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Author Topic: Magnetic loop TX antennas  (Read 1377 times)

KK4GMU

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Magnetic loop TX antennas
« on: September 06, 2022, 07:34:44 AM »

How far should a magnetic loop transmit antenna be kept from a metal chain link fence or from metal patio furniture?

I have a very constrained small patio area only 8' by 20' surrounded by chain link on two sides and concrete block walls on 2 sides.

I am looking at the Pracomm SML-1000-1.
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K5LXP

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Re: Magnetic loop TX antennas
« Reply #1 on: September 06, 2022, 07:58:39 AM »

Doesn't sound like you have much choice.   "As far away as possible", after that you get what you get.  Any chance of elevating the loop above the fence?

Mark K5LXP
Albuquerque, NM
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AC7CW

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Re: Magnetic loop TX antennas
« Reply #2 on: September 06, 2022, 08:03:10 AM »

I had one in horizontal orientation up about 20' on a metal pole the base of which was about 8' from a chain link fence. I liked it for working cw on 30 meters because not much frequency changing was necessary. It seemed to be somewhat nvis... sunspots were low and i don't recall hearing any dx but I worked hams in the southwest region from southern california
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K6SDW

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Re: Magnetic loop TX antennas
« Reply #3 on: September 06, 2022, 08:10:18 AM »

To quote KE0OG, the ARRL and QST new technical editor: When it comes to antennas "Everything affects everything." Dave has an excellent YouTube channel choc full of videos on simple to very complex technical subjects.

GL/73
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KK4GMU

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Re: Magnetic loop TX antennas
« Reply #4 on: September 08, 2022, 06:08:34 AM »

Here are some generalizations I've learned about magnetic loop antennas for transmitting so far.

1. They are best suited for portable use or for stealth/semi-stealth HOA situations. Deciding your primary purpose is key to the features you will need most.
2. They're not cheap:  Commercial ones range from $200 to thousands. The most popular ham versions are in the $300 to $400 range.  Home brew can be less than $100 with the limitations listed below.
3. The "big three" seem to be Alpha, Chameleon, and Alex. These are mostly in the +$300 to $400 price range.
4. They do NOT need to be mounted high up.  They can be mounted one loop height (36" to 48") above the ground.  They do not require a ground plane.
5.  Those less than $400 have limitations that many of the more expensive ones overcome, such as:
  • Skinny/thinner loops with not much "skin" surface.  The more conductive surface area there is on the main loop, the higher the efficiency up to a point. One to two inch diameter (or wide, flat surface) is ideal, with the tradeoff being bulk and weight.
  • Requires the operator being next to the antenna to tune. Lacks remote tuning.
  • EXTREMELY narrow bandwidth (high Q) requiring a geared tuning knob to achieve more precise tuning. Some units in this price range do NOT have gearing, making the tuning frustratingly touchy.
  • Having to tune at the variable capacitor that is mounted right next to the antenna results in the close proximity of the hand changing the tuning, making tuning that much more difficult.
  • Transmit power is generally limited to 15 to 25 watts due primarily to the limitations of the variable capacitor.  All or most mag loops under $1,000 use "air" variable capacitors - the kind in older radios.  The cheaper ones have fairly thin fins that are close together.  More than 20 or 30 watts transmitted into these potentially generates hundreds or thousands of volts to the capacitor which will produce arching for these smaller capacitors shooting the SWR to infinity.  Not good.  Higher power handling air capacitors cost $30 or more on ebay.  The highest power handling variable capacitors are "vacuum variable".  These range from $70 to several hundred dollars or more and capable of handling several hundred watts of transmission input in the mag loop application.

Since my use for such antenna would be stationary at home, I have no need for light weight, compactness, portability, foldability or carry bags.

I do have a need for the following:
  • Fairly stealthy (mounted low, not conspicuous)
  • Compact (no long stretched out wires).  I have a small space, no trees on my property)
  • Remote tuning.  The antenna will be outside, 15 feet from my indoor shack location.  Having to go back and forth outside to tune it is a non-starter.
  • Weather proof.  The cheaper ones are not. And I will not attempt to challenge my DW by dragging the loop into the house when it rains.  Even if there wasn't a DW, I wouldn't want that aggravation.
  • Higher power. Spending $400 on an antenna that can only handle 25 watts from my $1000 100 watt-capable radio makes no sense to me.
  • Commercially designed and built.  My priority is not an antenna building hobby. I understand there are hundreds of YouTube home brew mag loop designs and projects.  Some are tempting to try.  But my objective is to get something that works "out of the box." That potentially eliminates MFJs loop, but stay tuned. And by the time I get a high powered air variable capacitor and remote stepper motor hookup, I would be out several hundred bucks, dozens of hours of time, and a sketchy, dubious outcome.
In my opening post I mentioned the Pracomm SML-1000-1.  After two emails to them last week, and a phone call that went nowhere, I doubt they exist anymore, despite being reviewed by Dave Kasler a few years back.  But reconsidering that particular mag loop, while it can handle 100 watts, there was no remote tuning capability.

That brings me to MFJ.  Higher power handling (150 watts) and remote control.  Their MFJ-1788 gets mediocre to fair reviews, like 3.5 stars.  That non-stellar review is NOT due to the performance of the antenna once properly assembled.  I interpret all the reviews I have seen of that antenna as being at least a 4 or 4.5 star performer.  The great majority of negative reviews are due to MFJs crappy quality control.  Many reviews comment on bent variable capacitor fins, misaligned capacitor fins or loose shaft, loose or improperly installed screws and wires, and related fairly easily user-fixable items.  The performance of the antenna, once the "semi-kit" is properly put together, is generally praised.  But $700+.  Hmmm.  That is roughly $300 more than the "big three" mag loops.  For the extra $$$ you get higher power, remote control, and a semi-kit.

So, I have laid out my thinking to hopefully help those who are similarly HOA, space, and no-tree constrained, and who want an HF antenna that by most reports I have seen performs "almost/just about" as well as a multiband dipole up high.

If I got this, I would have to sell my MFJ-993B auto tuner and G5RV Junior antenna to help pay for it.

One of my favorite Florida expressions:  Is the juice worth the squeeze?

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KK4GMU

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Re: Magnetic loop TX antennas
« Reply #5 on: September 08, 2022, 07:49:04 AM »

I saw this on ebay for $300.  But local pickup only in California.  If it was in Florida I'd probably go for it.

MFJ-1786 Super Hi-Q Loop Antenna ~Now with REMOTE! LOCAL PICK UP ONLY 92647
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K5LXP

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Re: Magnetic loop TX antennas
« Reply #6 on: September 09, 2022, 10:39:21 AM »

"Everything Works" - N6BT

Expectation is a significant factor of your project here.  You've put a few unnecessary (in my opinion) constraints on the situation and in reading your message, there is only one possible conclusion.  That being the case, full steam ahead - get the loop, put it up and you'll decide if it works well enough for your goals.  Seems it might be on par with a mobile antenna, and in that respect it will absolutely make contacts and maybe well enough to keep your interest.  My observation though is that is the best it will ever be, where with other options performance and capability would be better, which allows for operating under more compromised conditions.  Everyone has their minimum point on the enjoyment chart in N6BT's article but when you start at the bottom, that's as good as it gets.  I wouldn't be "all in" financially for a solution that at best isn't all that great.  If mag loops at ground level were "almost as good" as dipoles you'd see more people using them.

Mark B.
Albuquerque, NM
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KK4GMU

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Re: Magnetic loop TX antennas
« Reply #7 on: September 09, 2022, 11:06:14 AM »

"Everything Works" - N6BT

You've put a few unnecessary (in my opinion) constraints on the situation and in reading your message, there is only one possible conclusion. 

That being the case, full steam ahead - get the loop, put it up and you'll decide if it works well enough for your goals. ...  I wouldn't be "all in" financially for a solution that at best isn't all that great.  If mag loops at ground level were "almost as good" as dipoles you'd see more people using them.

Mark B.
Albuquerque, NM

What do you believe the "unnecessary constraints" are?

Living with and understanding my constraints, I am not yet aware of other options for HF except the G5RV Jr I have mounted flush on my asphalt shingle roof. Not fully tested, but it looks like low SWR on only small segments of couple of lower bands.

I suspect more people would be using mag loops if more people were living with my constraints. (Please refer to first post under  "Antenna Restrictions.")
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K5LXP

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Re: Magnetic loop TX antennas
« Reply #8 on: September 09, 2022, 01:06:09 PM »

I used to live in the worst HOA ever - military housing.  After that I lived in a house on the national register, no visible antennas.  Then I lived in an apartment building with no yard and only a small community patio space.  I operated HF at all of them, and not one mag loop.  Fast forward to today and operating "portable" often results in people buying small loops and highly loaded antenna contraptions which might offer a "match" but especially QRP are frustrating at best to make contacts with.  You spend more time futzing around setting up and adjusting these antennas than making contacts in the field.  So from my perspective of over 40 years in this hobby operating HF in all manner of locations, you don't start with the poorest performing antennas, you go with the best performing ones you can possibly have.  Nearly always this will be a wire antenna.  Spending a bit of time figuring out how to orient some form of wire antenna within the space constraints will reap performance results over any "compact" reactance on a stick.  This isn't about breaking pileups or DXCC in a weekend running QRP.  Without an efficient antenna you run out of stations to work quickly, being more and more dependent on good propagation and strong stations that make up for your lack of signal margin.  Possible, and can be fun when it happens but you're locking in reduced opportunity with compromised antennas.  Sometimes you have no other choice but I wouldn't introduce limitations that don't actually exist.

Fairly stealthy (mounted low, not conspicuous)

What does that actually mean?  So a 3' loop on a stand is OK, but something like a nearly invisible wire or vertical isn't?

Compact (no long stretched out wires).

Would inconspicuous be OK? 

I have a small space, no trees on my property)

But there's a house, right?  Houses have height, length and depth, same dimensions as antennas.

Remote tuning.

Is in-shack tuning ruled out?

My priority is not an antenna building hobby.

This is a tight constraint.  You have a very specific set of operating conditions which demands a custom built solution.   You are binding yourself to find the lowest common denominator solution, eliminating even simpler, less expensive and better performing ones.  Yes, you or someone else might have to build and install it (somehow you managed to get a G5RV jr on the roof...).  You just have to weigh the option of an expensive poor performer that's plug and play, or stretch a bit and end up with something that will offer a better operating experience (read the N6BT article).

Mark K5LXP
Albuquerque, NM
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KK4GMU

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Re: Magnetic loop TX antennas
« Reply #9 on: September 09, 2022, 02:43:53 PM »

I find it amusing how some commentators can make assumptions about the physical and technical abilities of some of us asking questions about specific topics based on their own age, physical and technical experience and abilities.  Such advise would be helpful if I possessed their physical characteristics or knowledge.  Yes, if I had 40 years in the hobby I might have a solution.  But maybe not, considering the constrained physical environment in which I am located.
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WA3SKN

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Re: Magnetic loop TX antennas
« Reply #10 on: September 10, 2022, 05:21:54 AM »

The mag loop is a poorly radiating "speciality" antenna with a VERY HIGH "Q" (narrow bandwidth).
You seem set on trying one though, so go ahead and try it.  Some apartment dwellers love them!  I would not get rid of the tuner though... it is a useful device to have on hand.

-Mike.
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AI5BC

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Re: Magnetic loop TX antennas
« Reply #11 on: September 10, 2022, 08:02:28 AM »

This makes me think of golf course in the artic or a Jamaican bob sled team.
« Last Edit: September 10, 2022, 08:08:27 AM by AI5BC »
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KK4GMU

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Re: Magnetic loop TX antennas
« Reply #12 on: September 10, 2022, 09:54:11 AM »

Quote
I used to live in the worst HOA ever - military housing.  After that I lived in a house on the national register, no visible antennas.  Then I lived in an apartment building with no yard and only a small community patio space.  I operated HF at all of them, and not one mag loop.

You were HOW OLD when you had these experiences?  Did they even have consumer mag loops back then?

Quote
Fast forward to today and operating "portable" often results in people buying small loops and highly loaded antenna contraptions which might offer a "match" but especially QRP are frustrating at best to make contacts with.  You spend more time futzing around setting up and adjusting these antennas than making contacts in the field.

As I clearly understand, there is a lot of "futzing" designing, installing, testing, tuning, retesting, rinse and repeat several times ad nauseum to get a long wire or any other "home brew" in a highly constrained location to work wellish.  Or I could futz less with something that works reasonably well while transmitting and receiving, but obviously not up to your self-imposed standard.

Quote
So from my perspective of over 40 years in this hobby operating HF in all manner of locations, you don't start with the poorest performing antennas, you go with the best performing ones you can possibly have.

And that is the key. 1) Your perspective after 40 years experience in making antennas 2) Your assumption that your "best performing" solution is achievable in my environment with my set of circumstances and abilities.

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Nearly always this will be a wire antenna.

With emphasis on "nearly" while ignoring other impinging and critical factors.

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Sometimes you have no other choice but I wouldn't introduce limitations that don't actually exist.

Again, and this is key:  An assumptions that my "limitations...don't actually exist."  Do "age" and "physical abilities" count as "self-imposed?"  Is lack of 40 years in ham radio building antennas "self-imposed?"

Quote
What does "Fairly stealthy (mounted low, not conspicuous)" actually mean?  So a 3' loop on a stand is OK, but something like a nearly invisible wire or vertical isn't?

I was describing characteristics of a magnetic loop antenna.  Then you compared it to something I consider "infeasible" (long wire) due to my "self-imposed" (read allegedly "unreasonable") constraints.

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Would inconspicuous be OK?

Yes, that is one objective.  But not via something that I consider infeasible.

Quote
But there's a house, right?  Houses have height, length and depth, same dimensions as antennas.

Again, assuming that what is feasible for you is also feasible for me. My "house" is an attached multi-family condo in an HOA with heights that are a "no-go".

Quote
Is in-shack tuning ruled out?

No.  In fact, that is a requirement for any antenna I install.

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"Avoiding an antenna-building hobby" is a tight constraint.  You have a very specific set of operating conditions which demands a custom built solution.

"Demanding" a custom-built solution is based on your needs, expectations, objectives and capabilities.  Not everyone is you. The great thing about the ham radio or any hobby is that we don't need to react to the "demands" that are defined by someone else.

Quote
You are binding yourself to find the lowest common denominator solution, eliminating even simpler, less expensive and better performing ones.  Yes, you or someone else might have to build and install it (somehow you managed to get a G5RV jr on the roof...).

And that roof-mounted G5RV is not the type of project I care to repeat.

Again, "simpler" and "better performing" (based on my abilities and limitations) is from YOUR perspective and abilities. Such will not be "simpler" from my perspective. And while "better performing" is an ideal, it is irrelevant if not achievable based on my circumstances, abilities and constraints.

My "abilities" do not match yours.  My "limitations" do not match yours.  My limitations are not self-imposed, that is unless you consider age and level of physical abilities to climb and hang off ladders "self-imposed."

Quote
You just have to weigh the option of an expensive poor performer that's plug and play, or stretch a bit and end up with something that will offer a better operating experience.

Weighing I must.  Weighing I do.  All antennas are some level of compromise. "Poor performer" is relative to one's objectives and expectations. What is "poor performer" to an individual with 40 years in the hobby is not necessarily that to one who has different skills, abilities and unavoidable (not "self-imposed") constraints.

There are some here who would rather get on the air with more compromise than others rather than try to do what they are not capable of just because someone else believes differently.
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W9IQ

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Re: Magnetic loop TX antennas
« Reply #13 on: September 10, 2022, 10:52:04 AM »

Remote tuning.  The antenna will be outside, 15 feet from my indoor shack location.  Having to go back and forth outside to tune it is a non-starter.

I notice that the couple of loops you have considered require tuning at the antenna in order to use them - that would classify them as a "non-starter". Also check any antenna your are considering to see if it is rated to be left outdoors - not many are.

A remotely tuned version is relatively expensive to buy although there are plenty of articles on the web for constructing the circuitry and mechanics to remotely adjust the tuning capacitor. If this is within your ability and interest to construct, you may be able to convert a non-starter to have your required features.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KK4GMU

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Re: Magnetic loop TX antennas
« Reply #14 on: September 10, 2022, 11:26:52 AM »

Remote tuning.  The antenna will be outside, 15 feet from my indoor shack location.  Having to go back and forth outside to tune it is a non-starter.

I notice that the couple of loops you have considered require tuning at the antenna in order to use them - that would classify them as a "non-starter". Also check any antenna your are considering to see if it is rated to be left outdoors - not many are.

A remotely tuned version is relatively expensive to buy although there are plenty of articles on the web for constructing the circuitry and mechanics to remotely adjust the tuning capacitor. If this is within your ability and interest to construct, you may be able to convert a non-starter to have your required features.

- Glenn W9IQ
Yes, you noticed correctly.  The more I learn about loops the more I understand those that won't work for me.   I will only consider remote-tuned loops capable of higher (100 watt or greater) power and designed for permanent outside deployment.  The only non-home-brew I am aware of that meets all three criteria for less than $800 are the MFJ-1782, 1784, 1786, 1788, and 1789.  And even those, according to reviews, should be considered semi-kits (as I mentioned in my OP) due to requiring fairly easy to fix QC issues.

I also wonder about the MFJ-2389 8-band "stealth vertical, aka "Ugly Stick" performance compared to the MFJ-1788 loop.   While I wouldn't consider the ugly stick because of it being, well, "ugly" (too many protruding elements at 12++++ feet up) it won't pass "stealth" objectives.  And, according to most reviews, after challenging assembly and tuning, the upper bands (60 and 80) are very narrow - bands the loop doesn't get anyway.  But the loops 20 to 40 meter bands are probably "at least" as narrow.
« Last Edit: September 10, 2022, 11:50:08 AM by KK4GMU »
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