9:1 Balun (Dare I Ask)

[n3lrx]

I'm looking for some plans online to build a 2 Kw 9:1 balun. Although I can't use it now I figured I can at least do something constructive and put one together so that when I can use it I have it. But I can't find any online plans except for end fed long wires. Naturally I want to match 50 ohm coax to 450 ohm feedline.

My dipole is fed with 450 ohm (window wire) and I'd like to have a balun because I don't trust the one in my tuner to handle 300~375 watts of carrier and legal limit peaks. Besides I don't even know what the balun is in my tuner. I can't find any numbers in the manual. Even though it's a dual core and has balanced line connections I still don't trust it. Anyone know of a site that shows how to? Or should I invest in a book? Do I even need a 9:1? 450 / 50 = 9 So it was my assumption that a 9:1 was needed. I could be wrong and usually am so it won't be much new.

Thanks in advance.

[Steve - K4HX]

It's unlikely that the impedance at the end of your feed line will ever be 450 Ohms. So, you can go with a 1:1 or 4:1 balun. There a tons of good designs on the Web for those.

[flintstone mop]

4:1 balun is what is usually in the typical tuner for open ladder line.
So your gonna run full strap at your apartment........
Fred

[AB2EZ]

Randy

What Steve (K4HX) is saying is the following:

If you attach one end of a 450 ohm ladder line (or window line) to an antenna, whose driving point impedance is not 450 ohms (purely resistive), the impedance looking into the other end of the ladder line (where the tuner is) could be almost anything... depending upon: the length of the ladder line (in wavelengths at the operating frequency), and the actual driving point impedance of the antenna at that frequency. This is just the way the physics works out. If the tuner is designed to look into an unbalanced transmission line (e.g. coaxial cable), then using the unbalanced (SO-239) output connector of the tuner, with a balun added between the tuner and the balanced transmission line may be helpful (more below regarding practical problems)... but (depending upon the length of the ladder line, the frequency of operation, the driving point impedance of the antenna at that frequency, and the design of the tuner) a 4:1 balun, or a 1:1 balun may do a better job of helping the tuner produce a match than a 9:1 balun.

In general, using a tuner that was designed to work with an unbalanced load (e.g. designed to feed into a coaxial cable, with one side of the tuner's output connector connected to the tuner's chassis ground; and with the chassis ground connected to earth ground via a ground rod) with an antenna fed by a balanced 450 ohm transmission line (e.g. ladder line)... by including a balun at he point where the antenna plugs into the tuner... is not a good idea.

In general, the SWR on the ladder line will be very high at some (or all) frequencies of interest.

In the case of a transformer-type balun (i.e. a voltage balun):

The high SWR on the ladder line means that the voltage across the balun's antenna-facing winding (i.e. the winding that connects to the ladder line) may be much higher than it would be if the balun were looking into a 450 ohm impedance (or whatever impedance it was designed to look into). The nominal power rating of a balun (i.e. the maximum power handling capability listed in the balun's specification sheet) assumes that the balun is looking into the specific load impedance it was designed for (e.g. 450 ohms on one side, and 50 ohms on the other side, for a 9:1 balun; 300 ohms on one side, and 75 ohms on the other side for a typical 4:1 balun). At high transmitter output power levels (comparable to the nominal power rating of the balun), and in this type of high SWR situation, there are likely to be non-linearities introduced by the balun. That is, the magnetic field in the balun's core (which is proportional to the voltage across the balun's output winding) will be larger than the balun was designed to handle... leading to core saturation... that will generate harmonics... and possibly cause the balun to destroy itself due to overheating.

To use a 9:1 transformer-type balun (specifically designed to look into a 450 ohm impedance) in this high SWR situation, its nominal power rating will have to be significantly higher than the output power of the transmitter. More specifically, the balun's average power rating should be: the peak output power of the transmitter x the highest SWR that is anticipated will exist on the 450 ohm ladder line. For example, if the highest SWR that is anticipated to exist on the ladder line is 9:1, then, to accommodate a 1.5 kW output transmitter, the 9:1 balun should be rated at 1.5kW x 9 = 13.5kW.

If the balun was designed to look into a load impedance of Z ohms, that is not 450 ohms, and the ladder line has an impedance of 450 ohms, then you must further increase the average power rating of the balun by 450/Z. For example, if you use a 4:1 transformer-type balun designed to look into a 300 ohm line, then you need to further increase the average power rating of the balun by 450/300 = 1.5, in order to accommodate the larger line-to-line voltage that will appear across the output of the balun when it is connected to a 450 ohm impedance. In the example, using a 1.5kW peak power output transmitter in an application where the SWR on the 450 ohm ladder line could be as high as 9:1, and using a 4:1 balun designed to look into a 300 ohm impedance... you would require a balun whose average power rating is 1500 watts x 9 x 1.5 = 20.25kW

Similar, but different, arguments apply to the use of a choke (current) balun to feed a ladder line with a high SWR.

The principal purpose of using an antenna-facing balun in conjunction with an unbalanced tuner is to force the currents on the two ladder line wires to be equal and opposite (i.e. balanced, so there is no common mode current)... in order to minimize RF radiation from the transmission line (i.e. to prevent the transmission line from becoming part of the antenna). If the balun were working as an ideal balun, it would accomplish this purpose (provided there are no parasitic capacitance paths from either side of the balun to chassis ground or directly to earth ground). Connecting one side of the balanced transmission line to chassis ground at the output of a tuner would definitely unbalance the two currents that flow in the ladder line. However, there are tuner designs that accomplish the task of keeping the ladder line currents approximately balanced, without placing a balun on the antenna-facing side of the tuner. Such tuners include a pair of insulated (from chassis ground) output terminals, specifically for use with a balanced transmission line, but (again) they do not incorporate an antenna-facing balun at the output of the tuner.

If you look at the Palstar web site (they make good antenna tuners in my opinion), you will see that they do not include a balun on the antenna-facing side of the tuner (although they offer an antenna-facing current balun as an optional accessory for some of their unbalanced tuners... probably to satisfy market demand, whether using one is a good idea or not). Their other products include a 1:1 choke (current) balun, located on the transmitter-facing side of the tuning network, to provide better common mode isolation between the tuner and the coaxial cable leading to the transmitter.

As a separate matter, if you have an antenna that presents approximately a 50 ohm (resistive) load to the feedline, then including a 1:9 balun (rated conservatively for the power level you plan to use) between the 50 ohm antenna feed point and the 450 ohm transmission line (i.e. at the end of the feedline that connects to the antenna) is a good thing to do. Doing so (if the antenna really presents approximately a 50 ohm resistive load at the frequency of operation) will avoid a reflection at the point where the feedline connects to the antenna. However, it won't be very useful if the antenna does not present approximately a 50 ohm resistive load at the frequency of operation. Therefore it wouldn't be very useful to include a balun at the feed point to the antenna (might even be harmful in terms of the performance of the antenna system) if the antenna is something like a non-resonant doublet (i.e. each side is not approximately a quarter of a wavelength long at the frequency of operation) with a highly reactive component of input impedance.

Stu

[W1ITT]

Stu's comment about Palstar's use of a balun at the input is an adroit observation.  Baluns operate best at their design impedance...which generally involves a nonreactive load.  When you stick one on the output end of a tuner, you don't know what complex impedance it's going to see, and if you QSY it'll be something different again.
But, good engineering aside, many years ago I built the ARRL Handbook "Ultimate Transmatch"  It featured a nominal four-to-one balun on the output side.  There were three T-200-2 cores stacked.  I think the instruction called for 3M glass tape, and some sort of well insulated wire.  Not having any, I used black plastic tape over teflon plumber's tape.  For wire I used solid THHN.  I used that tuner, feeding an 80m dipole through about a quarter wave of real open wire line, and used it on all bands.  Yes, I smoked it (real smoke and small flames) a couple times running a 4-1000 on SSB at full strap, but mostly it ran reliably.  In the years long ago when K1RQG and I ran the Hosstraders net every Sunday afternoon on 75m for about 3 hours at a time, it was in the line.
If I were to use a balun on the output side for full strap AM, I'd probably go to three T-300-2 cores, use real 3M glass cloth tape, and better wire.  The Wireman sells much of this stuff.  Again, it's not the "best" engineering, but, based on experience, it will probably work just fine until you can build yourself a proper link coupled tuner.

[n3lrx]

Cool, thanks for the insight guys. Lot's of good information to ponder on.

I've got glass tape around here somewhere. It sat for years in one place because I had no use for it. Now that I need it I can't find it. Isn't that typical?  It's got to be around here somewhere. It can't have gone very far.

Thanks again, I learned something.

[W7TFO]

http://www.m0ukd.com/Magnetic_Long_Wire_UnUn/

73DG

[Jim, W5JO]

See if you can pick up the book by W2FMI,  It discusses what you want to know.  I have seen 6:1 and 9:1 baluns but they can be tricky.

[WD8KDG]

Addition to W1ITT's comments,

Love them or hate them, the "Ultimate Transmatch" worked. You can find it in a 1976 ARRL handbook. Each of the cores should be covered with Scotch 3M #27 glass tape. Then the cores are stacked and covered with another layer of said tape. The winding consists of 15 bifilar turns of #14 teflon covered wire. Built one in 1977, still going strong keeping a Junkston "500" happy.

A KW Junkston flashbox works great, but not the extreme matching capabilities of the above transmatch.

Craig,

[n3lrx]

Cool, in my search for 4:1 balun plans/kits I found a site that sells them for assembled for $55.

They've got a 2 Kw continuous, 10 Kw peak rating so that's more than my transmitter will ever put out so when the time arises I can just buy one. That's a lot cheaper than the 9:1 I thought I needed. That's the easy way out but I trust engineers design and assembly more than I trust myself at making one.

I just don't trust the balun in my tuner. It may very well be a 4:1 but It's probably rated for SSB/CW and not something with a 300 watts continuous carrier. The tuner itself is rated for 3 Kw but again, that probably means SSB/CW at 1.5~2 Kw PEP.

It says in the manual that it's still rated using the old transmitter input power ratings. so at 50% efficiency that would be 1.5 Kw. And since Class D is so efficient I'll probably never hit 3 Kw input and actually get more for less.

I am still surprised that I don't pop a breaker with the current I pull of one circuit. I have 2 complete computers including router, Vonage box, cable modem, speakers,  K7DYY transmitter. audio chain, and receiver all on the same line. There's a mess of power strips and extension cords under my desk. (Fire hazard waiting to happen I know.)

An no Fred I can't run a full gallon in the apartment. I'm having trouble not getting 'hot metal' even at the 60~70 watt tune setting. I've tried several antennas so far and each time I zap myself just by touching the transmitter. Way too much RF in the shack and the computers don't like it either, I get a nasty buzz from the speakers. I don't want to risk blowing up one or both of my LCD monitors or melting down a motherboard or power supply.

I draped wire all around my apartment the other day and it actually worked for receive (I could actually hear SW stations. But no hams.) So I got stupid and in my ultimate wisdom decided "Hey, let's see it this thing will load up" Bad move. My computers howled and I burned myself on the metal switch of the transmitter. And that was at 70 watt tune position. I can only imagine what 300+ watts would feel like. But, it didn't trip the SWR watchdog. The transmitter will shut down within a few milliseconds of the SWR is too high so it must have been close enough to tune in.

And thanks Jim, I'll check out the book. I would assume I can get it from amazon or the ARRL's bookstore. A book would be much better for me than reading it off a website.

Most hams must be color blind. Though they may very well be geniuses in their own right. The colors and background images don't do their site justice.. lol I think that somewhere hidden in part 97 that hams must have an atrocious website.  Mind you, I fall into that category myself without help.  I usually just use templates which are easy to modify once you get the basics in place, http://qsl.net/n3lrx was done with a template and help from some images found on the internet. I lost my password and can't get back in to edit it. Or I'll use portals or CMS and let someone else do the work.

Anyway, thanks guys for the invaluable input.

[Steve - K4HX]

Some info on baluns at the links below.

http://amfone.net/Amforum/index.php?topic=29558.msg229324#msg229324

http://amfone.net/Amforum/index.php?topic=12710.msg93883#msg93883

Plenty of reading in the two attached files also.

[Jim, W5JO]

Look for "Building and using Baluns and Ununs"  It is quite expensive but available throught Abe Books at
abebooks.com 

You may have to check all the listings to find one you may want to pay for including Amazon or any other book seller.

[AB2EZ]

Randy

See my earlier post regarding the need to use a balun whose average ("continuous") power rating is much larger than the peak power output of the transmitter.

Stu

Cool, in my search for 4:1 balun plans/kits I found a site that sells them for assembled for $55.

They've got a 2 Kw continuous, 10 Kw peak rating so that's more than my transmitter will ever put out so when the time arises I can just buy one. That's a lot cheaper than the 9:1 I thought I needed. That's the easy way out but I trust engineers design and assembly more than I trust myself at making one.

/quote]

[n3lrx]

Actually, I'll most likely only be running around 1 Kw PEP at the most. If I understand Bruce, K7DYY correctly 1.5 Kw PEP is the maximum for the transmitter and it would have to be running 400 watts carrier and close to clipping the final and causing nasty negative modulation. So I'll be running about 300~350 watts carrier at the most which should peak out at around 1 Kw PEP +/- a few watts.

So if I understand you correctly 1000 x 9 = 9000 that should still be under the peak limit of the fore mentioned 2 Kw continuous, 10 Kw PEP balun. Is that what you mean? I'm so confused. These things don't sink in like they used to. (I blame the meds for making me so dense but that's another story.)

These sorts of things is why I don't trust the internal tuner balun. I seriously doubt it can handle 350 Watts of carrier let alone the 1 Kw peaks. It might if I add about 2 or 3 more cores and re-wrap it with heavier wire, which would be about the maximum I can fit into the enclosure. I thought a wall mounted balun would be the best bet so I can run coax outside the house and window wire from the balun to the antenna.

I dug out the drawing from Frank, KA3AHE for his compact dipole and something along those lines is what I'll most likely be running if I can get a station set up at my moms. I've got a 160m dipole that I used to use for SWLing when I lived in Maryland, which is currently fed by window wire. I'll probably be chopping that down to a 75m dipole and folding it in half as the plans show. I'm pretty sure I can fit something like that from one end of the house to the big tree in the corner of the backyard. It's gonna take some work to convince my stepfather to let me do it though.

Time for sleep.. I've been up all night (again). Maybe this will sink in better after a few Z's.

[AB2EZ]

See my earlier post.

For this purpose, you should be comparing the peak transmitted power (1500 watts) to the average ("continuous") power rating of the balun.

To avoid overheating, the balun should have an average power rating that is at least equal to the average power output of the transmitter x the largest value of SWR you expect to have in the transmission line (at any frequency of operation) x the impedance of the balanced transmission line / the impedance the balun is designed to look into.

To avoid harmonic generation (a tougher requirement than avoiding overheating), due to saturation, it would be necessary for the average power rating of the balun to be greater than the peak power output of the transmitter x the largest value of SWR you expect to have on the transmission line x the impedance of the balanced transmission line / the impedance the balun is designed to look into.

For example: with a 1500W peak transmitter output power, a 4:1 balun designed to look into a 300 ohm impedance, a 450 ohm ladder line, and a maximum anticipated SWR on the 450 ohm ladder line of 9:1 ...  the 4:1 balun should have an average power rating of 1500 watts x 9 x 450/300 = 20.25kW

These tough requirements show why it is not a good idea to employ a balun on the antenna-facing side of a tuner (looking into a balanced line with a high SWR).

Stu

[n3lrx]

Grrr.. All this makes me want to just run coass and tune out the mismatch. But I don't even know if Franks design would work with unbalanced feedline and still be tuneable. God only know's how much it would cost me to build a 15 Kw balun to be on the safe side.

 <rant>
Can I do it without blowing something up is a better question. My luck it would be the transmitter that blows up and not the balun. Even though it's got an SWR watchdog that shuts the TX down on high SWR within a few milliseconds built in, it still scares me that it could fail and pop a final or two.

I really hate living life in a fog of anti-psychotic and anti-depressant medication but it's the only way life is tolerable for me. I can't live without modern chemistry, but I really hate sacrificing my understanding of technology. When most of what you've learned is corrupted data, and trying to learn more or the same over again is like talking to a brick wall it's very discouraging.

Since I had the real estate in Maryland and plenty of trees at my disposal and blanket permission from the landlord to 'do whatever you want with the property'  a real sweetheart deal. I was going to 'fold' the 160 M dipole and try a modified Bacon-Tenna. If it didn't work for 75/80 I had the land to make another for that purpose. But as usual, my life fell apart *again* so here I am.

Here that much acreage just isn't not possible. I'd be lucky if I can get an acre with some decent trees for a hefty price (if I could afford it). Being stuck in this apartment is even more depressing. Texas is looking like a better deal after all. At least I could get on the air.
</rant>

OK, I've vented. I feel better now.

[W3GMS]

Randy,

I think by now you have heard enough reasons to not use a balun on the output side of your tuner, especially when running high power.  So where to go from here?

You can build a twin synchronized rotary inductor tuner with a current balun on the input.  In that configuration the balun is always happy providing the tuner is adjusted properly. 

The second option would be to go to a more traditional tuner.  Link coupled no balun and the secondary coil having the capability of handling a series or parallel resonated configuration based on the reflected impedance that your dipole presents to the tuner. 

The third option is to go with resonated dipoles for the various bands you want to operate.  There are many techniques to make a short resonate antenna.  True the bandwidth does suffer and the losses tend to increase a bit but properly managed it gets you out of your other dilemma.

All of that has been discussed here and some searching will likely offer you some more idea's.  In addition, the "Antenna Handbook" and many other sources are available. 

If one solutions becomes to much of a pain, its time to move on to other concepts.   

Joe, W3GMS           

[W2VW]

Twin synchronized inductors are nice but not necessary. Save $ and construction labor. The drawback is more difficulty in initial tuning. Once the proper settings are found it's easy.

[Steve - K4HX]

More detailed info here.

http://audiosystemsgroup.com/RFI-Ham.pdf

http://audiosystemsgroup.com/CoaxChokesPPT.pdf

[aa5wg]

Hi Randy,

The link antenna coupler is a great way to go.

Chuck

[n3lrx]

Now why can't they make this for the legal limit+?
http://www.mfjenterprises.com/Product.php?productid=MFJ-974HB
It's compared to the Johnson spark box of yesteryear. I've got a Johnson Matchbox at my mothers house. t's in rough shape internally, and I was going to rebuild it. The plastic on the inductor is cracked and it flops around. But other than that it's in decent shape cosmetically. It would probably still work, the inductor doesn't flop enough to short it's just insecure.

[Jim, W5JO]

Now why can't they make this for the legal limit+?
http://www.mfjenterprises.com/Product.php?productid=MFJ-974HB

Is this what you mean?

http://www.mfjenterprises.com/Product.php?productid=MFJ-976

Wouldn't put 1 KW RF through it but for our "legal limit" it probably would do.

[n3lrx]

Whoa, that bad boy ain't cheap either.

[Steve - K4HX]

More on baluns.

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

I'd recommend repairing the Matchbox. With a little counseling, it will no longer be insecure.   

[Jim, W5JO]

Whoa, that bad boy ain't cheap either.

Nothing is any more, been to the grocery store lately?   

As far as I am concerned they could eliminate the darn cross needle wattmeter.  Those things are not accurate anyway and they are difficult for old eyes to see.