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

i8cvs domenico.i8cvs at tin.it
Sat Apr 20 15:28:38 PDT 2013

Hi Art, KC6UQH

Feeding a 50 ohm antenna with a odd numbar of 1/4 electrical wavelenght of
75 ohm coax cable you transform the 50 ohm antenna impedance into 100 ohm
and connecting two 100 ohm impedances in parallel you get again 50 ohm to
supply the system main fedline to TX and RX but using this solution the VSWR
into the 75 ohm coax cables is 75/50 = 1.5 and this VSWR represent losses
particularly at high frequencies from 435 MHz and up to 1296 MHz

On the other side feeding two 50 ohm antennas with the same lenghts of 50
ohm coax cables in parallel  you get a 25 ohm impedance to be transformed
again into 50 ohm using a matching line of  37.5 ohm 1/4 electrical
wavelenght long made with two pieces of coax cable 1/4 electrical wavelenght
long but in this solution the VSWR into both 50 ohm feeding lines will be
a good 1 to 1 and the losses are not very high so that this second solution
seems to be preferable.

The 37.5 ohm Zo impedance power divider can be professionally built in air
using two sections of round copper tubing 1/4 wavelenght long using the
following formula:

Zo = 138 log      (D/d)

where D is the inside diameter of the outer tubing and d is the outside
diameter of the inner tubing.

In this solutions the RF losses are less than using two sections of 75 ohm
coax cables in parallel each 1/4 electrical wavelenght long so that this
round copper tubing power divider solution seems to be preferred and it
is that I currently use up to 1296 MHz

73" de

i8CVS Domenico

----- Original Message -----
From: "Art McBride" <kc6uqh at cox.net>
To: "'i8cvs'" <domenico.i8cvs at tin.it>; "'Douglas Phelps'"
<dphelps1 at ameritech.net>; "'R.T.Liddy'" <k8bl at ameritech.net>; "'Amsat -
BBs'" <amsat-bb at amsat.org>
Sent: Saturday, April 20, 2013 7:47 PM
Subject: RE: [amsat-bb] Re: Mount an Arrow on rotator with 2m

> 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: 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
> 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" <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
> 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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