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Author Topic: Question on PVC supporting Magnetic Loop  (Read 1203 times)
KG7A
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« on: November 07, 2017, 06:32:43 AM »

I recently purchased a Active Magnetic Loop Antenna by DX Engineering, RF-PRO-1B.  DX Engineering owns the rights to Clifton Laboratories designs and formerly a Pixel Loop.  I want to use a 1.25 in. piece of sked 40 PVC for a mast and brace the middle section to the top of the loop.  The reason being is that the loop is only supported by 2 bolts at the top of the 90 deg. angle bracing plate.  I believe the extra mast going to the top of the loop will add more bracing for high winds.  I will also use a ground strap between the loop bracket and the rotor that is grounded.  Does it matter?  Is the Sked 40 PVC efficient as a RF insulator at frequencies between 100 KHz through 30 MHz in Receiving?  Will it change the receiving pattern?  Please advise.

Thanks,  Ronny  KG7A   
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VK6HP
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« Reply #1 on: November 07, 2017, 09:45:26 PM »

Hi Ronny

I'm not familiar with the DXE loop, although I had a quick look on line at the general arrangement.  I use a somewhat similar Wellbrook broadband receiving loop, turned by a small rotator. The loop is a little more than 1 loop diameter (1m) above a garden bed, the location and height being chosen to allow best overall noise nulling.

You don't mention how high your loop is mounted but I would think there is little problem using just the mounting provided.  It's a pretty light-weight element, with minimal wind loading.  My loop is mounted on a half metre or so of 35mm PVC reticulation pipe, via some thread adapters that allow the fittings to screw into a loop mounting flange.  With the Wellbrook system, the amplifier is at the bottom of the loop, and there is no vertical support within the loop. The PVC pipe/loop is turned by the rotator, which itself is mounted on about a 35mm galvanized steel pipe driven about 1m into the ground.  It's all quite mechanically stable, even in the high winds we get here in WA.

I've found it useful to put common mode chokes (which need to be effective at your lowest operating frequency) at both ends of the RG-8X feeder.  I'm about to upgrade the loop-end choke, and I'll ground one side of it (the side feeding the long coax to the shack) to the galvanized support pipe, just to see if the noise situation improves noticeably.  The loop currently works well, with some good, deep nulls on the low bands where I use it most.

If you feel you must brace the loop further, some PVC should be OK.  However, I would be a bit wary about introducing extraneous ground connections too close to the loop environment, especially if you haven't got a common mode choke at the antenna end.  It's easy to induce noise back into the loop from currents on e.g. the feeder coax.  My general advice would be to start with the best, cleanest system you can and then make changes one at a time.  

73, Peter.  
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KD8IIC
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« Reply #2 on: November 07, 2017, 10:24:48 PM »

   I have one of the very first Pixel Loops ever made and I helped with some perfecting that first version.
   I also have the second generation original Pixel, both have the Clfford Labs low noise pre-amps.
   Mine is supported using an aluminum 44" mast with a rotator mounted driven into the soil in my back yard.
   The only islation from the conductive and grounded mast is the PVC conduit supporting the Pixel loop from the rotor.
   It is quite steerable and show very good nulls below 3.5mHz. Top of loop is about 6' above ground.
   It is a pleasure to use 80 and 75m CW es phone during the summer months. Without the loop it is much less pleasant.
   Installation is very simple and I have not found that mounting it at 15' above ground to prove any improvement.
   Like I stated, the best use of this design is for rx of 150kHz to 7mHz.
   Not that it does not work @ 40m and above but the nulls are not as defined.
   I suspect this is due to natural propagation differences of the frequencies
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KD8IIC
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« Reply #3 on: November 07, 2017, 10:32:09 PM »

  Make sure you read and understand the instructions with this antenna.
  Do Not Ground Any Part Of The Loop Antenna. A PVC pipe goes between the rotator and loop mounting bracket.
  It needs isolation so as to "float" in the air.
 
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K8AXW
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« Reply #4 on: November 07, 2017, 11:07:49 PM »

FWIW:  My only experience with PVC and antennas is with two J-poles.  It was necessary to alter the length of the elements because they were installed in Sked. 40 PVC pipe.  Rather it has an effect on your loop, I have no idea.

I suggest you take some measurements or make some observations first and then go ahead with the PVC thing and see what difference, if any, it makes. 

Report back so we all can learn something.
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KD8IIC
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« Reply #5 on: November 08, 2017, 12:00:30 AM »

  The use of an insulated mast, i.e. PVC tubing is a must to isolate / insulate this antenna from the metallic support.
  It has already been designed around by the antenna's developer years ago. It's proven to work as specified.
  This is not a TX antenna.
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VK6HP
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Posts: 149




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« Reply #6 on: November 08, 2017, 02:21:06 AM »

I think the OP probably has the message that good installation practices are needed to get the best out of receiving loops.  If you start by achieving a good baseline performance involving good physical/electrical isolation and symmetry, then do some basic measurements, such as null depth on representative noise sources and broadcast stations, you're in a position to systematically refine the set-up with time.  Every installation is slightly different and will be influenced to a different degree by e.g. common mode coax current (which is of course reciprocal between transmit/receive analyses) and the resultant noise pick-up.  My own experience is that a point of diminishing returns is eventually reached but that, in a "medium" urban noise location, attention to the finer points has bought me 6 dB or so in null performance on 160 and 80m, relative to a baseline installation.  I'm hopeful of a little more with the next round of choke installation but admit that I'm now in the performance margins. However, with some VLF work in sight, every little bit helps.
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KD8IIC
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« Reply #7 on: November 08, 2017, 07:33:12 AM »

  Usually Joe Ham does not posses equipment to accurately measure gain and loss of a receive
  antenna system to start with. Install it per the instructions is always the best course of action.
  I'm sure the designer of this loop knows a little more about it than we may think.
  Why make this a burdensome task when it is basic plug and play with some attention given to
  some installation details? It's not meant to be hard to install or use as is & give excellent results.
  My two Pixels are installed per builder's instructions and work flawlessly.
  The wheel is invented in this case, no need for reinvention. Spend your time enjoying it.
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VK6HP
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« Reply #8 on: November 08, 2017, 04:45:44 PM »

While I take a slightly different view, we are in violent agreement that the most prudent course for the OP is to assemble the antenna per the instructions.

Beyond that, it's no deprecation of the antenna or its designer to optimize the installation in a particular physical and electrical environment.  As it happens I am an antenna/RF design engineer but, for the main optimizations one might consider, there is no need for complex test equipment; it's largely about doing the best you can in terms of achievable signal-to-noise ratio and with some patience, a station receiver and basic test equipment, relative measurements are not difficult.  Broadcast stations or aeronautical NDBs are good off-air test sources and, for quick pattern and null evaluations, add a receiver, pencil and sheet of graph paper.  For common mode checks beyond local noise sources, couple some low-level RF to the coax and use the receiver as a relative level indicator.  If you have an SDR or spectrum analyser, so much the better.

I have no doubt that the DXE loop works as well as the Wellbrook model and that good out of the box performance is likely, given care in the installation.  But my view is that ham radio contributes more when there's a level of understanding and associated informed experimentation, and my advice to newcomers is always to go beyond the level of being a consumer.

  
« Last Edit: November 08, 2017, 05:09:28 PM by VK6HP » Logged
KG7A
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« Reply #9 on: November 09, 2017, 10:55:14 PM »

     Thanks guys for your input on the info about the DXE magnetic loop.  I guess I forgot to tell you it will be about 20 to 25 ft abv. gnd. just above my TV antenna I use for on the air TV.  This is the West end of the roof on my house.  I live in the dessert of Mesa, AZ so I also wondered about ground conductivity effect performance on the loop.  I'm sure I could just mount it low to the ground but this is really the best place for it, up in the air, out of the way. 
     Thanks Peter for informing me.  I've also did some research and got info on how to test the PVC's conductivity.  CPVC has content of metel particles in the elements for making it.  But regular sked. 40 PVC an no metel particles in it.  The proof is to take a sample of PVC and place it in microwave oven for 30 seconds with a bowel of water for the load.  If it's warm of hot, it has metel particles in it but if it's cool it has none.  And a very good insulator for supporting the loop to the top. 
      Once again thanks.  73  Ronny  KG7A. 
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VK6HP
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« Reply #10 on: November 10, 2017, 12:40:36 AM »

Ronny, before committing to the high mounting I would do some tests in a few positions and heights.  I had a great, convenient location picked out for the Wellbrook loop but to my annoyance the best place in s/n terms turned out to be right in the middle of the front lawn, at about 1m high.  I couldn't quite put it at that best location but, after feigning an excuse for slaughtering a shrub in a nearby garden bed ("exotic weed, inconsistent with Australian native theme"), I squeezed the antenna into a good and inconspicuous spot.

If you do mount it near the TV antenna, be on the lookout for any house/power noise conveyed by the TV feeder.  The name of the game with these small antennas is s/n and, with small effective areas, to realize the full advantage relative to bigger systems you have to win substantially on the "noise" part of the quotient.  You really don't want to compromise that if at all possible.
« Last Edit: November 10, 2017, 12:47:27 AM by VK6HP » Logged
KG7A
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« Reply #11 on: November 10, 2017, 04:01:24 AM »

     Thanks Peter.  I didn’t relalize the S/N might be higher at feed lines.  I’ll try it 1 meter above the ground.  I guess it’s trial and error?  Thanks for that.  I’ll let you know.  73.
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N4MQ
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« Reply #12 on: November 10, 2017, 04:27:51 AM »

My loop is a 12' diameter, 1 1/8" copper coil 37 ' in length elevated ' above my rotor using a 16' mast of  2 1/2" PVC with a 2" x 2" wood support ( ripped 2 x 4 ) inside and braced with spray foam for stiffness.  Power rating is 1000 watts on 160 and 80 meters with 45 KV vacuum variable caps and it has been up for over a year.  I made an upper PVC  slip coupling to allow rotation and 3 guy ropes at the top.

PVC can support lots of weight if you guy it and consider the loading, I did not have schedule 80 pipe available, though it would have added an additional bit of safety margin overall.

Link to see it:   https://sites.google.com/view/n4mq-site/home

Enjoy, Woody Grin
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VK6HP
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« Reply #13 on: November 10, 2017, 05:32:34 AM »

Woody..that's some variable capacitor!  Interesting to see how you've tackled the design of the 160m transmit loop and tuner.  It's crossed my mind from time to time as a project and your video was very interesting.  Have you run the full 1 kW at this point?  Any WSPR or similar results to compare your loop and your other transmit antennas?

The small (1m dia) loops we've been discussing use integral low-noise, broadband amplifiers with excellent balance.  I noticed you said you were looking to complement your receiving antennas with something similar and, although it's fairly early days, I am quite optimistic about the overall value of my loop in signal-to-noise terms.  

Ronny, I was suggesting the signal-to-noise ratio might be lower near house wiring etc since the noise pickup from local sources can vary quite a lot with location.  Remember that if you can get, say, a 6 dB increase in s/n ratio by lowering the noise by an s-point or so, it's roughly the same effect on readability as Woody increasing his transmit power from 100W to 400W.  In my case, a payoff of that order was well worth the life of a nondescript shrub Smiley

(Of course, since we are never happy with second best, the ideal is for you to get your 6dB *and* for Woody to run QRO).

« Last Edit: November 10, 2017, 05:52:18 AM by VK6HP » Logged
N4MQ
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« Reply #14 on: November 11, 2017, 05:13:40 AM »

I have run ssb at 1 KW and mostly 350 w on AM local group (200 miles).  It gets out fine, better when on 40 meters before the upgrade to high power capable caps which limits resonance to about 5 Mhz for now.

The antenna shines as a receive loop and so so on transmit.  The resonance is tight and requires tuning due to the less than 5 Khz 3db points, BUT it limits noise being so selective.  Planning is to make more receive only loops using 1/2" hard line for material, aka small loops in the 6' Dia range.

Enjoy, Woody
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