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Author Topic: 2wl Balum  (Read 393 times)
N5WDD
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Posts: 23




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« on: March 04, 2007, 05:11:14 AM »

Thanks in advance for your help.

I have the space, supports and wire to put up a 2 wavelength 160m loop behind my house. I will be feeding it with ladder line from the antenna to a Balun under the eave of the house. From the Balun to the tuner in the shack I want to run a short (15') Coax.I want to use the antenna over all bands from 160m to 6m.

My question is this, where can I find a External Balun that will enable me to do this. I can find on several webb sites 80m to 6m Baluns and 160m to 10m Baluns but I am having a problem finding a 160m to 6m External balun for this use. Am I expecting to much, or am I not looking in the right locations. I may be thinking to big and need to scale down my ambitions for this antenna. but a 15 acre Hayfield and a roll of wire have just got me interested.

Any Help or Suggestions would be appreciated

N5WDD/AG

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N1BHH
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Posts: 7




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« Reply #1 on: March 04, 2007, 07:33:02 AM »

Does your tuner have a balun in it? You really don't need a balun if it has an internal balun. And if you feed with open wire or twinlead you really don't need one, just attach it to your balanced wire connections on the tuner. You will need a good station ground, which I would hope you already have. Good luck in your construction, I love building antennas.
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WA1RNE
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Posts: 824




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« Reply #2 on: March 04, 2007, 07:49:37 AM »


 Am I expecting to much, or am I not looking in the right locations.


 
 >> You're asking a lot from a loop and a single balun.
 
Assuming this is a horizontal loop and not knowing  the height above ground, it could work well on 160-40 but not with a 4:1 balun and an autotuner. The impedance presented at the balun will be very high and will likely be very difficult for the tuner to match. If you set up a link coupled balanced tuner and chuck the balun you will have a much easier time transforming the wide impedance swings to 50 ohms and the losses will be much lower - just by virtue of eliminating the balun and assuming the tuner is designed right.

 For 20-10, you will be much better off with a separate antenna, i.e. a tribander, quad, trap vertical and the multitude of high performance wire arrays that can be built for these bands which don't take up a lot of space, are easier to put up > a 1/2 wavelength above ground and for cheap $$.

 For 6, this antenna will be a big disappointment, and most "all band" baluns will not work well or at all on 50 Mhz. But given the size of 6 meter antennas - a dipole is only 9 feet long (round numbers)-  yagi's and quads are light weight and even small omnidirectional loops will work well.  Best of all, they are easy to build and something that you will be proud to own having built and adjusted it yourself.

 
 ....WA1RNE
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N5WDD
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Posts: 23




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« Reply #3 on: March 04, 2007, 08:05:17 AM »

This is a 2 Wavelength 160m Horizontal Loop, I have wanted to try a big loop and now is my chance, It will vary from 20 to 30 feet above ground and that is about the highest I can do. I understand that the loop may not be effective for DX on 160, but from my research I understand that the higher bands show some promise for low angle effectiveness. I have on hand everything needed except a external Balun to put up the loop and wanted to give it a go. and see how it worked. It may not be possible but I want to see what a loop this large could do. I may be expecting too much from this loop but I wanted some comment, advise before beginning construction.

N5WDD
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W5JI
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Posts: 146




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« Reply #4 on: March 04, 2007, 08:15:16 AM »

You will enjoy the loop. They are great, general purpose antennas and work well for locals and DX. Feeding it with ladder line is the best solution, but using a balun will cause several problems.

A balun is designed to step up or down in impedance as well as convert from balanced to unbalanced. Not sure which ladder line you will be using, but if it is 450-ohm the balun will need to be a 9:1, not a 4:1 or 1:1, in order to match 50-ohm coax to the 450-ohm ladder line. I do not know of any high power 9:1 baluns being offered commercially. You would need to wind your own. The reason you are not finding commercial baluns which are speced from 160m to 6m is that the toroid cores used do not cover this wide a frequency range. You could build a pair of baluns which would overlap and cover this range, but you would need to switch them somewhere in the range of 15 meters.

The main problem is that the impedance at the fed end of your ladder line will not be anywhere near 450-ohms on most ham band frequencies. It will vary from very low (10-ohms or so) to very high (4000-ohms or so) depending upon the frequency in use and the length of the ladder line. There is no way to insert a balun at this point to match the ladder line to coax. You can do it, but the stress on the balun will be very high and you will (may) experience balun failure unless the balun power capability is several times the power level you are running. In addition, the mismatch to the coax may result in your tuner struggling to find a match.

Yes, I know people run this setup all the time. Many have reasonable luck doing so, but only if they are running barefoot. The majority of tuners I see being sold at hamfests and on eBay have bad baluns from this type of operation.

If you really want to take advantage of the loop, you need to feed it with a balanced tuner such as Palstar, MFJ and EF Johnson make (or made). These tuners feed the ladder line directly and avoid the problem of baluns. They also will be more efficient in getting your RF into the ladder line and antenna. If you do not want to bring the ladder line into your shack, you can use two sections of coax in parallel to feed from your tuner output to the ladder line. The coax shields should be connected together at the tuner end and connected to the tuner ground. The coax section center conductors are connected to the ladder line. The shields should be left open at the ladder line end. You can run this coax section through walls and/or through conduit to get it into your shack without disturbing the balance of the line.

Purists will complain that the coax section will be about 100-ohms impedance and will not match the ladder line. They will complain that you will have higher losses in the coax section than if you brought the ladder line directly into the shack. True as these complaints may be, as long as the coax section is RG-8 or RG-213, and not more than 20 to 30-feet in length, you will have no real problems. I've run this configuration for years with legal limit power with no problems.

The key is the use of a balanced tuner as opposed to a tuner with a balun at the output. Without this, you will experience reasonable operation on some bands and poor (or no) operation on others.

One other issue, you want to avoid feed line lengths which would be resonant (or close to resonant) at your frequency of operation. The ARRL Antenna Handbooks list these lengths in the sections discussing guy wire lengths to avoid when guying a tower. For instance, the standard 100-foot roll of ladder line will cause all kinds of problems. This issue can be per hard to accomodate on the higher frequency bands such as 6m. One way around this issue is to have the capabilty of switching in extra length(s) of ladder line when the situation occurs.

Please do not let all the above deter you from the loop. It is all quite easy to accomodate and is very worthwhile to gain the advantages of the loop antenna.

73.....Jim  W5JI
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N4KC
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Posts: 287


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« Reply #5 on: March 04, 2007, 11:01:53 AM »

W5JI stole my thunder!  N5WDD, I envy you that hayfield and roll of wire.  Construct the loop!  You will not be sorry.  

I use a setup similar to the one W5JI describes, including the parallel coax cables to get the feedline inside the basement and to the shack alongside other coax, air conditioner ducts, and the like.  I do use an auto-tuner with an internal balun but agree that is not the best setup.  Still, it works well and, with only 100 watts, I see no heating going on at all with the balun.  I have a rough diagram of my loop on the website below.

Compared to my G5RV and a single-band 1/4-wave vertical with a great radial field beneath it, the loop works better 90% of the time.  I do have a problem tuning it on the lower 500 kHz of 160 and across about 1 mHz of 10.  It shines everwhere else!  I have not tried it on 6 but doubt it would be very effective there.  Mine is from 15 to 35 feet above ground and runs through tree limbs in spots (insulated wire until the limbs wear it off!).  It is also not even a full wavelength for 160.  I only had wire enough and convenient trees for about 470 feet, but I have been delighted with its performance.  I can reliably work out only about 500 miles at night on 160, but I do sometimes get the Europeans to hear me on 75 and 80, and get to South America often there.  Remember, that's with 100 watts.  It's not totally a cloud-burner!  It is absolutely fantastic on 30, 17 and 15...maybe on 12, too, if it ever comes back.  It loads fine there.

Anyway, with trees in the right spot, this is not an especially difficult antenna to erect, and it will work nicely for you.  Compromise where you must, but get the dang thing in the air.  Good luck!

Don N4KC
www.donkeith.com/n4kc.htm
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N5WDD
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Posts: 23




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« Reply #6 on: March 04, 2007, 01:38:33 PM »

Thanks Don you and W5JI for providing me with information that I really need. This information will make what I want to do a real antenna. I like your website, anyway thanks for the information this should help me get some wire up in a functional manner

Steve N5WDD
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N4KC
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Posts: 287


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« Reply #7 on: March 04, 2007, 03:45:20 PM »

I just added a few things to the web site that might help you at:

www.donkeith.com/skywire.htm

Drop me an email or look for me on the air when you get that beast in the air.  I'd love to hear what it sounds like!

Don N4KC
www.donkeith.com/n4kc.htm
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W3SCM
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Posts: 15




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« Reply #8 on: March 05, 2007, 08:16:57 AM »

I, too, am looking at a longer-than-full-wave loop at 160.  I have the wire, 600-ohn ladder feed line and balanced tuner but I was wondering if it was going to be worth it to string it through all the trees (I have nothing but!) and at heights that probably won't exceed 50 feet.  You've made me think this will work after all.

Be nice to have one of those personal jet-packs for about an hour with a site like mine!  Otherwise, I am going to have to shoot it through one tree after another.

Nice website, Don.  I will look for your books, as well.
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N4KC
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Posts: 287


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« Reply #9 on: March 05, 2007, 11:47:03 AM »

Scott:

I'd think it would make a fine antenna at 50 feet for most bands.  Trees are the problem...they seem to either be in the wrong place or not in the best place.  I doubt it's a big problem if the wire is insulated, other than the stress on the antenna when the wind blows.  If it is not insulated, it would probably be a deal-breaker, and especially if you are planning on running power.

Same offer: if you get it up there in the sky (or even if you don't), email me or look for me on the air and let's compare notes.

73,

Don N4KC
www.donkeith.com/n4kc.htm
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