Using a CB SWR for 2 meters?

[beamin]

I read that can do this by plugging the meter in as usual
press TX
set the needle
Reverse your radio plug and antenna plug
Read the SWR and it will be very close.


Would you also change the FWD switch to REV? Like when you would read it normally?
Why does this work?

 

[paulears]

CB VSWR meters probably won't work at UHF. The technical reason is to do with the diodes they use inside that have a frequency limit. VHF and UHF VSWR meters have much more precise internal construction as frequencies are much higher.

The point of a VSWR meter is to find out how much of your transmitter power gets out into the ether. If the antenna is not resonant, then if you squirt 10W out of your radio, then not all goes out - some is reflected back where it's wasted, and could of course damage the radio, which doesn't appreciate power coming back up the cable. The meter uses a diode which allows current flow only in one direction. So if you stick the meter on the output from the diode it reads forward going power. If you disconnect the meter and reverse the connections, then the diode is now facing the other direction and it reads the power going back towards the transmitter. Ideally it should read ZERO. Swapping these connections around every time you want to take a reading is a pain. So most meters have a switch which reverses the diode. So you set it to forward and take a reading, then switch it to reverse and take another. VSWR is the ratio of going out, vs coming back. Some meters have a single meter, and on forward you adjust a knob to read 'max', then flip the switch and read off the reverse reading showing the VSWR (V for voltage) as a ratio - 1:1 (perfect), 2:1 (just about liveable with) and 3:1 or worse (indicating lots of wasted power which means something seriously adrift with your system. Realistically - a 1:1 perfect reading is a kind of gold standard, and it's normal to have less than this. You often see it change when you change frequencies, as the perfect area can be quite narrow. People get really obsessed with getting a 1:1 reading but 1.2:1 is as good as my system gets, and that's a tiny amount of reflected power, and it doesn't bother me. My vehicle for work has a marine band antenna on it, and if I use this antenna on 2m, the VSWR is about 1.9:1. Fine with me. As far as I'm concerned I want it as low as I can get, but once it's in the 1's, I stop worrying - it's such a small difference I cannot hear it on receive, and the transmitter doesn't care!

Does this make sense?

 

[jaspence]

2 meters is not UHF, it is VHF (https://en.wikipedia.org/wiki/Ultra_high_frequency). No VHF SWR Meter? No Problem. - KB6NU's Ham Radio Blog provides simple instructions for what you want. If you are using 2 meters, chances are you also have 440, and the investment in a good meter for ham frequencies is worth considering.

 

[jonwienke]

Don't use a CB meter for VHF or UHF. Your readings will not be accurate.

 

[paulears]

Excuse the typo - I know 2m is not UHF. My apologies for confusing.

 

[k6cpo]

I have a Heathkit HM-2102 2 meter SWR and power meter. I had it hooked up to my 2m/440 radio and though it was working just fine until I stuck an antenna analyzer on the circuit. It turned out my SWR was within acceptable limits on 2 meters (1:1.2) but way off on 440 (1:3.5) I pulled the meter out of the circuit and it's now just sitting.

Using an in-line SWR meter not specifically designed for the band you're working can introduce high SWR into the line because of the mismatch. You're better off using one designed for the particular band, or, better yet, getting a dedicated antenna analyzer.

 

[KC4RAF]

Amateur radio is all about experimenting. Try a 11 meter swr meter and compare to a dedicated 2 meter swr meter and see how it operates.
Sticking a cb swr meter in circuit with a 2 meter system will not damage your radio if you keep the transmits short.
I experiment with the cb meter several times and it came very close to my dedicated 2 meter swr. Just follow the instructions in the second link that jaspence posted and you'll be fine.

 

[SCPD]

Yes, Beamin, like RAF says- by all means experiment with a CB SWR meter in a VHF circuit-- but ulimately get a real meter for those bands... a CB meter is made for lower wavelengths- its construction has portions that mirror falsely portions at the VHF wavelengths -- giving all sorts of incorrect readings. I will say, however, that a CB SWR meter can give pretty good results- ----maybe-- at 150Mhz-- but do you want to rely on it?... at 200Mhz plus,-- "Fa'get it !." Spin your wheels in more productive sands.....
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Amazon has (had?) several inexpensive V/UHF meters- Nissei is a brand I bought several of, to use in field kits (work related)... compared to our Bird's, they were remarkably close-- and if someone loses or destroys one, no paper work is involved.....
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.............................CF

 

[paulears]

We need to inject a little common sense here. All this doom and gloom about VSWR, damaging radios and warnings.

VSWR is about efficiency in the main - reflected power is wasted power, and it doesn't mean that the first time something comes back as reflected power things are terminal. We get people panicking that they accidentally left off the antenna from their portable and press the PTT, we get people worried they accidentally connected the VHF antenna to their UHF radio and stuff like that. The fact is that portable radios NEVER have anything like a 1:1 VSWR, because the antennas are always rubbish matches, and mobiles frequently get their antennas banged flat by trees or car park barriers and often you see taxis drive past with their vertical antenna virtually flat. They don't then spend ages getting a radio fixed, they just have terrible performance. Squirting 100 Watts down the cable and getting 10W back is quite a bit for the output stage to live with - and most high powered devices monitor VSWR and automatically throttle back to prevent damage. 5W portables can cope with the same ratio amounts coming back as normal everyday operation. In all the years I've had transmitters I have never blown anything up - no Vesuvius moments. The mismatches just mean nobody answers and eventually truth dawns. In fact, if a transmitter had objected, I'd take this as bad design - and no manufacturer wants piles of stock arriving back under guarantee for repair because their products couldn't manage high VSWRs. Shorting out a feeder running full legal limit on a peaked and tuned high performance home brew design is going to kill it, because when you build stuff yourself, you don't build in idiot features to these designs, but it is VERY rare for commercial products to suffer bad effects.

Next thing

It turned out my SWR was within acceptable limits on 2 meters (1:1.2) but way off on 440 (1:3.5)

1:3.5 is a BAD VSWR? Ratio wise it's small in terms of actual power being reflected. I'm quite comfy with any VSWR that's below 3. So 2.5:1 on the meter is fine with me. Looking at many of my various antenna systems on an analyser shows how unperfected they all are. It's chasing your tail stuff.

Of course each meter introduces losses and often changes in the very VSWR you are measuring - even Birds do it - But - this is all relative. If the best you can get on loads of commercial antennas is 1.5:1, then it's probably meter inaccuracy. Does it matter if the reading should be 1.24:1 or 1.9:1? No it doesn't because you are just looking for indications not specifics. If your good and decent performing antenna is 1.5:1 on your meter, and connecting a poorer antenna, or maybe one for a different band reads 2.5:1, then you can interpret what the meter tells you. With a meter out of it's band, the readings will be strange. They make no sense in many cases. My cheap old CB meter on 2 mtrs can hardly generate the full scale on the first meter, and nothing appears on the other one reading reflected at all. Trying it on UHF gives a half scale reading on both meters - it just doesn't work.

Experiment and test and then learn to interpret what makes sense and what doesn't!

Stop the scaremongering and pointless accuracy measurements. I don't have a Bird, so have no idea if my meter tells the truth. It does, however, give me a decent idea of what is happening.

 

[DaveNF2G]

The obsession with decimal point SWR is a CB thing. SWR is merely a ratio that allows one to calculate how much power is being dissipated by the finals in the transmitter, and the efficiency of the antenna system (two sides of the same coin).

 

[jonwienke]

The obsession with decimal point SWR is a CB thing.

Absolutely not. Minimizing SWR is one way te check antenna performance, and whether the antenna or feedline has a problem that could damage the radio or negatively affect range. It's not the only thing to look at, but it's a useful measurement.

1:3.5 is a BAD VSWR? Ratio wise it's small in terms of actual power being reflected.

Not true. It indicates more than 70% of your RF power is reflecting back from the antenna, AFTER the losses incurred by a round trip through the feedline. That's pretty bad by any rational definition.

 

[N8IAA]

I read that can do this by plugging the meter in as usual
press TX
set the needle
Reverse your radio plug and antenna plug
Read the SWR and it will be very close.

Would you also change the FWD switch to REV? Like when you would read it normally?
Why does this work?

First, as mentioned, a CB SWR meter will not work well on 2m frequencies.
You need to get one that has cross needles so you can see the Forward and Reverse at the same time. You would need a short jumper from the radio to the meter (with appropriate connectors on each end) then attach the coax from the antenna to the appropriate side of the meter. Then, when you TX, where the needles cross, is your SWR.
beamin, you seem to over think what your needs are.
Larry

 

[K4JGF]

Agree with what CF said. Is a CB SWR meter a good choice for 2 meters? No, but if that's what you have then use it. It's close enough. The coupling lines are too long at VHF so you have too much signal coupled to the detector diodes, but you should have enough range in the pot to set full scale. When I got into satellites (OSCAR 6 & 7 back in the day) one of the modes was 2 meters up/10 meters down. Being a poor college student at the time all I had to adjust the matching on my 2 meter homebrew 3 element Yagi was a CB SWR meter. It matched fine and I was able to make contacts through the satellites. I was able to later borrow a Bird wattmeter with the appropriate slug and check the antenna match. It was OK, less than 2:1.

 

[SCPD]

Its not to say that SWR is not important (too many double negatives?...sorry)- let me state that again...
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SWR is important- but often too over rated. Almost anyone with RF experience in the HF's and V/UHF's knows that with modern commercial equipment, readings of <2:1 are fine. Looking at the tables shows just how small a percentage of reflected power that represents. Also remember, if you have high enuff losses in your feed line, your reflect'd power will all be burned up well before it ever gets reflected back to register at the meter. Want an example?-- feed a 2GHz signal into a open end'd 1000 foot length of RG/58. No load, no termination- and a perfect 1:1 SWR-- or so it would appear !
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Modern commercial 2-way equipment have a number of fail safe ('idiot proofing') circuits to protect their PA's against thermal runaway and the likes.
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Usually, that is.
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Try placing an aluminum bowl in a microwave oven and see how long- if its protective circuits don't kicks in- the magnetron lives. I work with transmitters whose output far exceed the plate dissipation capabilities. A mismatch can reflects back enuff energy in such brief moments that the results are .... spectacular!...... We want to know accurately the SWR before going QRO.
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Back in the days of tube PA's- and the technology is far from dead- how many have tuned their equipment by plate colour?..that is, tuning so the PA plates went from a firery orange to just a dull red? And how often was there an adjustment necessary to effect that by lowering the SWR?... that reflected power is showing up directly in that glowing tube element.... ignore the glowing beast at your peril ....
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Personally, if its not microwave or high power HF- if the SWR is 2:1 or less, just "stick a fork in it and call it done." But use a good SWR meter.
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..................................CF
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[DaveNF2G]

Not true. It indicates more than 70% of your RF power is reflecting back from the antenna, AFTER the losses incurred by a round trip through the feedline. That's pretty bad by any rational definition.

Expressing the ratio in the normal sense (forward:reflected), 3.5:1 does not indicate a 70% power return. It indicates somewhere below 40%.

 

[N8IAA]

Just thought you all would like to know that the OP hasn't been on since posting his question. That's six days. Maybe wait to see when he gets back on to answer what we've all posted. Just saying.....instead of posting all kinds of stuff that has nothing to do with his original question.
Larry

 

[SCPD]

Smiles N8IAA..........
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I agree, the chances are he's moved off to other things- quite a common phenomenon on these forums. The question'er often only half heart'dly ask something in a single sentence, full of grammaticas, typo's etc.. in return he gets back a phethora of thoughtful, concise-well thought out responses. The original questioner is now gone- no thank you's, no howdy-do's- nothing.
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Ever had some Dumas ask you a question; you proceed to give them your full attention and think you've engage them with an answer--- only to look over and see they haven't heard a word you've said- paying no attention, having now drifted off into space, maybe starring at their iPhone?...
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I think any number of questions posed in these forums are like that- the people that do answer place infinitely more effort into what they write than the questioner ever merits- that questioner, by now who is absorbed in something on FaceBook --and could care less.
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But these questions open up many a colourful dialog- and I think that is what attracts people to these forums. Far from me to disparage it- I think it can be wonderful sport..... throw out a question, like a baited line in the water and see who will bite. Long after the orgininal poster has departed, the subject boils merrily along.
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Which evokes a story-.
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Last year we had a project in Nevada. It was at a government test site far out in the western deserts of that state, and required our own agency vehicles for logistics etc. A great chance for a road trip; the kind'a stuff that they say builds inter-work place camaraderie, etc... .... big lol
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We decided the drive out from New Mexico could be done in three days (slo drivers), and maybe make part of it a long drive the first overnight... that's the dullest portions -- drive it in the dark. We'd be switching off drivers while the others slept in the back of the Utes.
That was the plan, and it worked well. After stopping for dinner, we settled in for a long nights drive- the Utes strung out over the miles- with most of their passengers now fallen fast asleep in the backs of their respective vehicles.
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We have 2-way low band radios in those vehicles- wonderful for keeping everyone together- but best when not filled with chatter as people try to sleep (detect a devilish note creeping in here?... )
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At some point the squelch broke and it was a legitimate question about the road or something. This started an animated over-the-air conversation, but also set off a lot of grumbling from the sleeping passenger.
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Hmmmmmmmmmm-- if they were grousing like that in our vehicle, they must be pretty lively in the others- having to listen to all that chatter.
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One the engineers in one of the Utes was particularly talkative. He would go on for long periods of time talking about any and everything (he wasn't in our vehicle.) When he started talking, our passengers started grumbling (I was driving) so my 'co-pilot' and I would turn down the volume and half listen to him talk.
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Then the devil took over.
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"Ask Dave (not his name) a question" I said, "and let's see how long he talks"
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My co-driver did, and Dave went on for minutes- but we'd turn'd down the volume. Only the squelch light glowing told us he was transmitting.
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"Ask him something else" I said.
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The squelch light came back on, and continued for minutes- Dave meanwhile talking away in ernest..... we not hearing a thing.
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This went on for quite some time, until near dawn we turned up the volume and announced we were all stopping at a cafe for breakfast.
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I pulled in first, got out and was stretching when my friend Barbi, in her Ute pull up beside me in the parking lot-- jump out and began yelling at me---
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"What the Bloody Hell have you two been up to??...." indicating me and my 'co-pilot.'
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Laughing-- "whatever do you mean?" ....was our innocent reply.
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"You had Dave running his mouth all nite long on that radio !!- and none of us could sleep !"
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Does this sort of fit some of these Question'ers on the forum?...( smiles-)
Ohhh !.....But it can be so entertaining!
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............................................................CF

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[cmdrwill]

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"You had Dave running his mouth all nite long on that radio !!- and none of us could sleep !"

Ohhh !.....But it can be so entertaining....CF.

So, that turned out to be a good thing, no one fell asleep while driving. I would have liked to be there.

 

[SCPD]

Smiles Cmdr....
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We have an upcoming project out there this winter, called Tinkerbell. I can't promise any stories as colourful as the one I just post'd-- but it won't be for lack of trying..... stay tuned !.... lol....
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..........................CF