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!

3 Responses to No VHF SWR Meter? No Problem. »»


Comments

  1. Comment by David N8SRE | 2008/11/17 at 13:41:32

    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. Comment by Dan KB6NU | 2008/11/17 at 22:44:05

    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. Comment by Jack KG4MFJ | 2008/11/18 at 18:37:29

    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


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