Topband: question about RX splitter

[ZR]

----- Original Message ----- 
From: "VE2TZT" <ve2tzt at arrl.net>
To: <topband at contesting.com>
Sent: Monday, October 17, 2011 5:17 PM
Subject: Topband: question about RX splitter


> Hi Top band community,
>
>
>
>
> For a comming DXpedition I need to build a specific splitter with the
> following functionalities: 75 ohms input with common mode rejection, two 
> 50
> ohms outputs. The band pass will never need to be more than 1.8 to 15 Mhz.
>
>
>
> If I follow the rough way, I need a first core for the 1:1 common mode
> isolation, two cores for the splitter itself (the first for 2:1 input
> impedance lowering and the second for the splitting function including a 
> 1:4
> impedance transformation) and one core at each output for auto 
> transforming
> 75 ohms to 50 ohms. That's obviously too much cores.
>
>
>
>
> I have found on the internet an article from John Bryant and Bill Bowers
>
> ( http://www.dxing.info/equipment/rolling_your_own_bryant.dx)
>
> where they use only one core for the splitter. Additionally the primary of
> the splitter transformer is symmetric and isolated from the secondary so, 
> it
> seems that this splitter can at the same time provide common mode 
> isolation.
> If to end I adjust the number of turns of the primary to have an impedance
> transformation, It seems that I can get all the above functionalities with
> only one core.
>
>
>
> I know that there are no free meals so, what is wrong in my reasoning?
>
>
>
> 73, Gilles VE2TZT
>


I suggest the KISS principle Gilles.

Use either a 100 Ohm resistor for the splitter and call it 50 Ohms or use 
150 Ohms and call it 75. The antenna and the radio wont know the difference 
since all the interconnect coax is just a small fraction of a wavelength. 
Ive been using 75 Ohm feedlines for TX and RX for over 30 years and no rig 
from vacuum tube to modern has suffered. The resistors here are 100 Ohm.

One binocular core wound as a 2:1 transformer (50 to 25 Ohms or 75 to 37.5) 
for common mode isolation driving another bifilar (or a standard toroid) for 
the split. Use identical windings for either feed impedance. For high common 
mode rejection isolate the binocular windings in seperate Teflon or other 
sleeves; #28 wire works fine that way using BN73-202 cores.

Carl
KM1H