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Author Topic: Horizontal Delta Loop Dimensions  (Read 3301 times)
KD6NRP
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Posts: 19




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« on: December 17, 2012, 05:19:40 PM »

I have some questions about the dimensions for a horizontal 40m delta loop.

My current delta loop has a perimeter of 150.2 feet, is 12 feet above ground, and is fed with 300 ohm window line. The antenna system is connected to a Palstar antenna tuner in the shack.

1. The perimeter of my delta loop is 5.5 percent bigger than what it is supposed to be according to the formula.

    If I reduce the size of the loop to make it resonant for the desired 40m frequency, will it receive better? Will I get out better?

2. The standard formula for calculating the perimeter of a loop antenna is:

    Length in feet = 1005 / frequency

    How reliable is this formula? Is there a more accurate one?

73

Brian, KD6NRP
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WB6BYU
Member

Posts: 13335




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« Reply #1 on: December 17, 2012, 10:05:42 PM »

If you are feeding it with twinlead to a tuner, changing the length by 10' or so won't
make any noticeable difference, except perhaps in the tuner settings.

The resonant length of a loop depends on the wire diameter, the type and thickness of
insulation on the wire (if any), the exact shape, the height above ground, etc.

Let's see what EZNEC says.  For a triangular 150' in circumference (and trying to
make it as equilateral as possible) at a height of 12' the resonant frequency is about
6.8 MHz.

Just looking at difference in Earth characteristics, over poor ground the antenna is
resonant at 6.83 MHz with an impedance of 90 ohms.  Over very good ground it
shifts to 6.785 MHz and an impedance of 56 ohms.  The maximum signal strength
is almost 3dB better with good ground due to reduced ground losses.

If I assume poor (but not really bad, perhaps typical urban) ground, using uninsulated
#14 wire and trying to keep the loop triangular, I find a total length of 141.5' puts me
at about 7.225 MHz, and 144.5' resonates about 7.060 MHz.  In the latter case if
I just change the wire size to #22 while leaving everything else the same, it shifts
down to 7.035 MHz.

For the #14 wire at this height and ground type, the formula constant would be
about 1020 instead of 1005.  In free space the same loop dimensions resonate
at 7.140, for a constant value of 1037.

Unfortunately my old version of EZNEC doesn't handle insulated wire.

Of course, this presumes a perfect model antenna with no trees, masts, buildings,
power lines, or other objects in the vicinity that may affect it.  For example, I've
found that when I install a 40m loop on the roof of a house I often have to shorten
it by 10% or so from the formula length to find resonance.


So in summary, the formula gets you in the ballpark for many installations, but should
be considered just an estimate.  Even the computer models don't take into account
all the possible environmental factors.

(When I'm teaching the license class I tell them to remember 1006 instead of 1005.
That's because I can write "looP" on a transparency and flip it over to show "1006",
an easy way to remember the number.  Neither of them is perfect, but in the real
world either one is as good as the other, since you'll probably cut the antenna a bit
long and prune it to resonance anyway.)

The only reason it matters is if you want to feed the antenna directly with coax
without a tuner (or possibly with a quarter wave matching section.)  If you are
using twinlead to a tuner anyway, then the exact length makes little difference,
and you certainly won't be able to detect any difference between a resonant loop
and what you are using right now.

You could, on the other hand, leave your loop as it is and cut your 300 ohm twinlead
feeder to about 50' (for CW) to 43' (for the top of the band) and use coax from there
to the rig with a low SWR.  That works for 40, but for multiband operation using a tuner
is the simplest approach.
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W5DXP
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Posts: 3613


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« Reply #2 on: December 18, 2012, 05:39:57 AM »

Soapbox mode on: Dale said it above but a lot of people miss that point. Most large loops and single-wire dipoles that are fed with 300 ohm twinlead, 450 ohm ladder-line, or 600 ohm open-wire line are mismatched systems so it doesn't matter if the antenna by itself is resonant or not because the antenna system is most likely non-resonant (unless some particular  feedline length is chosen). That's why we use a tuner on such systems - to bring the system into resonance at our chosen frequency.

Varying the length of the feedline will change the resonant frequency of a mismatched antenna system and a random length of feedline on a mismatched system will invariably not be resonant on the desired frequency or on the frequency on which the antenna by itself is resonant (unless we use a tuner). Resonant frequencies can be detected with an antenna analyzer or grid dip meter. It is interesting to observe how different tuner settings change the resonant frequency of the antenna system and how the resonant frequency changes when the tuner is switched to bypass mode.

Antenna resonance is important when the feedline is flat, i.e. when the SWR on the feedline is 1:1 and the resonant feedpoint impedance of the antenna matches the characteristic impedance, Z0, of the feedline. Otherwise, antenna resonance by itself is of little importance. Note that a non-resonant Extended Double Zepp antenna has a gain advantage over a resonant 1/2WL dipole in the dipole's best direction and they have about the same radiation efficiency.
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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.
KD6NRP
Member

Posts: 19




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« Reply #3 on: December 18, 2012, 08:02:35 AM »

WB6BYU / W5DXP:

Thanks for your informative replies. My real issue has been that people can hear me, but I can't hear them as well. That's why I switched from an end-fed wire and counterpoise to a 40m doublet, and finally to a horizontal delta loop. Delta loops have the reputation of being good receive antennas.

A couple of days ago, I put up a 150 ft circumference delta loop using #24 wire with rather thick insulation. It got out and received well.

Yesterday, I put up an identical delta loop, but I used #24 copper weld wire and spray painted it to improve the aesthetics. The new delta loop is receiving even better.

My guess was the original 150 ft delta loop had a lower resonant frequency because of the insulation whereas the new 150 ft delta loop was working better because it wasn't insulated and resonated closer to the 40m band.

At any rate, the new loop appears to be working very well.

73
Brian, KD6NRP

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WB6BYU
Member

Posts: 13335




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« Reply #4 on: December 19, 2012, 01:05:07 PM »

What has been limiting your reception:  local noise pickup?

Most receivers have plenty of sensitivity for most HF work, so that the signal-to-
noise ratio is set by the on-channel noise from external sources.  A loop really
isn't any better at differentiating between desired and undesired electromagnetic
radiation - just as with any antenna, it will pick up whatever RF reaches it.

If you did notice a difference it most likely is because the loop is positioned
differently with respect to local noise sources (so it doesn't pick up as much
noise) or has less common mode current that causes the coax to act like an
antenna and pick up noise from devices in the house on its way from the
antenna to the shack.

My experience with resonant loops is that they don't seem to be as easily
affected by common mode currents as a dipole, and that can lead to lower
noise pickup on the coax.

The shift in resonant frequency due to the lack of insulation on the wire
probably makes no practical difference in the signal-to-noise ratio in itself.


But glad your loop is working well:  in that case I'd just leave it as it is
and enjoy using it.
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W0BTU
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Posts: 1703


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« Reply #5 on: December 19, 2012, 02:28:09 PM »

If you did notice a difference it most likely is because the loop is positioned
differently with respect to local noise sources (so it doesn't pick up as much
noise) or has less common mode current that causes the coax to act like an
antenna and pick up noise from devices in the house on its way from the
antenna to the shack.

My experience with resonant loops is that they don't seem to be as easily
affected by common mode currents as a dipole, and that can lead to lower
noise pickup on the coax.

Or, the loop is not responding as well to RFI at low angles. This is why I think so many people say they are quieter antennas. A low horizontal loop is a cloudwarmer, at least in all the models I've seen. And it works the same in receive: not much response to low angle signals.
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WB6BYU
Member

Posts: 13335




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« Reply #6 on: December 19, 2012, 02:50:17 PM »

But it wouldn't be significantly different than a dipole in that respect.
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