From: jfw@ksr.com (John F. Woods)
Newsgroups: rec.radio.cb,misc.misc
Subject: Re: Some More CB Questions
Date: 14 Aug 92 12:29:46 EDT

smash@oucsace.cs.ohiou.edu (Scott Mash) writes:
>Ok, just a few more questions for you CB pro's out there.
>My grandfather has a Pres. Zachery Taylor base CB and just put up a
>Archery All Band Antenna and cannot get the standing wave out.  There
>are no adjustments on the antenna and he has removed all the slack in
>the coax in hopes that would do it.  The standing wave is over a 3 on
>his meter (sorry I don't know what the 3 is), but my grandfather tells
>me it should be below a 2.  Any suggestions?

First, the length of the (50-ohm) coax has nothing to do with the standing
wave ratio (which is a measurement of reflected power; see below for further
definition).  If the meter is measuring correctly, and if the antenna has no
adjustments, you may have to settle for 3:1 SWR; this is unlikely to damage
the transmitter, though it may reduce its output power to compensate and avoid
damage.  You should also look over the antenna carefully for poor connections,
rust, or possibly paint keeping screws from making proper electrical contact.
It is possible to use "matching networks" to compensate for SWR to make the
transmitter see the simple 50-ohm load it wants to see, but it's more likely
that there's just something wrong with this antenna (assuming it was well-
designed for CB service, anyway).

>Second, I know it is legal to transmit 4 watts.  My grandfathers Pres.
>CB is only transmitting 2, as is my Realistic Car CB, how can these be
>set up to 4 watts?

Did you test your car CB on the same power meter?  It is possible the power
meter is faulty.  One quick way to check would be to borrow someone else's
meter; failing that, if you can buy (Radio Shack) or borrow a good 50-ohm
dummy load, you can check the SWR meter (the Radio Shack dummy load is $2 more
than their cheap SWR meter, though) -- with good 50-ohm coax, the SWR meter
should indicate 1:1 SWR with a good dummy load; any other value, and it's
broken.

If they really are putting out 2 watts, then you'll have to take them to a
2-way radio shop to have them adjusted.

Standing Wave Ratio:  if a transmission line is terminated by a load whose
impedance (resistance plus capacitive/inductive reactance) is not the same as
the "characteristic impedance" of the transmission line, the point of mismatch
will reflect power back to the source, which will have the effect of making
the source see a load impedance other than that of the transmission line's
characteristic impedance; modern fixed-tune transmitters tend not to like that
for a variety of reasons.  SWR is calculated as follows:  let the "reflection
coefficient" R equal the square root of ( the reflected power divided by
the forward power ); SWR is then (1 + R) / (1 - R).  Note that SWR is 1:1
when the reflected power is zero (match condition), and infinite when the
reflected power equals the forward power (open or short circuit load).  Note
that the length of the transmission line does not appear in this equation.

The belief that changing the length of the transmission line changes the SWR
probably stems from three things:  (1) It *is* possible to use a section of
transmission line to match impedances, ->but<- the transmission line section
will *not* be equal to either of the impedances -- I don't feel like working
it out, but *no* length of 50-ohm coax will match a non-50-ohm load to a
50-ohm source; this is used in some antenna designs where a section of 400-ohm
open-wire transmission line (or some other useful impedance) will be used to
match the antenna to the 50-ohm coax.  (2) If the SWR meter does not have the
same characteristic impedance as the coax line (it requires some care to build
a meter with a constant characteristic impedance), then it will introduce its
own mismatch, and it won't correctly register the SWR caused by the load
mismatch.  (3) If there is RF current flowing on the *outside* of the coax,
(generally a return current) it will flow right around the meter and not be
registered; the length of the coax can sometimes effect how much RF flows on
the outside of the coax, thus changing the SWR *reading* (but not the SWR
itself).

John, WB7EEL