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Author Topic: Delta loop  (Read 709 times)
2E0JHA
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Posts: 150




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« on: December 05, 2005, 02:23:35 AM »

I want to try a delta loop,iv read some articles on the subject and also read the RSGB book Backyard antennas . The diagrams in the book , and also on the web page i have been on doesnt give very good diagrams of the antenna. It looks like the antenna is a single piece of wire in a triangle formation ,the only gap in the wire being where the feeder joins the antenna.( so basically the wire is joined to the centre conducter, it then goes round in a triangle form and terminates being joined to the earth side of coax.) Is this correct?.I was told this would just create a short,
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WA4BLM
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« Reply #1 on: December 05, 2005, 03:57:24 AM »

Yes, you are right.  It will create a short but only with DC.  If you put an ohmmeter across your feedline of this antenna it would show zero ohms (or a few ohms due to wire resistance).  However, at RF frequencies impedance (resistance and reactance)comes into play.  Depending on the length of the wire and the frequency the impedance can vary over a wide range.  Never played with loops so don't know much about them.  It's the same with a folded dipole as far as the seemingly apparent short goes.
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N5LRZ
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« Reply #2 on: December 05, 2005, 05:04:22 AM »

Measure out the length of wire.  Using an insulator connect one end of the length of wire to one side of the transmission line.  Connect the other end of the antenna to the other side of the transmission line.

IF you use ladder line as your transmission line be sure to use a good tuner.  If you use coax you may or may not need a good ant tuner.  

The antenna will work on any band lower in harmonic to the fundamental cut.  Example a loop cut to 80 meters will work on 40, 20 and 10 meters.  

The antenna is broadsided.  IF you install the antenna in a vertical configuration the signals will radiate as if you were looking thru the antenna.   IF you install the antenna parallel to the earth the antenna will be a NV antenna with its signals being radiated almost strate up where they will bounce off the atmospnere in a downward direction giving you a range of a couple of hundred miles--good for short range HF communications.
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2E0JHA
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« Reply #3 on: December 05, 2005, 05:17:17 AM »

Thanks, 73.. billy
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W5DXP
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« Reply #4 on: December 05, 2005, 08:54:21 AM »

Take a look at my 40m triangular loop at:
http://www.qsl.net/w5dxp/40mtri.htm
--
73, Cecil
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WR6J
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Posts: 97




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« Reply #5 on: December 05, 2005, 11:39:05 AM »

The full wave vertical loop is an interesting antenna. It is balanced and doesn't require radials or other ground plane. As mentioned before, although it's a DC short it will resonate very nicely for RF.

The shape doesn't matter too much - a triangle is popular since it can often be done with a single mast or tree. A square or rectangle will work well too. In fact the larger the area enclosed by the loop the better the gain.

Feedpoint choice is interesting. If you feed the antenna on a vertical side (say the sloping vertical sides of the triangle) you will have a vertically polarized radiation pattern with two lobes coming off the two faces of the antenna ("broadside"). The radiation angle will be low and this would be good for long distance or DX work. Conversely, if you feed it along a horizontal side or at the apex of the triangle you will see performance more like a dipole with horizontal polarization. Assuming your loop is less than 1/2 wavelength off the ground, you will have much more high angle radiation that will be better for local communications (or NVIS).

Keeping your feedpoint higher off the ground will reduce ground losses. I would feed with ladder line if you have the choice. If you feed with coax then make sure you trim the loop very close to resonance with very low SWR otherwise your feedline losses will start to grow.

The reason that the loop when fed on a vertical side produces vertical radiation with low angles and a broadside pattern is very interesting (to me at least!) It turns out that the horizontal components cancel each other out and the vertical components reinforce each other. So the antenna ends up behaving like a pair of phased vertical antennas. It looks very similar to the Half Square antenna in terms of pattern. Try drawing a square loop and using arrows to show the direction of the current as you go around the loop. Keep in mind that half way around from the feedpoint the current is reversed (you are now 1/2 wl out of phase from the source.) You'll see why this behaviour happens. It turns out that this analysis holds true for other shapes that are harder to visualize.

Good stuff.

Have fun,

Richard
WR6J
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2E0JHA
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« Reply #6 on: December 05, 2005, 12:21:07 PM »

Thanks for that Richard, interesting stuff.You clarified a few extra points for me also. all the best..
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RobertKoernerExAE7G
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« Reply #7 on: December 05, 2005, 03:50:38 PM »

I bet you will have tons of fun with an antenna modeling program like EZNEC.com

One of the main reasons why I wanted to go from Mac into the WINTEL enviroment was antenna modeling.

Bob
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WR6J
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Posts: 97




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« Reply #8 on: December 05, 2005, 04:29:30 PM »

Bob - if that was directed to me then yes I already have a boat load of fun with EZNEC. The real world is always somewhat different - but it gets you close. I don't think it models ground losses very effectively - I'm not sure if that is a limitation of my modeling though.

Richard
WR6J
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N7DM
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« Reply #9 on: December 05, 2005, 04:37:31 PM »

Loops...'vertically'.. are my favorite antenna. Singly, or with parasites [quads].  If you are in rainy country, you can kiss rain static good-bye.  There is such small gain on a loop that it isn't worth talking about; but, the NULL on the edge [plane] of the loop is very sharp, and deep. You can orient the loop to help get rid of an area you don't care about...

The efficiency of a loop is governed by the amount of AREA the loop encloses. thus a circle is the BEST, an octagon or hexagon [who has those?] is next, followed by the square [that's most of us], and the triangle [delta].

Loops ARE balanced antennas and love tuned feeders [one of the reasons *I* love 'em]

During the High of the Cycle I work a lot of 17 and 12 meters, so I have two loops, 17 OVER 12... fed at a common feed point [insulator]. It looks somewhat like an upside down typewriter numeral "8"... and works super.

Good Luck..

DM
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KK7WN
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« Reply #10 on: December 05, 2005, 04:59:10 PM »

I use a delta loop cut for 80 meters, on 10 to 80 meters, using 450 ohm parallel line and a tuner. The apex is at 60 feet and the bottom at 12 feet. The loop is in a sloping configuration with approximately vertical radiation due to the feed line connection on one side. As far as I can tell from contacts, as a transmitting antenna it is about equal to a dipole mounted at about 45 feet. However, its real strength is as a receiving antenna. It is clearly much quieter than a dipole and signal strengths are usually on the order of 2 to 4 s units higher than a dipole with the same orientation at about 45 to 50 feet in height.Have fun experimenting!

   
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W5DXP
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« Reply #11 on: December 05, 2005, 05:42:44 PM »

>N7DM wrote: The efficiency of a loop is governed by the amount of AREA the loop encloses.<

A folded dipole is a loop with very little AREA. It's efficiency doesn't seem to suffer.
--
73, Cecil, W5DXP
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KB1LKR
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Posts: 1899




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« Reply #12 on: December 05, 2005, 09:28:59 PM »

But is not the folded dipole more a special case of [unfolded] dipole -- one w/ an impedance transformer attached -- than is is a loop (even though topologically it is still a loop?

If indeed area of the loop matters than the ideal would be a circle, with the hex being a close but easier to implement approximation, then  the square then th edelta or triangle... but is the two sided folded dipole still behaving as a loop or is it just a dipole w/ a DC short and 4x impedence (assuming constant wire diameter) of a regular dipole at resonance?

Just ruminating out loud...
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W5DXP
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« Reply #13 on: December 06, 2005, 05:52:31 AM »

Regarding the efficiency of a fullwave loop, I think some concepts of efficiency have spilled over from small loops. Given one wavelength of wire, I don't see where the inefficiency comes from as long as the loop is a one turn rectangle, fed in the center of a long side with the folded dipole being the limit. EZNEC can estimate the losses in a copper wire antenna.
--
73, Cecil, W5DXP
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N7DM
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Posts: 671




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« Reply #14 on: December 06, 2005, 02:56:57 PM »

Right Cecil.. I don't think any of us would ever 'feel' the difference between a circle and a delta... but... according to the studies I have had available, the statement is correct. Assuming you are using it in the vertical plane, where radiation is THROUGH the loop, as you finger through a ring.

A folded dipole is not a loop...Just a dipole arranged to increase the feedpoint impedance. The number of parallel 'parts' in a dipole increase the theoretical 70 ohms by a factor of " N Squared ", where N is the number of 'parts'. A folded dipole has two parts; thus free space feed of 280 ohms. Make it a THREE part one and the feed impedance goes up to 630.   Where this can be helpful is in the case of a Yagi, where the feed impedance can be in the area of 15 ohms. When I had a fixed, wire, three element, inverted config Yagi for 30 meters, I used a folded dipole for a D.E., and the thing bridged at around 70 ohms, real world. Fed it with 300 ohm twinlead... aimed at Eu, it was a GREAT antenna.
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