John Kolb <jlkolb@cts.com>

Signal distribution...


            ______________ B
           (      |
        L1  )      \
           (       / R1
  A  ------|       \ 50 Ohms
            )      /
           (       |
            )______|_______ C

A passive antenna splitter can be made from a handful of ferrite 
toroids and resistors. A signal applied at point A will appear
at points B and C. A signal applied at point B will appear at 
point A, and if A is terminated in a 50 ohm load, very little 
of the signal applied at B will appear at C; with proper 
construction, 20 db or more attenuation is possible. Signals 
applied at C, likewise, will not appear at B. The signal
applied at A will appear at B a little more than 3 db weaker, and
will vary very little in amplitude, whether point C is loaded
with 50 ohms, open circuited, or shorted. Thus an ideal method
of connecting two receivers to a single antenna. Half the signal
received on the antenna will be available to each receiver. If the
two receivers were tied directly to the antenna, one might be 
much lower impedance than the other at a particular freq, thus
loading down the antenna, and leaving very little signal for the
high impedance receiver. Likewise, a local oscillator signal
leaking out of the antenna jack of one receiver will be greatly
reduced at the antenna jack of the other receiver, which is the
second problem that occurs if two receivers are tied directly
to the same antenna.

The circuit above can be expanded. With three identical stages,
A of the first stage is the antenna input. A of stages 2 and 3
is connected to B and C of stage 1. B and C of stages 2 and 3
become outputs to drive 4 receivers, with a signal level
a little more than 6 db lower than the signal directly at the
antenna.

The above circuit can be used in reverse to combine two signal
generator outputs into a single signal for receiver crossmod
or intermod testing without the signal generators affecting each
other.

Modern receivers are very sensitive. The internal noise level
is much, much lower than the atmospheric noise picked up by
a reasonable antenna. Most of the time, you need to listen
to freqs near 30 MHz before the natural and manmade noise 
picked up by the antenna becomes low enough to be overcome by
receiver noise. Thus on freqs from VLF to the lower HF ranges,
the use of a passive splitter will not reduce the ability
to pick up signals.

Since a signal splitter is used to use one antenna with receivers
tuned to several different freqs, a broadband amp would be 
required if it were necessary to have a splitter with gain. 
Broadband amps are difficult to design that would not have 
much worse intermod distortion than the receivers with their
tuned RF stages.

The transformer T1 above is made by twisting two pieces of 
small gauge wire, such as 30 gauge, together and winding
5 - 15 turns through a ferrite toroid core. The start of 
one winding is connected to the end of the other to form the
centertap, point A. The type of core and number of turns
will determine the min and max frequencies before losses 
increase. The number of twists per inch, and overall winding
style will determine the balance and insersion loss. 

John Kolb  KK6IL  jlkolb@cts.com