Tunable citizen band antenna

An improved tunable citizen band antenna includes an adjustable tuning assembly located at the tip of a wire wound fiberglass core antenna to lengthen or shorten the effective electrical length of the antenna, so that the antenna can be tuned to a specific, desired resonant frequency, resulting in enhanced antenna radiation, reception, and power transmission.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to citizen band antennas and, more 
specifically, to a citizen band (CB) antenna including means for manually 
adjusting the electrical length of the antenna. 
2. Description of the Prior Art 
Citizen band antennas are designed to receive and transmit radio waves to 
and from a radio which operates within a specific range of audio 
frequencies. The purpose of the antenna is to achieve maximum radiation 
and reception of transmitted or received radio signals. Therefore, it is 
desirable to select an antenna whose natural or resonant frequency is 
tuned to the frequency of the radio waves that are to be transmitted and 
received by the user's radio. 
In the past, CB antennas have been factory tuned to a frequency 
approximately in the middle of the citizen band frequency range. As a 
result, as the user operated a radio at a frequency that was within the 
citizen band range but deviated from the factory tuned resonant frequency, 
transmitting and receiving performance faltered due to the cancellation 
effect of the non-resonant frequencies carried on the antenna. 
To help alleviate this problem, an improved type of antenna was designed 
wherein the antenna could be tuned to carry a specific harmonic within the 
operator's desired range of frequencies. This type of prior art usually 
comprised a metal whip antenna which could be physically lengthened or 
shortened by loosening a bolt and sliding a portion of the metal whip in 
or out of a coil housing. Whereas this type of antenna did indeed enhance 
frequency response, it exhibited loss of signal power due to material and 
series resistance along the length of the metal antenna. 
Another type of antenna was introduced that greatly improved the reception 
and transmission response. Signal strength was increased by using a 
fiberglass core around which was wound a transmission wire tuned, as 
before, to a frequency in the middle of the desired range of use. This 
type of antenna carried a signal to the receiver with very little loss of 
signal strength. However, this was still a non-tunable antenna and 
suffered from a deficient frequency response. 
The need existed to design a tunable CB radio antenna that would exhibit a 
desirable frequency response, while suffering minimal loss or compromise 
in signal strength. 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of this invention, it is an object of 
this invention to provide an improved type of citizen band antenna. 
It is a further object of the present invention to provide a tunable 
citizen band antenna which exhibits a good frequency response while at the 
same time exhibits minimal loss or compromise of signal strength. 
It is a still further object of this invention to provide a tunable citizen 
band antenna wherein the antenna is tuned manually to a frequency desired 
by the operator within the citizen band frequency range. 
Yet another object of this invention is to provide manually tunable citizen 
band antenna made of a material which is durable and exhibits low 
electrical resistance so as to efficiently transmit the desired frequency 
radio signals. 
Finally, it is an object of this invention to provide a tunable citizen 
band antenna which is easily fabricated using current antenna fabrication 
methods. 
Briefly described, a tunable citizen band antenna is disclosed wherein the 
antenna is comprised of a fiberglass core around which is wrapped a copper 
conductive transmission line, a brass tuning extender permanently mounted 
to the tip of the antenna core and to which the copper transmission wire 
is connected, a brass tuning screw threaded to fit into the tuning 
extender, a brass jam nut to lock in place the adjustable tuning screw, 
and a rubber "O"-ring mounted between the tuning extender and jam nut to 
reduce the detrimental effects of vibration. 
The brass tuning extender is epoxied to a fiberglass core that has had its 
normal physical length, as compared to a non-tunable antenna, reduced by 
the physical length of the tuning extender. The other end of the 
fiberglass core is then epoxied to a male threaded metal base. A copper 
conductive wire is then wrapped from the base, around the core, to the 
extender, and soldered to make electrical contact to both the base and the 
extender. The brass jam nut is threaded onto the brass tuning screw, 
followed by the insertion of the tuning screw through a rubber "O"-ring. 
The tuning screw is threaded into the tuning extender. In this manner, the 
operator can physically adjust the electrical resonance, or frequency 
response, of the antenna assembly. 
According to a broad aspect of the invention there is provided a tunable 
antenna, comprising: a flexible core; a conductive base member coupled to 
the bottom of the core; an upper conductive member coupled to the top of 
the core; a conductive transmission line wound around the core and coupled 
at opposite ends to the upper conductive member and to the conductive base 
member to complete a conductive path; and means coupled to the upper 
conductive member for varying the length of the conductive path. 
The foregoing and other objects, features and advantages of the present 
invention will be apparent from the following, more particular, 
description of the preferred embodiments of the invention, as illustrated 
in the accompanying drawings.

Referring to FIGS. 1 and 2, the tunable citizen band antenna is referenced 
generally by the number 10. The antenna is comprised of a fiberglass 
antenna core 12 to which is mounted a tuning assembly referenced generally 
by the number 11, a base 15, and a copper magnetic wire wrap 16. 
Referring to FIG. 2, the tuning assembly 11 is comprised of a brass tuning 
extender 14, a brass tuning screw 18, a brass jam nut 20, and a rubber 
"O"-ring 22. The brass tuning extender 14 is epoxied to the tip 19 of the 
fiberglass antenna core 12, which is, in turn, epoxied at its bottom 13, 
to the male threaded base 15. The copper magnetic wire 16 is then soldered 
to the base 15, wrapped around the epoxied base junction 13, wound around 
the core 12 at various pitches (depending on design criteria), wrapped 
around the epoxied tip junction 19, and then soldered to the brass tip 
extender 14, hence completing the electrical path from the tunable antenna 
tip to its base. The brass jam nut 20 is then threaded onto the brass 
tuning screw 18. The rubber "O"-ring 22, which acts to absorb vibration 
that is translated to the tip 19 of the antenna core 12, is then 
positioned against brass jam nut 20 as shown. The brass tuning screw 18 is 
then threaded tightly into brass tuning extender 14 until stopped by the 
jam nut 20 and "O"-ring 22. 
Referring to FIG. 3, by varying the depth that tuning screw 18 is threaded 
into extender 14, the effective length of the antenna 10 is increased or 
decreased. In this manner, the resonant frequency of the antenna, which is 
dependent on the effective electrical length of the antenna, is altered to 
fit the need of the user. 
The antenna can be tuned using any well known method. For example, the user 
can take a VSWR reading of the installed antenna. This test measures the 
reflected power of the radio signals transmitted on the antenna. The brass 
tuning screw 18 and the brass jam nut 20 are then adjusted to lengthen or 
shorten the effective length of the antenna until a minimum VSWR reading 
is obtained. At this point, the tunable antenna 10 is set for optimum 
performance at a specific audio frequency. 
The brass jam nut 20 and the shock absorbing rubber "O"-ring 22 assure that 
the tuning screw will not vibrate loose, thus preventing the loss of the 
desired tuned resonant frequency of the antenna. To further protect the 
tuning assembly 11 from detrimental effects of the environment, an 
optional tight fitting rubber tip 24 can be provided to seal the tuning 
assembly 11. 
As shown in FIG. 4, in addition to the rubber tip 24, the antenna 26 is 
sealed in a high impact, wide temperature range polyolefin 26 so as to 
protect the copper magnetic wire and reduce static along the antenna. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that the foregoing and other changes in form and 
details may be made therein without departing from the spirit and scope of 
the invention. For example, by fitting the tunable tip of this invention 
to a four foot long citizen band antenna as described above, a radio 
operator would be able to expand the usable range (usually 26.965 MHz to 
27.405 MHz) of a normal citizen band antenna to a range, for example, from 
26.280 MHz to 29.520 MHz which is nearly 3 MHz greater in range than a 
fixed or non-tunable antenna. Thus, in this application, the tunable tip 
antenna provides a radio operator with the option of transmitting or 
receiving signals at frequencies outside of the citizen band frequency 
range. As a result, the tunable tip antenna is not only more efficient, 
but also much more versatile than the normal fixed citizen band antenna.