daveharis
Member
Or are they frequency specific?
E.g. one designed for a cb radio will be no good for VHF?
E.g. one designed for a cb radio will be no good for VHF?
swr meter - are they all the same?
- Thread starter daveharis
- Start date
mikejames
Well-Known Member
I have one with two meters and a knob in the middle that is basically the same as a CB SWR meter, and it says something like up to 150MHz on it. I used it with 144 MHz amateur radio and it still works at 156MHz. But it is extremely sensitive, one simply moves the adjustment a touch off zero and the needles fly across the meters. Quite fiddly to use as a result.
A
Anonymous
Guest
CB is 27MHz Marine VHF is 150MHz Amateur VHF is a little bit lower. One of the amateur ones sold by Maplins is (just) rated into the marine band and should be fine. One of the others that Maplins used to stock was a bit too low. There is no point in using something outside its design range unless you have some means of checking the validity of the results. Using an SWR meter requires some knowledge and it is a bit of an art especially when you are on top of the mast and the Mrs is down below with the meter, using a PMR radio to talk to each other /forums/images/graemlins/wink.gif
G
Guest
Guest
[ QUOTE ]
Or are they frequency specific?
E.g. one designed for a cb radio will be no good for VHF?
[/ QUOTE ]
You are correct - they are designed to cover specific frequency ranges. If you are (or know someone who is) handy with a soldering iron - you can modify a CB SWR meter to cover the range 1Mhz to well above 1Ghz at low power.
See: http://ludens.cl/Electron/swr/swr.html for details.
Or are they frequency specific?
E.g. one designed for a cb radio will be no good for VHF?
[/ QUOTE ]
You are correct - they are designed to cover specific frequency ranges. If you are (or know someone who is) handy with a soldering iron - you can modify a CB SWR meter to cover the range 1Mhz to well above 1Ghz at low power.
See: http://ludens.cl/Electron/swr/swr.html for details.
earlybird
Well-Known Member
The design varies with frequency, but, and I'm no expert, I wonder if it makes much difference in practice for use over a brief period. I have both a cheap commercial CB meter, 27mHz, and a home made one for 150mHz. A swr meter is constructionally very simple. I use either as a "go / no go" test on my aerial from time to time, more looking for change rather than a precise SWR value and I can't see much difference in the result from either meter. I only connect them up temporarily and transmit a short burst. Since I don't want to tempt fate, I don't connect either in circuit permanently.
starfire
Well-Known Member
Check any Amateur Radio suppliers web sites or ebay for a VHF swr meter, well within limits for marine VHF.
Most useful would be a Daiwa crossed-needle type, nothing to adjust, just plug in & read.
No reason not to leave it connected all the time, at least you know
1. the set transmits
2. there is something like a reasonable antenna on the the end of the coax.
3. the coax hasn't corroded into green or black mush.
Most useful would be a Daiwa crossed-needle type, nothing to adjust, just plug in & read.
No reason not to leave it connected all the time, at least you know
1. the set transmits
2. there is something like a reasonable antenna on the the end of the coax.
3. the coax hasn't corroded into green or black mush.
A
Anonymous
Guest
I'm surprised that you say that ham VHF SWRs are "well within limits" for marine VHF. The ones I checked out were not (not far out, actually but still out of spec). If you have just checked, could you share the model number and supplier with us as I am about to order one from Maplins but it is big, expensive and heavy so would like an alternative.
As for leaving the SWR meter attached, I'm not sure that's a good idea. The environment is not necessarily benign on a yacht and if the SWR meter fails in rough seas when maybe you need the VHF the most, the last thing you need to be doing is to fiddle around with patch leads and connectors. Actually, some VHFs do blow up if Tx into an open circuit (which is a disgrace, totally avoidable) so you could end up blowing your set. I would suggest that it is done periodically, maybe before serious passages, at the beginning of the season and once or twice during the season?
As for leaving the SWR meter attached, I'm not sure that's a good idea. The environment is not necessarily benign on a yacht and if the SWR meter fails in rough seas when maybe you need the VHF the most, the last thing you need to be doing is to fiddle around with patch leads and connectors. Actually, some VHFs do blow up if Tx into an open circuit (which is a disgrace, totally avoidable) so you could end up blowing your set. I would suggest that it is done periodically, maybe before serious passages, at the beginning of the season and once or twice during the season?
daveharis
Member
Thanks for all that eveyone.
Maplin sell 2 vhf meters.
140-525MHz called "VHF-UHF"
1.8-150mHz called "HF-VHF"
Marine band is 156 - 162 MHz
So the vhf - uhf is the correct one isn't it? Other one just misses doesn't it?
Maplin sell 2 vhf meters.
140-525MHz called "VHF-UHF"
1.8-150mHz called "HF-VHF"
Marine band is 156 - 162 MHz
So the vhf - uhf is the correct one isn't it? Other one just misses doesn't it?
A
Anonymous
Guest
Yup, that's where I am at. But the VHF/UHF one is quite a big thing, I seem to remember and is VERY expensive. I was hoping that someone might know of a lower cost ham one that is spec'd for the range. In all possibility, the 150MHz one would be OK but one would need to see the frequency response before being able to comment. Anyway, yes, the VHF/UHF one is fine for marine. Don't forget to order the patch leads and connectors and a dummy load.
daveharis
Member
Cheers
PilotWolf
Well-Known Member
[ QUOTE ]
140-525MHz called "VHF-UHF"
[/ QUOTE ]
Don't know which model this is but my vhf/uhf meter is actually switched between the vhf/uhf bands - so you maybe find it doesn't cover the whole range.
W.
140-525MHz called "VHF-UHF"
[/ QUOTE ]
Don't know which model this is but my vhf/uhf meter is actually switched between the vhf/uhf bands - so you maybe find it doesn't cover the whole range.
W.
john_morris_uk
Well-Known Member
There are several different answers to this question - depending on what you are really asking!
1. The most common design of cheap VSWR meters is frequency dependent. In other words and with all other things being equal, it gets more sensitive as the frequency increases. The designer will have made the length of the lines inside appropriate to the design frequency, power being used to test the VSWR and the sensitivity of the meters being used. (Off subject slightly, this means that it makes it very difficult to use such a meter as an accurate measure of power over a wide frequency range. Furthermore any such power measurement would have to be done into an accurate 50 ohms impedance load. Also note that because of the non-linear characteristics of the diodes used in these sorts of meters they are not very accurate at very low power levels.) However, you weren't probably asking this question so...
2. Assuming that the dimensions are reasonably for the frequency in use, the diodes inside must be suitable as well! Its no use trying to use a VSWR meter at frequencies that the diodes are not capable of rectifying. In some of the examples offered (Amateur VSWR Meters rated up to 150 Mhz) I don't believe for one moment that the diodes inside aren't capable of use at the 156 Mhz of the Marine VHF Band. (You can always check that a simple design of VSWR meter is working more or less correctly by connecting in the antenna cable backwards. On the two meter designs, the meters just reverse rolls. On the one meter designs with a switch that says Forward/Reverse then the switch will be 'backwards'. Check that the readings look the same with the meter in either way round. On the two meter models, you will have to guess the scale as the meters will doing the opposite jobs...)
If you want to know what I really think - try an Amateur Radio VSWR bridge rated up to 150 Mhz or so. It will almost certainly work at 156 Mhz, but normal caveats apply to the advice!
Remember that VSWR is not the whole story and that a good VSWR doesn't guarantee that you have a good antenna. Conversely a bad VSWR (Above 2:1 or so) DOES tell you that something is wrong. If the VSWR is 1.2 or 1.5:1 then leave well alone as any fiddling will not improve matters to ANY noticeable degree.
1. The most common design of cheap VSWR meters is frequency dependent. In other words and with all other things being equal, it gets more sensitive as the frequency increases. The designer will have made the length of the lines inside appropriate to the design frequency, power being used to test the VSWR and the sensitivity of the meters being used. (Off subject slightly, this means that it makes it very difficult to use such a meter as an accurate measure of power over a wide frequency range. Furthermore any such power measurement would have to be done into an accurate 50 ohms impedance load. Also note that because of the non-linear characteristics of the diodes used in these sorts of meters they are not very accurate at very low power levels.) However, you weren't probably asking this question so...
2. Assuming that the dimensions are reasonably for the frequency in use, the diodes inside must be suitable as well! Its no use trying to use a VSWR meter at frequencies that the diodes are not capable of rectifying. In some of the examples offered (Amateur VSWR Meters rated up to 150 Mhz) I don't believe for one moment that the diodes inside aren't capable of use at the 156 Mhz of the Marine VHF Band. (You can always check that a simple design of VSWR meter is working more or less correctly by connecting in the antenna cable backwards. On the two meter designs, the meters just reverse rolls. On the one meter designs with a switch that says Forward/Reverse then the switch will be 'backwards'. Check that the readings look the same with the meter in either way round. On the two meter models, you will have to guess the scale as the meters will doing the opposite jobs...)
If you want to know what I really think - try an Amateur Radio VSWR bridge rated up to 150 Mhz or so. It will almost certainly work at 156 Mhz, but normal caveats apply to the advice!
Remember that VSWR is not the whole story and that a good VSWR doesn't guarantee that you have a good antenna. Conversely a bad VSWR (Above 2:1 or so) DOES tell you that something is wrong. If the VSWR is 1.2 or 1.5:1 then leave well alone as any fiddling will not improve matters to ANY noticeable degree.
William_H
Well-Known Member
Just to add to John's excellent story. A long cable at VHF especially the thinner coax RG58U can dissipate (waste 50% of the transmitter power.
This means that while a perfect cable (short) with an open circuit or short circuit on the end (bad antenna) will reflect back all of the transmitted power
giving a very high VSWR reading. However with the losses of a typical mast height cable of the cheaper type only 25% of the reflected power will arrive back at the meter (transmitter) so indicating an antenna that while not good is perhaps acceptable. (even with no antenna connected)
In other words cable loss gives an error indicating better results than what it really is. So put the VSWR meter near the antenna to test antenna.
Testing from the set will however give some indication of integrity of cable.
The best bet then is to test the system when it is new / working well so you have an idea of what to expect if you have doubts.
http://ludens.cl/Electron/swr/swr.html might help Note this design is not suitable for more than 1watt of power and not suitable to be left in circuit when the radio is in use. olewill
This means that while a perfect cable (short) with an open circuit or short circuit on the end (bad antenna) will reflect back all of the transmitted power
giving a very high VSWR reading. However with the losses of a typical mast height cable of the cheaper type only 25% of the reflected power will arrive back at the meter (transmitter) so indicating an antenna that while not good is perhaps acceptable. (even with no antenna connected)
In other words cable loss gives an error indicating better results than what it really is. So put the VSWR meter near the antenna to test antenna.
Testing from the set will however give some indication of integrity of cable.
The best bet then is to test the system when it is new / working well so you have an idea of what to expect if you have doubts.
http://ludens.cl/Electron/swr/swr.html might help Note this design is not suitable for more than 1watt of power and not suitable to be left in circuit when the radio is in use. olewill
rogerdog
Well-Known Member
another alternative is contact local Ham radio club and see if they have an antenna analyser like an MFJ-249B or simerlar. Usually some knowledgeable people who will talk /sort out / analyse radio antennas (results do for max information need a bit of explaining) all day long -than system can be set up optimally and coax antenna checked thoroughly - it gives everything that an SWR (a VSWR in reality) meter gives and more.
Posts above just occurred the analyser is basically an automated bridge - and as to coax 100% in agreement most RG 58 is utter rubbish the RG 58U is a bit better (but variable in quality) but another 100% vote for RG 213U
Posts above just occurred the analyser is basically an automated bridge - and as to coax 100% in agreement most RG 58 is utter rubbish the RG 58U is a bit better (but variable in quality) but another 100% vote for RG 213U
Strathglass
Well-Known Member
I have installed a VSWR meter at the chart table as a part of tne instrumentation. It has a stated frequency range of from 1 to 150 Mhz.
It's primary purpose is for the 1.8 - 30 Mhz amateur radio (TS50S) as there is a remote ATU which I won't be able to see when when using the rig.
But It is normally left connected in the VHF path but can be easiy be transfered to the HF rig.
I have no doubt that the power reading accuracy may not be very high over and beyond the stated frequency range but the repeatability will be more than adequate.
It is only a hobby type instrument which is far removed from the professional Bird VSWR meters I used to use when in defence electronics. It has three power ranges of 2, 20, and 200 watts and works well on on all the frequencies I use it.
The internal construction is to quite a high standard and the RF bit's are even silver plated.
I can easily and quickly bypass it if I wish but it is quite encouraging to see the power meter rise to the same points each time when transmitting on the VHF.
Perhaps it is a bit overkill but I like to know what is going on and it was relatively innexpensive.
I will post a photo of the chart table when I get back from the laptop to the main PC.
[image]
[/image]
Added the picture the VSWR meter is second from the right. Just a very basic unit with no meter illumination. The panel folds down and gives assess without the use of toolsl
I think the post by John Morris answers your question fairly precisely.
Cheers
Iain
It's primary purpose is for the 1.8 - 30 Mhz amateur radio (TS50S) as there is a remote ATU which I won't be able to see when when using the rig.
But It is normally left connected in the VHF path but can be easiy be transfered to the HF rig.
I have no doubt that the power reading accuracy may not be very high over and beyond the stated frequency range but the repeatability will be more than adequate.
It is only a hobby type instrument which is far removed from the professional Bird VSWR meters I used to use when in defence electronics. It has three power ranges of 2, 20, and 200 watts and works well on on all the frequencies I use it.
The internal construction is to quite a high standard and the RF bit's are even silver plated.
I can easily and quickly bypass it if I wish but it is quite encouraging to see the power meter rise to the same points each time when transmitting on the VHF.
Perhaps it is a bit overkill but I like to know what is going on and it was relatively innexpensive.
I will post a photo of the chart table when I get back from the laptop to the main PC.
[image]
Added the picture the VSWR meter is second from the right. Just a very basic unit with no meter illumination. The panel folds down and gives assess without the use of toolsl
I think the post by John Morris answers your question fairly precisely.
Cheers
Iain
vyv_cox
Well-Known Member
Thanks John, that seems to cover it!
My SWR meter was given to me by an ex-colleague who was an amateur radio enthusiast. It's quite cheap and cheerful so I doubt very much that it is anything other than a low-cost device intended for the Ham Radio frequencies. I have used it with great success for many years. Maybe it doesn't give me the precise values of reflection but it's plenty good enough to ensure that all my connections are good.
My SWR meter was given to me by an ex-colleague who was an amateur radio enthusiast. It's quite cheap and cheerful so I doubt very much that it is anything other than a low-cost device intended for the Ham Radio frequencies. I have used it with great success for many years. Maybe it doesn't give me the precise values of reflection but it's plenty good enough to ensure that all my connections are good.
daveharis
Member
Cheers chaps.