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View Full Version : What's the power limit and useful frequency range for this cheapo CB SWR meter?



WA3LKN
06-16-2008, 06:49 PM
I'm trying to decide if this has any use in Ham Radio other than 10m QRP work. I don't have the spec sheet for this. It's obviously a dedicated CB item but was wondering what its capabilities are.

It's a Realistic (Radio Shack) Model #21-576

See jpeg

AG3Y
06-16-2008, 06:53 PM
I think you posted a thread about this same device just a week or so ago. It is an AM modulation meter, as well as a CB SWR meter. I notice the power scale reads 10 watts full scale. I sure wouldn't try putting more than that into it !

Being a CB device, I wouldn't expect it is very accurate below 15 and above 10 meters, and I sure wouldn't vouch for its accuracy between those two extremes!

You asked; that's my opinion!

73, Jim

K9STH
06-16-2008, 09:40 PM
Ignoring the watts scale the meter should be good all the way down to 160 meters where the SWR readings are concerned. However, the amount of power needed to get a full scale deflection (to calibrate the SWR scale) is going to rapidly increase the lower you go in frequency. This is because of the relation of the length of the "pickup" assembly to the wavelength of the signal. The strength of the signal depends on the length of the "pickup" in relation to the actual wavelength of the signal. The less the ratio of length of the "pickup" to the wavelength of the signal the more power that is required to get the same meter deflection.

On the high side it should be fairly accurate up through the 6-meter band. Now the accuracy of the SWR reading is going to "drop off" considerably as you get higher and higher in frequency. That is because to be accurate the relation of the "pickup" assembly has to be fairly short when compared to the wavelength of the signal. For a "relative" reading (i.e. detecting the lowest SWR but not knowing what the actual value is) it may be usable on 2-meters and even 1.25-meters. It probably will not work on the 70 cm band and higher although you might still be able to get a "relative" reading on the 70 cm band.

The opposite is true when the frequency increases. The longer the "pickup" in relation to the wavelength the less accurate the device becomes. That is why I say that it may work for a "relative" reading but definitely not be accurate in terms of the SWR value.

The wattmeter reading even if accurate on the 11 meter band (will be fine on the 10 meter band and probably on the 12 meter band) will generally not be accurate on both higher and lower frequencies. The calibration in terms of power when designed for the 27 MHz band will probably be with specifications between 25 MHz and 30 MHz. Above and below that it will read lower and lower for a given amount of power as you go lower in frequency and higher and higher for the same amount of power as you go higher in frequency.

Glen, K9STH

W6TMI
06-16-2008, 10:00 PM
To paraphrase Glen, basically it would (at lower power settings) be ok for relative SWR readings, comparing power output and the like.

I wouldn't mess around with the modulation meter, it's only for AM. If you are messing with AM use whatever you radio recommends.

AG3Y
06-17-2008, 12:19 AM
I would like to mention that I do have a couple of SWR bridges that were designed for the 11 meter CB band, myself! I find them to be quite adequate to get me in the ballpark when I am careful not to "bury" the needle in the forward position!

Since it is not a good idea to measure SWR with more power than necessary to get a full scale "forward" reading when set to or near the maximum sensitivity setting, the meter should be quite adequate to serve on just about anything between 40 and 6 meters! Maybe even 80 . Just don't expect laboratory grade precision!

73, Jim

K9STH
06-17-2008, 12:53 AM
If you want "laboratory grade" readings you really shouldn't be using an SWR bridge anyway! Most commercial two-way radio technicians and laboratory people use a high quality wattmeter to read forward and reflected power.

Glen, K9STH

WA3LKN
06-17-2008, 01:45 AM
Thanks very much for the info. gents.

WA7KKP
06-17-2008, 08:36 PM
If you want a GOOD wattmeter, invest in a Bird 43 and the proper slugs.

You can get an SWR bridge for $10-20 at most hamfests that are far better than this RS item. Most of them should tickle prettty well with QRP at 10 meters; it is at the lower frequencies you need upwards of 50 watts or so to set the forward sensitivity to full scale.

Gary WA7KKP

W4HAY
06-17-2008, 08:53 PM
I've had numerous CB SWR meters pass through my hands and found they were close enough for government work on HF as long as power was kept under 100 Watts. Most used the "parallel line coupler" circuit and so sensitivity increased with decreasing frequency. I wouldn't trust the "power" function, though.

K3WRV
06-18-2008, 12:27 AM
What you have is probably based on the "monimatch" design described in the ARRL handbook over the years. If you look at the innards, you probably have a "stripline" etched in the PC board (two parallel traces on the outside and a fairly wide trace down the center connecting the two antenna ports. I've got something similar (tho mine has two meters, and it's l;abeled "Swan" (but the same thing came also labeled MARS, Realistic, Lafayette and other names). Mine came with a calibration chart and covers 10 W to 1KW. In terms of accuracy, it was actually pretty good from 80 up thru 2 meters, but I never relied on it for any critical power measurements. It also did a nice job of relative SWR measurements for setting a tuner or adjusting a dipole or a vertical. But the calibration control was VERY TOUCHY to set repeatedly.

When considering any instrument, you need to decide how much accuracy (or precision) you need. If you want to measure 1/32 inch, you probably won'r want to use a yardstick. And if you want to measure a mile, you probably won't use a one foot ruler.

Most modren SWR/PWR meters these days use a "Breuene" circuit which is less frequency sensitive and requires less adjustment, but if you alrady have the thing, or it's cheap, just use it. Do you really care if your SWR is 1:1 or 1.3:1? Most of us don't.

If you need to increase the power measurement range of the thing, try replacing the pot that adjusts the power level. Normally it's about 1K Ohms. But you'll lose some resolution at low power levels.

K4LSX
06-18-2008, 01:43 AM
Good previous posts. I say forget accuracy - ham radio isn't about mil-spec details, unless you seek to bloviate about your 1.100321 to 1.000 VSWR with a $300 aftermarket microphone. Decrease power, decrease sensitivity of the VSWR meter (so to not wrap the needle in forward) and tune for minimum reflected, then go about enjoying your radio. If you can't find a spot where your tuning devices will produce a min VSWR, it is more likely to be caused by what you have connected to the bridge than by the bridge itself.

KC4UMO
06-18-2008, 01:52 AM
If you want "laboratory grade" readings you really shouldn't be using an SWR bridge anyway! Most commercial two-way radio technicians and laboratory people use a high quality wattmeter to read forward and reflected power.

Glen, K9STH
I have both types of Bird meters slug and slugless But I find mosdt of the time with HF antennas using my old cb meter. They seem to work fine and get you close enough. And if you drop it or leave it out in the weather no big loss.

But when I am designing or trying something new I use the bird meter to read reflected power.

Just keep the power low and you should get by.

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