FRS Radios

FRS Radio
Family Radio - License Free Communication That Works!
Originally Published in 1998

One of my newest hobbies - or areas of interest - is FRS Radio.  Surprisingly - not too many people are familiar with this relatively new radio service.  The FCC authorized this class of service slightly over a year ago and by now a number of equipment manufacturers have their products on the market.  I own a set of Kenwoods which I have found a number of good uses for.

First - let me go over some of the basics of FRS and then we will get a little deeper into the technical aspects of the service.  FRS was intended by the FCC to provide short range - license free radio products for families and small businesses to use.  The range is limited to about two miles under good conditions - more over water and less if the terrain is dense.  My experience would conclude that a range of about .75 miles can be expected.  I am not surprised when a greater range is achieved - but I don't plan to establish reliable contact for much over a mile.  The power level of the radios is limited to 500 mW or less and external antennas cannot be connected.  Most of the currently available radios use 3 or more AA or AAA cells.  Some come with rechargers and external accessories such as speaker microphones and headset boom microphones.

All of the radios available share the same 14 channels within the 462/467 mHz FM [UHF band] - so they are inherently quiet and have good penetration through objects.  All of the radios have some form of squelch control that keeps the speaker off unless a signal is being received.  Some of the top radios have additional squelch tone circuitry such that your radio does not open to traffic other than your other unit(s).  The Kenwood radios feature a method of tone squelch known as CTCSS which requires that both units be set to the same RF Channel,  1 thru 14 and that each unit be set to the same CTCSS number,  1 thru 38.  This gives over 600 possible combinations of channel number/ctcss number combinations to insure that only your other radio can call into yours.  Many of the FRS radios also feature some type of calling signal - such as a ringer to alert your other party.  At least one manufacturer has provided an optional vibrator function as well.

The radios are quite compact - easy to use and most feature some form of battery saver circuit.  My Kenwoods get over 60 hours of operation on a fresh set of batteries.  Some of the other models don't do quite so well.  One of the manufacturing secrets of the radios - is that they all use SMT -[Surface Mount Technology] and LSI [Large Scale Integration] of the components.  This makes the radios fairly easy to manufacture in quantity and attributes to the relatively low cost for these sophisticated products.  Top featured units cost slightly over $100 each - but low end units can be sold for less than $50.  Some of the lower cost units are less than full power or work on just one channel [usually channel one].  However;  even the cheapo units can talk to the others under the right setup.

There is a Usenet Newsgroup dedicated to the discussion of FRS which is linked below and below the line - I have reposted here a couple of articles that I contributed to that newsgroup.  FRS is working quite nicely for me as my wife and I find new uses for the radios.  I use my FRS radios on most of my jobsites to assist in wire pulls and/or system adjustment.  In order to subscribe and participate in our FRS Newsgroup - you would need to configure your newsreader to access:

But First - here is a sketch of my Kenwood - UBZ-LF14-YK[yellow]

From the Manual:


Here are some typical postings to that newsgroup:

Here is an article that deals with just exactly how the Kenwood Battery Saver Circuit works ....

My Kenwood Current Consumption Test
August 20, 1998    by Allen Barnett
      rev. 8/21 to include Caution:

Here is the Test Condition:

First - let me tell you a bit about the test setup - in case you are interested.  Prior to doing this test - I knew that the radio went into a 'pulsing the receiver' condition - six seconds after no buttons are pushed and no signal is received.  I had also seen my mA meter pulsing an erratic reading during an initial test a few days ago.  This is one of Kenwoods' battery saving features - you'll see how it works in a minute. 

I tapped the battery circuit by inserting between one of the cells and the clip - a thin double sided aluminum tape sandwich from which I had removed the lower loop of the aluminum and left the paper backing in place.  This effectively opened the battery and gave me two aluminum tangs to connect to the meter.  But I did not use a meter - I used a scope.  A scope can accurately measure DC Voltage as well as pulse time duration.  In order to measure current I inserted a precision one ohm resistor between the two aluminum tangs such that current through the batteries would pass through this 1 ohm resistor as well.  This gave me a way to read and calculate actual current [via the scope vertical scale] and time duration [via the scope horizontal scale].

Here's the math:  I=E/R - so with a 1 ohm resistor - my volts on the scope would equal actual current being drawn at the time [because the value would be E/1(one)].  So if my scope reads 1 volt - the current would calculate to be 1 volt divided by 1 ohm or exactly 1 amp.  I decided to use regular alkaline batteries for this test rather than a controlled power supply so that the voltage and current would 'float' exactly like it does in real world operation.

Here are the results:

Radio Off -             Zero - No Measurable Current
Push 'On' Button -       3 mA [visible for .3 sec]
Radio Comes On -        50 mA with backlight on
Radio Stays On -        40 mA with backlight off
Transmitting w/tone -   320 mA [slightly less w/voice]
Receiving Signal (1) -  80 mA with speaker loud
Receiving Signal (2) -  60 mA with speaker lowered
Receiving Signal (3) -  50 mA with no audio [dead carrier]
Radio Enters Standby -  40 mA for .3 seconds
Radio 'Pulses' -         3 mA for .7 seconds
* Cycle Continues -
Radio Wakes Up -        same as receive 1,2,3 above
Radio Powers Down -      3 mA in standby (spade icon showing)
                        * Please see Caution: Below
Battery voltage at start of test = 4.4 volts / end of test [2.5 hours] 4.3 volts.  These were not fresh batteries.  All above readings were on Chan 2 / CTCSS 38.

Additional Test:
Before leaving the bench - I put the radio on a variable power supply and lowered the voltage to 3.6 volts [as if using 3 freshly charged cells].  The above readings correctly went down by 15 to 20 percent.  I also lowered the voltage to the point where the radio would no longer 'turn on'.  This occurred at 2.85 volts.  I assume this means that the button logic needs at least this level to function.

PS,  The above 'actual' readings are very close to the values given in the Kenwood Manual Specs.  My test includes some conditions that are 'not listed'.

I believe the figures speak well for one of the top battery performers.  Of special interest to me - was how deep the 'standby pulsing' reduces battery drain.  In Receive Standby - the radio draws only 3 mA for 70 % of the time.  That's really good design!  Your mileage may vary - but I think I did this test right!  If you are using a meter rather than a scope - you will probably see your meter display 'pulsing' during your radios' standby cycle.

It occurred to me after the original posting - that since the Kenwood - if allowed to power down automatically - it continues to draw 3 mA - eventually your batteries would go dead in storage.  Therefore;  radio owners are cautioned to properly store your radio after use - [by manually turning it off].

Allen Barnett  23 Years in Small Business
A B T E C   -  Lawrenceville, Georgia USA
  {=Commercial Sound & Video Systems=}
    ( Newest Hobby - 'FRS' Radios )

Here is an article about Speaker/Microphones and Kenwood Remote ...

Here you go:

The SMC-34 aux mic for the Kenwoods seems to use what we called transistor/resistor logic some time ago.  Anyway - the 3 switches select one of 3 resistor values across sleve and ring of the small plug.  Tip carries the speaker audio.  The large plug is also a three circuit plug.  The tip is not used - the microphone connects to sleve and ring.

The Speaker inside is:       The Microphone Element:
   Rated at 1 watt              is an Electret-Condenser
   produces 83 db               -58 db sensitivity
   8 ohm impedance              impedance of 2K ohms

The Schematic below does not show - Switch 1 is Monitor,  2 is Scan,  3 is Call Tone.

Allen Barnett  23 Years in Small Business
A B T E C   -  Lawrenceville, Georgia USA
  {=Commercial Sound & Video Systems=}
    ( Newest Hobby - 'FRS' Radios )


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