CB/Radio Antenna

[john wright]

Glenn,
I am looking at the packing and the Radio shack cat# is 21-990B. It may
or may not be still available.

J.R. Wright
GMCGreatLaker
77 Eleganza II
Michigan

>

>
> will do both radio and CB. I got mine from Radio Shack.>>
>
> J.R., That sounds like an ideal antenna. I looked at Radio Shack but
> couldn't find one that would do both. Do you have a catalog number? TIA.
>
> Glenn
> 78K OR

 

[richard missett]

Have one of those combo antennas on my
Chevy {Jayco) conversion van and it works great.
I would like to know if it is possible to change the roof antenna on my
73 without dropping the inside cap? I understand the lead runs down
inside the windshield divider. Does the inside of this divider come off
and is it necessary ? Or is it as simple as attaching the replacement to
the old antenna lead and fishing it down from the roof ? (I can dream
can't I)
The nut that holds the mast to the lead is cracked and I can't locate a
replacement that will work.

Dick Missett
73 ex-Sequoia
Wyoming, PA

 

[john wright]

Yes it does have the splitter and tuning pot to adjust the SWR for the
CB side. I had to put a 90 degree coax conector on the ant. to provide
clearance to the front inside cap. Did you go to the store and have the
Radio Shack guys check on there computer if the item was still
available? Sometime it still might be in the warehouse.

J.R. Wright
GMC GreatLaker
77 Eleganza II
Michigan

>

>
>
>
> J.R., Thanks for taking the trouble. The 1999 Radio Shack catalog doesn't
> show that number. Close... but not close enough. So, guess I'm SOL. 21-990B
> must have been discontinued. The catalog shows several 21-xxx numbers but
> they are all for CB antennas, or for CB/10M Ham (with a loading coil). All
> longer than needed for FM.
>
> Does your antenna have a splitter like Paul Bartz has? A box like he
> described with a tuning device for the CB side is probably a good way to go if
> you want a good looking exterior with only one (short) antenna. A 31" to 32"
> length is ideal for FM reception; can be shorter with wire windings or a coil.
> However, anything less than the ideal length for either FM or CB will be a
> compromise in performance, but should be within tolerable limits ... if you
> can stand less than perfection. {
>
> I was glad to hear Paul say the splitters are avaiilbale from JC Whitney.
> Been lookin' for one a them dudes for quite awhile. Want my coach to look as
> sleek as possible.
>
> This net never ceases to amaze me; it's absolutely the greatest for finding
> out everything about anything!
>
> Glenn
> 78K OR

 

[edgar t. kremer]

A word of caution about CB antennas. A full length CB antenna is considered to be
a quarter wavelength long.
The math: (300,000,000/27,000,000) x (39.37/4) = quarter wavelength = approx. 109
inches.
Which means for maximum efficiency the antenna must be 109 inches long. There is
no magic to make a shorter antenna perform as well as a quarter wavelength. "Black
boxes" fool the radio into thinking a short antenna is long. If a short antenna is
to be used, a top loaded antenna (the coil mounted at the top of the antenna) is
best.

Another consideration: if one antenna is mounted on each side view mirror, they
should be 109 inches apart for maximum efficiency. Obviously the coach is not 109
inches wide; therefore, efficiency suffers. Also, the mirrors must be well
grounded to the metal body/frame to supply the ground plane for the antenna.

The best place to place the antenna is as high up as possible. Did you ever see a
broadcast TV or radio antenna not mounted on a tall tower? Thus, the roof is the
desired platform. Maximum radiation will be in the direction of the most metal.
Therefore, if the antenna is mounted toward the front of the coach (the most
likely spot), maximum radiation will be to the rear.

Esthetics, height considerations, cable run, etc. will all play a part in antenna
position. "There ain't no free lunch". Whatever you do to shorten the antenna
and/or limit its height will decrease its efficiency.

 

[john dolan]

I agree with Ed's comments on the CB/FM/AM antenna now discontinued by
Radio Shack.

It does depend on what your objectives are, since every thing in life is
a compromise, only more so in antennas. If you are looking for the
lowest profile, neat installation you must be willing to sacrifice
efficiency. The 32 inch antenna is a far cry from optimum on 11 meter
CB, but so is the 32 inch AM/FM combination not the best choice for
AM/FM.

Many folks have noticed that AM radio used to come in better in the old
days. That is not just nostalgia, they are right. Many AM/FM radios
have one trimmer capacitor near the antenna connection and you either set
it for best FM, best AM or neither!

Now add CB to the compromise and with proper tweaking you will get it to
work equally poorly on all three frequencies (1.2 khz, 27 mhz, 102 mhz)
But if most of your CB is contacting another GMC that you just passed on
an interstate, and AM radio is used mostly for city traffic reports, I
would just optimise your system for FM and take what you get on the other
two and you will probably be happy.

- --
Regards,
John 74 Glacier near Washington, DC.