75 OHMS -- IT'S NOT JUST FOR TV ANYMORE

by Don Irving

-> Note: View this using a fixed font like Courier 10 for things to line up.

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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?

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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

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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

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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?

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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?

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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.

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