Hello Domenico, Art and the -bb, My comment here is with the ARROWS antennas.
I think one thing is overlooked here. We are dealing with two antennas on different frequencies.
And yes in this it may work since 435 MHz is 3 times the 145 MHz. So 1 quarter wavelength on 145 MHz becomes 3 quarter wavelength on 435 MHz. Therefore it may work impedance wise.
However there may be a problem with coupling between the two frequency bands.
73 OZ1MY/Ib
-----Oprindelig meddelelse----- Fra: [email protected] [mailto:[email protected]] På vegne af Art McBride Sendt: 20 April 2013 19:47 Til: 'i8cvs'; 'Douglas Phelps'; 'R.T.Liddy'; 'Amsat - BBs' Emne: [amsat-bb] Re: Mount an Arrow on rotator with 2mverticalorhorizontal
Domenico, There are two solutions to use coax to combine two 50 Ohm antennas. 70.7 Ohm cable (50 to 100) [Two 50 ohm antennas transformed by two lines to 100 Ohms each, in parallel = 50 Ohms] and your suggestion using 35.4 Ohm cable section (25 to 50) = [Two 50 ohm antennas connected in parallel = 25 Ohms, transformed to 50 Ohms]
The interesting part is you do not save anything when using 75 Ohm coax as 1/4 wave sections (two in parallel =37.5 Ohms) VS two separate 75 ohm sections one to each antenna.
Art, KC6UQH
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of i8cvs Sent: Wednesday, April 17, 2013 5:00 AM To: Douglas Phelps; R.T.Liddy; Amsat - BBs Subject: [amsat-bb] Re: Mount an Arrow on rotator with 2m verticalorhorizontal
Hi Douglas,
Using two identical 50 ohm coax lenght you can't transform each 50 ohm antenna impedance into 100 ohm but if you connect both 50 ohm coax cables to a T connector than the resulting impedance at the common output of the T connector will be 25 ohm i.e. two 50 ohm impedances in parallel and your radio will see a VSWR of 50/25 = 2 and this is the reason it will not work !
BTW if you connect the above 25 ohm impedance to the input of a 1/4 electrical wavelenght long coax cable of 36 ohm impedance than at the output of it you will get an impedance of 36^2 / 25 = 52 ohm toward the main feed line with a good VSWR of about 1 in TX and RX
To build a 1/4 electrical wavelengt long of a transmission line of 36 ohm impedance you can connect in parallel two 1/4 electrical wavelenght of 75 ohm coax cable like RG-59 or RG-11 i.e. 34,13 centimeters or 13,38 inches long for 145 MHz
BTW in this situation if both antennas are mounted in parallel one Vertical and the other one Horizontal and if the elements are mounted on the same plain with respect to the space than the resulting polarization of the radiated field can be at 45 degrees like \ or at 135 degrees like / depending at wich side of the dipoles the inner conductors of the 50 ohm feed lines are connected.
73" de
i8CVS Domenico
----- Original Message ----- From: "Douglas Phelps" [email protected] To: "R.T.Liddy" [email protected]; [email protected] Sent: Tuesday, April 16, 2013 8:31 PM Subject: [amsat-bb] Re: Mount an Arrow on rotator with 2m vertical orhorizontal
Any reason why, using identical coax lengths, you could not transform the impedance to 100 ohms and then use a T connector to sum both antenna signals and achieve 50 ohms to the radio? I know Transmit power will be 3 dB down at either antenna but is there any reason it will not work? Any antenna gurus out there?
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_______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb