1/4 vs 1/2 wavelength antenna

I read in sci.electronics.design that Tom Ring wrote (in ) about '1/4 vs

1/2 wavelength antenna', on Thu, 3 Mar 2005:

See the last sentence, about the effect of an **8 ohm** source impedance on damping.

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Hi Thomas,

10 seconds to adjust all 6 stubs?

73's Richard Clark, KB7QHC

Richard was asking how long it took you to tune the triple stub filters during devolpment.

I am curious about the exact nature of the impedance transmformation these devices provided.

jk

Hi Thomas,

I thought 10 seconds was awful quick. How long would it take to flatten the response when manually adjusting the triple stub tuners?

What merit did you find with triple that could not be found with double stub tuners?

73's Richard Clark, KB7QHC

Hi Thomas,

Certainly not as conventional Triple Stubs. However, care to provide some of the cogent details of that particular project? Any interesting insights?

73's Richard Clark, KB7QHC

Response is flattened through gain controlling the pre-amp from a look-up table held in the micro's EEPROM. The alignment procedure is automated using the HB-IP bus from the spectrum analyzer and a computer. The computer/analyzer also looks for harmonic content and spurious emissions during this procedure. Think it takes about ten seconds to do this.

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I doubt you understand what he wrote. I can't fathom why you would be concerned with the OP when your own difficulties are so acute.

How would you know?

How long will it take you to figure out that he wrote not a wisp of a word on what the "output-Z" of the amplifier is? He did write that he determines how the amp was loaded to acheive power, something I've been saying is the prime concern.

No the triple stub tuners are only for development. Production boards have discrete components to form the match network. Power levelling or "flattening the response" is computer adjusting the output power to compensate for the reactive components to ensure a constant output power over the entire band of the radio. We also put in a small temperature compensation coefficient into the EEPROM because the PA tends to put out more power when it is cold.

using

Hi gwhite,

I would have to agree with you on most everything you have said through this thread. I once saw my boss (with his "PHD") try to model and match a power amp based on the small signal parameters off the datasheet. He insisted that the stated input and output impedances were characteristic parasitics of that amp and wouldn't change between a small or large signal. It was kind of pathetic to watch him struggle for over a month on the matching network, and I think he had resorted to guessing in the end.

I've often questioned why manufactures put small signal parameters on their datasheets? Makes no sense to me. Even if they do publish some large signal parameters it is unlikely to be the exact same mode of operation that you need for your project.

Playing with triple stub tuners on PA's has shown me that there are many combinations of input and output impedances that appear to give similar results at any one frequency, but give different results at others frequencies. So it's a matter of finding the input and output impedance that give you adequate performance over the entire scope of your project.

Thomas

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Get a sense of humor. Or maybe more ice and mixer.

tom K0TAR

Oh, and going from 8 ohms output impedance to 10e-7 (unless I miscounted) would take the damping factor from 1 to 8e7, which is a bit more than 2. Ignoring the speaker wires of course.

tom K0TAR

If your amp has to operate over a wide frequency range it is not likely that you can flatten the response just with stubs. Stubs should be looked at as more single frequency devices than broadband networks. But you can use the stubs to plot out the appropriate impedance curve on the Smith Chart to ensure a flat response when you model in the discretes.

I usually just try to get the flatness of the response as close as possible and rely on a software calibration routine to flatten it off. Saves a lot of time.

It's my understanding that a triple stub tuner of the right length can reach anywhere on the Smith Chart where as a double stub can not.

They might be of some use for specific cases. For example, if the PA is class A, is used well backed off because of high PEP-to-avg ratios of the signal, and you've managed to get the output load dialed in, s-params can be useful for a first cut at the amplifier *input* match. I've always still had to do some tweeking though. Also, with some work and considering the load-line match, they can give you an idea of what gain can be accomplished. This might already be in the data sheet though, as you mention.

One of the large signal parameters I like best is how much power the device can dissipate. ;-) Voltage breakdowns and Imax are nice too. ;-) ;-)

I read in sci.electronics.design that Tom Ring wrote (in ) about '1/4 vs

1/2 wavelength antenna', >

Also ignoring the ***voice-coil resistance***. If that is included, as it must be for a correct analysis, you get 2.

F Langford-Smith 'invented' the concept of damping factor, and around

1949 accepted the point made by James Moir that, by not properly taking into account the effect of the voice-coil resistance, it was a seriously misleading concept. Yes, 60 years later, people are still being misled.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Good point. I stand corrected.

The only nit I would pick would be that impedance be used, since you need to measure it at the frequency(ies) in question, not DC. And then there is that pesky crossover in most systems. Personally I like biamping.

tom K0TAR

'OM' is radio ham speak for 'Old man'. 8-)

Leon

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Leon Heller, G1HSM
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I read in sci.electronics.design that Tom Ring wrote (in ) about '1/4 vs

1/2 wavelength antenna', on Sat, 5 Mar 2005:

So do I.

-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited.

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