70cm SWR

[thedeerhunter270]

I need to check the SWR on my dual band mobile (2m 70cm) antenna, but I only have an SWR for 11m. I have been tempted to try it and see if works, not really knowing much about the electronics of SWR meters.

[Spike]

Yes you can (sort of) for 2m anyway (this wont work on 70cm's)

(From K5LXP)

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.

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.

[thedeerhunter270]

Yes you can (sort of) for 2m anyway (this wont work on 70cm's)

Very interesting read, thank you.

[M3HIM]

I like it!

I suppose using a better diode (Silicon Pin or Schottky perhaps) would help but at 440MHz the pick-up is likely to affect the feeder characteristics and throw the reading out considerably.

[M3HIM]

Or you could build one: Homebrew SWR meter for VHF, UHF and SHF (including Wifi ISM Band)

[G8YMW]

Dont forget that 70 cms is the 3rd harmonic of 2m so if 2 is OK then no reason to think there will be a problem with 70 cms.

[M3HIM]

I take it you mean that, with a dual-band antenna, if it is resonant on 2m then it should be okay on 70cm too.

I bought a used glass-mount 2m/70cm mobile antenna a few years ago from a guy who said that the VSWR on 70cm was always high (>2.0:1) but it didn't seem to cause a problem. It has a quirky counterpoise arrangement that might not be making good contact with the co-ax outer shield and I wondered if that had something to do with it. Also, the capacitive coupling for a glass-mount would not necessarily follow the 3rd harmonic rule without careful adjustment. However, I eventually came to the conclusion that he had probably not been using an VHF/UHF VSWR meter, hence the high reading on 70cm. In the end I opted for a new much larger tri-band antenna for 6m/2m/70cm but I will fit the glass-mount at some point and see if it can be adjusted to be as good on 70cm as it is on 2m.

I have just bought a basic meter for 2m/70cm from Moonraker and, once I get the ladder out to do come antenna maintenance, I'll let you know how I get on with it. I'll check out my 'big' mobile and my mini mag-mount too!

[Dagnabbit]

I experimented with a 70cm dipole with a meter with claimed coverage up to only 150MHz.
Even if the readings couldn't be relied upon to be absolutely accurate, it was still good enough to tell where, as I snipped bits off, the best tuning was. The dipole turned out exactly the length I was expecting, and then the readings got worse again as I continued chopping to make it even shorter. And the reading at the best point? Just under 1.5 - as expected too.

There's still going to be some response to a mismatch, so it's still handy for relative measurements. I wonder what it would be like on 23cm, but there's still a lack of cheap-ish equipment for that band, and I can't face the effort to build anything

[G8YMW]

M3HIM wrote

I take it you mean that, with a dual-band antenna, if it is resonant on 2m then it should be okay on 70cm too.

Yes. Odd harmonics will come back down to be a reasonable match with coax. Even harmonics will have a high impedance so bad match to coax (Can be done but a story in itself)
What antenna manufacturers do is fiddle about to get the radiation pattern right.
Eg 1/4 wave on 2 is 3/4 wave on 70. 3/4 wave has a high angle of radiation.
Turn the 2 metre part into a base loaded 1/4 wave (If done right) will give a 5/8 on 70, low angle of radiation

[M3HIM]

I experimented with a 70cm dipole with a meter with claimed coverage up to only 150MHz ... And the reading at the best point? Just under 1.5 - as expected too.

Bear in mind that the nominal impedance of a half-wave dipole is 73 Ohms and so won't match a 50 Ohm feed at it's correct resonant length without a matching stub.

I just noticed an advertisement in PW Magazine this morning for the Midland Alan KW520: Up to 400w 1.8-200 + 140-525Mhz Power SWR Meter £79.95.

[G8YMW]

M3HIM wrote

Bear in mind that the nominal impedance of a half-wave dipole is 73 Ohms and so won't match a 50 Ohm feed at it's correct resonant length without a matching stub.

Why bother, that is 1.5:1 SWR The radio will be happy enough with that (Assuming you get the 73 Ohm side rather than 32 ish Ohms) .

[RD250]

There might be a few models of CB SWR meter that would give some kind of reasonable reading but realistically you need to get a meter designed for 70cms.

From my experience of looking inside cheap CB SWR meters the RF grounding is poor and the quality of the main 50 ohm transmission line isn't good. So by the time you get to 70cms the meter could be presenting quite a poor match to the radio even with a dummy load on the other side of the meter.

Eg I'd expect the meter itself to present a return loss of ballpark 13dB (about 1.6:1 VSWR)

So it would be pretty hopeless as a SWR meter on that basis alone. The other factors are the directivity will most likely be extremely poor at 70cms.

Just make or buy or borrow a proper meter rated for 70cms

Even on 2m I'd expect the directivity of a typical CB SWR meter to be very poor. eg ballpark 15dB. This would be hopeless for getting a reasonable level of accuracy.

[G8EAD]

@RD250 assume 0 financial budget.
In that case the CB SWR meter switched around tx/antenna method works sufficiently well IMNSHO

Your mileage may vary.

[paulears]

There seems to me little point in a VSWR meter that isn't reliable. A bit like using two pairs of bathroom scales if you're so fat it exceeds the label for maximum weight - so you borrow another and you put one leg on each and then attempt to get an accurate reading of your total weight. You might get an accurate reading, but how do you know? The divides that give accuracy at 27MHz might be or might not be accurate at 145MHz but that's doubtful, otherwise the meter would proudly proclaim 145MHz and sell a few more to the hams as well as the CB folk. UHF is pushing it severely. The electronic meters seem pretty accurate at 2m, but mine differs quite a bit from my analyser with a VSWR bridge. I see little point in using any tool outside of where it's design puts it.