VSWR (SWR) meter for VHFs - good Ebay supplier

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I've been having terrible trouble finding a good source for an SWR meter that is rated for the marine band, patch leads, connectors and a dummy load but found a shop on Ebay who supplied me quickly and reliably.

9' GOOD QUALITY RG58 COAX WITH PL259s + SHROUDS FITTED
ZETAGI DL50 50w DUMMY LOAD - DC TO 500 MHz,
PL258 DOUBLE FEMALE COUPLER FOR CB RADIO AND HAM USE
ZETAGI COMPACT SWR METER FOR HF - CB - VHF

From truck-kingradio a UK Ebayer (commercial shop). I was very happy -- excellent communications and the best price of the lot much cheaper than Maplins and is speced for the marine band.

I have no connection with these people.
 
No, this one is for higher frequencies and does not have a power meter -- he might have something in his shop or email him. Same basic idea, tho'. With my SSB I rely on checking the power drawn from the batteries (lights dim! and volts drop) and I assume that the ATU works. I get out as far as Africa when I am in the mood.
 
What was wrong with the supplier Richard posted, I bought from them and had excellent service..

much cheaper too.
 
I tried to by from them and after four phone calls and two emails, then one week when the closed the shop "for staff training" I still did not get a price. I had to go elsewhere because Mrs L was in the UK and I needed delivery. The online price of that chap was actually a lot higher than the price from the Ebay shop. Still, I'm glad you got a good service so now folks have got a choice of two.
 
I would say it would be ok.

Only limiting factor would be the 50W rating of the dummy load. Likely to get a bit hot if you leave full power from your HF set on for any length of time.

But you can live without a dummy load.

Not sure what the double coupler is for but if its rated for "ham" use it is ok for your SSB.
 
[ QUOTE ]
I would say it would be ok.

Only limiting factor would be the 50W rating of the dummy load. Likely to get a bit hot if you leave full power from your HF set on for any length of time.

But you can live without a dummy load.

Not sure what the double coupler is for but if its rated for "ham" use it is ok for your SSB.

[/ QUOTE ]
/forums/images/graemlins/confused.gifThe frequency response of the SWR meter is 3MHz to 200MHz Not much use for a marine SSB.
 
ZETAGI DL50 50w DUMMY LOAD - DC TO 500 MHz,
PL258 DOUBLE FEMALE COUPLER FOR CB RADIO AND HAM USE
ZETAGI COMPACT SWR METER FOR HF - CB - VHF

No mention or 3-mz here

I have been using this type of thing - including ones I have built myself - for > 40 years.

Trust me

It will work perfectly well - SWR meters are not especially frequency sensitive, particularly if you have 100W or rf "up" them.
 
[ QUOTE ]
ZETAGI DL50 50w DUMMY LOAD - DC TO 500 MHz,
PL258 DOUBLE FEMALE COUPLER FOR CB RADIO AND HAM USE
ZETAGI COMPACT SWR METER FOR HF - CB - VHF

No mention or 3-mz here

I have been using this type of thing - including ones I have built myself - for > 40 years.

Trust me

It will work perfectly well - SWR meters are not especially frequency sensitive, particularly if you have 100W or rf "up" them.

[/ QUOTE ]

I gave you the shop details and the specs are there. The spec on their website says 3MHz, the instruction leaflet says 3MHz. I was asked if it was suitable for a marine SSB and I said 'no' as the frequency range is wrong -- it needs to go down to 2MHz. 2182 kHz is the distress frequency and the most 'important'.

It might work fine and it might not be very accurate. It could be misleading. This manufacturer (Zetagi) offers other products that are specd for marine SSB ranges Without taking this thing apart and/or checking the circuit or doing lab tests on it, I am certainly not in a position to say that the device with a quoted bottom frequency of 3MHz will be fine down to the lower marine SSB frequencies of around 2MHz and a little lower.

Always buy test and measurement equipment that is specified for the job. The manufacturers generally put limits on a spec as wide as they 'dare' to sell more product so never go outside that range with measurement equipment unless you have working knowledge of that specific model, or some means of calibrating it.
 
The frequency response of the SWR meter is 3MHz to 200MHz Not much use for a marine SSB.
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What frequencies do you use???????

It will be perfectly adequate for using on 2182 which is just out of range and I doubt if he is going down to 500 KCs fpr morse.

Most yachties chat on around 4 to 8 mhz or herb on 12 so exactly what is wanted though I prefer the twin needle type
 
[ QUOTE ]
It will be perfectly adequate for using on 2182

[/ QUOTE ] Please explain how you can be so certain, given that the bottom of the range is 3MHz? Something a wee bit technical would be appreciated......we've already had the 'trust me' /forums/images/graemlins/wink.gif
 
VSWR meter works by indicating the difference between power in one direction and power in the other direction.

This ratio is measured by comparing the two.

There are no units

The only calibration is to adjust sensitivity so that forward power gives FSD.

The measurement depends on a voltage being induced in a sensing wire. The voltage induced is proportional to frequency. Your post speaks of VHF so presumably around 200Mhz

The range from 3 MHz to 200Mhz is a factor of almost 70

The range from 3 Mhz to 2 Mhz is 1/3

Add to that the machine will give fsd on 25 watts from a vhf set

A typical SSB will have 200 watts

All that is necessary is that the SSB can produce fsd at 2 Mhz

If it can do that then it will work.

I cannot conceive of any such instrument that would give a meaningful reading at 3 MHz but not at 2. To do this would involve fitting a high pass filter - and no manufacturer is going to do that to no purpose.

Remember all these things do is indicate a minimum - it does not give a measurement in any units
 
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Please explain how you can be so certain, given that the bottom of the range is 3MHz? Something a wee bit technical would be appreciated......we've already had the 'trust me'
___________________________________________________


TRUST ME...... /forums/images/graemlins/laugh.gif

If you were to take the model you have.....The model 3 to 500 and the model the cbers use You would be surprised how accurate they all are for your purposes over ssb and vhf.

Yes maybe not calibrated for accuracy of power settings however there are so many other varianbles that this will be so with even the one supposedly for the range in use unless you paid big bucks. WHen it comes to measuring forward and reverse you will have no problem with any of them, especially if you have been relying on the dimming of lights and battery voltage.

These cheap meters give a fair indication however like most things you get what you pay for so don t expect the performance of meters costing 4/5 times as much.

If a meter is calibrated supposidly from 3mhz to 500 mhz then you can be certain it isnt going to be wildly inaccurate at 2mhz or 550mhz certainly not so much as would concern you.

I have used a CB vswr meter (27MHZ) for ssb and marine vhf when nothing else was available and it gave accurate enough readings to decide when the antenna was OK or not.

These meters will give a good idea of power adjustment when tuning SSB however only the more expensive ones can be expected to give reasonably accurate power readings and may even have frequency range settings to improve accuracy.

If it works for you be happy however if ever caught short in a foreign port then don't discount the grotty SWR meter offered by an itinerent CBer.
 
You seem to be assuming that the swr meter is a purely resistive device which is almost certainly won't be. The meter itself has an input impedance of Z=R+jX which will only be ZO over a limited frequency range, the cable (antenna lead) has reflections (this is what we are looking for), the connector to the antenna has reflections, and the antenna itself is highly frequency dependant and has reflections. The reflections are in antiphase with the forward voltage but due to their physical displacement are not all in phase with each other. Mismatch between the ZO of the transmitter and the Zin looking into the cable is the combination of those mismatches which, as you will know, can tend to REDUCE the perceived problem (i.e. make the SWR look better than it really is). The meter itself Z=R+jX actually alters the circuit, and this is one of the problems. Really wide bandwidth meters such as this are a wee bit suspect because they can disguise a mismatch by causing their own phase change. It isn't the insertion loss (which you are focusing on) that is the problem -- you can usually wind up the Tx power to get some kind of reading -- the problem is that the (out of phase) difference between the Tx voltage and the reflected current (which is what we are measuring when we look for SWR) is affected by the (complex) impedance of the meter. In other words, what you see with the meter in circuit is NOT what happens when the meter is removed. I'm not sure if I have explained that very clearly but it is difficult to do explain without going into the maths and in any case I haven't worked through the maths from first principles for years.

Another way of looking at it, if you like, which is perfectly valid and might be more intuitive, you have a piece of equipment specd over around 2 decades - imagine the Bode (or Nyquist if you prefer) plot - phase and amplitude. Think what is happening to the phase at either end of the passband! It is the phase that causes the problem when measuring SWR with these kinds of meter, not the amplitude.

Furthermore, there are many different designs of SWR meters and some of them are quite cunning with transformers and capacitors. Neither you nor I have any idea of what is in the Zetagi box - I have one but it is in a fairly thick steel box which has been folded over and stamped in a press so is effectively sealed, so I can't even peak inside. This model is specd over nearly 2 decades, which is huge for an SWR meter. The fact that they have put a lower limit of 3MHz suggests to me that they didn't feel comfortable about specing it down to 2MHz -- Zetagi are quite big in this field, they would probably have taken it down to the bottom of the marine SSB band if they could have done so.

As for Bilgediver's comments about my using power measurements for the SSB, marine SSBs are fitted with an ATU, unlike many ham sets. Maybe he is a radio ham with a home station, SWR and power meter. Hams often don't use an ATU and tune the antenna before transmitting, in the classic way. You just don't DO that on a marine SSB. The regs wouldn't allow it. The ATU tunes the antenna for you instantly every time you Tx. If the ATU fails then you don't launch - which you notice on a boat as the volts don't drop (believe me, you know when you are launching). You'd probably also fry the output stage unless you are very quick.

So firstly an SWR meter is generally unnecessary for a marine SSB with ATU and secondly, if you wanted to check the SWR over the band, you would be advised to buy or borrow an SWR meter that is designed to cover the whole frequency range that you want to check over. It is a technical and mathematical subject, but reading the specification is easy for everyone. If it says 3MHz to 200MHz, believe them. Don't jump to the conclusion that you can use it at 2MHz.

More importantly, don't advise other people on the forum that they can operate the equipment outside the manufacturer's specified range unless you have some specific technical knowledge about THAT specific piece of test equipment, not assumptions based on incomplete knowledge.

Now it could be that this piece if kit is FINE at 2MHz but until the manufacturer or a technically competent person who I trust has actually tested it in a lab then I certainly won't use it outside the specd range or recommend that others do.
 
You are over complicating the issue.

The principle of these things is that they do not affect in any way the transmission line between xmitter and ant.

They should - and effectively do - look like just a few inches of coax. They should not present any reactive component in either direction.

They work by elecromagnetically sensing forward and reflected power by a very short length of wire running parallel to the inner conductor of a length of coax.

Typically each wire has a diode at one end and a matching resistor at the other. the wires are orientated in opposite directions.

The reflection coefficient, Cr is then given by Vr/Vf.

This equals Root Pr/Pf

In a constant resistance P is proportioonal to V

VSWR then = (1+Cr)/(1-Cr)

The only frequency sensitive bit is in the voltage induced in the sensing wire.

At any given frequency this will be proportional to the transmit power.

Certainly there are more sophisticated machines available, but they are not cheap, and even then the principle is the same.
 
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The principle of these things is that they do not affect in any way the transmission line between xmitter and ant.

[/ QUOTE ]Yes, exactly, that is essential otherwise they can measure their own reflection (i.e. the mismatch due to the Zout of the meter being different to the Z0 of the cable) and the 'genuine' reflections are themselves reflected and attenuated by that same mismatch.

When a manufacturer quotes a frequency range for a piece of RF equipment he has tested the frequency response over a range. From that plot he decides what to claim as an operating frequency range. Anyone with no actual knowledge of the circuit, having not seen the lab test results, to state categorically that the equipment will operate outside the specified range is working from guesswork.

The problem is that it will not be obvious to the user that the measurement is incorrect and it could be very misleading. Comments in this thread have suggested that provided there is enough signal to get a reading, the reading will be meaningful. I hope that I demonstrated in my previous post that that is not correct. Once the Zout of the box is significantly different from the Z0 of the antenna system and cable you have a problem and we can reasonably expect this to happen at and beyond the frequency limits specified.

If people want to guess on their own account, that's fine. But to guess and declare in public that a piece of equipment will work outside its specified frequency range is not fine.

I really don't think I have any more to say on this matter -- we don't want a design study on SWR meters in this thread because the issue is whether a particular product - about which we have no knowledge other than the manufacturer's spec -- will work, not whether some other product will work. Will this particular Zetagi model be good at 2MHz? Don't know, but the spec gives a cutoff at 3MHz.

When using electronic test and measuring equipment stay within the manufacturers' specifications because it is likely to be unreliable or inaccurate outside the specified range.
 
Lemain - as I understand it you now have a SWR meter and a 50W dummy load.

In the interests of science why don't you hook up the dummy load to your ssb through the SWR meter and transmit a carrier into the dummy load in the 4 MHz marine band (or in the 80m amateur band if you are an amateur, it being closer to the "magical" 3MHz) and note the SWR.

Then transmit a carrier into the dummy load in the 2MHz marine band and note the SWR.

You might like to do so at higher frequencies too within the specified frequency range of the dummy load.

Then come back and post your findings.
 
I tried the kit on the VHF the day before yesterday and while I can get a VSWR of 1.25:1 on my emergency aerial when I connect the dummy load I read about 2.8:1. With the bits I have on board I cannot determine whether I have a problem with patch leads or the dummy load. Have written to the mfr who happens to be Italian, which is where I am at the moment. When I am sorted out, I will do that test.
 
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