GM300 antenna differences/recommendations

tn1999 · Post by **tn1999** » Mon Dec 10, 2001 12:55 pm

For a GM300 UHF 8ch 438-470 split model.
Can someone explain the differences/advantages of the three recommended antennas
hae4003 1/4
Hae4011r 3.5dg gain
???????? 5db gain
TIA

Post by **wavetar** » Mon Dec 10, 2001 4:06 pm

Well, without delving too deep into antenna theory, the gain numbers you gave sum it up. The 1/4 wave (no gain) antenna is by far the smallest, measuring about 5 to 7 inches, depending on where you are operating in the UHF band. No gain simply means that the radiation pattern coming off the antenna is circular, therefore some RF is wasted going up into space & down into the ground. The 3 & 5 dB gain antennae flatten the radiation pattern, directing more of it outward around the antenna, and less of it upwards & downwards. This results in a measurable "gain" in the effective radiated power coming off the antenna. Picture it like flattening out a tennis ball, same amout of material, but the flattened ball has a larger circumference than the unflattened ball. An antenna with a 3dB gain will radiate 10 watts as effectively as a 1/4 wave no-gain antenna would radiate 20 watts, by directing the RF to where it's more useful. In general, an antenna with a gain will result in better performance in a given system, but there are many variables to consider, including antenna size. The higher the gain, the larger the antenna. The 3.5 db gain antenna you mention will measure out at around 3 feet, compared to the 1/4 wave 5-7 inches.

Todd

<font size=-1>[ This Message was edited by: wavetar on 2001-12-10 19:07 ]</font>

raymond345 · Post by **raymond345** » Tue Dec 11, 2001 2:47 pm

This is true about gain but it depends on what u want gain for.If it is 3 db gain and u have noise.
Then the noise is 2 times more also.
On cellular the more gain u have and the closer to the cell site the control channel will turn your power down to aslow as 100 milliwatts .Most do not think of this.
This can play havoc around tall bldgs.

Raymond eMail [email protected]

kens · Post by **kens** » Tue Dec 11, 2001 2:54 pm

I agree with Todd's info and would like to add a couple of items:
Don't forget the 2.5" helical that customers often request. This is by far the least desirable for performance and has at least 3db loss over the 1/4 wave.
Check on the gain figures for dbi or dbd. This indicates how the gain computation was obtained as it is a comparison with another antenna. The I stands for isotropic which is a theoritical antenna in free space. D is dipole and the gain figure will be 2 db less than the same antenna rated dbi. Therefore the same antenna rated 3 dbd gain will have 5 dbi gain.

tn1999 · Post by **tn1999** » Wed Dec 12, 2001 10:41 am

Thanks Todd, Raymond and Kens for sharing your knowledge. I will be using the mobile radio in a race car and be comunicating to hand helds around the track. The largest track is just under 5 miles with severe elevation changes, trees and other obstructions. The shortest track is a flat 1.5 miles with line of sight all around. Clarity is a necessity. Coverage around the 5 miles would be optimal but if there is some loss in the out laying areas that shouldn't be a problem. Based on your comments I'm leaning on the 1/4 antenna. The shorter shaft will not get whipped around as the 3 footer will. Comments on this approach are welcome. Thanks
Tony