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T-Match vs L-Network

N

Naysayer

Well-Known Member
Mar 6, 2020
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New York
I've been using a Dentron T-Match that's inefficient and it's increasingly noticeable. It appears that an L-Network is more efficient and very do-able using the same Dentron case. So it prompts the question, 'Why Dentron or other manufacturers always made T-Matches instead of L's given the increased efficiency of the L-Match?' Is it component size? Is my idea doomed for having a case too small? 10-80 only and I need to use coax as I'm a city person (coax upgrade coming soon).
neil
 

T matches (being two L matches back to back with two components in the middle combined into one) offer a much wider range of impedances that can be matched compared to a single L match. The company clearly decided it wanted the extra tuning range of the T match.

At that point they had to decide on high pass or low pass. Low pass requires two inductors and those inductors need to have a good amount of adjustment, so to simplify the manufacture, they instead go with high pass as it only requires one inductor.

If you were to choose an L-match, would you go with high pass, or would you go with the low pass version that also helps remove harmonics? I will assume low pass. And in that case, the impedance you are matching to needs to be in this red area.

smith3.png


If you happen to have an impedance in the white area, you can reminisce about those good old T-tuner days as you add a quarter wavelength of coax to rotate the red area 180°
 
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That sums up my situation spot-on. Thank you for the added insights. I will replace my coax (>3 decades) and reassess the ATU afterwards. I find it hard to replace coax because it looks OK from the outside lol. HF is so quiet lately, low noise level just not hearing any activity.
 
Naysayer said:
That sums up my situation spot-on. Thank you for the added insights. I will replace my coax (>3 decades) and reassess the ATU afterwards. I find it hard to replace coax because it looks OK from the outside lol. HF is so quiet lately, low noise level just not hearing any activity.
Click to expand...
You could test your coax before replacing it. Good coax can last a long time if water don't get in it.

With a dummy load and SWR/power meter, you can find out how good it is. Start by measuring the SWR with the meter at the beginning of the coax and the dummy load at the end of the coax. If that checks out, write down the wattage then move the power meter to the end with the dummy load and see how much the power reading dropped. Checking SWR alone is not enough as loss can eat up a reflection before the SWR meter can measure it. Measuring the power loss lets you confirm with the cable specs and verifies the SWR reading were trustworthy.

If you have a nanoVNA, you can use the transform function in either step mode or impulse mode to look for impedance discontinuities along the coax. If you had an oscope and a fast schmidt trigger square wave generator or an avalanche transistor pulse generator, you could do the same. Deviations from 50Ω as the signal front propagates will cause reflections a fast enough scope will see and locate.

The nice thing about using TDR to test coax is that you don't have to climb the tower. You can leave it connected to the antenna.
 
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I removed the ATU after identifying bands each antenna works alone w/ analyzer. This was part of my effort to have amp work on antenna with cover attached instead of on bench into DL without any cover attached.

It's progress albeit baby steps as it's still into DL. I did not expect the cover to throw-off the Inductor settings despite it being a screen mesh. I managed to tune-up on 10 & 40.

I'm going to make an L-tuner when I find a suitable enclosure, I have most everything else for simple test setup. Any suggestions on what max values I can expect/aim for on inductor and cap?

I read somewhere that by reversing the input/output of an L Network it can better suit certain bands high/low. Well I'm off to scour flea bay to see what there it's been awhile.

neil, NYC
 

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After having studied many possible configurations, I explain why I chose a modified version of the FRI-Match for my homebrew antenna tuner.
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