SWR Meter

[Stemar]

My VHF worked perfectly last year. Since then I've taken the mast down and that meant cutting the antenna cable. Now I can't raise Solent Coastguard from Hardway.

I remade the cut using a soldered socket of the same type as plugs into the VHF mounted in a box on the end going to the radio and a plug on the end going to the antenna. The problem could be here or at the connection to the antenna itself. Nothing else was disturbed.

It seems that an SWR meter should allow me to troubleshoot my connections and help me decide if I need to find a volunteer to go up the mast, but I know nowt about 'em. Fleabay has no end of cheap and cheerful meters for CB systems. Would one work for a VHF antenna, or do I need a "proper", ie expensive, one?

Is there anything else I need to know about using one? (current knowledge approximately zero)

 

[30boat]

MEASURING SWR

Measuring SWR shows you how much of your radio's transmit power is
reflected from the antenna back into the antenna cable, generating heat
and wasting power. With this information, you can adjust the length of
your antenna or antenna cable so your antenna and radio combination can
produce the maximum power they are capable of producing.

For more information about measuring SWR, see the ARRL Handbook, available
at your local library.

CONNECTING THE METER

To connect this meter to your CB or amateur radio and your antenna, you
need RG-58U coaxial cable (not supplied) with a PL-259 connector on both
ends (such as RadioShack Cat. No. 278-968). The cable must reach from
the back of the meter to your radio's antenna jack.

Follow these steps to connect the meter to your radio and antenna.

1. Turn off your radio. Then disconnect the antenna from the radio and
plug it into ANTENNA on the back of the meter.

2. Plug the coaxial cable into the radio's antenna jack and into
TRANSMITTER on the back of the meter.

CALIBRATING THE METER/MEASURING SWR

1. Set CALIBRATION/FS/SWR on the front of the meter to CALIBRATION/FS.

2. Turn on your radio. Then select a channel or frequency on your
transmitter and hold down its transmit key. Do not speak into the
microphone.

Notes: An SWR reading will be different for different frequencies
(channels). If you transmit on one channel more often then
any other, select that channel. If you transmit on several
channels, choose a frequency in the middle of the range of
channels you use. (For example, if you transmit on all 40 CB
channels, choose Channel 20, because it is midway between
Channel 1 and Channel 40.)

If you use a CB that has sideboard modes (SSB), do not select
any of these modes. (Use only AM for SWR measurements).

If you have an amateur radio, select the continuous wave (CW)
or tuning mode to check the SWR.

3. While you hold down the transmit key, rotate the meter's knob until
the needle points to CAL.

4. Release the transmit key, then set CALIBRATION/FS/SWR to SWR.

5. Press the transmit key again, then read the SWR by noting the needle's
position on the upper SWR scale. A meter reading of 1 on the upper
scale is ideal.

The figure shown between the upper and lower scales on the meter
(% REF POWER) indicates the percentage of power that is wasted
through reflection of the signal from the antenna back into the
antenna cable. For example, an SWR reading of 1.5 also means that 4%
of your signal power is lost. However, 96% of the radio power is more
than enough for almost all applications.

6. Repeat Steps 1-5 for each channel or frequency you want to get an SWR
reading for.

Interpreting SWR Readings

An ideal SWR reading is 1.0, but this reading is usually possible only
under laboratory conditions or with a dummy load. Actual antenna
installations have higher readings. The information below will help you
interpret the readings you get.

SWR Efficiency Interpretation

1.0 to 1.5 Excellent The antenna cable and the antenna length
match the transmitter's output
requirements almost perfectly.

1.5 to 2.0 Very good The antenna, the cable, and the
transmitter operate very efficiently.

2.0 to 3.0 Acceptable The antenna, the cable, and the
transmitter operate with some loss. If
possible, adjust your antenna or antenna
mounting system to improve.

Above 3.0 Inefficient Adjust your antenna or antenna mounting
system to improve efficiency.

IMPROVING SWR

There are several ways to improve the SWR of your radio/antenna
combination. Try these first.

Be sure you are using the type of cable recommended for your
equipment. If the manufacturer recommends a 50-ohm cable, do not
substitute another type that has a different impedance.

Confirm that you mounted your antenna according to the manufacturer's
instructions. The angle and the base arrangement can affect the SWR
reading.

Adjust the length of your antenna according to the instructions
provided by the manufacturer. A change of as little as 1/8 inch can
make a measurable difference.

See your radio's and antenna's owner's manuals.

MEASURING FIELD STRENGTH

Your meter can measure the low levels of radio frequency (RF)
generated by a radio transmitter. You can use these measurements to
map the transmitting direction and coverage pattern of your antenna, or
to compare your radio and antenna's efficiency before and after an
adjustment. You do not need to connect the meter to the antenna or the
radio to measure field strength.

It takes two people to measure field strength. For an accurate
measurement, you will need to map the site around your antenna.
Then, while someone presses the radio's transmit key, you will need to
walk around the site around your antenna with the meter, marking the
readings you see.

2. Set CALIBRATION/FS/SWR to CALIBRATION/FS.

3. To calibrate the meter, have your helper select a channel or frequency
on your transmitter and hold down its transmit key. Tell the helper
not to speak into the microphone.

Then, while the helper holds down the transmit key, walk around and
adjust the meter's knob until the meter's needle moves to 3 on the
FIELD STRENGTH scale.

Notes: If you use a CB that has sideband modes (SSB), do not select
any of these modes. (Use only AM for field strength
measurements).

If you have the amateur ratio, select the continuous wave (CW)
or tuning mode to check the field strength.

4. Have the helper release the transmit key, then make a change to your
radio or antenna (such as adjusting the antenna or transmitter or
adding a new antenna).

5. When the change is completed, have the helper hold down the transmit
key again while you watch the meter's needle.

If the field strength reading is higher, you have probably improved
the power output and efficiency of your radio and antenna. If the
field strength reading is lower, repeat Steps 4 and 5.

Note: If you move the physical location, or change the transmission
direction, of your antenna, you might have to recalibrate the
meter and measure the field strength again.

See "Calibrating the Meter/Measuring Field Strength" above.

(BR/tc 5/26/98)

 

[Alpha22]

All of which is greatly helpful.....???

A CB SWR meter is of no use to you at all. You must obtain one that is made for the frequency's you wish to measure. In amateur radio terms that is 2 Meters or 144MHz - Give or take a little.

You are looking for maximum transmitted power and the minimum of reflected power, that gives you the ratio.

 

[Salty John]

The newer reconnection is the most likely to be faulty if the antenna connection uses the same connector as before. So I'd disconnect at the radio and at this new connection. check with a multimeter that there is no continuity between the outer shell of the PL259's and their centre pins. If you can get the two ends of this cable close enough to each other also check that there is continuity pin to pin and shell to shell but never between shell and pin.

The most common problem with these soldered connections is that when you push the stranded centre core through the centre pin you snag a single strand and bend it back unnoticed. This will cause a short in the line.
The cable to the antenna cannot be checked this way from deck level because when connected the antenna will show a dead short anyway. To perform the same test you need to disconnect at the antenna and radio ends of the cable.

The trouble with SWR meters is that you can't buy one that has a competent operator attached to it!

 

[FullCircle]

I have an Avair AV-201 with PL259 flyleads you can borrow for the cost of postage and return. I will include a copy of the manual, which is easy-peasy.
Send me a PM if you want.

 

[Stemar]

...The trouble with SWR meters is that you can't buy one that has a competent operator attached to it!

That's always the problem with any claver bit of kit!

FullCicle, thanks for the offer. I'll try Salty's checks first, but may well be back. Unfortunately, I've a lot on my plate at the moment, meaning boat passes are few and far between, so it may be a while

 

[Alpha22]

An SWR meter will confirm the correct operation of an antenna, or not. In your situation it won't tell you a great deal, other than there is a fault. But you already know that!!
I'm with Salty John..... if all else fails, cut the new plugs off and try again.

 

[VicMallows]

For more information about measuring SWR, see the ARRL Handbook, available
at your local library.

I wish it was! Two copies in Hampshire and those are Reference Only and a long journey away.

The theory in my 1967 copy probably hasn't changed much; but the valves (sorry: Vacuum Tubes) might be difficult to get hold of I must remember to put a new copy on the Christmas list.

G3TSM

 

[William_H]

Antennae

As said DC measurements can be useful to check connectors and cables. But as said many antennae have a DC short circuit within the antenna. So measurements at DC are best done with antenna disconnected. Not easy with mast up and antenna on top. You might do well to make or buy another antenna to be mounted on pushpit as an emergency antenna until you get mast down.
One of the difficulties with VHF and VSWR measurements is that the typical thin RG 58 cable that is most often used has quite significant loss. Some cables can lose half the transmitted power in 60 ft of cable. This is not a big problem as loss of half power is hardly noticed. However when measuring VSWR if you disconnect the antenna on the end of 60 ft of cable and measure VSWR from the transmitter the power that comes back representing a bad VSWR is only 1/4 of what is sent up to the end of the cable. This looks like an antenna that is not really bad but not good. While actually it is no good at all. Indeed a long enough cable can look to a VSWR meter like a perfect antenna with nothing connected or a short circuit.
So best to check VSWR at the antenna or at least near to it rather than at the transmitter. Which of course defeats your purpose of checking from below. good luck olewill

 

[tim_ber]

My VHF worked perfectly last year. Since then I've taken the mast down and that meant cutting the antenna cable. Now I can't raise Solent Coastguard from Hardway.

I remade the cut using a soldered socket of the same type as plugs into the VHF mounted in a box on the end going to the radio and a plug on the end going to the antenna. The problem could be here or at the connection to the antenna itself. Nothing else was disturbed.

It seems that an SWR meter should allow me to troubleshoot my connections and help me decide if I need to find a volunteer to go up the mast, but I know nowt about 'em. Fleabay has no end of cheap and cheerful meters for CB systems. Would one work for a VHF antenna, or do I need a "proper", ie expensive, one?

Is there anything else I need to know about using one? (current knowledge approximately zero)

These guys sell a cross needle type. Very, very simple to use. The needles cross and you simply read off the ratio. It comes with instructions. Usually delivered in a day. I installed mine permanently in the boat. Type "swr meter" into the search box, because the link doesn't work properly. I bought the AV 40 type for VHF frequency, £40

http://www.kcb.co.uk/shop2/index1.html?lang=en-uk&target=search.html&lmd=0.17240215052341323

Salty john sells the best aerial and his advice is always spot on.