----- Original Message ----- From: "nader omer" st2nh@yahoo.com To: amsat-bb@amsat.org Sent: Friday, April 20, 2007 4:44 AM Subject: [amsat-bb] Best arrangemet and best distance

Hello all I would like to know what the best arrangement and the best distances between vhf cross yagi antenna uhf cross yagi antenna l-band helical antenna s-band helical antenna over a one cross boom 73 de Nader -st2nh

Hi Nader, ST2NH

Any antenna has an aperture area that is the area around the antenna in wich the energy of the radio wave enters and is extracted from free space.

For a dish the capture area is just the area of the mouth and obviously you don't want to put a dish inside to another dish

For the same reason the capture areas of different antennas must be spaced so that they do not overlap.

For a yagi the aperture area A can be "approximately" calculated using the following formula:

/ 2 G x /\ A = ------------- [ square meters] 4 x 3.14

Where:

G = the isotropic gain expressed in factor (not in dBi ) / / \ = wavelenght expressed in meters

Example for a 2 meters yagi with G= 13 dBi ( 20 time in factor)

2 2.07 x 20 A = ----------------- = 6.79 [square meters] 4 x 3.14

A circle with area A= 6.79 square meters has a radius R= SQR( 6.79 / 3.14) = 1.47 meters.

This means that no other capture area generated by a different antenna must overlap the above 6.79 square meters extending up to about a distance of 1.47 meters from the boom.

Since the aperture area A of any antenna is proportional to it's gain G it follows that for the same wavelenght greater is the gain and greater must be the distance from any other antenna in the system even if the antennas are designed for different frequencies.

With this in mind and knowing the gain of each antenna it is possible to calculate the best distance from each antenna to the next one.

Supposing that the gain of a 70 cm antenna is 16 dBi or about 40 time in power it follows that using the above formula A= 1.50 square meters and R= 0.69 meters

If the 2 meters and 70 cm antennas are to be placed over the same support in the array than the distance between their booms must be not less than 1.47 + 0.69 = 2.16 meters and greater is better.

For a yagi the capture area is not exacly a circle but an ellipse by the way the small error introduced by the above formula in the calculation of A can be accepted.

For accurate calculation of the aperture area A in a yagi the equations discussed in the following article are recommended:

" Staking Yagis is a Science " by Steve Powlishen, K1FO, Ham Radio Magazine May 1985

If someone is interested on this matter and cannot find the original article I can send by email a 5 MB + zipped file.

73" de

i8CVS Domenico