BoatBanter.com - How to use a simple SWR meter and what it means to your VHF

How do you know that you have a better signal when you tune your
antenna for lowest SWR?

An SWR meter is good for telling you when your transmitter is seeing a
flat 50 ohm load. It is useful as a tuning device. Not much more.

Solid state finals don't like complex impedance's. It causes them to
draw excessive currents. High SWR goes along with reflected complex
impedance's. Also the output circuit is usually fixed at 50 ohms. If
you don't match that you will not get maximum power transfer from the
transmitter.

Your solid state transmitter usually has a power reduction circuit
that reduces power out when it sees a high SWR. So having the proper
match at the transmitter will ensure full power out.

On tube transmitters you will usually not find any high SWR protection
circuits. Tubes are very tolerant on complex impedance loads.

As explained in the ARRL HANDBOOK, reflected power does not reach the
finals in your transmitter. See below again.

Regards
Gary

On Mon, 24 Nov 2003 18:23:34 -0500, "Doug Dotson"
wrote:

So what is the use of an SWR meter if SWR has no affect on
radiated power or stress on the finals? I know for a fact that if
I tune my antenna for lowest SWR, I have a better signal.

Doug

"Gary Schafer" wrote in message
.. .
On Sun, 23 Nov 2003 14:43:26 GMT, (Larry W4CSC) wrote:

On Fri, 21 Nov 2003 18:32:56 GMT, Gary Schafer
wrote:

You need to tune your tuner only to provide the proper impedance match
to your transmitter. A 50 ohm SWR bridge at the transmitter will tell
you when you have achieved that.

WHOA! There you go again! If I tune my transmitter to 50 ohms
RESISTIVE, non-reactive, it will ABSORB any reflected power because
it's acting like a RESISTOR, as it should, converting reflected power
into HEAT in the final amp.....(c;

Larry W4CSC

NNNN

I see you haven't read your Bird manual yet about power delivered to
the load have you Larry. :)

ARRL HANDBOOK CHAPTER AND VERSE:
Here is some stuff right out of the 2000 ARRL Handbook. Chapter 19:
Transmission Lines. Page 3, Mismatched lines: Look at the 5th
paragraph there if you don't want to read the rest. It tells you what
happens to reflected power traveling back down the line.

It says that if it encounters a mismatch at the load, part of the
energy gets reflected back down the line. "It (reflected energy)
encounters another mismatch at the generator. Reflected energy flows
back toward the load. After a few such journeys it diminishes to
nothing. Partly because of loss in the line but mostly because it is
absorbed by the load. If the load is an antenna that energy will be
radiated."

FURTHER:
Look at page 19.7. There is a table there showing the line losses and
SWR encountered with a 100 foot flat top antenna at different
frequencies. With that antenna at 7 mhz the SWR on RG8 would be
49.1:1. The loss would be 5.8 db on 100 feet of cable. With open wire
feed line the loss would be .3 db.

If all the reflected power got dissipated in the finals of the
transmitter at an SWR of 49:1 you wouldn't have much left to radiate.

STILL NOT A BELIEVER ?:
Do the simple experiment with your watt meter and the tuner that I
described before. If you have 2 watt meters, one to put on each side
of the tuner, all the better.

Don't forget to read that little paragraph in the Bird manual about
"power delivered to the load".

It is forward power minus reflected power. But note that THE FORWARD
POWER READING ON THE WATT METER WILL BE THE TRANSMITTER POWER PLUS THE
REFLECTED POWER. As the mismatch at the load end of the line is
increased so does the "apparent forward power" on the watt meter in
that line increase.

If anyone that is interested doesn't have an ARRL handbook or can't
find it in their version let me know on the news group and I will scan
the page for you. Let me know here as I don't look at this email
address very often.

Regards
Gary