Re: How to use a simple SWR meter and what it means to your VHF

Discussion in 'Boat Electronics' started by Brian Runyard, Nov 14, 2003.

  1. Larry

    That's the best description I've seen, brilliant.

    "Larry W4CSC" <> wrote in message
    > I've seen many posters talk about antennas and know lots of boaters
    > with antenna troubles and no clue how to see how it's doing, way up
    > there, so thought I'd stick my neck into the guillotine and give some
    > basic instructions on what an SWR meter is, what it does, and what it
    > means after you learn how to use it. This will be all about VHF
    > marine band, but is the same for any frequency the meter is made for.
    > FIRST, let me say not all SWR meters are suitable for VHF marine band
    > use. The reason for this has to do with the directional coupler, the
    > part that senses power going this way and power going that way built
    > into your meter. A CB SWR meter is NOT suitable for VHF work. VHF is
    > out of its design range and its directional coupler is too long. Put
    > using a CB SWR meter out of your mind. It's not a good reading. A
    > ham radio VHF SWR meter IS acceptable as its range is usually from 140
    > to 170 Mhz, which includes our VHF marine band. Most boaters will get
    > the little white Shakespeare VHF power and SWR meter from Waste Marine
    > or some other overpricing boat shop so that's what I'll use for my
    > example.
    > These little passive SWR meters use the RF power of your radio to
    > power the meter and require no batteries or power source. One guy I
    > know got no reading and through the meter had dead batteries in it.
    > There aren't any. His transmitter power amp was output.
    > The controls on it are quite simple. There's a switch that switches
    > from POWER OUTPUT to REFERENCE (SETUP) to SWR, a 3-position slide
    > switch. The POWER OUTPUT meter is useless unless you have the ANTENNA
    > jack plugged into a 50 ohm dummy load like the guys at the factory did
    > to calibrate it. Depending on the position of the meter in the line
    > of a defective antenna system, it might read way low or it might peg
    > on 1 watt. Consider it fairly accurate if the SWR of the antenna is
    > quite low (below 1.5 to 1)
    > So, What's SWR??.....
    > SWR (pronounced as three letters, unless you're on CB where it's
    > called "swur" for some reason noone knows) means Standing Wave Ratio.
    > The keyword there is RATIO, a measurement of the peak voltage found on
    > the transmission line in one place, compared to the minimum (trough?)
    > voltage found 1/4 wavelength from that peak in either direction.
    > These peaks and valleys are caused by reflections of an imperfect or
    > off-tuned antenna, bad connectors, kinked transmission lines bent too
    > sharply and a lot of just poor luck. You can watch these waves out by
    > the seawall. If you toss a rock into the water (transmitter) it
    > creates waves that expand out in all directions. When the wave
    > bounces off the seawall, watch what happens. The wave coming in from
    > the stone bounce off the REFLECTIVE seawall and go BACK towards the
    > transmitter, a "reflected power" that wasn't absorbed by the wall. As
    > the reflected waves pass through the incoming waves that haven't
    > reflected, yet, notice how there is a wave that doesn't move.....a
    > Standing Wave that has PEAK positions that stand still a set distance
    > from TROUGH positions, that also stand still.
    > The same exact thing is going on in your antenna system, every time
    > you press that button. In electronics, there are two simple devices
    > that STORE electrons....capacitors that charge (electrostatically) and
    > inductors that store energy (magnetically). If you doubt this, go out
    > and pull the spark plug wire off a running Seagull outboard to test
    > this theory....we'll wait. Ah, I see you're back? Why do you look so
    > "shocked"? Did it work?
    > A perfect RF transmission system perfectly transfers the power from
    > the transmitter to the perfect antenna, which radiates all the power
    > the transmitter put out into the air, blasting all the recievers with
    > your fantastic signal so they can hear your pleas for help. These
    > systems do not exist. The antenna is never tuned to your channel,
    > only close to your channel (we hope) and the transmission line is that
    > cheap white crap from Wasted Marine or RatShack, not rigid coaxial
    > line used by broadcast stations, made to exacting standards. To keep
    > it short, the line and antenna have "reactance", like that wall. And
    > it's that reactance (inductance and capacitance) that cause anomolies
    > that make reflections, like that wall.
    > What can we do? How can we measure how bad it is?...............
    > Most marine antennas are sealed up and "pretuned" for open areas. Not
    > much we can do to "tune" them to the middle of the band. I like old
    > Metz antennas, made by an old ham company, just because I can trim
    > that element for best results. It's tunable. That fishing pole of a
    > fiberglass antenna is actually a little, specially bent wire embedded
    > inside fiberglass to keep it straight (and disintegrate reliably in
    > sunshine so you can replace it, often). "Pruning" the tunable antenna
    > requires us to measure SWR at different frequencies so we can center
    > its curve up on the band we want for best results.
    > What "curve"??
    > An antenna "resonates", where the inductance balances out the
    > capacitance and acts like a radiating resistive load, over a fairly
    > large range of frequencies, not just one. Lucky for can be
    > made to pass channel 1 and channel 72, fairly reliably, without
    > retuning like you have to do to change channels on the HF SSB antenna.
    > The curve looks kinda like this:
    > | * *
    > SWR| * *
    > | * *
    > | *
    > Frequency>>
    > If we center the best (lowest) SWR up in the middle of the band, it
    > will have an acceptable SWR (low) on channels on the bottom and top we
    > want, and are allowed to use. Part of the testing, we will test
    > different channels across the marine band to get an idea of what YOUR
    > plot would look like.
    > So, how do we measure SWR??.............
    > For the little meter to measure the antenna, it has to be located
    > INLINE with the RF power, between the transmitter and antenna.
    > IDEALLY, we'd like to adjust the antenna with the SWR meter located
    > between the transmission line, antenna end, and the antenna's coax
    > connector. Obviously, sometimes, this is not practical for a simple
    > test. However, the further you are from the antenna, back down the
    > transmission line, the less the reading is about the antenna SWR and
    > the more the reading is about the cable losing the signal (attenuation
    > and leakage) and the reactivity of the cable, itself. If we measure
    > the SWR at the antenna end, the SWR we measure is only about the
    > antenna. If we measure it where it's easy, at the radio, the reading
    > is about the antenna AND the cable, so you can't tell which is at
    > fault if it sucks.
    > OK, let's assume you're like me, hate heights, weigh too much to haul
    > up on a winch with less than 6 strong arms on a winch handle and the
    > bos'n's chair might not like the load, anyways. So, we'll measure the
    > antenna at the radio end, at least until we find it's all screwed up.
    > Disconnect the antenna from the radio, with the radio off so you don't
    > inadvertently transmit into an open which might do harm to the
    > transmitter.
    > Now you need a "coax jumper" that didn't come with the meter. Radio
    > Shack has them, so get one that's just long enough to hook the meter's
    > RADIO port to the radio so we can still read it and switch the
    > controls. If you'd like to MOUNT the meter on your panel, buy two
    > right-angle UHF 90 degree adapters so we can mount the meter on the
    > front of the panel and the L-shaped connectors will go back through
    > the panel to connect the cables to. That would let you see power
    > output every time you keyed the VHF so you'd be SURE it was
    > transmitting, instead of calling out for a radio check so often. I
    > leave them in SWR to watch the antenna, here.
    > Hook the antenna to the antenna jack and the jumper between the Radio
    > jack and the radio. Turn on the radio and tune it to a commercial
    > channel not monitored by the DEA or USCG around 40-something. Put the
    > meter's little switch in the REFERENCE or SET position and turn the
    > set control all the way to the left, to keep from pegging the meter.
    > Test at FULL POWER so you can see if something up there is arcing at
    > FULL POWER (the meter jumps up in SWR if it is).
    > Key the transmitter and don't talk into the mic. Turn up the SET level
    > "volume" control until the meter reads FULL SCALE, all the way to the
    > SET mark. This sets the reference level of the meter to the power
    > coming out of the radio "under these conditions". Once set to full
    > scale, flip the switch to SWR and pray it drops all the way to 1 on
    > the SWR scale (no reflected power) indicating I was a liar and there
    > IS a perfect antenna system.....Read the pseudo-accurate meter SWR
    > scale. 1 is the left edge (1:1 standing waves - there aren't any
    > standing waves because the antenna is perfect). The next mark up is
    > probably 1.5 with hashmarkes for 1.4, 1.3, 1.2, 1.1, which is silly.
    > Then it's 2:1 then the middle of the meter is 3:1 SWR and there's no
    > marks higher because 3:1 is BAD, BAD, BAD....way too high. Unkey the
    > mic before the cops start looking for you.
    > What does this mean??
    > Here's the relative power levels of the major points.
    > 1.0 reflected power....all 25W is going out on the air
    > 1.5 SWR.....4% reflected power....1 watt is reflected back, 24W goes
    > out and noone notices anything because you couldn't measure 4% out of
    > the lab.
    > 2.0 SWR......10% reflected power...2.5W is reflected back and 22.5 W
    > goes out on the air and STILL noone notices anything unless they are
    > magicians.
    > 3.0 SWR......25% reflected power....6.25W is reflected back and 18.75W
    > goes out on the air. Someone comparing this antenna with your perfect
    > antenna just notices a little movement in his S-meter on the other end
    > if you're weak. 3.0 and above is considered "bad SWR" and something
    > needs to be fixed.
    > CB myths........
    > 1.01 SWR is good. 1.1 SWR is a disaster. What nonsense. Where do
    > they get this from? ANTENNA MANUFACTURERS selling new antennas,
    > that's where. They made millions from this myth. Wanna see a real
    > broadcast TV station's huge UHF antenna SWR LIVE on the net? Look at:
    > This a real readout of a powerful +megawatt TV transmitter from WZPX
    > on Channel 44 (with a nice new digital TV transmitter, too!) The
    > software company puts it on the net. SWR tonight on the
    > beast-on-the-mountain is 1.3:1 but I've seen it read 1.8:1 which is
    > really high at these power levels. On your boat, it's not. You don't
    > have thousands of watts coming back down the pipes at you!
    > Ok, now always turn the SET control back to the left before unhooking
    > the meter or changing channels. Do it now.....
    > Ok, make the same measurements on a few channels (not 16, 22A please)
    > across the marine band. Record your SWR results and make a crude
    > chart of them plotting SWR measured against channel (frequency) number
    > like I did above.
    > Is the lowest SWR near the middle of the channel numbers? No? Does
    > the curve at least have a low point (dip) inside the marine band?
    > Yes, but the dip is around Channel 3 and SWR is much higher at Channel
    > 72 (why they could hear you on 16 but not 72 way off). The antenna is
    > tuned too LOW. If it's tunable, we need to shorten the element to
    > raise the resonant frequency. If 1 is high SWR and 72 is low, we need
    > to lengthen the antenna element. Ideal is a curve with its low point
    > somewhere in the middle of the band with less than 2:1 SWR on any
    > channel. The curve shows you where the antenna tuning is and how
    > broadbanded (how many channels will it radiate well).
    > If you measure this curve up at the antenna before all that cable
    > attenuates the SWR reading, it will simply be much more pronounced
    > because the cable attenuates power up as well as it does power down
    > the mast....making our reading weaker by a bit. AS you can see,
    > tuning an antenna ISN'T rocket science. If the channels you use are
    > all less than 2:1, it's fine. If they're less than 1.5:1, it's great.
    > If they're all really low....SOMETHING IS WRONG WITH THE ANTENNA, THE
    > CABLE OR THE MEASUREMENT, because every antenna has a curve.
    > Ok, we'll now haul a, ah, volunteer....up the mast to
    > trim the antenna the way it shows in the instructions......
    > You all should be able to measure SWR just fine with the little
    > meters, now, and have a vague idea of what it means.
    > Please leave the classroom quietly so's not to wake the four students
    > in the back row we lost. (Class quietly leaves, instructor slips out
    > and puts lights out with them still asleep. One once slept right
    > through lunch....(c;)
    > Larry W4CSC
    > "Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"
    Brian Runyard, Nov 14, 2003
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  2. Brian Runyard

    Jim Woodward Guest

    "Larry W4CSC" <> wrote in message


    > Thanks for your comment. Tomorrow, someone can come explain how the
    > hand-pumped, marine toilet functions and what it means when it refuses
    > to flush and makes that gurgling sound, instead. We'd ALL like to get
    > a little instruction on that piece of engineering!

    Aw, sheeeet....

    Jim Woodward

    Jim Woodward, Nov 14, 2003
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