Accurate PEP Meter?

[CbRadiosPRONTO]

2222dbfl. I hear that on the LP.


I bought the rat shack 4 weeks ago. Last week took it back because it started showing stock cobra 29's at 8 watts carrier. The replacement is showing similar wached out readings. Arg!

I think pep is 200-300% higher than avg. somebody smarter in here knows for sure.



CbRadiosPRONTO

 

[543_Dallas]

2222dbfl. I hear that on the LP.


I bought the rat shack 4 weeks ago. Last week took it back because it started showing stock cobra 29's at 8 watts. The replacement is showing similar wached out readings. Arg!

I think pep is 200-300% higher than avg. somebody smarter in here knows for sure.



CbRadiosPRONTO

8 watts avg or pep? 8 pep isn't too far off for a bone stock radio. 4 watt carrier @ 100%mod would be 16 pep. The old rat shack meter I've had for 10 years is close to a bird on avg...not sure about pep. There might be an adjustment in there if you want to see some big numbers. How much higher pep is than average depends on a lot of variables. Perfect textbook sine wave will show no swing on avg and at 100% mod the pep will be 4 times average power. Once the waveform is anything but a sine wave all bets are off.

I use an Ameritron AWM-35 on the base. It has a remove head and the meter is connected with cat 5 cable. I really like being able to easily move it from the radio room to the work bench in the garage.

I've used a palstar 2000am in the mobile for years and it's held up very well. It also has a remote head but uses a 3 pin mic plug and a power plug vs the more convenient and easy to find cat 5 cable.

Burd makes fine instruments but I don't need that kind of quality. I really don't see the point unless you are running serious power and need a serious meter.

 

[CbRadiosPRONTO]

8 watts carrier


CbRadiosPRONTO

 

[543_Dallas]

8 watts carrier


CbRadiosPRONTO

That's too bad. Those used to be a good meter for the price.

 

[CbRadiosPRONTO]

That's too bad. Those used to be a good meter for the price.

I know. Frustrating. I've heard nothing but good about them.


CbRadiosPRONTO

 

[222DBFL]

Yeah seems like the quality of the rat shack meters isn't what it used to be that is for sure! Mine does okay, use it mostly for swr readings anymore as I don't much pay attention to power output. Set it and forget it. More concerned with keeping my VSWR reading in order. They work for that at least. But I know what you mean about the different readings. Had one start acting funny not too long ago and just replaced it rather than worry about it. And 543 I hear ya on the expensive meters, a man can dream at least lol. Maybe one day. I travel too much these days and am not at home a lot so that would be a hard one to justify. Now the Dave made digital meter is one that I wouldn't mind having or something like what you mentioned. Have any photos that you could share, curious as to what they look like and where to get them? Sounds right up my alley as most of my time is spent in the mobile. Thanks for ant replies and god bless.

 

[543_Dallas]

Yeah seems like the quality of the rat shack meters isn't what it used to be that is for sure! Mine does okay, use it mostly for swr readings anymore as I don't much pay attention to power output. Set it and forget it. More concerned with keeping my VSWR reading in order. They work for that at least. But I know what you mean about the different readings. Had one start acting funny not too long ago and just replaced it rather than worry about it. And 543 I hear ya on the expensive meters, a man can dream at least lol. Maybe one day. I travel too much these days and am not at home a lot so that would be a hard one to justify. Now the Dave made digital meter is one that I wouldn't mind having or something like what you mentioned. Have any photos that you could share, curious as to what they look like and where to get them? Sounds right up my alley as most of my time is spent in the mobile. Thanks for ant replies and god bless.

Ameritron Flat Mount Mobile SWR/Wattmeters AWM-35H - Free Shipping on Orders Over $99 at DX Engineering

Palstar - PM2000AM

There are several online stores that sell them. These are just the first I found with decent pics.

 

[tba02]

I use a Diamond SX-200 but the only thing I have to compare it to is an old Dosy that reads higher and doesn't have a PEP option.

 

[space cowboy]

according to cbradiomagazine the radio shack swr power meter is accurate, just a little low on the pep. it's an easy thing to add 10% to your reading if you need to.

Another good point about this meter is it's ability to measure peak output giving it the ability to show SSB output. Although not an active peak reading meter it does a pretty good job of showing your peak output in SSB modes. I'd say in most accounts when measured against an active peak wattmeter I found the Radio Shack meter to show about 15% lower on the peak output readings.

 

[GnG8d]

according to cbradiomagazine the radio shack swr power meter is accurate, just a little low on the pep. it's an easy thing to add 10% to your reading if you need to.

That's exactly what I would expect actually. Meters like the Radio Shack, Dosy etc., which use passive components in the peak detector circuit will generally read lower than a circuit with active components.

When it comes to SSB average power, I've read numbers indicating 10-20% of peak and with heavy processing and compression, maybe as high as 20-30%. But I have no way to verify any of those numbers and don't play with SSB enough to have a real feel, but those numbers seem high compared to what I've seen.

Lets take what I think I know about an AM signal and dissect it to a SSB signal. If we modulate a 100w carrier with a sine wave, the average power will swing up to 150w and the peak envelope will grow to 400w. Now lets subtract the 100w carrier from the 150w forward modulation which leaves us 50w, or 25w in each sideband. That leaves 25w of average SSB power in a 400w peak envelope indicating that the SSB average power is 6.25% of the peak envelope.

I don't see any way possible for the average SSB power to be any where near 60-70 or especially 80% of the peak envelope. Removing the carrier alone eliminates 25% of that power and then one of the sidebands needs removed as well. I would suggest any meter showing average SSB power at or above 25% of pep might be suspect .......... assuming that the signal is clean.

 

[Justme]

The only way to read PEP accurately is introducing an active circuit that samples the PEP peaks and shows them on the meter like the CN 801 of Daiwa.
On average your meter without that circuit will show 25 to 33 % of the PEP peaks that are produced.
An 100 watt transmitter will have PEP of 100 watts in the peaks, an normal meter will show 25 to 35 watts operated within the transmitter limit as specified by the maker of the transceiver.

My 200 watt FT 2000-D shows 180 watt in PEP reading mode, whistling shows 225 watt, that is how I set up the modulation looking at the scope and ALC.
The 225 watt is how the FT 2000-D came from the factory....
Using the EQplus that still gives good audio loud without overdriving the P.A.
Need more power? I switch on the Heathkit SB-1000, that one needs 45 to 65 watts in for 600 -1000 watts out with the new 3-500 ZG tube, and on a 230 volt 16 amp line.

 

[Robb]

There is yet another means to find out if the watt meter you are using is correct.

Excerpt from:
Dummy Load Construction

Now we'll find out how to use the optional diodes and binding posts that we've included in the dummy load..

Measuring Power Accurately

The signal from your transmitter or transceiver is an almost perfect sine wave. We know this because the harmonics are at least 40 dB down from the carrier. When we measure the peak voltage from the detector (the BAV21), we are measuring within one percent of the true peak value of our carrier, not including the diode drop; we will add that back in later.

For power measurements, I recommend that a 0.01uf disk ceramic of at least 250V rating be connected between the binding posts. This will charge to the peak voltage applied to the 50-Ohm load, less the diode drop. You can then measure this voltage with your DVM.

Let's assume you measure 99.6V with your DVM. Add 0.4V for the forward drop across the BAV21 for a total peak voltage reading of 100V. The diode drop is a constant, always add 0.4V to your reading! (This assumes you are using a DVM or scope with an input impedance of 10 Megohms. For 100V DC, the forward current will be 10ua, for a forward drop of 0.4V)

Since this is a peak voltage, we need to divide by the square root of two to get RMS voltage. Take your calculator and divide by 1.414.

100 divided by 1.414 equals 70.72 Vrms.

To calculate power, we take the RMS voltage, square it, and divide by the load impedance, which in our case is ALWAYS 50 Ohms!

(70.72)^2 / 50 = 100W

So the output power, dependant on the accuracy of your DVM, is nearly 100W. If your DVM is accurate, say within 1% on DC voltage measurements, you have nailed your rig's output power within 2%, or 2W!

Which - BTW - is more accurate than a Bird 43P (5%).
Easy to understand how he derived this solution - and the math is very easy to grip too.

So anyone that wants to modify their cantenna can - with the use of a DMM - can get a reasonably accurate figure of their radio output and can at the same time gauge their own watt meter's accuracy. Of course, your DMM will also need to be reasonably accurate too, and a Fluke 87 will cost about $300. But for that much money, you can buy a used LP100 if you are fortunate enough to find a decent used one.

 

[GnG8d]

On average your meter without that circuit will show 25 to 33 % of the PEP peaks that are produced.
An 100 watt transmitter will have PEP of 100 watts in the peaks, an normal meter will show 25 to 35 watts operated within the transmitter limit as specified by the maker of the transceiver.

I never considered this in the past and always used a peak reading meter on SSB, but now my curiousity is piqued. Can you show some theory or math supporting this? I poked around a bit and couldn't find much more than blanket statements.

 

[GnG8d]

You are warned. Here is a link providing 4 pages of discussion with additional links to numerous other pages. I read the first 2 pages and then book marked it for later.

LINK

 

[loosecannon]

There is yet another means to find out if the watt meter you are using is correct.

Excerpt from:
Dummy Load Construction



Which - BTW - is more accurate than a Bird 43P (5%).
Easy to understand how he derived this solution - and the math is very easy to grip too.

So anyone that wants to modify their cantenna can - with the use of a DMM - can get a reasonably accurate figure of their radio output and can at the same time gauge their own watt meter's accuracy. Of course, your DMM will also need to be reasonably accurate too, and a Fluke 87 will cost about $300. But for that much money, you can buy a used LP100 if you are fortunate enough to find a decent used one.

i just did this exact mod to my homebrew dummy load last week.

it seems to work great!
used the math to calibrate my cheapo para dynamics meter, and it was actually pretty darn close before my slight adjustments.

i believe that adjusting for accuracy using the middle of the needle movement is the best way to go.
LC