 40 degree angle; great if you want to communitcate with the birds.

 

[conve36]

ok, so if i cut a rod to about 6 1/2 inches that shoud work right?

 

[w8jjr]

The best bet is to used a swr meter and start at 7 " trim alittle off at a time till you get a reading of 1.5:1 or better.

1:1 is perfect and most of the time theoretically unobtainable, but we can get close.

1.1:1 is almost perfect, thus this is very good

1.5:1 is good enough for almost every purpose, and is not to bad

2:1 is getting into the questionable zone. The value is getting high enough to cause
problems in some cases.

3:1 is the generally top accepted limit 3:1 SWR will cause problems in many cases. And may indicate a short.