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Receive Antennas (below 30MHz) For all topics related to receive antennas used on HF, MW, LW, etc. For transmit antennas use the Amateur Radio Antennas forum.

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Old 08-12-2012, 3:35 PM
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Default Pixel Pro 1B loop antenna

I'm adding this strictly for informational reasons. I've read and have been asked about loop characteristics in the past. This isn't intended for those in the know. Just for those learning more about loops. I plan to mount this loop 35' on my roof and cut the cable down to 25'. Right now it's 12' up behind the garage with 100' of quad shield RG6. I want to see what if any effect this will have getting it up and in the clear. Mostly for noise and low band.

Pixel Pro 1B active loop antenna nulls - YouTube
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Old 08-12-2012, 5:17 PM
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Hi i use the wellbrook active loops mounted 5 metres high im in a double story house when i mounted them 8 metres high performance wasnt as good took me a while to get the right height and direction dont have a rotator yet using 20 metres RG213 low loss coax cable,these are my youtube videos monitoring ndb,aero..

NDB 350khz AM - YouTube
HF AIRCRAFT - YouTube
Australian Marine Weather Broadcast - YouTube
HF AIRCRAFT - YouTube

Regards Lino..

Last edited by carmelof; 08-12-2012 at 6:56 PM..
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Old 08-12-2012, 6:44 PM
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Quote:
Originally Posted by BDM123 View Post
I'm adding this strictly for informational reasons. I've read and have been asked about loop characteristics in the past. This isn't intended for those in the know. Just for those learning more about loops. I plan to mount this loop 35' on my roof and cut the cable down to 25'. Right now it's 12' up behind the garage with 100' of quad shield RG6. I want to see what if any effect this will have getting it up and in the clear. Mostly for noise and low band.

Pixel Pro 1B active loop antenna nulls - YouTube
Rotor adds considerable improvement, 49m would be of interest if you happen to do any more video's.

Look forward to your Loop height observations.

Thanks for sharing.
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Old 08-12-2012, 9:56 PM
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Getting a small loop high and away from your local noise is always a good idea. But going too far can be detrimental, say on a 50 foot tower at 10mhz or above. The elevation look angle rises sharply.

Of course line-of-sight reception is improved so it is a tradeoff.

I'll plot a 3 foot square loop for 8.867, 12.365, and 13.261 mhz up at various heights in EZnec later and maybe save some footwork.
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Old 08-12-2012, 11:18 PM
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Quote:
Originally Posted by hertzian View Post
Getting a small loop high and away from your local noise is always a good idea. But going too far can be detrimental, say on a 50 foot tower at 10mhz or above. The elevation look angle rises sharply.

Of course line-of-sight reception is improved so it is a tradeoff.

I'll plot a 3 foot square loop for 8.867, 12.365, and 13.261 mhz up at various heights in EZnec later and maybe save some footwork.
That would be interesting on your findings. Let me know
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Old 08-13-2012, 12:12 AM
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Default 1mhz at 100 feet

This is pretty much the canonical hemispherical elevation plot shape one encounters with a small loop, such as this 9-foot square (36 inches each side) circumference model.

This is at 1mhz at one-hundred feet high! AM BCB dxers luck out. Great line of sight, and no real degradation of the elevation angle. The pattern looks the same at only 3 feet high.
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Last edited by hertzian; 08-13-2012 at 12:32 AM..
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Old 08-13-2012, 12:16 AM
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Default 7 Mhz at 10 feet pattern

Things will start to get squirrely the higher in frequency and the higher we go physically. This is still the 3-foot square loop.

At 7 mhz mounted above your roofline, it is still pretty much the canonical elevation pattern so no troubles here.
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Old 08-13-2012, 12:19 AM
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Default 7 mhz at 50 feet

This time, we put the small loop on top of our 50-foot tower. But careful listening may reveal some weird stuff not experienced before:
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Old 08-13-2012, 12:22 AM
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Default 14 mhz at 10 feet high

Totally freaked out, we drop the height back to 10 feet, but try our hand at 14mhz:

Not too bad actually, but your close skip contacts will be a bit weaker. This may or may not be an advantage depending on your requirements.
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Old 08-13-2012, 12:24 AM
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Default 13.261 for Carmelof

Here is what you may have experienced on 13.261mhz at 8 meters high:
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Old 08-13-2012, 12:26 AM
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Default 13.261 at only 5 meters high

Here we go on 13.261 at 5 meters. Things are improving as you reduce the height:
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Old 08-13-2012, 12:28 AM
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Default 13.261 at only 1 meter high!

Dropped back to nearly just 1 circumference high above ground, here is the elevation pattern on 13.261 mhz:
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Old 08-13-2012, 12:31 AM
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Default Wrap up

So what we see here is that when you go higher in frequency, AND higher in height, your elevation pattern can actually become much worse than if you had it just above ground.

You'll have to decide if that makes a difference to your operations - for some it might be ok, but I have a feeling that most want the nearly hemispherical coverage. For me, anything much beyond 10 feet and 10 mhz high starts to go bonkers.
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Old 08-13-2012, 12:42 AM
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Great info and thanks for posting that. Anything above say 20 meters I would rarely use the loop for. I have other antennas for those bands. I can keep the loop no higher than 30' (9.1 meters). That is about the height at my peak. Low side of SW (30/m and lower), MW/LW are very important regarding the loop.
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Old 08-13-2012, 10:55 AM
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I think I read somewhere? that 15' is the ideal height for the Wellbrook. Interesting observations. I was not aware of the height/freq relationship. Thanks for sharing
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Old 08-13-2012, 10:47 PM
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For MW / LW, you can mount the small loop very high and not distort the elevation pattern. But that may not matter so much since most AM/LW signals don't arrive at a very high angle - I think. So height adjustments on these bands may have more advantage when ground absorption losses and the like are a consideration.

And since the deepest null-depth is obtained perpendicular to the loop at low angles, you should still be able to get a deep null on AM/LW stations, as well as local noise sources.

For HF, where relatively high incoming skywave signals are present, you may not be able to obtain a deep null, since the null falls off the higher it looks. Fortunately, local noise down low can be put into the null.
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Old 08-13-2012, 11:29 PM
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Default Azimuth null pattern over perfect ground

Here is the typical plot of those great figure-8 nulls on a small loop (1/10th or less in wavelength circumference) loops. Looking downwards on it from a birds eye view.

If you live near saltwater, or have other great ground, nice null depths can be had:
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Old 08-13-2012, 11:31 PM
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Default Azimuth nulls for the rest of us.

Here is what most of us can probably expect over average ground. Instead of a 30+ db null, we're looking at maybe 15db attenuation in the null.
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Old 08-13-2012, 11:37 PM
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Default Good nulls only at low angles!

The null depth is best at low angles. We really start to lose the null when the noise / signal source is not perpendicular to the loop. Here we are at 22 degrees above the horizon. Barely a -6db null!

Fortunately, while the null is going away, we have better coverage for skywave signals at that angle.

So if you have a ground-mounted loop with a neighbor on the 2nd floor generating noise, either move the loop further away, or try to get it perpendicular to the noise to put it into the deepest null depth.
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Old 08-14-2012, 12:07 AM
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Default Elevation standing off to side of loop

I guess to wrap it up - earlier on we saw what a small loop's elevation pattern looks like when you are perpendicular to it - like looking through a magnifying glass.

This pattern is what it looks like when standing off to the side of it, so you are looking at it edge-wise. For low angle signals, rotation may improve the signal.

So the small loop isn't really perfectly hemispherical with a low-angle null, but more of an oblong pattern to skywave reception.
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