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PI vs PI-EL output




 
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Author Topic: PI vs PI-EL output  (Read 841 times)
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KK7UV
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« on: February 13, 2019, 07:03:14 AM »

Playing around with matching network software (Tonne's PI-EL) I was looking at a Pi-network for the 2x 813 rig where the plate load is 4000 ohms (2000v at 250mA, k=2).   I noticed that second harmonic attenuation at reasonable Q's of 11-14 is only around -25dB to -30dB, not even close to the required -43dB, which isn't attainable until the Q is around 48.    Or does the second harmonic count as a "spurious emission"?   

The -43dB is attainable with the Pi-L network however, but not without the added complexity of bandswitching, etc.   


I also notice that on some rigs (broadcast rigs too) the tank circuit is enclosed in it's own cage, and in others it is not.   How critical is this when all of the transmitter workings are inside a big metal cabinet?  Do we just make sure any other RF-sensitive circuits such as input/grid networks are shielded in their own enclosures within the cabinet?

- Steve KK7UV
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PA0NVD
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« Reply #1 on: February 13, 2019, 08:28:21 AM »

What value did you enter as input for the second harmonics? The tube output contains much less second harmonics than fundamental, so that difference you should add to the attenuation to get the second harmonics output.
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kb3ouk
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« Reply #2 on: February 13, 2019, 06:17:50 PM »

What are you going to be driving it with? The cleaner the input is, the less the output has to filter out. With my 2814 rig, the second harmonic output when using my Millen 90800 to drive it was much higher than using my FT-901DM, the Millen just being link coupled with no filter on its output and the Yaesu having a pi net.
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KK7UV
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« Reply #3 on: February 13, 2019, 07:54:17 PM »

I will probably use my modern transceiver to drive it at first, and then ultimately a homebrewed DDS-VFO followed by a buffer/amp stage.

So then, assuming proper tuning I have nothing to be concerned about with a Pi-net as long as my drive signal second harmonic is at least -18 dB, (such that combined with the -25dB in the Pi-net meets the -43dB regulation).

That should be easy enough.   Thanks!
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K1JJ
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« Reply #4 on: February 13, 2019, 08:41:23 PM »

Hi Steve,

What kind of antenna will you be using?  If it's say, a 75M dipole fed with coax, the second harmonic will be greatly attenuated due to the huge mismatch on 40M.

Or, if it's open wire fed with a tuner, the tuner will add significant attenuation.  So much in these cases that you will easily exceed the -43 DB specs in the real operating world, though maybe not on the bench into a broadband dummy load.

I remember running a 1.5 KW PEP solid state linear 75M mobile but never worried about not having a bandpass filter because the 11'  loaded whip was totally mismatched anywhere other than a narrow spot on 75M.

The only problem is when running a G5RV, trap antenna or any other multi-band antenna such as a log periodic that is designed for broadband use. Then there may be minimal additional attenuation.

Yes, input / output networks and components need to be shielded from each other for stability and for best filter isolation.   And as with any filter, the individual components should be at right angles to each other. This includes the plate choke.

T
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KK7UV
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« Reply #5 on: February 14, 2019, 05:41:45 AM »

That should be no problem with my antennas:

82-ft. top-loaded T-vertical with coax-feed and L-network at feedpoint for 160m.
132-foot an 73-ft. balanced-fed dipoles with balanced tuners for 80m and 40m.

Shielding/spacing input from output should be easy enough.  I was thinking more in terms of the P.A. tank tuning network itself and shielding it from the outside world.   I was perusing through photos of broadcast transmitters to get some layout and design ideas and noticed, for example, that a Collins 20v has an enclosed output tank network but a Gates BC250GY does not.

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Patrick J. / KD5OEI
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« Reply #6 on: February 14, 2019, 07:58:02 PM »

I noticed in pi-el that the Zin at the 2nd harmonic was only 203 Ohms (4K, 50 Ohms Q14). Is that 'near short' taken into account when the 2nd harmonic is calculated?
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KD6VXI
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« Reply #7 on: February 14, 2019, 09:52:19 PM »

You can play with the image R value to get different 2nd harmonic reduction values, too.

I like pi-El, especially since it will interface with ELSIE......

But honestly, I use Ian Whites spreadsheet a lot more.  It takes a LOT more variables into account.

Google g3sek spreadsheet for the download.  It takes stray C, stray L, the plate choke inductance as well as any parasitic suppressor values into account.  It DOES require Excel the first time to run.  After that you can use open office, etc.

Another way to skin a cat.

--Shane
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