Bob:
The antenna analyzer SHOULD have a source impedance of ~50 ohms. If it didn't
it would cause a reflection (or possibly reduce or cancel a reflection) if it
presents an odd conjugate condition other than 50 ohms real.
Maybe I am not clear on what to do: You're attaching a load impedance to the
input of the tank that emulates the desired tube operating load impedance.
Chuck said that this is ~1800 ohms. Given that a resistor of that value is
placed from that node (plate cap) to RF ground (along with the shunt
capacitance of the tube's output, Cp-g for grounded grid), and the tank is
properly adjusted, you should measure 50 ohms or a 1:1 SWR at the output of the
amplifier at desired frequency. There is no other termination at the output,
only the source impedance of the antenna analyzer, 50 ohms.
You could go the other way, measure from the tank input and hang a 50 ohm
termination at the amp output, but most impedance bridges are too inaccurate or
won't resolve > 1K ohms.
This whole thing reminds me a bit of WG0M in Minneapolis a few years ago who
wanted me to design a matching network for his 160m "L" antenna. I used NEC2
to ballpark it. He hooked it up and was mad because he still had 3:1 SWR. I
went over to help him, and it was immediately obvious what the problem was:
The vertical portion of the "L" ran parallel and 3" away from his 60' tower. I
never knew (nor did he tell me) he had a tower. A little fiddling and we had a
match.
So if there's anything you're forgetting to tell me Chuck ...
73, Steve, K0SF
----- Original Message -----
From: "Bob Sullivan" <robert@isquare.com>
To: "STEVEN & NANCY FRAASCH" <sjfraasch@embarqmail.com>
Sent: Tuesday, July 17, 2007 2:09:52 PM (GMT-0600) America/Chicago
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
Steve,
Been reading your very informative comments - thanks for taking the time to
post.
The idea of using an antenna analyzer is a great one but I thought about
what you said below and was wondering if I should place a 52-ohm
termination at the output of the tank before utilizing the analyzer (MFJ)?
Thanks!
73, Bob
WÃYVA
Personal site: http://www.isquare.com/personal_pages/ras-hardware.htm
At 08:22 AM 7/17/2007, you wrote:
>Chuck:
>
>I just think it is something easy. For example, is your bandswitch
>shorting the section of inductor that should be in play ? Is your plate
>choke much smaller than planned (that would have the effect of a pretty
>strong inband zero) ? Perhaps your plate blocking cap is too small a
>value, or you're not setting the output T/R relay for transmit ? It just
>seems like too much work to take apart again. These are all the things I
>have had to straighten out with Bob, K0DD, but I assume you're way ahead
>of him.
>
>Hey, believe me, we (and especially Bob) have made plenty of mistakes (I
>can think of one at work last Thursday). So, I guess, I would go check
>the not-so-obvious obvious and make sure.
>
>If you use your antenna bridge, remember, that it IS the source impedance;
>therefore, you're not putting a 50 ohm resistor at the tank output, the
>antenna analyzer is sourcing 50 ohms and you're placing the 1800 ohm
>resistor (if that is what you think the design plate impedance should be)
>at the tube plate connection to ground.
>
>If you give me the tank component values and the config, I can quickly
>calculate the input impedance.
>
>Another tip: when I do PI-L networks I usually set the middle node to the
>conjugate mean of the input and output impedance. Therefore, if the input
>impedance is 1.8K (real), and the output is 50 ohms (real), in between the
>two L networks should be: (1.8K*50)^.5 = 300 ohms real. There may be Q
>and efficiency benefits depending on the components used to use a
>different intermediate impedance, but generally, the middle of the PI-L
>should be at conjugate mean.
>
>Anyways, send me your data if you want some help.
>
>73,
>
>Steve, K0SF
>
>----- Original Message -----
>From: "Chuck Curran" <ccurran@wi.rr.com>
>To: "STEVEN & NANCY FRAASCH" <sjfraasch@embarqmail.com>, amps@contesting.com
>Sent: Monday, July 16, 2007 7:00:04 PM (GMT-0600) America/Chicago
>Subject: RE: [Amps] Grid Dipping the Pi network in a new amp?
>
>To All:
>
>Thanks for the many valuable posts in regards to my dipping the Pi-Net
>output circuit. I have read each and every one and learned a bit in the
>process.
>
>Steve, this last post of yours has caught my attention, since I just
>purchased a MFJ-259B about 6 weeks ago. I have already tried most of the
>other suggestions, with no success. The MFJ-259B was picked up for another
>issue, now resolved due to the ease of use of the antenna analyzer.
>
>I did not use any cleaners on the silver plated contact sections of the
>vacuum variables, but last time I checked, silver oxide was conductive.
>However, In order to resolve this issue by the weekend, I will dis-assemble
>all three key components, check everything again and then test -- I expect
>to discover my error during that endeavor! I am considering completely
>removing the tune and load caps and the B&W 852 tank coil and then
>re-connecting all on the bench, and then try the analyzer. I will do the
>fully assembled version first, since it is already there and ready.
>
>I am new to this list, and at least one of my posts seems to be missing,
>hope this one goes through OK.
>
>Thanks a million to all,
>
>Chuck
>
>
>
>-----Original Message-----
>From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
>Behalf Of STEVEN & NANCY FRAASCH
>Sent: Monday, July 16, 2007 7:06 AM
>To: amps@contesting.com
>Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
>
>I'd dispense with the grid dip and get an antenna analyzer. Terminate as I
>suggest (leave the tube in socket so that the tube parasitics are in place)
>and look at the output port with the analyzer. Don't forget to key the
>output T/R relay (amp power off of course).
>
>This method works fine in HF and beyond. I am doing 20 GHz work now at work
>and using the same method. The terminations are 0402 resistors, not leaded
>ones.
>
>Also, not terminating the output will shift the result significantly, that
>is why I prefer the terminated method with a grid dip and I'll take the
>broader measurement. But I don't use my grid dip any more. Pay to get an
>analyzer, it is well worth it.
>
>73,
>
>Steve, K0SF
>
>
>----- Original Message -----
>From: "Tom W8JI" <w8ji@w8ji.com>
>To: "STEVEN & NANCY FRAASCH" <sjfraasch@embarqmail.com>, "Chuck Curran"
><ccurran@wi.rr.com>
>Cc: amps@contesting.com
>Sent: Sunday, July 15, 2007 4:27:14 PM (GMT-0600) America/Chicago
>Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
>
>The general condition where a grid dip meter dips the
>deepest is with the highest possible Q, NOT when the tank is
>loaded with a resistance.
>
>If you are not getting a dip and you are using a real grid
>dip meter, it is because you are looking at the tank
>incorrectly or have something wired wrong.
>
>Think of the tank as a transmission line. A normal
>pi-network tank when terminated acts like a line section
>with an electrical length of 130 degrees or so. In fact a
>tank perfectly tuned on 7MHz for a 3000 ohm tube and 50 ohm
>load resonates when open circuited about 6.9 MHz, so you
>should see a dip near the operating frequency WITHOUT
>termination.
>
>Of course this is still largely a useless test, sine it does
>not mean the tank is working as a matching network.
>
>The most useful test is to terminate the tube anode with a
>resistor that looks like the operating load impedance you
>want (use short leads to the resistor). Then you look at the
>output port and look for a low SWR.
>
>
>I'm going to side with Ian on this one. The largest dip is
>ALWAYS when there is no resistance, and the dip would be
>very close to the working frequency. You will get the
>poorest dip when you follow the advice to terminate the
>tank.
>
>You have something else wrong, or have a bad dip meter.
>
>73 Tom
>
>
>
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