Since nobody believes a 148/grant will do 25+ watts...

[Geekster]

Its only CB guys, not rocket science or NVLAP accredited sanctions here (that was last month here at the shop). The Bird wattmeter is fine for lay measurements. If you are really that concerned about your power measurements send it off to an accredited lab with NIST traceability.





[/sarcasm off]

Fact of the matter is that most companies (Vectronics, Para Dynamics, Palstar, MFJ, et al.) use the Bird Thruline for their calibration bench. [Well at least that was the case when I was working for them].

Geekster edited [added info]
From the Bird 43 Manual:
Attenuation (power lost by heat in a line) as well as VSWR may be measured by inserting the unknown line between two Thruline wattmeters, or between a Thruline wattmeter and a Termaline absorption wattmeter.

I have never tried this with the Birds but have performed a similar test when calculating ERP for a coordinated repeater (OARC).

 

[ken white]

...Its only CB guys, not rocket science or NVLAP accredited sanctions here (that was last month here at the shop). The Bird wattmeter is fine for lay measurements. If you are really that concerned about your power measurements send it off to an accredited lab with NIST traceability...

agreed...

 

[Moleculo]

Hehe...yeah plus at the power levels we're talkign about +/- 5% isn't really that much of a swing.

Anyways, with regards to C2's comments about the IMD products on this mod or the 50 watt cobra 29 mod: What we really need is for those comments to be meaningful is to see a before and after snapshot. That way we have a point of reference of where we started. Nobody can tell us whether or not any of these mods cause more or less IMD until we know what we started with. So your analysis is the 50 watt Cobra mod (or this one), while a correct interpretation of what the Spec analyzer is telling us, doesn't necessarily tell us if it introduces any worse (or better) readings than before.

We have plenty of techs on this forum that have this kind of tech equipment that could perform this analysis for us and report back. I would really be excited for someone to perform this 148 mod and tell us the before/after results, and perhaps even the 50 watt cobra 29 mod. I'm sure Freecell, DTB, NomadRadio, Richard, etc. could all do it if they were so inclined.

The other part of the equation that everyone seems to miss is whether or not the IMD products (or "splatter" if you want to call it that) are discernable by other CBs or other radio services. At the power levels we're talkign about, if other radios adjacent channel rejection is still able to filter any bleedover out because it's at such low levels, then it's not really a problem. Likewise with other radio services. When you introduce amplifiers, then you have a new problem all together which is probably worth worrying about. And then if you amplifier introduces it's IMD products and distortion, you could have a much worse problem.

Like the guys said...it's just the CB radio service. With all the technical discussion, we can't lose sight of the practicality side of the equation...

 

[C2]

I see what you are saying. Kind of like, if my car is putting out a lot of toxic smog, but souping it up does not make it put out any more toxic smog than before, then it is OK.

But, if it did then put out a lot more smog, it would be OK as long as nobody else was choking in it.

In addition, we have keep this all in perspective. Car fumes are supposed to make people's eyes water, so it is all good.

I think I have some measurements of a clean CB laying around someplace. That would be a good starting point. It is not a 148 or 29 though.

What I remember, off the top of my head: third order IMD was below 32 dBc and the 2nd harmonic was -68 dBc. Unfortunately that was just with a 1 kHz tone just a hair under 100% modulation from a 4W carrier.

 

[Moleculo]

I see what you are saying. Kind of like, if my car is putting out a lot of toxic smog, but souping it up does not make it put out any more toxic smog than before, then it is OK.

Hehehe...well not exactly. What I'm saying is that it it doesn't put out any more toxic smog, then the mod didn't make it better or worse. Whether or not the baseline smog is "OK" is a different analysis altogether.

But, if it did then put out a lot more smog, it would be OK as long as nobody else was choking in it

Again, no that's not it at all. If it put out a lot more toxic smog, but it was still below the baseline of what everything else in place found acceptable, then it's still acceptable. The issue is what is the acceptable baseline?

 

[preacherman]

[quote="Moleculo
Again, no that's not it at all. If it put out a lot more toxic smog, but it was still below the baseline of what everything else in place found acceptable, then it's still acceptable. The issue is what is the acceptable baseline?[/quote]

And...who gets to establish that baseline for "everyone else"? 8)
preacherman

 

[C2]

Ooh, Ooh, Ooh...I think I know who that is.

 

[Happy_Hamer]

I find this hilarious, because I've been getting 45-50 watts out of a Cobra 29 chassis for years. And no, not by cutting and / or jumpering parts. By making engineering changes and taking into effect how a transistor's impedance changes when the VCC and drive changes can make a difference. Might I add that the Cobra 148 mod that Mole did is completely possible, I've been doing that for years as well. Typically 28-30 watts output. And NO we never "volt" the finals, it's not needed.

 

[Geekster]

Hehe...yeah plus at the power levels we're talkign about +/- 5% isn't really that much of a swing.

Anyways, with regards to C2's comments about the IMD products on this mod or the 50 watt cobra 29 mod: What we really need is for those comments to be meaningful is to see a before and after snapshot. That way we have a point of reference of where we started. Nobody can tell us whether or not any of these mods cause more or less IMD until we know what we started with. So your analysis is the 50 watt Cobra mod (or this one), while a correct interpretation of what the Spec analyzer is telling us, doesn't necessarily tell us if it introduces any worse (or better) readings than before.

I agree, It would be nice to see before and after snapshots.

The other part of the equation that everyone seems to miss is whether or not the IMD products (or "splatter" if you want to call it that) are discernable by other CBs or other radio services. At the power levels we're talking about, if other radios adjacent channel rejection is still able to filter any bleedover out because it's at such low levels, then it's not really a problem. Likewise with other radio services. When you introduce amplifiers, then you have a new problem all together which is probably worth worrying about. And then if you amplifier introduces it's IMD products and distortion, you could have a much worse problem.

Like the guys said...it's just the CB radio service. With all the technical discussion, we can't lose sight of the practicality side of the equation...

Selectivity and image rejection would play a significant role on the (other) CB’s. This of course varies by mfg, but we also have to account for the narrow bandwidth separation of (CB) 11 meters. (0.44MHz)

11meter band is used by broadcasting, fixed, mobile, and maritime stations, many of them low powered units in trucks, taxicabs, small boats, etc. USB and AM are mainly used.

Some frequencies: (in MHz)
26.100
26.175
26.480
26.950
26.960
27.230
27.410
27.540

 

[C2]

I find this hilarious, because I've been getting 45-50 watts out of a Cobra 29 chassis for years. And no, not by cutting and / or jumpering parts. By making engineering changes and taking into effect how a transistor's impedance changes when the VCC and drive changes can make a difference. Might I add that the Cobra 148 mod that Mole did is completely possible, I've been doing that for years as well. Typically 28-30 watts output. And NO we never "volt" the finals, it's not needed.

I agree. More lip service.

BTW, "engineering changes" sounds gay. Never heard it put that way before. Just wondering, are you and engineer?

 

[Logan]

My head hurts

 

[Geekster]

28 watts can be possible with a 2SC1969 according to the data sheet.

However, in AM how does the audio portion (waveform) have room to breath (sinusoidal peaks) without clipping once it goes above the 28 watts, unless going into AB (push-pull) operation with 2 finals?

Cornfused...?

 

[ken white]

28 watts can be possible with a 2SC1969 according to the data sheet.

However, in AM how does the audio portion (waveform) have room to breath (sinusoidal peaks) without clipping once it goes above the 28 watts, unless going into AB (push-pull) operation with 2 finals?

Cornfused...?

The biggest issue with overmodulation is that the PEP should never exceed four times the carrier power (100% Modulation) or the envelope will have square wave harmonics generated in the area between the enevelope peaks. This creates severly distorted audio audio at the receivers envelope detector and envelope generates high harmonic frequency content in the radio spectrum. As for clipping of the signal in the radio, class "C" will be less likely to clip since tha amplifier is biased below cut-off, while any of the other classes will clip sooner.

 

[radioreddz]

with out a doubt a Grant XL will do 25 watts. have used one in my suburban and with a 2.5 watt dead key swing is right at 25 watts. Grant XL best Cb radio ever. to bad they did not make one with a built in freq counter like the Cobra 148F GTL. add an amp and a couple of switches and a freq counter and go dxin, a true 2950 killer

 

[davegrantsr]

sheesh....i'm sorry i started this. assuming it worked right, a 30 watt grant xl would serve minimal purpose at best. for me, it serves no purpose. it just needs to excite my driver, nothing more. and even there, it far outtalks any single final radio around. no matter the claimed output. my 1 tube will take care of any 30 watt xl or 50 watt 29, so....
can't agree more with radioreddz, though. my xl kills EVERYTHING