What's easier to match, low R or low X?

[K6JEK]

What’s easier for a Johnson KW Matchbox, 51 +j464 or 15 +j0?

I want to feed my 75M dipole with 300 ohm window line and use it on 30M too. The usual rule of thumb, odd multiples of 1/8 wavelength at the lowest frequency, makes for a feed line that is not great for 30M. In particular, a 3/8 wavelength feed line for 3880 is a 351.4 degree feed line for 10.1 MHz. That isn’t much help for the 163 -j934 feed point impedance. It brings it almost back where it started.

So I’m looking at some other impedances that occur along the 180 degree length of the 300 ohm feed line thinking something else must be better. But what’s better? Those two I mentioned above are some interesting choices among many.  But what should I look for?

 And yes everything would work better with open wire at 600 ohms and in the end there will be some cut and try.

[W7TFO]

R is always easier to match than J, at least for me.

73DG

[Opcom]

The ancient 180L-3 collins automatic tuner uses a roller type RF transformer to match the R once the reactance is tuned out.

[K6JEK]

The ancient 180L-3 collins automatic tuner uses a roller type RF transformer to match the R once the reactance is tuned out.

I wasn't familiar with the 180L-3 so I Googled it. It's yet another of Collins' amazing electro-mechanical devices. It's a beautiful thing. Thanks for bringing it up.

[Opcom]

A good example of a manual tuner using the same concept is the BC-939 tuner for the T-368.

entire -35 manual, see page p.46
http://www.pestingers.net/pdfs/other-radios/military-manuals/tm11-809-35.pdf

anyway, roller inductor L5 is used as a variable autotransformer as stated. Its function in the tuner is called 'coupling' This method is apparently rare today, whether for good reason or bad I don't know.

The BC-729 tuner, used with the BC-610A through D transmitter, uses a variometer setup L5 to accomplish the same result.
The later BC-610E through I uses the BC-939 or B.

manual showing both tuners:
BC-610A through D, E:
 http://www.bunkerofdoom.com/lit/bc610/TM11-4057.pdf   page 69, 78



Back to the original question regarding the Matchbox specifically, I've no experience with that unit and shouldn't have gone much into this R-matching thing. I tend to agree with Dennis on that point of the R adjustment.

The Matchbox is quite different from the BC-939 and -729 in that although it does have a variable-ratio transformer made up by the link coupled coils circuit and the band switch, that same transformer is also part of the reactance tuning function, so they seem combined.

Is it true to say that the R adjustment and j tuning can not be individually addressed in the Matchbox? If that makes any sense. With any tuner there will be some loads that are difficult or impossible.

You mentioned 180 degree length of the 300 ohm feed line, and I remember someone who also had that happen, in his case making his tuner strain because of the high impedance.
What he did was to add enough feedline length to get away from that 180 degree point to where the tuner handled it more easily, running the 450 Ohm (in his case) ladder line in a sort of a big circle spaced on insulators on the side of the shack, and using a DPDT knife switch to short out the extra length except when needed for that particular band. The switch was where the ends of the ladder line circle met, so it was simple physically. The extra line was 'in' on 80M, and bypassed on all other bands.

[N5RLR]

I want to feed my 75M dipole with 300 ohm window line and use it on 30M too. The usual rule of thumb, odd multiples of 1/8 wavelength at the lowest frequency, makes for a feed line that is not great for 30M. In particular, a 3/8 wavelength feed line for 3880 is a 351.4 degree feed line for 10.1 MHz. That isn’t much help for the 163 -j934 feed point impedance. It brings it almost back where it started...

Add feedline?
 
Quick-&-dirty calculations here show that if the feedline* were taken out to 107' 2" (450 degrees, [1.25 or 5/4 wavelengths] at 10.100 MHz), 552 ohms would be shown at the radio end, from 163 at the antenna feedpoint.  Would the tuner be happier at this impedance?  The additional 23' 5-11/16" of window line could be switched in for 30m and bypassed everywhere else (a la what Patrick described, above).
 
* -- Assuming 88% Velocity Factor, and 83' 8-5/16" (3/8-wave, as stated) length for 3.880.
 
(Emphasis on "dirty," I'm up late, battling joint/back pain.  )

[ka1bwo]

What’s easier for a Johnson KW Matchbox, 51 +j464 or 15 +j0?

I want to feed my 75M dipole with 300 ohm window line and use it on 30M too. The usual rule of thumb, odd multiples of 1/8 wavelength at the lowest frequency, makes for a feed line that is not great for 30M. In particular, a 3/8 wavelength feed line for 3880 is a 351.4 degree feed line for 10.1 MHz. That isn’t much help for the 163 -j934 feed point impedance. It brings it almost back where it started.

So I’m looking at some other impedances that occur along the 180 degree length of the 300 ohm feed line thinking something else must be better. But what’s better? Those two I mentioned above are some interesting choices among many.  But what should I look for?

 And yes everything would work better with open wire at 600 ohms and in the end there will be some cut and try.

Jon,
Here is the range of impedances that the Johnson Matchbox will transpose to 50 ohms for 80, 40 and 20 meter bands. Hope this helps with your conclusion
Joe   

[K6JEK]

Half answering my own question, for T and L networks, not the Johnson Matchbox, low R is a killer. You're better off with higher R even if that means higher X. Here's a link to an article by the late, great, G3TXQ, mostly about balun misconceptions but look at the graph for losses vs R with various values of X. At 10 +j0 you lose 20% of your power. At 100 +j250 it drops to more like 5%. That's for a T network.  

http://www.karinya.net/g3txq/tuner_balun/

But the KW Matchbox is a different beast. Some testing shows it won't even match impedances below about 100 ohms on 75 meters. I'm pretty sure I've done that so I don't know what to make of the "Matchbox Shootout" which is a compilation of  ARRL tests put together by the not-late, still great DJ0IP. This shows no match below an impedance of 100, less than 10% loss for 100 - 400, 10% loss  for 800 - 3200.

https://www.dj0ip.de/antenna-matchboxes/matchbox-shoot-out/

Beautiful Smith charts, Joe.

[KD6VXI]

The big flashbox will go lower than a hundred ohms. I've used it on 75 meters with an Inverted V 36 ft at the apex.  I had about 70 feet of 400 ohm line in it.

It took a LOT of fiddling to get it to match it. And with 500 watts pep (it was a 100 watt carrier and 120 pct mod) I could easily arc the left side cap.

With a full size dipole I've used that same tuna with a 3cx3000, no problems.  I've //heard// guys have run 4k pep into them....

With a full size 160 dipole in a true V (however, the ground fell away VERY quickly on both sides) I was able to load 1.3kw pep with the 275 watt flashbox all bands.

I'd MUCH rather have a higher feed point R than X.  The low R really put a strain on a big Johnson matchbox.

--Shane
KD6VXI

[K6JEK]

The big flashbox will go lower than a hundred ohms. I've used it on 75 meters with an Inverted V 36 ft at the apex.  I had about 70 feet of 400 ohm line in it.

It took a LOT of fiddling to get it to match it. And with 500 watts pep (it was a 100 watt carrier and 120 pct mod) I could easily arc the left side cap.

With a full size dipole I've used that same tuna with a 3cx3000, no problems.  I've //heard// guys have run 4k pep into them....

With a full size 160 dipole in a true V (however, the ground fell away VERY quickly on both sides) I was able to load 1.3kw pep with the 275 watt flashbox all bands.

I'd MUCH rather have a higher feed point R than X.  The low R really put a strain on a big Johnson matchbox.

--Shane
KD6VXI

Thanks, Shane. I'm amazed by the things I don't know sometimes. Just today after being at this for decades did I learn this bit of wisdom.

[ka1bwo]

The big flashbox will go lower than a hundred ohms. I've used it on 75 meters with an Inverted V 36 ft at the apex.  I had about 70 feet of 400 ohm line in it.

It took a LOT of fiddling to get it to match it. And with 500 watts pep (it was a 100 watt carrier and 120 pct mod) I could easily arc the left side cap.

With a full size dipole I've used that same tuna with a 3cx3000, no problems.  I've //heard// guys have run 4k pep into them....

With a full size 160 dipole in a true V (however, the ground fell away VERY quickly on both sides) I was able to load 1.3kw pep with the 275 watt flashbox all bands.

I'd MUCH rather have a higher feed point R than X.  The low R really put a strain on a big Johnson matchbox.

--Shane
KD6VXI

Evaluating the Johnson KW matchbox for impedance range that can be transposed to 50 ohm for 80 meters.  The minimum real for 3.5Mhz: 47.7 ohms, for 4Mhz: 41.6 ohms

[ka1bwo]

Half answering my own question, for T and L networks, not the Johnson Matchbox, low R is a killer. You're better off with higher R even if that means higher X. Here's a link to an article by the late, great, G3TXQ, mostly about balun misconceptions but look at the graph for losses vs R with various values of X. At 10 +j0 you lose 20% of your power. At 100 +j250 it drops to more like 5%. That's for a T network. 

http://www.karinya.net/g3txq/tuner_balun/

But the KW Matchbox is a different beast. Some testing shows it won't even match impedances below about 100 ohms on 75 meters. I'm pretty sure I've done that so I don't know what to make of the "Matchbox Shootout" which is a compilation of  ARRL tests put together by the not-late, still great DJ0IP. This shows no match below an impedance of 100, less than 10% loss for 100 - 400, 10% loss  for 800 - 3200.

https://www.dj0ip.de/antenna-matchboxes/matchbox-shoot-out/

Beautiful Smith charts, Joe.

  Range of impedances that can be transformed to 50 ohms using a T network.
  Input/ output  caps max 250pf,  L max 30uh, Q=100
Typical range of Z  for a L networks   

[ka1bwo]

Half answering my own question, for T and L networks, not the Johnson Matchbox, low R is a killer. You're better off with higher R even if that means higher X. Here's a link to an article by the late, great, G3TXQ, mostly about balun misconceptions but look at the graph for losses vs R with various values of X. At 10 +j0 you lose 20% of your power. At 100 +j250 it drops to more like 5%. That's for a T network.  

http://www.karinya.net/g3txq/tuner_balun/

But the KW Matchbox is a different beast. Some testing shows it won't even match impedances below about 100 ohms on 75 meters. I'm pretty sure I've done that so I don't know what to make of the "Matchbox Shootout" which is a compilation of  ARRL tests put together by the not-late, still great DJ0IP. This shows no match below an impedance of 100, less than 10% loss for 100 - 400, 10% loss  for 800 - 3200.

https://www.dj0ip.de/antenna-matchboxes/matchbox-shoot-out/

Beautiful Smith charts, Joe.

L match low pass, L input