Miata Mailing List: January 1997, Message #218
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   From: (none)

Subject: Re: Jeff's CB and antenna splitter & non-OEM radio

   Date: (none)



This concerns CB antenna splitters in general and also the ones I've

been offering to those on the Miata list.  So for those not interested

please forgive me for taking up this space on the list.  



Because of Jim Creer's comment I believe that those of you who have

gotten one of my antenna splitters would like to know more.  So I've

spent a little time putting together the follow that is meant to be more

descriptively correct than technically rigorous.



On Sun, 22 Dec 1996 04:06:43 -0500 Jim Creer  writes:

>Just about any type of splitter will work but the most crucial

>step is matching the xmitter to the antenna length.  JC Whitney

>sells an inexpensive splitter with a built in SWR meter. 



I can conditionally agree with Jim about any type of antenna splitter

working.  Cars are much the same way.  A Yugo may get you there the same

as a Miata -- but then again it may not.  In any case they sure don't

work with the same effectiveness.  In the case of antenna splitter

performance the "there" may not be as far away.

 

J.C. Whitney does sell three types of splitters, all under $20 plus

shipping.  None viably work with the Miata's antenna(s) when installed

at the convenient plug-in location in the trunk.  Two of the three types

will somewhat adequately perform when installed at the dash location

behind the radio.  



The third type may too depending on if you get one that was properly put

together when it was made and was adequately internally adjusted.  This

one comes with a short black rubber antenna.  This antenna is a great

antenna that contains an internal metal spring to keep it standing up

straight over time -- some others in the Miata-market bend over after

use and stay that way.  The splitter itself has some quality electronic

features but it is put together with very low quality control and

components.  About 25% of these that I've observed will put about half

of the CB transmitting power into the AM/FM radio instead of to the

radiating antenna.  This can burn out the AM/FM radio and certainly

reduces the talk-range of the CB.  I've seen no two of these built the

same -- some were assembled with parts on the circuit board improperly

interchanged.   BTW, I have a number of short black rubber antennas that

screw into the fix-antenna Miata that I could sell at a very low price

-- ask.



The other two types in the J.C. Whitney catalog are really the same

thing but one includes, for $3 more, what they say in an internal VSWR

tuning light.  That feature would be well worth the additional cost if

its indicator enabled precisely setting the VSWR matching to the

standard 50 ohm impedance of CB radios -- it doesn't.  Its VSWR CB

tuning control adjustment can be set just as accurately, and maybe more

so,  simply by listening to a weak CB channel 19 signal and setting the

splitter's CB tuning control for best reception.  I don't believe the

VSWR tuning light is worth any additional cost (the splitter will

perform better without this power consuming circuit).  A precise best

setting can be obtained with an external VSWR meter (available at Radio

Shack).  Although this splitter installed behind the radio can achieve a

best setting, its performance may be less than ideal.    



In the past J.C. Whitney has advertised an additional variety of antenna

splitter for splitting a CB antenna into also AM and FM (this was

offered at a lower cost).  I've examined those and found them internally

to be exactly the same as the antenna splitter they offer for use with

the AM/FM antenna.  The only difference between the two units is the

printing on the case's top.  One has the letters "CB" painted over so it

says "Antenna" and no longer says "CB Antenna".  So, if you want to get

a splitter directly from J.C. Whitney you might as well buy this one if

they offer it again.      



All of the splitters available from J.C. Whitney use some minimal

quality internal parts.  Of particular concern is the VSWR matching

capacitor that's used for the tuning adjustment.  These are mechanically

inadequate and will easily fall apart if the screwdriver adjustment is

not made very carefully.  When making the adjustment if you take great

caution to assure that the screwdriver's turning does not wedge against

the access hole in the case the adjustment can successfully be make a

few times.  However, making this adjustment more than a few times will

cause the capacitor's internal pivot contact to become electrically

loose and the splitters operation will become erratic.  These capacitors

also have an inadequate voltage handling capacity for operating when the

splitter's antenna matching circuit is optimized for best efficient

operation -- the capacitor tends to electrically arc between its

plates.  This may be the reason why these devices are made with a design

that's less than optimum for some installations.  The J.C. Whitney

splitters also noticeably attenuate the weak AM radio signal reception

of the Miata's radio.  



CB/AM/FM antenna splitters are notorious for their poor performance and

all of them are a compromise from the best antenna-type performance.  An

AM or CB antenna on a car is also a serious compromise from the natural

quarter wave radio resonant frequency.  Ideally an AM radio quarter wave

length long antenna would be about 219 feet tall and a CB antenna would

be about 9 feet tall.  An ideal FM antenna would be about 2.5 feet tall

and this is around the height of the Miata's antennas (the Miata's power

antenna is a bit shorter than the fixed one).  



The operational theory of an antenna splitter that also tunes a

non-ideal length antenna is a bit technical to briefly explain.  So,

I'll just simply say that the antenna splitter's job is to electrically

fool the way the CB radio signal is processed along the coaxial cable

feeding the too-short antenna.  Adding circuits (inductors and

capacitors) between the CB radio and the too-short antenna allows the CB

radio to see an antenna load that appears to be from a full length

properly matched antenna -- it's only under this condition that all the

available CB power can be extracted from the CB radio.  



For best efficiency these CB antenna splitter circuits should be as near

the antenna as possible to eliminate losses in the transmission line. 

With this type antenna splitter the low loss proper match is 

only obtained between the CB radio and the splitter.  The coaxial cable

line between the splitter and the antenna is operated severely

mismatched.  Coaxial cable transmission lines losses become significant

when the cable is not properly impedance matched to the antenna load.  

This is especially a problem in the Miata when the splitter is placed

behind the radio because Mazda uses an undersized small coaxial cable to

the Miata's antenna that has a higher loss than most coaxial cables. 

Also, the antenna line in the Miata is longer than in most other cars

that have front fender antennas and every inch of this undersized cable

adds to the CB's signal loss.  So, if the splitter is installed behind

the Miata's radio the Miata's inadequate coaxial antenna line will

produce a CB signal loss situation that reduces the efficiency and

talk-range 

of the system.  

 

The way to avoid this excessive coaxial line loss, and maximize

communication range, is it to install this impedance matching type

splitter as close to the antenna as possible.  This is related to the

reason why most short CB antennas include a coil built into the antenna

mast -- this gets the impedance at the base of the antenna to match the

50 ohm impedance of the coaxial line and the 50 ohm impedance of the CB

radio -- this maximizes the amount of CB power delivered to the antenna

and thus, the talk range. 



Jim does state, the way to get these J.C. Whitney CB antenna splitters

to function in a Miata is to pull the radio so that they can be plugged

into the Miata's antenna line near the radio.  I've said that before on

this list too. Having the lossey long length of coaxial cable feed the

CB signal to Miata's antenna does allow these splitters to function. 

Although this installation will enable the splitter's operation, it is

not as effective as is possible.  



I get the splitters (for some of their parts) I've been offering from

J.C. Whitney.  I get them, take them apart, strip out and trash the low

quality components.  I then rebuild them with different tuning circuits

and high quality components and resell them for more.  The cost of the

high quality tuning capacitor I use would likely exceed the cost of all

of the other components used in the splitter if it were included at the

time of original manufacture.  The J.C. Whitney splitters use only a

series tuned CB matching circuit, I rebuild them using a series and

parallel circuit that more precisely matches the CB radio's 50 ohm

impedance while more effectively coupling and matching the Miata's

antenna(s).  This can be done because unlike the J.C. Whitney offerings

that must be tolerant to a wide range of difference type car

applications, the ones I do are specifically optimized for only one

application -- the Miata's trunk installation and the Miata's

antenna(s).  Because they are peaked for this application they will not

as effectively work if they are installed at the dash location behind

the Miata's radio or if a non-standard Miata installation condition

exists.     



Because these splitters more effectively process the CB's transmitting

power to the Miata's antenna it becomes more critical that these

splitters also effectively isolate the CB's transmitted signal from the

AM/FM radio.  So I also custom tune them for maximum AM/FM radio

isolation from the CB radio's transmitting signal.  This tuning is

somewhat dependant on the length of the coaxial line to the AM/FM radio

and the type of radio.  Therefore, I optimize this isolation to maximize

it with the factory radio and installation.  Because it's difficult to

do and critical that it be maintained, I provide no user AM radio

peaking control -- that's preset too for maximum AM radio reception for

the Miata radio.  



These are the reasons why I do not assure proper operation of my

splitters if a different AM/FM radio or especially if another type

antenna is used.  For those installations the coaxial line or antenna

lengths may be different -- these parameters are extremely critical with

my splitters because the peak tuning is less forgiving for changed

radio, antenna, or antenna line conditions.   It's conceivably possible

that the connection of an aftermarket radio with some different antenna

impedance could disrupt the balance of the splitter system and result in

more of the CB signal being delivered to the AM/FM radio -- if the

aftermarket radio is not well protected in its design it might suffer

some damage caused by too much CB power entering the aftermarket AM/FM

radio.  Some are apparently using my rebuilt splitters with an

aftermarket radio and to date no one had let me know that they have had

a problem.   However, caution shoud be taken if the OEM Miata antenna

system or radio is not used. 





-- 

Jeff Anderson                 @   @     @     @   @@@@@   @      

Arlington, VA                @ @ @ @    @    @ @    @    @ @

91 Silver, 92 Blue          @   @   @   @   @ @ @   @   @ @ @ 

MCA, Capital Chapter       @         @  @  @     @  @  @     @

M Club #97

 1) Curer of the Miata's bad head rest sound.

 2) Miata special high performance antenna splitter for CB use too. 

    Ends scratching your Miata with magnetically mounted CB antennas. 

  e-mail for info.