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Of SWR and Tuners




 
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Author Topic: Of SWR and Tuners  (Read 7870 times)
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Steve - WB3HUZ
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« on: December 16, 2007, 03:02:54 PM »

With all the talk last night of SWR and tuners, especially with some station's antennas icing up, and since Walt, W2DU has recently joined this forum, I thought it would be a good time to delve into this subject. In short, especially on the lower bands and with relatively short runs of coax and/or low-loss coax, SWR is generally not a big concern.

If you are running a rig that requires you to tune and load the final, just tune and load the final. Don't worry about the SWR. If you can tune and load your rig to the proper plate current value and no arcing occurs in the final tank, you are probably good to go - NO TUNER needed. Make an old buzzard transmission. If you notice little or no heating in the coax, you are most certainly good to go. Don't believe me? Read this chapter below from Walt's "Reflections II" book, especially section 7.5

http://www.w2du.com/Chapter07.pdf


I recommend you read the entire book, since it will put everything in context and make clear the basics.


My 80/75 meter antenna is a dipole, resonant around 3820 and fed with 100 feet of RG-8 coax. I have used it without any problems over the range of 3700 to 3900 kHz. I'm sure I could cover a larger frequency range, if desired.

Using GNEC to simulate the antenna and W9CFs transmission line applet to look at the feedline, I checked things at 3700 and 3900 kHz. Here's what I found.

 - 3700 kHz

Dipole feedpoint impedance: 80.1 - j54.042 (SWR: 2.57:1)
Impedance at the end of 100 feet of RG-8: 36.59 - j35.65 (SWR: 2.33:1)  * SWR is slightly lower than at the feedpoint due to loss in coax.
Loss in the coax: 0.5  dB   * Matched loss (with no SWR) is 0.36 dB, so only an additional 0.14 dB of loss is incurred due to the SWR.

 - 3900 kHz

Dipole feedpoint impedance: 94.183 + j34.495 (SWR: 2.22:1)
Impedance at the end of 100 feet of RG-8: 67.21 - j40.39 (SWR: 2.07:1)  * SWR is slightly lower than at the feedpoint due to loss in coax.
Loss in the coax: 0.44  dB   * Matched loss (with no SWR) is 0.37 dB, so only an additional 0.07 dB of loss is incurred due to the SWR.


Notice the additional loss in the feedline due to the SWR in both cases is a tenth of a dB or less! That would never be noticed on the air and will not cause heating of the coax. Putting a tuner in line would only produce additional loss.

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flintstone mop
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« Reply #1 on: December 16, 2007, 06:29:12 PM »

OK FB Steve,
Thanks
It's worth a try to tune and load the transmitter/linear and let 'er rip. It saves that extra step of tuning the tuner.For Ham use, there should be no danger of coax heating or major crap outs. Stations running a few thousand watts or 50KW definitely have to worry about excessive SWR.
I feel for the FM stations on a budget that do not have radoms or heaters on their ants with the freeze and ice we are experiencing now. We always lowered the RF out for those few critical hours to protect the feedline/antenna/transmitter
Fred
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Fred KC4MOP
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« Reply #2 on: December 16, 2007, 06:44:40 PM »

I was going to post a new topic but the subject here is about what I would have called it, although it address different issues.

There is an article in World Radio about the evils of the new tests for hams and specifically about Extra Class.

The gripe was that there was nothing in the exam about the importance of where the wattmeter/SWRmeter is located in the transmission line, i.e. before or after the tuner.

As the commercial Swr/tuners I am aware of have "antenna" terminals, one would be lead to believe that the tuner is always just preceding the antenna.  In my Heathkit tuner, the antenna is connected to its output also so the meter/bridge would always be on the transmitter side.

Am I missing something?

js

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KF1Z
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Are FETs supposed to glow like that?


« Reply #3 on: December 16, 2007, 07:10:49 PM »



The meter would naturally need to be between the transmitter and the 'tuner'... of course...



Not sure why anyone would complain that it isn't a test question though...

If it did need to be on a test, I would put it in the Technician pool, not the Extra!
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KA8WTK
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« Reply #4 on: December 16, 2007, 07:20:27 PM »

I don't think you are missing anything....

Xmitter ------> SWR Bridge ---------> Tuna -----------> Antenna

At least, this is what you might need to do with a new type solid state xcvr.

What Steve is saying is that with a tube final, that will properly dip and load into the antenna, there is no need for the tuner and the coax losses from the SWR don't amount to much. If it dips and loads, use it!
However, if you get any arcing or heating, you may need to resort to the tuna and SWR bridge.

Heck, half of these new rigs have auto-tuners built in. The new-fangled appliance operator doesn't know how to adjust a tube final or a tuner. He just pushes a button.
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Bill KA8WTK
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« Reply #5 on: December 16, 2007, 07:59:59 PM »

  '  Heck, half of these new rigs have auto-tuners built in. '

Smost of 'em  only  'tuna' up to a 3:1 SWR.  Me Icom706duce/ with the outboard tuna works ok to extend the mobile ant frequency range a bit. My icom 746 has a more robust built-in tuna, but its nothing to go hog wild over.  The hunk of B&W and breadslicer werks much better............   klc
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What? Me worry?
KA1ZGC
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« Reply #6 on: December 16, 2007, 09:43:00 PM »

I agree with you, Steve. I don't watch my VSWR, I watch my plate tank. The SWR bridge is out in the doghouse, not in the shack.

Where I employ a tuner is where I instinctively feel it does the most good: as a link-coupled, tunable, unbalanced-to-balanced impedance transformer. Since I'm converting the impedance of the unbalanced line from the shack (which is essentially constant if I pre-tune the transmitter to the dummy load) to the impedance of the balanced line to the antenna (which varies wildly from one band to another), that's where a link-coupled tuner makes the most sense. When I'm changing bands at the tuner is the only time I ever look at the SWR bridge, and even if I can only get it down between 1.5:1 and 2.5:1, that's good enough. The transmitter can take it from there with ease.

What I will watch on an icy night like this is my plate current. If I'm halfway across the band from where I set the antenna at tune-up time (say if I tuned for 3885 and I'm on 3705), sudden climate changes will yield sudden current changes. One night the 4-250 shot up from 350mA to 400mA in 20 seconds in that exact scenario during an ice storm. Mind you, I'm still using the 420 ohm stuff because I've been too lazy to build an open wire line, which is really to blame for that.

On the other hand, if I'm tuned at the antenna end for 3885 and I'm operating on 3885, the current usually doesn't budge one iota, no matter what the weather does.

If I don't see sudden current changes, I really don't care what the VSWR reads (and I'm not going to walk 80 feet through 18 inches of snow in the middle of the night to find out). If the plate tank hits resonance in the usual spot and stays there, then there's no impedance shift that I need to worry about. If it's a little funky, I'll lighten the loading as long as I'm not abusing the screen too much.

I haven't read Walt's work yet, but my gut has always told me pretty much what you just said (without the math, of course: my gut was never very good with math).

FWIW.

--Thom
Kilowatt Amplifier One Zero Grid Current
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W1UJR
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« Reply #7 on: December 17, 2007, 11:06:26 AM »

Good post Steve!

For it is on one of the subjects most misunderstood, I shudder at things I hear on the air. (and things I used to think!)
Crazy statements, made by well meaning hams, about SWR.
Our friend Walt has graciously shared a good bit of his vast knowledge on his website, and a great article on SWR, entitled "Too Low An SWR Can Kill You", can be found here --> http://w2du.com/Chapter01.pdf

Here is a great quote from Walt's book, sure to raise the rankles of SWR purists:
"From the viewpoint of amateur communications, it can be shown mathematically, and easily verified
in practice, that the difference in power transferred through any coaxial line with an SWR of 2 to 1 is
imperceptible compared to having a perfectly matched 1:1 termination. Furthermore, the coaxial feed lines
we use in the HF bands with an SWR of 3 or 4, and often as high as 5 to 1, have an equally imperceptible
difference at the receiving end. When feed-line attenuation is low, allowing such higher values of SWR
permits operating over reasonably wide frequency excursions from the self-resonant frequency of the
antenna with the imperceptible power loss just described, in spite of the prevalent impression to the contrary
."

Steve, you're on point, Walt's work in Reflections, I and II, is the Bible for this, and quite straightforward reading.
Took me a weekend, but aside from some of the more complex math, I breezed through the book, and I'm a public school grad.  Grin
The basic fact is simple, when employing a proper conjugate match, energy is not lost, it is reflected back up the antenna.

As you mentioned the beauty of a tube rig with its own adjustable output network, is that often the matching can be handled right in the rig itself.
Solid state rigs are a different story, and I use my Johnson Matchbox for these, 50 year old technology still works, the laws of nature have not changed in the last half century.

In fact, dollar for dollar, I find the Matchbox better than any tuner I've used. I have a Nye Viking MBVA, works but that is much more temperamental than the Johnson. And talk about construction, remove the 47 screws and take a look at the components in the KW Matchbox, makes the tuners from a popular company located in MS look like toys. For use on balanced line, yet again another old technology which still works very well, the Johnson is the way to go if one wants a commercial tuner. Of course, you can always home-brew your own tuner out of some copper tubing and a couple of air variables.

The only fault I have with the Matchbox is the lack of 160 meter coverage. A few years back, Bowie Bill WC3K, now a Silent Key, had an excellent write up in Electric Radio addressing this matter. I plan on installing Bowie Bill's Matchbox 160 meter mods on KW unit this Christmas vacation, and will report back the results.


-Bruce W1UJR


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K3ZS
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« Reply #8 on: December 17, 2007, 11:20:39 AM »

I got an autotuner when I purchased my first solid state transceiver a few years ago.   I mainly needed it to match into the grounded grid linear.   It had an untuned input,  and needed a tuner so that the transceiver would fully drive the linear.  That's about the only time I used it.
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k4kyv
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Don
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« Reply #9 on: December 17, 2007, 01:40:56 PM »

I have to watch SWR with the Gates BC1-T on 160.  It is almost as bad as a ricebox for not wanting to load into anything outside of 50-70 ohms.  It simply wouldn't load into my feedline running very far off the resonant frequency as tuned with the L-network at the base of the quarter wave vertical tower.  I have it set to 1:1 at 1900, and use it without  retuning, across the entire band.  At 1800 and 2000 the SWR in the 140' of feedline that runs from the transmitter to the matching network in the doghouse is about 2.5:1.  In over 25 years I had never worried about it. But for the Gates I had to build an extra outboard L-network between the transmitter and the feedline going out to the base of the tower, in order to make it take a load. A problem I later discovered the hard way was that if the feedline gets accidentally disconnected from the output of the L-network in the shack allowing it to run without a load, the rf circulating current between the Gates and the L-network soars high enough to burn out the thermocouple in a 0-5 amp rf ammeter.  I lost two very nice meters before I figured out what was going on. 
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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flintstone mop
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« Reply #10 on: January 04, 2008, 09:33:45 AM »

Back to the top with this one.My Titan linear does not like any far out mismatch. I tried again and the zorching sound inside the amp was not good. I even reduced the power down to 100 watts. SOME finals may have enough range to accept the way out loads.
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Fred KC4MOP
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« Reply #11 on: January 04, 2008, 10:37:47 AM »

Hi All,
I was reading in the JAN Q Street, "The Doctor Is In" and there was an interesting question about the numbers we see with + and - J. The "doctor" gave an answer that reminded me of my experience with my MFJ analyzer and the Xs reading. When I slowly move the frequency up the Xs increases. It's a + J and if the Xs goes down it's a minus J.
If it's minus,  that would mean it's capacitive reactance and I could add a variable coil in series to counteract the capacitive reactance. I wonder if that could be my solution??

My dipole is out of commission (normal Winter antenna job) and I'm trying to make the best with the vertical until I can afford $100 to replace (upgrade)the feedpoint and replace the ladder line. Somewhere in that system, the line is open. Since the rains have been frequent, the antenna is reading an open circuit. The ladder line has seen rough service from the Oxon Hill days rubbing and scraping on trees at various points of the line and has taken on water. It also has about 3 splices, so I'm gonna trash it and replace. I have a pully system on the dipole and the entire system can be lowered in 5 minutes for repairs.

Fred
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Fred KC4MOP
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« Reply #12 on: January 04, 2008, 11:25:41 AM »




I have to watch SWR with the Gates BC1-T on 160.  It is almost as bad as a ricebox for not wanting to load into anything outside of 50-70 ohms. 

Don...My Gates will load into a coax fed dipole that is resonant at 1945kcs.....  from 1885 to 2000 with only a slight tweak of the tuning knob .........I never have more than 20 watts reflected....  Dipole is a flat top @ 60 ft.   coax is RG-? military 50 ohm stuff with a solid #12 center conductor...I removed the "L" coil from the gates tank...  Steve
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flintstone mop
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« Reply #13 on: January 21, 2008, 10:32:22 AM »

It's Back
I must be missing something here tuning a linear into a high SWR load. I try to disregard the far out readings of the in-line wattmeter and try to obtain the original readings when the linear is tuned into the 50 ohm dummy. And all I get is flat topped audio and the the grid current meter has movement when I speak. The Titan zorches inside on the high SWR and the Ameritron just gives the above mentioned results. At least there is no zorching!!!
In a 50 ohm load the current meter shows little movement.
WAZ going on here?
I imagine a plate modulated Tx wouldn't care about the SWR, except Don's Gates....hi
I'm just a few days away, whether it's 5 F or 40 F outside, to get the dipole back on the air. Waiting for replacement open ladder line.
Next radio season I might motorize the tuning for the 160M vertical.
Thanks for reading again

Fred  Shocked
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Fred KC4MOP
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« Reply #14 on: January 21, 2008, 12:13:39 PM »

I dunno Phred. In my experience pi net output circuits just dont like hi swr. Swinging links could care less. I could load my HB rig into almost anything with the BUD swingin link, but I've never been very successful at loading a pi net into anything higher than a 2.5 to 1 or so swr.


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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #15 on: January 21, 2008, 12:38:04 PM »

Fred

A pi network is very good at some things... but it has its limits.

If the load at the antenna port is, for example, 100 ohms resistive, then by increasing the loading capacitance and readjusting the tuning capacitor, you can match it (assuming you have enough loading capacitance available), while still placing the correct, resistive r.f. load on the tube.

However, keep in mind that the objective is to place a resistive load on the tube itself, having the desired value (i.e., the correct r.f. load on the tube), e.g., 2500 ohms or whatever is the right value for the tube you are using and the way you are using it.

For some types of antenna loads (the impedance looking into the cable from the r.f. output connector of the amplifier), the pi-network may not be able to convert that load into a resistive load of the correct value (e.g., 2500 ohms or whatever).

If the impedance of the load on the tube is purely resistive, but too high a value... then you will see flat-topping of the output of the linear amplifier when you try to modulate the input to the amplifier. An example of how this might occur is if the resistive part of the antenna load were 10 ohms of less. Even if you were to tune out the reactive part of the antenna load, the pi network would probably not be able to match a 10 ohm resistive load. The transformation from the output to the input is (roughly) proportional to L x L /R, where L is the inductor in the pi network and R is the resistive load. If R is too low, then the load on the tube will be too high (non-intuitive... but that's the way the math comes out).

If the impedance of the load on the tube is not purely resistive, and also too high,  then the flat topping will occur at an r.f. output power that is lower than the r.f. output power you were able to obtain into a purely resistive dummy load.

If the impedance on the tube is too high, and also not purely resistive, you may find yourself turning up the input drive power to get the output power you want... and, as a result, causing grid current to increase (or start to flow, as the case may be).

To get the amplifier to put out the most power it can, with the best linearity, you should try to place a load at the antenna port that is not too reactive, and not too far off from 50 ohms (e.g., 100 ohms should be ok).

Therefore, you may need to use a good, high power tuner between the amplifier and the load... even though a tuner would not be necessary if the load were not so reactive.

Best regards
Stu

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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
Steve - WB3HUZ
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« Reply #16 on: January 21, 2008, 02:33:24 PM »

When you say "high SWR", just exactly what do you mean Fred. Give us some numbers.
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flintstone mop
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« Reply #17 on: January 21, 2008, 05:45:06 PM »

Ok Steve
It looks like the sweet spot for the vertical is 50 ohms impedance and 5 ohms of some sort of reactance Xs. 1.5:1 SWR at 3760. At 3885 the Z is 200 ohms and about 85 ohms of Xs and 5:1 SWR. I can't tell if the J is + or -.
There's supposed to be a trick to see the + or- J using the MoFo Junk 259 analyzer.
Both of my linears do not like a big mismatch past 3:1.
I'll have my dipole back together soon and this headache will go away.
Fred
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Fred KC4MOP
Steve - WB3HUZ
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« Reply #18 on: January 21, 2008, 08:58:27 PM »

OK, makes sense you may have some arcing. Most modern day amps have minimally spaced caps.
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flintstone mop
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« Reply #19 on: January 21, 2008, 09:03:59 PM »

Yea Steve,
Both have minimally spaced caps just enough to work into a 50 ohm load. A Ranger or Viking or other tube transmitters had heavier caps.
Fred
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Fred KC4MOP
N3DRB The Derb
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« Reply #20 on: January 21, 2008, 09:45:52 PM »

they should go buy specially made Russian Capacitors leftover from the glory days of the Soviet Union!  Smiley
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