Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Tue, 27 Aug 2002 06:30:25 -0400
Message-ID: <redirect-1712576@logan.com>
From: "C Krouse" <kc16@qwest.net>
References: <list-1712246@logan.com>
Subject: Re: [LML] Re: LNC2 tail comm antenna
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

Here ya go.

If you don't want to spend a lot of money.....and you don't know someone
with a antenna analyzer or a spectrum analyzer, or a communications
analyzer, etc...., then you can use the Radio Shack SWR meter this way.

1.  Learn how to read or use the meter to make a reading.
2.  Determine what the center frequency of the radio band that you are in.
3.  Buy or get the neccesary connectors to hook up the SWR meter in line
from the output of your device to the antenna.
4.  Determine how to calibrate or determine if the meter is in calibration
in order to set the meter to zero.
5.  Set your communications equipment (radio or nav equip) to the center
channel or frequency in the bandwidth.
6.  Make sure that the antenna is connected.
7.  Set the device to transmit.
8.  Observe the meter reading.

You have now measured the SWR of the antenna.  What do you do if it is over
2:1 VSWR?  Well, the answer to that is simple and understanding it isn't too
hard.  The RF energy being transmitted is done so in a sinusodal manner at
the frequency of the transmission.  If there is an impedence mismatch in the
antenna, which there always is, then some of the energy is reflected back
into the transmission line.  This is what you have measured; the reflected
power expressed in a ratio.  The reflected power will be out of phase with
the forward signal, so the length of the cable will affect the SWR.  If the
power is out of phase, then changing the length of the transmission cable
will affect the phase thereby tuning the cable to the SWR.  You can get
fairly close to a match by shortening the cable an inch of a couple of
inches at a time.  Or you could try a foot at a time.  You could also make
up different lengths of cable and add to the existing cable.  Whichever way
you choose, you can always remake the cable and run it again.  This is a
good reason to always install antenna cables with a mindset that you will
have to remove them someday and replace them.

Curtis Krouse
N753K


> Okay, I'm convinced.  I want to test the VSWR of my antennas before I seal
up
> the tail and install rear windows.  How do I do that?  Do I buy a meter
from
> Radio Shack for $29 or something?
>
> All I know is that VSWR stands for Voltage Standing Wave Ratio and that
> numbers less than 2 are okay, 1.5 is better, anything over 2 is bad.  But
I
> have no idea how to actually test it.  Can someone tell us (in words of
one
> syllable) how to do it?  Do I need to have the coax cable attached or can
I
> connect the meter directly to the antenna.  (I would think the coax should
be
> attached since it's part of the antenna "system".)
>
> Thanks in advance.
>
> - Rob Wolf
> LNC2 MkII SFB  55%