Miata Mailing List: January 1997, Message #275
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   From: (none)

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

   Date: (none)



Here is a short insight into Jeff's lengthy explanation of why his

splitters work better than generic ones:



I bought one and tried it out ....   It worked so well that I bought a

second one for my family's other Miata.  If I had three Miatas I'd buy a

third splitter.



'Nuff said!



Wayne Tibbit

Dallas, TX

Lone Star Chapter, MCA

'93 Black'n'Red L.E. (Shadow)



----------

> From: Jeff Anderson 

> To: Multiple recipients of list 

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

> Date: Friday, January 03, 1997 9:50 AM

> 

> 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.

> 

>