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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 08-03-2018, 6:23 PM
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Default Loop questions

I am thinking of buying a receive only loop to compliment the Alpha Delta 66' Sloper connected to my Drake R8B receiver. I would prefer to have a one size fits all for LW/MW/HF - ie NDBs to 30M.

Some questions:

I understand a loop has best performance along the plane and nulls signals perpendicular to the plane.
Is the best signal off either end of the plane (90' rotation would suffice) or does one need to rotate through 180' to get the best signal?

Are there any 12 volt light duty rotators available?

I will need to mount the loop outside as my house has a steel roof.
I've seen discussion about height but wonder if there is any conclusive evidence as to the effectiveness of higher up (say 7 metres) vs ground level (say 2 metres)? I can do either.

To minimise shipping cost to New Zealand I'm leaning towards the W6LVP Loop Kit with just the amplifier and power inserter and I will provide the loop and 12 volt power supply locally.
I've read nothing but good news about this antenna. Anyone care to comment?

Many thanks,
Rick
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Old 08-03-2018, 7:21 PM
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I guess I'll tackle the height question - everything I've read on the subject (and people's installations) suggest that it's better to keep the loop closer to the ground - around 1-2 meters off seems to be where everyone puts it. And of course, keep it as far from the home as you can.

I suspect that steel roof would really mess the loop up; keeping it near the ground seems to be the way to go. And if you have a HOA or similar that says 'No Antennas', it's easy to put that in a garden and hide it.

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Old 08-03-2018, 7:27 PM
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This is widely considered to be a very fine reference on the subject of loops - of course, the shipping to NZ would be hell, but I suspect that you will be able to order it locally...

https://www.universal-radio.com/catalog/books/0016.html

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Old 08-04-2018, 1:19 AM
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I second Mike's recommendation about the Joe Carr loop antenna book.

One thing Joe and some other publications don't really cover is the mounting height of such a small loop. When you mount it high, as seen by antenna design programs like EZnec, you'll see that the reception pattern is nearly a perfect sphere. Down a bit low, like a loop diameter or so above the ground, the pattern *down low* is squashed somewhat for low-angle dx along the plane of the loop. Yet you still have the horizontal null, and the slightly oval higher angle for skywave reception.

Mounted high however can have it's benefits depending on what you want to do - down low you are relying solely on skywave propogation, but higher up is better for also receiving direct-wave or possible even more groundwave signals depending on frequency. Ie, hearing local amateurs when the band is not open, but chatting away locally is much easier up high than down low.

I'd say plan on trying it *both* ways to see what you like best.

Re the steel roof - as long as you are at least a loop diameter higher than the roof, AND you can have it centered above the steel, that can work to since balance is maintained, and you are using an RX-only loop that can be retuned. or is merely wideband already. Mounting it at the edge of a steel roofline, even if well above it, would not be recommended since that would be a very unbalanced surface underneath.

Remember that these tiny loops worked very well on top of older aluminum-skinned aircraft too when they would rely on NDB's and local am stations to get a bearing (usally the null, and not banging the s-meter) - as long as a general sense of balance underneath the loop was maintained. Mount one on the edge of a wing, and um, no go. . Just saying that above the steel roof would be ok if you got it a reasonable distance away, and somewhere in the middle of that mass. Maybe not perfect, but usable.
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Old 08-04-2018, 5:25 AM
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Wouldn't having the loop showing a 'perfect sphere' as a response be detrimental to one of it's more powerful components - being able to null out signals (at least on the lower bands - as I understand it the higher in freq you go, the more it starts acting more omnidirectional).?

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Old 08-04-2018, 1:49 PM
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One of the factors to keep in mind with a loop is what is referred to as the Antenna Effect (AE)-- and its how this effects what the antenna is mounted to and around.
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AE is the unequal capacitance exhibit'd between sections of the loop to ground. As the loop becomes more unbalanced by this effect- ie: it's proximity to the earth etc, the antenna begins to exhibit more of the properties of a short vertical. This vertical pattern, now imposed on the 'figure-eight' of an ideal loop, will distort its pattern by filling in its null portions -- this undoes the loop's useful properties.
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Using a conductive non-ferrous shield (like aluminium) about loops will offset this AE. Though it may decrease the receiving efficiency of the loop somewhat, the sharpness of the nulls are greatly preserved and enhanced. It also permits effective use of the loop over various ground surfaces. It is imperative this not form a continuous shield about the loop- that there be a break, usually opposite the feed point. And at this feed point, to keep the loop balanced, a balun of sorts is necessary.
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I guess that's enuff from me on this, and altho the theory of loop operation is quite fascinating- I will spare everyone...........
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Old 08-04-2018, 5:06 PM
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I see this question was overlooked Tuihill-------
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"I understand a loop has best performance along the plane and nulls signals perpendicular to the plane.
Is the best signal off either end of the plane (90' rotation would suffice) or does one need to rotate through 180' to get the best signal?"
__________________________________________________ __________
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When the loop is perpendicular to the signal, the induced voltages on both sides of the loop cancel and the response is minimal. When the loop is parallel to the flow, there is an induced voltage difference between the sides, and the response is maximum. The size of the loop and the wavelength will influence the induced voltage, but you have it correct.
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In theory only a 90 degree rotation should go from full null to full signal, but loops can be quirky. If the feed point is not balanced, then the optimal response may be found in that extra 90 degrees. Also don't be afraid to tilt the axis of the loop as well... we'd like to assume nice classical groundwave polarizations (I'm talking long wave here)- but in reality?.....
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(I tried to avoid the theory stuff ...........smiles)
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Old 08-04-2018, 5:29 PM
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Interesting. I've also been reading the posts from my friend Mike (Invergordon) about his condtruction of a loop which he says works very well.
I recall from my sea-going days in the 60's that all ships had some form of direction finding equipment - all Marconi equipped had a "Bellini-Tosi" up above the bridge deck on the very centreline of the ship. There were two crossed loops which fed coils inside the receiver where the signal 'field' was 'reconstructed' and a rotatable coil picked up the signal to feed the receiver headphones and signal strength. The loop antenna was constructed from copper shield which was insulated at the top with the antenna wiring inside - a Faraday shield in effect but not continuous.

Interesting article and pictures of the receiver and loops here....

Marconi UK Electronics

The 'sense' antenna added the signal from a vertical wire into the 'figure-of-eight' to produce a cardiod (heart-shaped) pattern to alleviate the ambiguity of two 'nulls' - the null is much sharper than the peak to deduce the direction of the signal.
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Old 08-05-2018, 1:01 AM
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Quote:
Originally Posted by ka3jjz View Post
Wouldn't having the loop showing a 'perfect sphere' as a response be detrimental to one of it's more powerful components - being able to null out signals (at least on the lower bands - as I understand it the higher in freq you go, the more it starts acting more omnidirectional).?
Glad you asked, because that question is totally logical, and my earlier post not so precise!

In elevation angle, it appears to be mostly a sphere when mounted high, but in *azimuth* you still retain that classic deep null parallel to the loop, and the peak inline with the plane of the loop.

That peak inline with the plane is what suffers when mounted high having a higher look angle than when mounted down low. That may or may not matter to some.

And of course, we're talking small-loops - that is loops that are 1/10th wavelength or less in circumference typically. Medium and large loops take on different reception lobe patterns, with both voltage and current nodes, whereas small loops with only a single current node have this classic pattern we talk about.

Another nod to Joe Carr's loop book!

Lauri - another good point about *slight* tilt - I have had to use that before when a local noise source was located in an apartment a few floors above me. The small loop has those deep nulls, but the deepest part is horizontal to the horizon. Thus to null out that nearby apartment across and above from me, a slight tilt was needed to being back my happiness.

Heh, truly hard core loop fanatics might consider both azimuth AND elevation rotators if they have noise slightly above or below the horizon of the loop and are surrounded by an rf jungle above and below, and not just directly across.

Lauri - great point that I've hardly seen anyone ever mention before.
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Old 08-05-2018, 1:36 AM
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Tuihill - does it have to be a small rotatable loop? Can it be a medium-to-large size loop?

Other than some of our mighty fine loops found in the wiki's, another broadband untuned loop might wet your interest if you want to start slinging some wire:

loop

Even though the project uses a 9:1 isolated transformer, I personally would additionally put a common-mode-choke inline with the feedpont. And, you may just get away with the preamp in the R8, (if you actually feel you need it), rather than a dc powered outboard one.

And anything higher than 10mhz or so might have too much of a squirrely pencil-beam lobe(s) to it, but might do for what you want.

As I learned first hand, for low-band use, the quality of the ferrite material makes a large difference - random computer toroids can be disappointing.
Of course there are commercial offerings too...
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Old 08-05-2018, 1:45 AM
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Majoco - I remember seeing those awesome loops bolted to a ship, but later got news that they were seldom used backups.

But yeah - being fixed in place to withstand ocean-going conditions - can you imagine a rotatable loop just getting hammered and corroded out there?

Here, the two loops are fed to a goni-ometer type of phase circuit, rather than swinging the small loop.

I've been over the whole faraday thing before, so won't go into it unless asked. I was so disappointed when I built my first "shielded faraday loops", only to find out that unless you are in the near-field, say 1/20th or less away from a noise source, that the loop actually acts like most normal antennas for far-field use - which is what I was most interested in. In that instance, I was bummed that my shielded loop was NOT filtering out the electrical fields and grooving on the magnetic field. W7EL and others set me straight.
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Old 08-05-2018, 10:48 PM
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Having had a rethink... The loop that Mike and other DX hunters have made is a different animal to the ship DF loop. The ship's shielded loop perhaps deliberately reduces the sensitivity for a good reason, whereas the hunters loop needs all the signal it can get.

We are looking for the DX but the ship DF loop doesn't need to receive distant stations, indeed the opposite. Long distance reception of the LF beacon stations for the purposes of position finding is fraught with problems, especially at night. Ideally the path to the transmitter should be totally over water because as soon as the path crosses over a coastline it suffers from "coastal refraction", the path bends like a stick placed at an angle in a pond. Also after dark "night effect" comes into play, where reflected signals from the ionosphere are received along with the ground wave (sea wave?) via an unknown path and will result in false bearings. Any DF bearings that appears to be 'wandering' or fading are to be treated with suspicion!
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Old 08-06-2018, 1:05 AM
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Quote:
Originally Posted by Lauri-Coyote View Post
....Using a conductive non-ferrous shield (like aluminium) about loops will offset this AE. Though it may decrease the receiving efficiency of the loop somewhat, the sharpness of the nulls are greatly preserved and enhanced. It also permits effective use of the loop over various ground surfaces. It is imperative this not form a continuous shield about the loop- that there be a break, usually opposite the feed point. And at this feed point, to keep the loop balanced, a balun of sorts is necessary.
Great stuff in general. I believed that too in the beginning. But I did a test to determine if the inner "shielded wire" is the actual antenna element, or is it the shield itself being the antenna element via common mode?

Do this test - for coax loops or small tubing loops, put a snap-on ferrite or two over one of the arms, at least a little bit away from the bottom 0-volt point, like half way up, and watch the sensitivity and directivity go absolutely nuts. If the loop was using the center shielded wire as the antenna, then the ferrite choke should make no difference.

Thus, for a shielded loop with the gap at the top, this gap IS the feedpoint from a common-mode shield antenna, to the transmission line internal mode like normal.

But the point here is not to win this age old argument - IF one can keep their transmission line and simple wire antenna balanced and isolated, there is no need of shielding - because in reality, one is using the common-mode of a shield to emulate a wire anyway! Just more complicated.

Anyway the snap-on ferrite test beats throwing Tierman, Maxwell and others around endlessly like when I first encountered this on usenet many years back. It was the "oh wow, no" moment for me when I did it.

Last edited by hertzian; 08-06-2018 at 1:09 AM..
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Old 08-07-2018, 10:22 AM
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Majoco and Hertz- I could have used your guy's expertise years ago as I stumbled thru this topic.
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My lab was providing the technical assistance to a VLF "Whistler' project- we were trying to come up with antennas for the extreme ELF regions. Loops seemed best suited for what the researchers wanted , and from that experience I drew my knowledge about shield'd v.s non shield'd antennas. We were looking at resonant loops for directivity, but as immune to man made noises as possible.
As you point'd out, Hertz- there is more to it than simply shielding the coil when it comes to common mode currents... In the end it was decided to move the whole project high above the Arctic circle to escape certain noise limitations.
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Ahhh....today I do miss those field projects..... .........
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To you Tuihill, you can see that this loop question gets quite deep. But if you are willing to experiment, it can be quite an interesting learning experience.
You might try winding your own loops and seeing how they fare.
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On a side note, maybe give a thought to listening to the VLF "whistlers'- these are really truly long distance LW DX signals.
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Old 08-07-2018, 3:24 PM
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VLF 'whistlers' - hmmmm....somewhere in my pile of old magazines must be that article that I made a 'whistler receiver' from, probably 'Practical Wireless' but maybe 'Wireless World' - at least 45 years ago. ISTR that it was an audio amplifier that went up to about 50kHz, in front of that was a balanced demodulator and a tunable oscillator - I taped all the noises onto a battery cassette deck. Had to be miles out into the UK countryside to get away from power lines and TV radiation and I had a spool of wire that I reeled out over the grass. Some fascinating noises that I couldn't attribute to anything I could see except for one night there was a Perseid shower - all sorts of ascending and descending tones, great explosions, almost birdsongs and twitters - amazing. I wonder where the tapes are....

That's got the grey matter stirring - I see this is "perseids week".....

https://www.space.com/32868-perseid-...wer-guide.html

https://www.stuff.co.nz/science/8292...ucky-few-Kiwis
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Old 08-07-2018, 4:06 PM
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I always wanted to do a "whistler" antenna project, but never got around to it. Gotta' cross that off my bucket list so I guess I should start building....

Forgot - the beautiful thing about a shielded loop, is that is naturally incorporates it's own balun . How?

Quick definition of the loop - shields connected together at the bottom - junction box - zero rf volt potential. Out of phase rf signals cross the common-mode shield of the loop at different times, and produce a differential voltage across the gap, and that signal travels down the inside transmission line mode of the loop arms.

BUT, this also means that the equally distributed loop arms effectively cancel any reception from the transmission line itself. If you have any problems with say a 100 foot run of coax hanging in the air, with a lot of common mode reception on it, once that pesky noisy current reaches the loops junction box, it splits equally up the loops' outside shield, and hence no differential voltage at the gap, thus no common-mode noise problems from the transmission line.

Basically it's hard to go wrong with a shielded loop, no matter which surface one believes is doing all the reception!

Propeller-head info: because there is a LOT of distributed capacitance when you make a broadband shielded loop, an antenna tuner in the shack helps. This usually means settings that incorporate a lot of inductance, and it is very sharp. So choose coax / hardline etc, that has the lowest distributed capacitance spec.

Here is a classic design for the 80m band - and the loop is tuned with a capacitor in the junction box. For a wide-band version, I forego the variable capacitor and transformer in the junction box, and just tune the whole thing with a tuner in the shack. Make it larger in circumference if you want to go VLF - although at some point there will be so much distributed capacitance that the tuner won't be able to handle it and now you are looking at a preamp instead. Don't get hung up on SWR and the like - we aren't transmitting on it.

http://seed-solutions.com/gregordy/A...ion/3805er.htm

Another good alternative is to use about the same 12 feet of coax, but if you look at these simplified diagrams you don't even need a junction box (just enlarge this project!) by making a pigtail at the very end of the coax with the center conductor and braid shorted together, and then brought back to a little bit of exposed braid on the transmission line itself while trying to keep the gap and arms symetrical in length. I've used this technique in the field with junk coax laying around, no junction box, no soldering iron, or other fancy materials, and just used duct tape *tightly* wound. For a couple of day's fun maybe.

http://emcesd.com/tt120100.htm

Tuihill -- despite all this, a coaxial shielded loop is very simple really. Make your cuts as balanced and symetrical as you can - although it doesn't need surgeon like accuracy, throw an antenna tuner in the shack and you are done. I always marvel at it's inherently interesting engineering beauty.

Last edited by hertzian; 08-07-2018 at 5:06 PM..
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Old 08-07-2018, 4:56 PM
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This is probably going off topic, but I'm good at doing that....
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The Whistler projects I was involved with were plagued with all sorts of unresolvable man made noises- That became the 'why' the project people ended up taking us up into the frozen Far North of the Canadian Northwest Territories. For receivers they used basically high gain audio amplifiers connected to our various antennas. I am sad to say our best loops (according my lab) were not the prime performers- It actually turned out short verticals worked the best. The challenge was to provide a very high input impedance so as not to attenuate the very low frequencies- in these cases, well into the audio range.
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For us, the support team, the logistics was 'fun'- for a lack of a better term. The receiving station was located about a mile from our base camp- and nothing electo-mechanical was allowed any closer. That meant tents heaters were portable oil stoves, (-20 to -30deg. F) tents were lit with pressure lamps-- and every thing electronic was powered by batteries. When a support team went out to the receiver site it was by 'mushing' out on snowshoes, pulling sleds- Roald Amundsen and Robert Scott , would have been proud of us
Even though we took precautions, the static discharges generated from simply walking out there were heard by the receiving team hundreds of yards before we ever arrived at the site.
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If I ever longed for the nomadic life of an Inuit, (Not !) it ended that season... still, the cold clear black skies, the stream auroras- and the eerie songs of the "Whispers" made it all worthwhile.... and no one got frostbiten.

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Oh, least I forget it, thanks for the heads-up on this weekend's meteor shower- it had slipped completely past me....
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Old 08-07-2018, 5:06 PM
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Old 08-07-2018, 6:09 PM
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I am very grateful to have received all of these responses from you.
Much of the information is way over my head but I get the ideas behind most of it.

I've ordered the W6LVP kit from Larry Plummer. Just the amplifier and power inserter.

The radio is sitting beside a window on the outside wall. I have a 50' length of RG8/U cable that will run straight out of the house as far as I can to one of the posts on a nearby fence. On the post I will rig the mounting mast to hang the loop about 2 - 3 metres above ground for the first go at it anyway. I will try different materials and diameters for the loop itself.

I did find a portable 12 volt rotator out there but it is not meant for permanent outdoor installation. Portable Rotation

I might play with two lengths of string through the window for a low-tech solution. I may visit a hobby store to check out R/C servos. Someone suggested getting a small car windscreen wiper motor from a wreckers yard. We'll see how well the strings work first.

Once again, my thanks to all for your interest and advice. I will let you know how I get on.

Cheers from Te Pahu.
Rick

ps: I will also order the Loop Antenna Handbook by Joe Carr.

Last edited by tuihill; 08-07-2018 at 6:14 PM.. Reason: added ps
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Old 08-07-2018, 7:44 PM
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I hope that's a flask of your favourite VSOP on your left hip.... Oops, sorry, nothing stronger than Coca-Cola in the US Mil! At least we had RUM to keep us warm!
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