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Author Topic: Horizontal Loop Antennas  (Read 4697 times)
KL3HY
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Posts: 117




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« on: August 15, 2011, 02:13:36 PM »

I just got passed my general class exam, so I'm trying to plan my first HF antenna.  Based on what I've read about it through searches of these forums as well as lots of Google searches, I'm looking into a horizontal loop, sized for 80m.  I've also seen this type of antenna referred to as a German quad.  A lot of what I'm seeing is causing some confusion though.

One thing is that some people recommend using nothing but ladder line for the feed line, all the way to the antenna tuner.  How would you do that if the only antenna connector on the back of the tuner is an SO-239?

Also, I've read that the ladder line radiates RF and essentially is part of the antenna--is that correct?  I've seen lots of conflicting info on this one, so I'm pretty confused about it.

If I were going to build it today I'd run the shortest length of 75 Ohm coax I could get away with to a balun, and then 450 Ohm ladder line to the feed point of the loop.  Am I way off on this plan?

Thanks,
Mike
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WB2WIK
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Posts: 20603




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« Reply #1 on: August 15, 2011, 02:35:40 PM »

I just got passed my general class exam, so I'm trying to plan my first HF antenna.  Based on what I've read about it through searches of these forums as well as lots of Google searches, I'm looking into a horizontal loop, sized for 80m.  I've also seen this type of antenna referred to as a German quad.  A lot of what I'm seeing is causing some confusion though.

One thing is that some people recommend using nothing but ladder line for the feed line, all the way to the antenna tuner.  How would you do that if the only antenna connector on the back of the tuner is an SO-239?

That likely means the tuner does not have an internal balun and is set up for unbalanced loads only.  You could step up to a better tuner, or add a balun outboard to the tuner, preferably very close to it, for the balanced line connection.

Quote
Also, I've read that the ladder line radiates RF and essentially is part of the antenna--is that correct?  I've seen lots of conflicting info on this one, so I'm pretty confused about it.

I'd vote on the other side of this.  Ladder line normally hardly radiates at all; the line is "balanced" and the radiated field outside the line is pretty small.  However, you can make coax "not radiate" at all! -- which you can never fully do with ladder line.  Normally, it's not a big deal.

Quote
If I were going to build it today I'd run the shortest length of 75 Ohm coax I could get away with to a balun, and then 450 Ohm ladder line to the feed point of the loop.  Am I way off on this plan?

No, that sounds right.  However if you build a resonant 80m loop, e.g., 285' in perimeter or circumference, and it's laid out well, it will also be resonant and an easy match for coaxial cable on 40m, 20m, 15m and 10m, and "almost" resonant (not quite) on 30m, 17m and 12m.  As such, I've found just feeding such a loop with 75 Ohm coax often works just fine without any ladder line or baluns, and sometimes even without a tuner!  The big advantage of the balanced line is when you feed something that's a very poor match, especially if the load impedance is way on the high side of 50 or 75 Ohms.  But a full-wave loop shouldn't be.  It's normally about 100 Ohms on the fundamental and all harmonics, and is a better match to coax than it is to ladder line.

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K9IUQ
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« Reply #2 on: August 15, 2011, 06:54:35 PM »

It's normally about 100 Ohms on the fundamental and all harmonics, and is a better match to coax than it is to ladder line.

I have a 3 side (delta) horizontal 277 ft 80 mtr loop. When I first put it up 4 yrs ago I fed it with coax. It was ok on 80m but SWR was not very good on 40mtrs or 30 mtrs. I took the coax down and installed 600 ohm openwire feedline. It made all my problems go away.

I use this antenna on 80 and 30 mtrs. My 40 mtr 1/4 vertical outperforms it on 40m. It will work all bands 80-10m.

To Mike:
The horizontal loop is a difficult antenna to put up. Since this is your first antenna I suggest a 130 ft doublet fed with open wire feedline. This is a much easier antenna to install and will perform well.

Here is another tip that will serve you well. Do not believe everything you read about antennas on the internet. Much of the info is false. Instead buy an antenna book by a reputable publisher like the ARRL.

Stan K9IUQ
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KL3HY
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Posts: 117




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« Reply #3 on: August 17, 2011, 09:19:07 AM »

Thanks guys, I appreciate the feedback.

I'll have to put some more thought into this.  The difficulty of the antenna doesn't really discourage me much, but it does mean more planning effort, and I might put it off until the spring.  I'll probably go ahead and do it though. 

One other question that comes to mind--is there a risk of arcing to nearby objects (trees, etc) when transmitting at 100 watts or less?  I've read that a dipole can sometimes arc from the ends of the wires, but since the loop is a DC short (dunno if that's the correct terminology), my guess is that risk of arcing is almost nothing.  Would that be correct?

Thanks again,
Mike
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K9IUQ
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Posts: 1832




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« Reply #4 on: August 17, 2011, 09:50:29 AM »

One other question that comes to mind--is there a risk of arcing to nearby objects (trees, etc) when transmitting at 100 watts or less?

I have always used insulated 12-14ga stranded wire available at your favorite Home Depot/Lowes.. it will stretch a little the first year but it does not matter when using open wire feed.

I have run many wire antennas thru trees with insulated wire.

I run legal limit when required, my antennas do not arc.

Stan K9IUQ
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KL3HY
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Posts: 117




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« Reply #5 on: August 17, 2011, 10:22:00 AM »

Thanks Stan.  I apologize--I should've mentioned that I have over a thousand feet of uninsulated aluminum wire, so I was thinking in terms of that.  I can certainly buy some insulated wire, but I since I already have the uninsulated wire I was hoping to be able to make use of that.
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K9IUQ
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« Reply #6 on: August 17, 2011, 11:23:57 AM »

I have over a thousand feet of uninsulated aluminum wire, so I was thinking in terms of that.  I can certainly buy some insulated wire, but I since I already have the uninsulated wire I was hoping to be able to make use of that.

Not sure I would use aluminum wire as it will probably break swaying around in the wind especially if it is solid wire. Uninsulated wire will arc if you get it close to conducting obstacles including trees. Arcing is detrimental to your radio.  Wink

Since the loop takes some effort to get installed properly, I would spend the $$ and get copper stranded insulated wire.

Stan K9IUQ

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W4VR
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« Reply #7 on: August 17, 2011, 12:19:49 PM »

You could use a remote 1:1 balun to the open-wire line but keep the coax as short as possible.  If you are running low power you should not experience arcing over the tree branches, but it's best to use insulated wire for this application.  I assume you intend to use this antenna as an all-band antenna, so the high voltage points on the wire will vary from band to band.
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AE5JU
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Posts: 229




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« Reply #8 on: August 17, 2011, 09:24:50 PM »

Author and ham Don Keith N4KC ("The Antenna Party") has some great articles on his website,

http://www.donkeith.com/n4kc/article.php?p=12       "Skywire Horizontal Loop Antennas"

and

http://www.donkeith.com/n4kc/article.php?p=13       "Feed the Beast - Transferring Radio Frequency Energy from Your Transmitter to Your Antenna"

that pretty much cover the subject.

73,
Paul - AE5JU
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W3HKK
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Posts: 596




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« Reply #9 on: August 18, 2011, 05:11:08 AM »

Horizontal loops are fun,  and do a great job for rag chewing NVIS contacts within 1000-1500 miles .  Getting them up as high as possible  makes a significant difference but they work pretty well at 15-25 ft if thats the best you can do.

The classic 80 or 160 m loop  up at 45 ft or so  is a terrific radiator on all bands that are multiples of the fundamental freq.  ie  160-80-40-20-15-10.    SWR is  in the 2:1 range on most of those bands so a tuner helps the rig. 

The weight of the wire is a factor but being wire, if it falls down its usually not a big deal to shoot another string or rope up and rehang it. 

Larger loops  with lighter insulated wire go up easier but go much below 14 gage and you may have breakage/stretching/ issues. But again, no big deal in fixing it.

The 40-45 ft ht is especially nice for multiples of the fundamental freq.  since you get a lot better lower angle radiation and work  more DX on those bands.    So the higher you go the better  DX radiator it is, with many small lobes some with gain.

Ive fed it with OWL, ladder line and coax.  All worked fine.  No problem  with  9:1 balun to coax for entering the shack from outside.   Coax  all the way is ok but your losses will climb on higher hf frequencies ut are no big deeal on 160-80-40 for most lengths.  Multiples of 100 ft runs makes  owl/LL more appealing.

No real sweat with number of sides.  Jut fit it to your lot and the best trees,  and GO!  Ditto  horizontal, or sloping.  Use the highest trees.
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WB6BYU
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Posts: 13287




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« Reply #10 on: August 21, 2011, 09:29:36 AM »

Quote from: KL3HY
I just got passed my general class exam, so I'm trying to plan my first HF antenna.  ...I'm looking into a horizontal loop, sized for 80m.  ...  A lot of what I'm seeing is causing some confusion though.


There are two basic approaches to feeding a horizontal loop.  If the length of the wire is pruned to
resonance at an appropriate point (usually around 3.55 MHz, roughly 280 feet long) then it can have
a reasonably low SWR using coax feed on most HF bands.  The impedance ranges from around 100 ohms
on 80m to 400 ohms on 10m, so using a 4 : 1 balun should give you 2 : 1 SWR or better at resonance.
Even at the edges of the bands the SWR rarely will be worse than 5 : 1, so coax losses are low and the
tuner requirements are not too difficult - even most internal autotuners should handle it in most cases.
Such a combination should cover at least 80, 40, 20, 17, 15 and 10m, and you might get 12m depending
on the exact wire length, but the SWR will be higher on 30m and 60m.  However, to get the higher
band resonances to line up properly you have to tune it for the low end of 80m:  the SWR may be over
5 : 1 at the top end of the 75m phone band,  which might be a problem with a limited-range tuner.
If your primary interest is 80m you can feed it with a quarter wavelength of 75 ohm coax, which
should give a pretty good match there, at the expense of a somewhat higher SWR on the coax on
the higher bands.

The second approach is to use a low loss feedline (twinlead or open wire line, etc.) all the way from
the antenna to the tuner.  In this case the tuner must have a wider operating range and should be
balanced, but the exact wire length isn't critical - the tuner adjusts for whatever wire and feedline
length you happen to use.  This approach will cover all HF bands with low losses (depending on the
tuner.)  If your tuner doesn't have a balanced output, you can add an external balun and a VERY
SHORT length of coax.  Either a 4 : 1 or 1 : 1 balun will work here:  I'd generally choose a 1 : 1
unless the wire and feedline lengths have been optimized for a 4 : 1 balun.

The latter approach is a good choice when it is difficult to adjust the antenna length, while the
former may allow the use of the antenna with a limited-range tuner, or sometimes without a tuner.


Problems may appear, however, when you try to mix these two methods by running balanced
feedline to a remote balun and a length of coax from there to the tuner.  The antenna itself will
still work,  of course, but you can have higher losses in the coax and in the tuner even though
the tuner will match it.  Ideally such a system would be designed with a combination of wire
and feedline lengths (typically a resonant loop with a half wave of feedline on 80m) that gives
a reasonable impedance on the bands of interest.


Quote

Also, I've read that the ladder line radiates RF and essentially is part of the antenna--is that correct?  I've seen lots of conflicting info on this one, so I'm pretty confused about it.


No.  The ladder line is part of the feedline, not part of the antenna.  If the antenna is balanced,
radiation from the line is very low.  There is a strong electromagnetic field around the balanced
line that may cause coupling to other wires very close to it.  While the inner conductor of coax
is shielded, connecting it to a balanced antenna can cause common mode currents to flow on the
outside which will radiate more than the equivalent open wire line.  In practice, this shouldn't be
a major issue with either type of line.


Quote
If I were going to build it today I'd run the shortest length of 75 Ohm coax I could get away with to a balun, and then 450 Ohm ladder line to the feed point of the loop.  Am I way off on this plan?

It depends on how short of a length you can get away with, and the matching range of your tuner.
I'd suggest a coax length of less than 1 foot.  If it has to be more than, say, 5 to 6 feet, it may be
worthwhile to optimize the loop and feedline lengths to provide a reasonable impedance to the balun
to reduce losses in the coax.
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