[amsat-bb] Re: Mount an Arrow on rotator with 2m verticalorhorizontal

Art McBride kc6uqh at cox.net
Sat Apr 20 10:47:10 PDT 2013

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.


-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] 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

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" <dphelps1 at ameritech.net>
To: "R.T.Liddy" <k8bl at ameritech.net>; <amsat-bb at amsat.org>
Sent: Tuesday, April 16, 2013 8:31 PM
Subject: [amsat-bb] Re: Mount an Arrow on rotator with 2m vertical

> 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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