User's Manual - VFD Series RF Wattmeters
Digital RF Power/ V.S.W.R. Indicator
INTRODUCTION
Thank you very much for purchasing the RF Applications, Inc. VFD Series RF Power/V.S.W.R.
Indicator. The VFD
Series represents a breakthrough in flexibility for measurement of the performance
of your transmitting system.
CMOS microprocessor control insures fast capture time and low power dissipation.
With the ability to monitor your
V.S.W.R. (and power levels) at all times, you will know that your antenna system is
performing within its design
specifications.
IMPORTANT
The VFD is designed to operate from a 12 to 16 volt DC power source. Do not
connect it to AC (especially line
voltage). As with all electronic equipment, keep the VFD out of direct sunlight and
keep it dry.
IF YOU HAVE PROBLEMS
We want you to be satisfied with your purchase. If you have any type of problem,
please check the Trouble
Shooting section of this manual. If your problem persists, call our Customer Service
Department at 440.974.1961.
Please have the VFD and this manual at hand when you call.
UNPACKING AND SETUP
INITIAL SETUP
Unpack the VFD. The box should contain:
 | One Display Head |
| One Directional Coupler |
| One 12VDC Power Cable or Optional Power Pack |
| One Warranty Card |
| This Manual |
If anything is missing, give us a call and we'll remedy the problem. Please take
a few moments now to send in your
Warranty Card.
The VFD consists of two major components: the Display Unit and the Directional
Coupler. The Directional Coupler
is installed in series with the output of your transmitter and antenna. A six-foot
(1.8m) cable is provided for
connection between the Display Head and the Directional Coupler. You may use a
longer cable if you wish (consult
the Special Applications section later in this manual).
Most users want to see how the unit works as quickly as possible after receiving it.
If you want to do this, read on. If
you would prefer to install the VFD permanently now, move ahead to the Installation
section of this manual.
If you are currently using a wattmeter in your setup, you have everything you need
for a test. If you are not using a
wattmeter, then you will need a suitable jumper to connect the Directional Coupler
to your station.
Connect the Directional Coupler as follows:
| Using RG-8 or RG-58 cables:
 | SOURCE goes to your transmitter (directly) |
| ANTENNA goes to your antenna (or tuner) |
|
| Plug the mini-phone connector into the Directional Coupler and connect 12VDC
to the power cable provided. The
black wire goes to ground, and the wire with the white stripe goes to plus. |
The VFD is protected against polarity reversals, but be careful just the same.
Apply 12 volt DC power. You should
observe that the V.S.W.R. lamp on the Display Head comes on. The firmware version
number will be displayed and
then the unit will display the message "IDLE." You are now ready for a test.
Simply use your normal procedure to transmit. The VFD will give you an instant
indication of your current status. If
your transmitter has an internal tuner, you might want to turn it off. The internal
tuner will cause a 50 ohm impedance
to be presented to the finals of your transmitter, thereby changing the
characteristic impedance present at the
antenna jack on the transmitter. This will affect the accuracy of the VFD (just as
it would any other measuring
device).
If you are having other problems, refer to the Trouble Shooting section of this
manual. Otherwise, you are now
ready for a permanent installation.
INSTALLATION
DISPLAY HEAD AND COUPLER LOCATION
The Display Head is designed to be either table-top or wall-mounted. Once the
position is chosen, mount the
Display Head using the supplied Velcro strips if desired. Choose a suitable location
for the Directional Coupler and
mount it as well. Be sure that you mount the Directional Coupler close enough to the
Display Head so that the
supplied cable can reach. If you want to use a longer cable, this is fine--any
shielded three wire jumper cable will
suffice, up to 100 feet.
Connect the 12VDC cable to the Display Head. If you purchased the optional power
pack, go ahead and plug it in
now. If you are using the power cable, connect the black lead to ground and the
white striped lead to plus. The
Display Head is protected against polarity reversal, but try to get the polarity
right anyway.
DIRECTIONAL COUPLER WIRING
Since the purpose of the VFD is to provide an indication of the match of your
antenna system to your transmitter, it
is critical that the Directional Coupler be installed as the first element after the
antenna jack on your transmitter (or
amplifier if you are using one). If you install it after your tuner, you will be
quite frustrated by your inability to change
the V.S.W.R. readings on the VFD. With this in mind, run a suitable jumper cable
from your transmitter's antenna
jack to the SOURCE connector on the Directional Coupler. The standard Directional
Coupler comes with SO-239
connectors. You can order optional "N" connectors if you prefer.
If you are using an amplifier, you want to install the Directional Coupler after
your amplifier. Connect the cable that
goes to your antenna system to the ANTENNA jack on the Directional Coupler.
You may ground the Directional Coupler to your station grounding system if you
desire. There is a further
discussion on this in the Technical Discussion section of this manual.
Recheck your cables and fasten them down appropriately. Turn on the power to the
Display Head (you should see
the firmware version number displayed, then the VFD will go to its idle state).
OPTIONS
If you ordered the V.S.W.R. Relay Option or the Power Monitor Option, consult the
documentation supplied with
that option to make the proper connections to your amplifier.
TECHNICAL DISCUSSION
RF POWER AND V.S.W.R. MEASUREMENT
The purpose of this section of the manual is certainly not to go into great
detail on the theory of the measurement of
RF power and voltage standing wave ratio (V.S.W.R.). There are many more
publications on the subject dating
back 50 or more years. But we do want to discuss the most important issues
pertaining to this matter.
For the purpose of this discussion, RF power refers to the peak voltage measured on
the line, multiplied by itself,
and divided by 50 (voltage squared divided by the impedance of the load, which is
watts).
V.S.W.R. is calculated by finding the sum of the forward and reflected voltages on
the line and dividing by the
difference of the forward and reflected voltages on the line:
You can see that if there is no reflected voltage, you will be dividing the forward
voltage by the forward voltage,
which is 1.0. A V.S.W.R. of 1.0 is ideal (and actually unobtainable because there is
always a small amount of
reflected voltage on the line). A V.S.W.R. of 2.0 indicates a power loss of about
10%, at 3.0 the loss is about 25%,
and it gets worse from there. By loss, we mean that, for example, a transmitter
looking into a 3.0 to 1 V.S.W.R. will
only be able to deliver 75% of its output power to the load (the antenna). To
summarize, the transmitter must see an
exact impedance match (50 ohms in this case) to deliver full power to the load.
Again, to achieve maximum power transfer from the transmitter to the antenna, an
impedance match between the
antenna system and the transmitter is required. Normally, the desired impedance is
50 ohms. Antennas need to be
designed and built to match this impedance.
In any antenna system, the object is to get the most power delivered to the actual
antenna. Feedlines, antenna
switches, connectors, and yes, wattmeters all contribute to losses between an RF
source and an antenna. You can
conclude from all this that the best place to make a power or V.S.W.R. measurement
is actually at the antenna, and
this is indeed the best place. In most installations, however, power and V.S.W.R.
measurements are made in the
shack near the transmitter. This is generally not a problem, as long as the user
realizes that the readings that are
being taken represent those of the entire antenna system, not just the antenna. And
in some cases, the readings
can be misleading.
For example, you might have a perfectly resonant (50 ohm) antenna running on 10
meters. You connect it with 150
feet of RG-58 cable. Your station-mounted wattmeter shows that you are delivering
100 watts from your transmitter
into this antenna system.
Here's the problem: the RG-58 cable exhibits a loss of 2.0 dB per one hundred feet
at 30 MHz. This means that if
you mounted your wattmeter at the antenna, you would read only 50 watts!
Your V.S.W.R. measurements can be affected, because losses in the feedline reduce
the level of the reflected
voltages as well. Therefore, you can be lulled into a sense of security about your
V.S.W.R.; when, in fact, it is higher
at your antenna than you see on your meter.
Wattmeters like the VFD allow you to mount the Directional Coupler at the antenna
(or at least near it, with some
restrictions). By doing this, you can see what is happening near the antenna.
However, additional installation costs
are incurred, because you must bring the sensing cables back to the shack.
In most installations, it is still most practical to install the measurement
equipment in the shack. The best solution to
the above problem is to use high quality feedlines, connectors, etc. to make your
installation. By high quality, we
mean components that are rated for the power, frequencies, and distances you will be
using. Then, use a 50 ohm
dummy load to test each feedline system after you have installed it. You can verify
that your feedlines have the
predicted losses and low V.S.W.R. readings. With this knowledge in mind, you can now
use the information your
shack-mounted power and V.S.W.R. meter gives you.
As mentioned above, each element in the connection system between the transmitter
and the antenna introduces
some amount of loss and increased V.S.W.R. The goal of any design is to minimize the
losses and V.S.W.R.,
while maximizing instrument accuracy over the specified frequency range.
As in any engineering exercise, tradeoffs must be made between performance and cost.
You will find that most
commercially available instruments do not introduce significant losses or V.S.W.R.
However, you can find wide
variations in accuracy among several watt/V.S.W.R. meters. Therefore, you have to be
cautious in comparing
readings between various meters.
In addition, your station grounding might affect your power and V.S.W.R.
measurements. When you connect all of
your equipment together, you introduce new paths for RF energy to flow (in addition
to your coax shields).
As an interesting point, however, many users find that the absolute value of the
displays on their meters doesn't
really matter that much. For power, the idea is to maximize it. For V.S.W.R., the
idea is to come as close to 1.0 as
possible (below a certain minimum, of course).
Most measurement systems for high frequency (HF) power and V.S.W.R. measurement
consist of two major
components. The first is the sensing system (Directional Coupler), the other is the
display system. There are
commercially available systems that integrate both of these components into one
package (a conventional
watt/V.S.W.R. meter) and some that separate the two (such as the VFD).
USING THE VFD
GENERAL OPERATION
In addition to the 2x16 vacuum fluorescent display, there are two buttons on the
front of the VFD. The top button is
labeled "TUNE/OPERATE" and the lower one is labeled "SET ALARM LIMIT." There is an
LED that is used to
signal you when your V.S.W.R. exceeds a preset limit.
THE VFD DISPLAY
When no RF is present, the VFD will indicate the selected operating mode (Tune or
Operate), the letters RFA on
the right side of the top line, any vanity text (if the Vanity Option was ordered)
on the lower line and IDLE on the right
side of the bottom line.
As soon as RF is applied, the top line will indicate, in bargraph mode, the relative
amount of power you are
applying. The RF power will be displayed on the second line.
RF POWER DISPLAY
Your RF power is displayed on the second line. There is a "sharp attack, long
decay" peak detector used for the
numerical RF power display. You will find that the displayed value ramps up quickly
and holds your peak value for a
certain amount of time. This time is about 3 seconds when in "OPERATE" mode (the
default) and about 1/2
second when in "TUNE" mode. You can choose which mode you would like to use by
pressing the top button on
the display.
In order to provide a pleasing display regardless of CW or SSB operation, we have
carefully selected the update
interval for the "OPERATE" display mode. This can be a problem, however, when you
are trying to set an antenna
tuner. For this reason, the display update interval is about six times faster when
you are in "TUNE" mode.
IMPORTANT
The display will occasionally indicate a small wattage at the end of a
transmission. This is normal. The
data capture time is fast. One dash (or whistle) will provide enough signal for you
to get a reading. This
is a nice feature that helps to reduce QRM, too.
The VFD indicates the mode you have selected whenever there is no RF present. If you
notice the power indication
"bouncing," you have probably left the unit in TUNE mode. Simply switch it back to
OPERATE mode to correct this
problem.
V.S.W.R. DISPLAY
The V.S.W.R. display gives you continuous readout of your V.S.W.R. The display
indicates V.S.W.R. from 1.0 to
29.9. Above 29.9, the display reads "MAX." When your preset V.S.W.R. limit is
exceeded, the display will indicate
"ALARM!" and "VSWR HI." The red LED on the front panel will light. If you have the
V.S.W.R. relay option, the relay
will be de-energized also.
The power-on default for the V.S.W.R. alarm is 3.0:1. By pressing the "SET ALARM
LIMIT" button, you can set this
trip point to 1.5:1, 2.0:1, 2.5:1 or back to 3.0:1. This value is not "remembered"
when you remove power from the
unit, therefore, it must be set each time after the unit is powered up.
AUTORANGING BARGRAPH
The VFD incorporates a unique bargraph display. This display answers one of the
major complaints about digital
wattmeters. Many users are used to and feel most comfortable with analog meters,
because they can "see what
they are sending." Digital displays remove this feature (who wants to see a bunch of
meaningless numbers
bouncing around?). The VFD updates the bargraph much more quickly than the numeric
displays. The resulting
display is pleasing and quite useful.
The display uses 13 of the character positions on the top line of the vacuum
fluorescent display. Each of these
character positions has five vertical lines. Therefore, the resolution of the
bargraph function is 65 elements. The
microprocessor in the VFD uses all 65 positions up to about 500 watts, then it "autoranges"
above 500 watts to use
the 65 positions over about 3000 watts (300 watts on the VHF version). Once it
ranges up, it does not "range down"
until RF is removed for a few seconds (long enough for the unit to return to the
idle state).
In normal use, however, you can use the digital display to find out what your actual
power is. The bargraph is very
handy for a relative indication of what your station is doing, and you can "see what
you are sending."
The bargraph is particularly useful for tuning up amplifiers. Simply adjust your
amplifier for the longest bar.
ACCURACY AND CALIBRATION
The VFD uses an eight-bit analog to digital converter. This results in a display
that cannot indicate every single
power value from zero to 3,000 (there are only 256 codes available from the analog
to digital converter). You will
notice that this display will indicate power in steps; for example, 100, 104, 109,
113 watts, etc. The general
specifications for the VFD call for accuracy that is plus or minus ten percent; plus
or minus the analog to digital
converter resolution (which is one power step increment).
We calibrate the VFD at 500 watts at 14.100MHz and then verify its operation on 1.8
and 28MHz. If your application
calls for better accuracy on a certain band, you can easily recalibrate the unit to
suit your needs. In this case, you
can enjoy plus or minus five percent (or better); plus or minus the resolution
accuracy.
You will be adjusting the potentiometers on the PC board. Be careful not to blast
the board with static electricity.
The calibration procedure is as follows:
- Open the VFD.
- Press and hold "TUNE/OPERATE" while applying power to the VFD. Hold until
"CALIBRATE" appears on the display.
- Apply a known power, ideally 500 watts, to the Directional Coupler.
- Set potentiometer R10 (upper) to display the correct power next to the "F" on
the display.
- Turn off the RF source.
- Reverse the RF cables on the Directional Coupler.
- Apply a known power, ideally 500 watts, to the Directional Coupler.
- Set potentiometer R11 (lower) to display the correct power next to the "R" on
the display.
- Turn off the RF source and DC power to the VFD.
- Reassemble the VFD.
The most accurate V.S.W.R. readings are made with at least 25 watts of forward
power. You will sometimes notice
a small change in V.S.W.R. as you change power levels. This is normal. Cables,
connectors, traps and other items
can respond differently under high power conditions. Variations of 0.2 in V.S.W.R.
are not uncommon. If your
variation is higher, you might have a problem with one or more of the items
mentioned above.
If you are having problems, give us a call.
REMOTE COUPLER MOUNTING
There is no reason that you cannot mount the Directional Coupler remotely. You
will need to use a high quality,
shielded, two conductor (plus shield) cable. In addition, the Directional Coupler is
not designed for outdoor use, so
you will have to put it in another enclosure.
You can expect changes in accuracy with extremely low temperature. We have tested to
-10 degrees C, and the
changes are minimal at that temperature.
We have tested the Directional Coupler with 100 feet of cable, and it performs well.
Be aware though, you will be
reading power at the other end of a feedline. Maybe you don't really want to know
what's out there!
SPECIAL DIRECTIONAL COUPLER CONNECTORS
If you like to use "N-" type connectors, contact the factory. "N" connectors are
available.
IN CASE OF TROUBLE
The VFD is well designed and should give you many years of trouble free service.
Most of the problems you will
encounter usually occur during installation and startup. If the items below aren't
sufficient to help you solve your
problem, please give us a call.
| Unit will not come on |
Check power connections. Is the unit on? Polarity OK? |
| High V.S.W.R. displayed; no forward power
displayed |
Be sure that the SOURCE input to the Directional Coupler goes to
your transmitter (or amplifier). |
| V.S.W.R. doesn't change when adjusting a tuner |
Be sure that you have installed the Directional Coupler AHEAD OF
the tuner. If your rig has an internal tuner, you should turn it off to make
your measurements. |
| Power display grossly inaccurate |
Someone might have recalibrated the unit. Perform the
calibration procedure in this manual. Your internal antenna tuner might be
turned on. Turn it off. |
If you are experiencing other problems, give us a call. We'll help you out as
quickly as possible.
|
