No VHF SWR Meter? No Problem.

On the HamRadioHelpGroup mailing list, a ham asked:

Will a CB SWR meter work on a 2 meter ham radio??

Mark, K5LXP, replied:

Yes and no.

I use a CB SWR meter to check 2M antennas all the time. There is a trick to it however.

If all you want to to is check SWR on your 2M antenna, you don’t necessarily have to buy a dedicated VHF/UHF SWR meter. What I keep in my toolbag for that is a cheapie $5 hamfest special CB SWR meter. They really don’t work well on 2M but there’s a trick you can do that will net a reasonably accurate SWR reading on 2M with one of these meters. What you do is connect up the meter as usual, key the rig with the switch in the forward power position, set the adjustment for full scale. Now, without touching anything, swap the coax connections so that the rig is connected to the “ANT” side of the meter, and the antenna is connected to the “XCVR” side. The reading you see on the meter will be very close to your real SWR. The closer to 1:1 your SWR is, the more accurate it will be. It would be more convenient to have a real SWR meter or antenna analyzer if you do a lot of testing, but for a quick antenna check after a mobile install or whatever, the $5 CB meters work OK.

When I asked his permission to use this, Mark said, “I don’t get the credit, I picked it up from some OF years ago.” (E-mail me if you don’t know what an OF is.) Well, Mark, you might not get the credit for thinking this up, but you certainly get the credit for passing it on. Thanks!

Take a look at these related posts:

Comments

  1. David N8SRE says:

    Very clever. Thanks for passing that on. I never have understood why HF SWR meters are a dime a dozen but VHF ones are so hard to track down.

  2. A further comment from K5LXP gives a little deeper technical explanation:

    The theory is relatively simple.

    A basic SWR bridge is comprised of two couplers, each of which consist of a stripline or a pickup loop, and a detector diode. One coupler is used to detect forward power, the other reflected.

    At 27 MHz, the precision of the components required isn’t too stringent. Just about any diode will work, and minor imperfections in the stripline or pickup coils won’t impact the accuracy that much.

    But at 2M suddenly minor differences between the striplines, stray capacitance, and type of diode starts to matter. By using just one of the couplers for both the forward and reverse readings any error that exists in that coupler is the same for both readings and thus cancels out. It’s unlikely it’ll work at 440 however, it’s just too much to ask to expect a true 50 ohm network and zero bias or hot carrier diode in a cheap CB meter.

    The absolute reading on 2M may still be somewhat inaccurate, especially at high SWR but odds are you don’t care about that. All you’re interested in is 1:1 or as close to it as you can get, and for that the $5 meters will be good enough.

  3. Jack KG4MFJ says:

    Dan,

    I have found that the old Micronta CB 3 range power/modulation/SWR meters work quited well on 2 meters (probably better than they did on 11 meters). These are the 3 dial meters left to right power 5/50/500 watt range, modulation and SWR. The SWR and power meters are accurate within about 3% when compared to my Bird meter on 2 meter FM. Never tried it with SSB though. I have two of these and both read the same, so it is probably not just a coincidence.

    Jack KG4MFJ

  4. thomas clay says:

    One quick question.When setting the hf swr meter up in the forward power position is the swr meter connected to the antenna of unknown swr or to a 50 ohm dummy load as usual?
    Thanks
    Thomas

  5. You connect it to the antenna. That sets your reference for the reflected reading.

  6. thomas clay says:

    Thanks for the reply.
    73′s
    KB4CJD

  7. Larry Mergen says:

    Thank you very much for this information. Money is tight and I have a little HF meter but didn’t want to buy a separate VHF meter.

  8. Mike Kuhlmann says:

    Thank you it worked for me.

  9. Daniel Sands says:

    I’m interested in the air band, 118-137MHz. Can a CB SWR meter be used as suggested on this page with any expectation of accuracy?

  10. If you use it in the manner described, yes, it will be accurate. Are you going to be transmitting on this band? If not, then you don’t really need an SWR meter.

  11. Daniel Sands says:

    In my case yes, I recently bought a plane and discovered that it has a wingtip antenna built in. Turns out to be a mod that a guy named Bob Archer sold back in the 60′s. It’s not a typical dipole design, not a dipole at all in fact. It integrates the balun into its design. According to Archer, who I was able to track down, it should have a VSWR of about 1.5. Anyway with age and such I’m not so sure about it. It seems to have decent range but the reception is not always as clear as expected and occasionally I also have to repeat.

    So short answer, yes :)

    • Sounds like some testing is in order then. :) I’m sure you know this, but I’d suspect bad connections first.

  12. Daniel Sands says:

    Update: I measured the wingtip antenna and got about 1.5:1 as advertised. That was kind of shocking so I measured the whip dipole and got 2.5:1. This is consistent with new antennas, so I guess I got an accurate measurement. If signal loss is within specs, does that disqualify the antenna, coax, and connectors as possible problems?

  13. No. Lossy coax will actually make the SWR reading look better than it should. Think about it. If the feedline is lossy, the meter will measure less reflected power.

  14. Roger Peterson says:

    I have an old Lafayette 99-26395 Power/SWR. It has a power meter, a SWR meter, and a tuning knob that goes from 0 to 10. It does not have a forward/reverse switch and the coax connectors are not labeled. I have no idea how to use it! I was unable to find any information online as to how to properly use it. Are you familliar with this meter??

    Thanks!

    • Dan KB6NU says:

      Hi, Roger–

      I have a very similar meter, the Quement SWB-3. So similar, in fact, that I wouldn’t be surprised if the same Japanese company made both meters. Basically, to measure the SWR, you adjust the pot so that the power meter reads full scale. Then, you can read the SWR directly from the SWR meter.

      To measure the power, you have to set the pot to a pre-set position. This is different for each band. Needless to say, this measurement is not very accurate.

      To get the manual for the SWB-3, go to http://bama.edebris.com/manuals/quement/swb3.

      73, Dan KB6NU

  15. I will guess “OF” Was short for Old Fart.. Meaning some old guy back when..
    LOL. Best guess I could come up with.

  16. Dave N6DLH says:

    Great little tip! Been testing out all the VHF antenna’s I built. Do not want to invest in a dedicated VHF SWR meter since I am going to get an antenna analyzer. But I have this old hand me down HF meter here. Thanks
    73, Dave N6DLH

  17. So will this method work for dialing in an FM antenna for a Low Power FM transmitter?

    Thanks,
    Mike

  18. Paul KC9UZV says:

    Just got my first 2m rig. Got the antenna up a day before the snow. With no way to test it, I have just been listening. Used your method with an old HF meter, and see that it is OK to transmit. Thanks for this tip.

  19. I dont understand, we are susposed to swap the connections so they’re backwards while still transmitting? Isnt that really bad?

  20. Dan KB6NU says:

    You would think so, but not really. Take a look at the schematic at http://freecircuitdiagram.com/2010/12/29/simple-swr-bridge/. This is similar to the circuit you’ll find in most of these cheapie SWR bridges. It’s pretty symmetrical no matter which way you hook it up.

  21. Ok, I get the theory now, just need some help on the procedure… So the correct step by step procedure would be to
    -attach transmitter to transmitter, and antenna to antenna.
    -Make sure switch in on forward, transmit and adjust to read full.
    -stop transmitting and swap antenna and transmitter connectors
    -switch to ref and transmit to see swr

    Is this right?
    Thanks for the help!

  22. Dan KB6NU says:

    No! You always leave the FWD/REV switch in the FWD (forward) position. When you connect the antenna to what is normally the transmitter connector and the transmitter to what is normally the antenna connector, you’ll be reading the reflected power only if the switch is in the FWD position.

  23. I received my tech. license about a month ago. I put up a discone antenna with the stinger and 4 ground plane radials removed for use on 2 meters. I used the good 50 ohm coax. I have a cheap Radio Shack SWR meter and cannot get the meter to calibrate. The needle will go about 3/4 of the way to the calibration mark but no further with knob turned completely clockwise. Help. What am I missing?

  24. John KD6VKW says:

    This trick can be extended to 440 but I got fooled badly while tuning a 440 J-pole by moving the connection points up and down. I couldn’t get a low VSWR no matter what I did.

    After hours of frustration, I disconnected the j-pole and substituted a dummy load on the ANT connector of the meter. It read about 3:1!

    Obviously, that particular meter, though fine for HF and okay on 2m, could not read 440 reliably. As soon as I got a different meter (and tested it into open, short and dummy load) I had my 440 j-pole ready to go very soon after.

    Most of my meters are now clearly labeled with the bands on which they will provide useful indications.

Speak Your Mind

*