-> note: view this using a fixed font like courier 10 for things to line up.
------------------------------------------------------------------------------
traditionally, scanners have used 50-ohm cable and connectors. the
reason for this is historical rather than technical. my assertion in
this article is that 50-ohm hardware is often not the best choice.
what is the "ohms" measurement associated with antenna hardware?
----------------------------------------------------------------
without getting more technical than necessary for this article, it is a
measure of the characteristic "impedance" exhibited by antennas, cable,
and connectors to radio signals passing through them. signals pass
through best when the antenna, cables, and connectors all exhibit the
*same* degree of impedance to signals. common values are 50 ohms and 75
ohms.
2-way radio systems generally use 50-ohm hardware
-------------------------------------------------
radio systems used by police, fire, etc. typically transmit on a single
radio frequency or a small number of frequencies that are very close
together. the most common antenna type used for these systems is called
a ground plane antenna. ground plane antennas characteristically
exhibit about 50 ohms impedance at their "resonant" frequency (the
frequency they are designed to transmit on). 2-way systems use 50-ohm
cable and connectors because this matches the 50-ohm impedance of the
ground plane transmitting antenna at its resonant frequency.
the tv antenna wiring in your home uses 75-ohm hardware
-------------------------------------------------------
tv antennas are not ground plane antennas. they consist of several dipoles.
dipoles exhibit 75 ohms impedance at their resonant frequency. for
this reason, tv hardware uses 75-ohm cable and connectors. 75-ohm tv
hardware is plentiful and cheap because it is mass produced for consumer
use. it is not suitable for the 2-way radio systems described in the
previous paragraph because 75 ohms is not a good impedance match with the
50 ohms of the resonant ground plane antennas. also, it is not designed
to handle the high power of transmitting; it is for receiving.
why has the scanner world traditionally used 50 ohm hardware?
-------------------------------------------------------------
just cuz -- no legitimate reason. the scanner world grew up around 2-
way radios, so 50-ohm hardware seemed the natural thing to use. perhaps
the thinking was that since scanners often use ground plane antennas, 50
ohms would be the right choice. whatever the reason, the fact is that
there is no advantage to using 50-ohm hardware for scanners over other
kinds of hardware available.
this is true because ground plane antennas exhibit 50 ohms impedance
only at their single *resonant* frequency. the minute you stray from the
resonant frequency the impedance varies rapidly. while 2-way radio
*transmitters* transmit only on the resonant frequency (or a few very near
it), scanning receivers sample hundreds of different frequencies in widely
separated bands. the impedance presented by a ground plane scanner antenna
during normal scanning may range from a few ohms to several thousand ohms.
for this reason, 50 ohms is no more desirable than 75 ohms or any other
value. impedance is not a worthy consideration with scanners. the choice
of 50-ohm hardware is just tradition.
is there any reason *not* to use 50 ohm hardware for scanners?
------------------------------------------------------------
well, yes, or i wouldn't have written all this. as it turns out, 75-
ohm tv hardware is much cheaper and more plentiful than 50-ohm
communications hardware. consider the ready availability of tv antenna
cable, signal splitters, a/b switches, and the common crimp-on f
connector. you can find them in every hardware store, and they are
ideal for scanner use. the cheap, crimp-on f connector is one of the
lowest loss connectors in the world. rj-6 coax cable for tv use
exhibits lower loss and greater interference shielding than most 50-ohm
cable costing several times as much. if you want to route your signals
among different scanners using a/b switches, or if you want to switch
things (like filters) in and out, then cheap, tv hardware is ideal.
(good luck finding a 50-ohm a/b switch suitable for uhf.)
adapters to go from f connectors to your scanners and antennas are
available at radio shack, and the signal loss in the adapters is no more
than the losses present in the 50-ohm connectors that would have gone
there otherwise.
for all these reasons, 75-ohm tv hardware is the ideal choice for home
scanner wiring. it is cheap, easy to install, and has excellent loss
and shielding characteristics.
i recently converted all my home scanner wiring to 75-ohm tv hardware to
take advantage of all this. one company, grove communications, has
decided to buck tradition and offer scanner devices (filters, pre-amps,
etc.) with 75-ohm f connectors instead of the expensive 50-ohm bnc
connectors or the lossy 50-ohm pl-259 connectors. (as a matter of fact,
i learned much of this from a book by bob grove who owns grove
communications.)
maybe over time other manufacturers of scanner equipment will learn the
error of their ways and switch over. it's an idea whose time has come.
--
don irving (don@irving.org)
