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L matching network




 
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WA2SQQ
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« on: December 02, 2017, 09:52:28 PM »

For a simple L matching network I’m going to use for a 160m inverted L, is there any difference if I feed the RF into the inductor, with the capacitor after the inductor to ground, vs. feeding the capacitor with the inductor to ground?

Also, to handle 1kw, what would the recommended voltage rating be for the capacitor?
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N1BCG
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« Reply #1 on: December 02, 2017, 11:20:04 PM »

It depends on the impedance of the antenna at the feedpoint and that will determine whether the component to ground is at the xmtr or antenna feedpoint. Once you know the impedance you can calculate the voltage and current throughout the circuit to determine component values.

Also consider that the series inductor with shunt capacitor to ground will act like a low-pass filter and have better harmonic filtering characteristics than if the components were swapped.

The short answer would be: "depends"
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K1ZJH
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« Reply #2 on: December 03, 2017, 11:42:05 AM »

Do you have an antenna analyzer?  What is your feedpoint impedance? If low, you can lengthen the antenna and use a series cap to increase the impedance at the base. That will also elevate the high current point on the vertical portion of the L antenna. Inductors are in general more lossy than a good quality cap.

http://www.eham.net/ehamforum/smf/index.php?topic=111769.0

Pete
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WA2SQQ
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« Reply #3 on: December 03, 2017, 08:41:14 PM »

Thanks - the project begins next weekend. I’ll have some numbers to work with.
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W3RSW
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Rick & "Roosevelt"


« Reply #4 on: December 03, 2017, 09:27:58 PM »

Capacitor shunted to ground after the coil feeds a higher impedance antenna than the radio's 50 ohm output.

Capacitor shunted to ground before the coil feeds a lower impedance antenna.

Typically a L network tuner will have a switchable stator on the capacitor from
both the input side and the output side, the rotor of the cap being grounded.
Be sure the switch can carry high current and voltage and is not far from the
input and output terminals with short as leads as possible to the cap.

As an example if the antenna is a little shorter than natural resonance, a few turns of the coil will load it with the capacitor cancelling additional reactance.  
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RICK  *W3RSW*
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« Reply #5 on: December 03, 2017, 11:03:47 PM »

Well said, Rick -

One potential problem with adjusting an L network... What if the antenna is ~near 50 ohms and the transmitter is ~near fixed 50 ohms but we still want the match tuned to 1:1?

Tapping the cap on one side of the coil or the other may not do the trick. (pi)    IE, a close 50 ohm to 50 ohm match is sometimes hard to tune/match.  I've run across this problem when driving a ~50 ohm linear tube input with another 50 ohm amplifier in cascade.  The solution was to hang the coil from the input to ground (T-match) and put the caps in series on each side. There is always a sweet spot that gives a 1:1.

As for in-field antenna tuning, my favorite method is to get out the MFJ-259B ant analyzer with the coil and capacitor and a pile of clip leads. Play around with positioning and coil taps until 1:1 is found. I would favor the cap going to ground to help reduce harmonics, (pi) but as said, sometimes a T-match is the only one that will work, but has less harmonic suppression since the caps are in series.  In the case of a 50 ohm to 50 ohm  antenna situation, it would equate to a vertical radiator of slightly longer than 1/4 wave to produce 50 ohms. So, it might be wiser to purposely make the antenna longer than 1/4 wave  (>50 ohms) to be able to use a FB pi match using a higher impedance feed.

T
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W3RSW
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Rick & "Roosevelt"


« Reply #6 on: December 04, 2017, 10:32:14 AM »

Your close case sure does happen, especially if the antenna is cut to Natural resonance right in the middle of the band.  If close to 1:1 then simply add a tuner bypass circuit. This usually calls for a double deck or similar switch though.  You don't need the tuner for several KHz perhaps around that node.  Grin.

When adding all that wiring and new switch contacts, still be sure to keep stray capacity as low as possible or 10 meters, etc. may be only close to match with main cap set fully unmeshed.  Switch may have to be at rear of cabinet next to in/out connectors with shaft extension to front of cabinet to achieve this.

I've seen reactance approach infinity / zero on one side of the 1:1 point, then wildly swing to opposite infinity / zero on the other side.  When I first encountered this I added parallel capacitance up the woz as I approached that node before realizing what was happening.

Don't ask what 'infinity / zero' is; you just have to experience it. Convient way to describe a wild unknown reactance that won't behave.  Grin
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RICK  *W3RSW*
WA2SQQ
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« Reply #7 on: December 04, 2017, 11:16:19 AM »

So to answer the question, yes I have an analyzer. Am I capable of using it to its full potential – no.

I’m not embarrassed to say that smith charts and complex impedances have always been my black hole. I’m sure it can be simple once you find the explanation that flips the switch in your brain and makes the light go on. To get around this Inverted L project I decided to make a simple L network, measure the values that give me the match I need, and substitute fixed values at the feed point. From the info you’ve all shared I just need to experiment to see what configuration I need. I wish I could look at the complex impedance and know exactly what I need. I appreciate the explanations which I have learned from. The only stupid questions are those we don’t ask!
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KA2DZT
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« Reply #8 on: December 04, 2017, 11:33:38 AM »

Rick

Didn't your teacher ever tell you that you can't divide by zero?? Grin

Don't try to figure out Smith Charts, you'll just be going around in circles. Sad

Fred
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W3RSW
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Rick & "Roosevelt"


« Reply #9 on: December 04, 2017, 11:46:43 AM »

Nor can infinity plus, or times, or divided by, or integrated by, or differentiated by anything still equal anything but infinity. That's the beauty of a very powerful concept.

If your divide by zero observation is directed,  the "/" symbol didn't mean divide by, but meant a coupling of concepts, e.g., I'm a Jack of all trades, fisherman/astronomer/amateur radio operator ...etc.  Grin
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RICK  *W3RSW*
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« Reply #10 on: December 04, 2017, 11:52:36 AM »

From the info you’ve all shared I just need to experiment to see what configuration I need.

That's by far the most fun and educational approach!  Start with an "L match" of any configuration. Either choose a frequency on the analyzer and experiment with the matching components or sweep your matcher by changing the analyzer frequency to see where you get signs of resonance. You can then make component adjustments to improve the match or "walk" the frequency to where you want.

This is a magnificent way to see how C and L affects your match at various frequencies!
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K1JJ
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« Reply #11 on: December 04, 2017, 02:01:09 PM »

Your close case sure does happen, especially if the antenna is cut to Natural resonance right in the middle of the band.  If close to 1:1 then simply add a tuner bypass circuit. This usually calls for a double deck or similar switch though.  You don't need the tuner for several KHz perhaps around that node.  Grin.

Yep, a 3-position tuner selection circuit is exactly what I had to do with my Class A linear amplifier chain.  I found that unless the 50 ohm to 50 ohm driver to amplifier match was near perfect, I would lose the fine-tuned 10dB extra IMD bonus.  I could see the IMD swing from -45dB to -55 dB 3rd just by fine tuning. The problem was that it required a T-match on some bands and a Pi-L match on other bands to be optimum.  

So I now have a switch on the big amp that reads "Pi"  or "T-Match" or "Bypass."   It wasn't that hard to figure out the routing to get there.

Hey, we hams will do anything to chase down perfection, right?

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
WD8BIL
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« Reply #12 on: December 04, 2017, 05:19:58 PM »

For most inverted L's we homeowners deal with the feed point is usually a low résistance, as with all short verticals, with some ungodly reactance, usually capacitive.

So an L network, and it has been true more times than not here, will most likely be a series inductor with the shunt cap on the radio (higher impedence) side.

The shorter the verical section... the lower the feed point resistance.

Dowload the EZNec demo program. It is very limited but simple antennas can be worked out on it.

https://www.eznec.com/demoinfo.htm






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Steve - K4HX
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« Reply #13 on: December 04, 2017, 08:30:51 PM »

If you can make your Inverted-L on the order of 5/16 to 3/8 wavelength, all you will need is a cap.
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KB2WIG
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« Reply #14 on: December 04, 2017, 10:04:38 PM »



I cut mine to at 164 feet. I'll let ya know how she werks when the feed line gets out there.

Any one have  hints how to stop the goo that's leaking out of my orange Commscope rg 11?? It looks messy, and how  am I gonna stuff it into a ' pl 259??


klc
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Making Amplitude Modulation GREAT Again!


« Reply #15 on: December 04, 2017, 10:55:15 PM »

That's flooded coax, if it's what I'm visualizing.

You cut a good 8 inches to a foot off, and use a compression connector right then and there!

Then, use an F to so239 or PL259.

If the critters get to that outer pvc cover, the oozing will keep it from having water ingress.

--Shane
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WD8BIL
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« Reply #16 on: December 05, 2017, 02:35:36 PM »

Quote
If you can make your Inverted-L on the order of 5/16 to 3/8 wavelength, all you will need is a cap.

Good point Steve! I was assuming 1/4 wave!
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NA3CW
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« Reply #17 on: December 05, 2017, 03:34:23 PM »

A fun and painless way to get familiar with the Smith chart is a software package cleverly named "Smith" by Fritz Dellsperger:

(http://www.fritz.dellsperger.net/smith.html)

The demo version is free.  It's limited to 5 elements and the resulting file can't be saved but you're only looking at 2 elements and a couple of numbers. 

Using your analyzer get the starting R and jX numbers to insert them and the frequency of interest into the program.  From there you click on a shunt cap and a series inductor to try to get to the center of the chart which is 50 ohm resonance.  Don't worry-- you'll figure it out.  It's like a video game.  You can also insert sections of feedline and play with other matching component combinations.  It's easier than real L's and C's for initial experiments.  Then when you move into hardware you will have a good starting point. 

My two cents,
Chuck
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KB2WIG
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« Reply #18 on: December 05, 2017, 07:24:05 PM »



S,

Yes it's flooded cable. I asked the cable guy what he did with the 'ends' , as I had a 300 foot run to the pole. He said, " I throw it in my truck, and ride around with it until I need some." So  he left the end, and some 75 ohm stuff he pulled out of a house with me. ( the 'house' cable has a CC steel center conductor, but I don't mind - its free).   I'll probably use the Orange stuff ( I live near Syracuze) to feed the eL until next spring.... Depending on if this location works out... I was going to put it out back, but I didn't get started clearing brush until late, and It looks like I'll need the chain saw, so I'll try it by my driveway.

I have a  nice Rushkin vac variable, and we'll see what happens this Friday.

The Smith Chart. I still have a few sheets of it lying around, but it brings back bad memories and desperation from skool.


klc
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WA2SQQ
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« Reply #19 on: December 06, 2017, 11:10:28 AM »

A fun and painless way to get familiar with the Smith chart is a software package cleverly named "Smith" by Fritz Dellsperger:

(http://www.fritz.dellsperger.net/smith.html)

My two cents,
Chuck
NA3CW

Thanks, I'll check it out
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DMOD
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« Reply #20 on: December 06, 2017, 01:28:25 PM »

Here are some MatLab calcs for an L Net (Cshunt at source - Lseries to Antenna ) in which

Rsource = 50 ohms

Rant = 15 ohms

Total Ant capacitance of 35 pf (- j2.42k)

Zant = (15 - j2.42k)

Fc = 1.88MHZ

CShuntSource = 260 pF

Lseries = 207 uH


Q = 1.5275

Notice the Q.

Highly recommend a "T" match where you can control the Q.


Phil - AC0OB
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