Experiences using a portable HF loop

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Experiences using a portable HF loop

k6sdw
Greetings all, I'm curious to hear from anyone using a portable HF magnetic loop. I've read the theory behind them so I'd like to hear your opinion about them? How efficient these antennas are, especially running qRP.


Thanks all,


73


ed
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Re: Experiences using a portable HF loop

Elecraft mailing list

I have never used one but knew someone who did, he basically said a end fed wire out performed it.



      From: Eddy Avila <[hidden email]>
 To: Elecraft <[hidden email]>
 Sent: Friday, April 21, 2017 9:36 PM
 Subject: [Elecraft] Experiences using a portable HF loop
   
Greetings all, I'm curious to hear from anyone using a portable HF magnetic loop. I've read the theory behind them so I'd like to hear your opinion about them? How efficient these antennas are, especially running qRP.


Thanks all,


73


ed
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Re: Experiences using a portable HF loop

Phil Hystad-3
In reply to this post by k6sdw
The May 2017 QST not only does a review of the Elecraft KX2 but it reviews the Alpha Antenna, 10-40 meter support.  This is a magnetic loop antenna and I think it retails (basic model) for $299.

73, phil, K7PEH

> On Apr 21, 2017, at 6:24 PM, Eddy Avila <[hidden email]> wrote:
>
> Greetings all, I'm curious to hear from anyone using a portable HF magnetic loop. I've read the theory behind them so I'd like to hear your opinion about them? How efficient these antennas are, especially running qRP.
>
>
> Thanks all,
>
>
> 73
>
>
> ed
> ______________________________________________________________
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Re: Experiences using a portable HF loop

Augie "Gus" Hansen

On 4/21/2017 9:00 PM, Ron D'Eau Claire wrote:
> The very BEST magnetic loops are incredibly inefficient, yet like any
> "incredibly inefficient" antenna one can make amazing contacts on them when
> conditions are right.

If you use the suggestion that the loop perimeter (circumference for a
circular loop) be 1/10 of a wavelength, then efficiency is quite low -
on the order of 10%. This is the guidance given for small receiving
loops to get the best nulls.

However, a transmitting loop can have a perimeter of up to 1/3
wavelength and still have a constant current throughout its length
(within 1% per John Kraus). This constant loop current is what defines a
magnetic loop.

> The problem is with resistive losses. The day we have room-temperature
> superconductors, we will have efficient small transmitting loops. Until
> then, only a few percent of the power applied is radiated.

My loops are typically a bit short of 1/3 wavelength at the high
frequency of the tuning range, and no less that 1/10 wavelength at the
low end. The efficiency of a 3/10 wavelength loop, for example, is in
excess of 90% provided the resistances are kept low. I typically use the
outer (shield) of hard line for the loop conductor and a series vacuum
variable capacitor for remote tuning.

It is also possible to use multiple turns to reduce the size of a
magnetic loop even further. There is an example of a 40-80 meter mag
loop on my QRZ page that uses 7/8-inch hard line in a two-turn
configuration. Even though only 4.5 feet in diameter, it works quite
well when compared to a typical back yard inverted vee or low dipole,
and it can be hidden in an attic, the back of a garage, or tucked out of
sight among trees or bushes.

73,
Gus Hansen
KB0YH

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Re: Experiences using a portable HF loop

Michael Babineau-2
I agree with almost everything that Ron says in the post, except I would substitute the
statement  "The very BEST magnetic loops are incredibly inefficient” with “The very BEST magnetic loops are incredibly inefficient on the lower bands”.

Pretty much all of the Magnetic Loops on the market are around 1 meter in diameter, which means that circumference of
the radiator is about 10 feet.  This size of loop if well constructed can have efficiencies approaching 90% on 10m and something
in the range of 30 to 40 % on 20m, so they will work reasonably well on these higher bands.
The wheels start to fall off on 30m and especially 40m, where typically you would be looking at single digit efficiencies.  
This is really not surprising, if you put a 10 foot vertical on 40m you are going to find that it is not very efficient either !
If you had a 2 meter diameter loop then efficiencies on 40m and 30m would be significantly improved, but you would lose the
ability to resonate it on 10m/12m and likely on 15m and 17m too.


What a 1 meter diameter Magnetic Loop has going for it is extreme portability (at least for those with a foldable radiator),
quick deployment (typically < 5 minutes for something like the Alex Loop), ground independence (no need for radials), some bi-directionality
in the plane of the loop but more significantly a very deep null broadside to the loop which can be effective at killing a noise source.
It also typically provides continuous band coverage from 10m through 40m with 2:1 SWR bandwidths for a well
constructed loop around 100+ Khz on 10m dropping to something around 10 Khz on 40m, all in a very small package. It is both a reasonable DX antenna
on the higher bands as well as a usable NVIS antenna on 40m. As with any antenna they work better if higher, but as a minimum they must be 1 loop radius above ground, so
mounting on a lightweight camera tripod works ok.

It is possible to make NVIS QSOs on 40m using only a few hundred mW of power, often with surprising signal strength, so putting 5 or 10 watts into an
antenna with  5% efficiency can still yield a usable signal.


Cheers

Michael VE3WMB

P.S. The secret to squeezing out the best performance when building a loop is having a solid conductor that is welded/soldered to
the tuning capacitor (you need to try to avoid mechanicalconnections as they add resistance) and using either a split stator or butterfly air variable
capacitor or a vacuum variable capacitor. Cheap capacitors with wiper contacts on the rotor will kill your efficiency.

The radiation resistance of a Magnetic Loop is low (typically less than a few ohms) and it drops as you press it into service on lower frequencies. Even small additional
resistances add up and become significant when compared to the radiation resistance on the lower bands and this is what causes the Loop efficiency to go down the toilet.


>From: "Ron D'Eau Claire" <[hidden email] <mailto:[hidden email]>>
>Subject: Re: [Elecraft] Experiences using a portable HF loop
>Date: April 21, 2017 at 11:00:09 PM GMT-4
>To: "'Phil Hystad'" <[hidden email] <mailto:[hidden email]>>, "'Eddy Avila'" <[hidden email] <mailto:[hidden email]>>
>Cc: "'Elecraft'" <[hidden email] <mailto:[hidden email]>>


>The very BEST magnetic loops are incredibly inefficient, yet like any
>"incredibly inefficient" antenna one can make amazing contacts on them when
conditions are right.

>The problem is with resistive losses. The day we have room-temperature
>superconductors, we will have efficient small transmitting loops. Until
>then, only a few percent of the power applied is radiated.

>For now, they are great when simplicity of setup allows operation where
>otherwise nothing could be done.

>73, Ron AC7AC
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Re: Experiences using a portable HF loop

David Orman
In reply to this post by Phil Hystad-3
I would agree with the portable lossy versions. Definitely not the properly
constructed versions. The loop on the left is for transmitting, and is 2"
diameter tubing. Yes, I have some weed removal due. :) Not all loops are
created equal, with minimal resistive losses and a well designed
capacitor/tuning mechanism they can be relatively efficient. This one is
quite good on 20, and better on 40 than any traditional wire antenna I
could setup with the yard and lack of trees. A halfwave dipole a halfwave
up on 40m would outperform it by 4dB or so. No way I could get an antenna
like that setup. The trade-off is of course being extremely high q, a very
narrow bandwidth. Hence the RX loop on the right.

https://goo.gl/photos/PExWyEYZmtVQDSgf7

The downside is the weight and bulk, a small transmitting loop made out of
some coax is going to be down on efficiency significantly, and even low
wire antennas probably will outperform them. Sure are easier to setup when
you don't have trees around, however. Just wanted to make sure people
understand STLs are not necessarily low efficiency, just certain designs
are, and the portable models I've seen all fall in that category.

On Apr 21, 2017 22:02, "Ron D'Eau Claire" <[hidden email]> wrote:

> The very BEST magnetic loops are incredibly inefficient, yet like any
> "incredibly inefficient" antenna one can make amazing contacts on them when
> conditions are right.
>
> The problem is with resistive losses. The day we have room-temperature
> superconductors, we will have efficient small transmitting loops. Until
> then, only a few percent of the power applied is radiated.
>
> For now, they are great when simplicity of setup allows operation where
> otherwise nothing could be done.
>
> 73, Ron AC7AC
>
> -----Original Message-----
> From: Elecraft [mailto:[hidden email]] On Behalf Of Phil
> Hystad
> Sent: Friday, April 21, 2017 7:45 PM
> To: Eddy Avila
> Cc: Elecraft
> Subject: Re: [Elecraft] Experiences using a portable HF loop
>
> The May 2017 QST not only does a review of the Elecraft KX2 but it reviews
> the Alpha Antenna, 10-40 meter support.  This is a magnetic loop antenna
> and
> I think it retails (basic model) for $299.
>
> 73, phil, K7PEH
>
> > On Apr 21, 2017, at 6:24 PM, Eddy Avila <[hidden email]> wrote:
> >
> > Greetings all, I'm curious to hear from anyone using a portable HF
> magnetic loop. I've read the theory behind them so I'd like to hear your
> opinion about them? How efficient these antennas are, especially running
> qRP.
> >
> >
> > Thanks all,
> >
> >
> > 73
> >
> >
> > ed
> > ______________________________________________________________
> > Elecraft mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:[hidden email]
> >
> > This list hosted by: http://www.qsl.net Please help support this email
> > list: http://www.qsl.net/donate.html Message delivered to
> > [hidden email]
>
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> delivered to [hidden email]
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Re: Experiences using a portable HF loop

WD4SDC
In reply to this post by k6sdw
I've had good results using LMR400 (outer conductor) and an air dielectric tuning cap which will handle the 10-15W power of a KX3.  In some informal testing comparing a 6 foot diameter loop and a 1/4 wave wire antenna (40 meters) using a car chassis as a counterpoise, the loop beat the wire by 5dB (NVIS path) (C/N on spectral display).

How efficient is the loop? I don't know - not as efficient as a 4" diameter copper pipe 6 foot loop, but I do know it's relative ;)

A mobile whip is *very* inefficient.  The loop in this case is generally *less* inefficient :)
You can make contacts on both.

K4HKX did a very detailed comparison of loop antennas with full dipoles and other HF antennas.  He posted his results here:

qrz.com/db/k4hkx

Very interesting read.

I'm playing around with a mobile loop idea for 80 and 40 meters using the KX3.  If it works as good as I think it will, it should give me about the same performance as adding a KXPA100 - or not... that's why playing with ham radio is fun.

73's
Steve
WD4SDC
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Re: Experiences using a portable HF loop

Grant Youngman-2
In reply to this post by k6sdw
A wire will not always outperform a loop.  It depends among other things on whether or not you have a way to hang the wire at a reasonable height.  Typically useless on the beach, for example, unless you pack a pole or two to get it up in the air, which means guys and all the rest.  Doable, of course, but a lot of junk to tote around even if you have relatively compact extendable poles, plus the time to put it up.  Poles/guys can be a problem when there are a lot of people around (beach/park/etc) or children who will be attracted to them like a magnet to trip over, etc..  You get the picture.  It’s these kinds of situations where the loop shines.  Stick it on a table or light tripod and you’re on the air in 5 minutes.  

With my KX3, depending on where I’m going and what the landscape looks like I use one or more of various wires, Buddipole hardware (light weight mast, shock cord whip, miscellaneous bits), or an AlexLoop.  I’m currently waiting delivery on a W4OP loop.  The best I can say about any of these options is that if it’s what you have, use it.  They all work, sometimes surprisingly well.  After a couple of years of this, I can’t say definitely which is always “best”.  I”m getting too old and decrepit to still be much of a hiker, so I’m generally driving somewhere and walking a relatively short distance to a picnic table in a park, down to the neighborhood dock, or the beach, etc.

The AlexLoop is very light to carry, but a downside is that it does NOT have any sort of tripod or table mount.  You’re on your own jury rigging something out of PVC and clamps to hold it up, or conjuring something that can screw into a tripod, etc. (there are plans on the web).  None of it is rocket science, but it isn’t just a standard part you can purchase.  

> Greetings all, I'm curious to hear from anyone using a portable HF magnetic loop. I've read the theory behind them so I'd like to hear your opinion about them? How efficient these antennas are, especially running qRP.
>
>

Grant NQ5T
K3 #2091, KX3 #8342



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Re: Experiences using a portable HF loop

donovanf
The electric field of a horizontally polarized antenna is parallel to
the earth and it induces significant current into lossy earth if
its less than about 0.1 wavelength high (14 feet on 40 meters).


If you can install a horizontally polarized antenna 0.1 wavelengths
high or higher, its efficiency is superior to field expedient
vertically polarized antennas such as "magnetic" loops. This
is why end-fed half wavelength (EFHW) horizontally polarized
antennas are so popular with QRP back packers where efficiency
is very important.


On a mountaintop, a relatively low EFHW antenna can produce
astounding QRP results. These affects are very easily observed
(and measurable) with a 200 milliwatt WSPRlite transmitter and
a USB battery that will easily fit on your pocket.


On the other hand, vertically polarized antennas -- such as small
"magnetic" loops -- produce significant low angle radiation
even at very low heights. Unfortunately they also suffer from
proximity to lossy earth. They really excel at a salt water beach
(but you must be very close to salt water) or in a salt marsh (not a
particularly pleasant place to take your family or girlfriend,,,).


73
Frank
W3LPL





----- Original Message -----

From: "GRANT YOUNGMAN" <[hidden email]>
To: "Elecraft" <[hidden email]>
Sent: Saturday, April 22, 2017 5:36:33 PM
Subject: Re: [Elecraft] Experiences using a portable HF loop

A wire will not always outperform a loop. It depends among other things on whether or not you have a way to hang the wire at a reasonable height. Typically useless on the beach, for example, unless you pack a pole or two to get it up in the air, which means guys and all the rest. Doable, of course, but a lot of junk to tote around even if you have relatively compact extendable poles, plus the time to put it up. Poles/guys can be a problem when there are a lot of people around (beach/park/etc) or children who will be attracted to them like a magnet to trip over, etc.. You get the picture. It’s these kinds of situations where the loop shines. Stick it on a table or light tripod and you’re on the air in 5 minutes.

With my KX3, depending on where I’m going and what the landscape looks like I use one or more of various wires, Buddipole hardware (light weight mast, shock cord whip, miscellaneous bits), or an AlexLoop. I’m currently waiting delivery on a W4OP loop. The best I can say about any of these options is that if it’s what you have, use it. They all work, sometimes surprisingly well. After a couple of years of this, I can’t say definitely which is always “best”. I”m getting too old and decrepit to still be much of a hiker, so I’m generally driving somewhere and walking a relatively short distance to a picnic table in a park, down to the neighborhood dock, or the beach, etc.

The AlexLoop is very light to carry, but a downside is that it does NOT have any sort of tripod or table mount. You’re on your own jury rigging something out of PVC and clamps to hold it up, or conjuring something that can screw into a tripod, etc. (there are plans on the web). None of it is rocket science, but it isn’t just a standard part you can purchase.

> Greetings all, I'm curious to hear from anyone using a portable HF magnetic loop. I've read the theory behind them so I'd like to hear your opinion about them? How efficient these antennas are, especially running qRP.
>
>

Grant NQ5T
K3 #2091, KX3 #8342



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Re: Experiences using a portable HF loop

WD4SDC
In reply to this post by k6sdw
Last post didn't go thru...trying again...

I've had good results using LMR400 (outer conductor) and an air dielectric tuning cap which will handle the 10-15W power of a KX3.  In some informal testing comparing a 6 foot diameter loop and a 1/4 wave wire antenna (40 meters) using a car chassis as a counterpoise, the loop beat the wire by 5dB (NVIS path) (C/N on spectral display).

How efficient is the loop? I don't know - not as efficient as a 4" diameter copper pipe 6 foot loop, but I do know it's relative ;)

A mobile whip is *very* inefficient.  The loop in this case is generally *less* inefficient :)
You can make contacts on both.

K4HKX did a very detailed comparison of loop antennas with full dipoles and other HF antennas.  He posted his results here:

qrz.com/db/k4hkx

Very interesting read.

I'm playing around with a mobile loop idea for 80 and 40 meters using the KX3.  If it works as good as I think it will, it should give me about the same performance as adding a KXPA100 - or not... that's why playing with ham radio is fun.

73's
Steve
WD4SDC
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Re: Experiences using a portable HF loop

Dan Presley
In reply to this post by k6sdw
Eddie-I have both the AlexLoop and the W4OP loop and enjoy using them both in the right situations. It takes some patience to tune but I’ve worked a fair amount of both domestic and dx with these loops. The AlexLoop is best on 20 and higher, and I use an inexpensive Vivitar tripod VIV-VPT-1250 and it works fine to elevate the loop, and is super lightweight. I use the loops on SOTA outings especially if there’s no easy way to put up a wire. They can be as effective as a vertical in the right environment and a lot easier to put up. They also exhibit some directivity an as they can be rotated to null noise or unwanted signals out.  One more nice trick in the antenna bag.

Dan Presley  N7CQR
[hidden email]


> On Apr 21, 2017, at 6:24 PM, Eddy Avila <[hidden email]> wrote:
>
> Greetings all, I'm curious to hear from anyone using a portable HF magnetic loop. I've read the theory behind them so I'd like to hear your opinion about them? How efficient these antennas are, especially running qRP.
>
>
> Thanks all,
>
>
> 73
>
>
> ed
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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Re: Experiences using a portable HF loop

WD4SDC
In reply to this post by k6sdw
 Tried posting w/ web interface....that didn't work, so will try
here....please excuse dups if that occurs.
-------------------------------------------------------------------------------------------------------------------------------


I've had good results using LMR400 (outer conductor) and an air dielectric
tuning cap which will handle the 10-15W power of a KX3.  In some informal
testing comparing a 6 foot diameter loop and a 1/4 wave wire antenna (40
meters) using a car chassis as a counterpoise, the loop beat the wire by
5dB (NVIS path) (C/N on spectral display).

How efficient is the loop? I don't know - not as efficient as a 4" diameter
copper pipe 6 foot loop, but I do know it's relative ;)

A mobile whip is *very* inefficient.  The loop in this case is generally
*less* inefficient :)
You can make contacts on both.

K4HKX did a very detailed comparison of loop antennas with full dipoles and
other HF antennas.  He posted his results here:

qrz.com/db/k4hkx

Very interesting read.

I'm playing around with a mobile loop idea for 80 and 40 meters using the
KX3.  If it works as good as I think it will, it should give me about the
same performance as adding a KXPA100 - or not... that's why playing with
ham radio is fun.

73's
Steve
WD4SDC

On Fri, Apr 21, 2017 at 9:24 PM, Eddy Avila <[hidden email]> wrote:

> Greetings all, I'm curious to hear from anyone using a portable HF
> magnetic loop. I've read the theory behind them so I'd like to hear your
> opinion about them? How efficient these antennas are, especially running
> qRP.
>
>
> Thanks all,
>
>
> 73
>
>
> ed
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>
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Re: Experiences using a portable HF loop

Paul Stoetzer
In reply to this post by Michael Babineau-2
I made well over a thousand 40m QSOs with my KX2 and AlexLoop during
NPOTA. I was getting true 57 and 59 reports at times with 10 watts to
the loop and even worked a couple of stations on the West Coast on 40
meters one morning.

As you said, it was very effective on NVIS with easy QSOs up and down
the eastern seaboard during the daytime hours.

73,

Paul Stoetzer, N8HM
Washington, DC

On Sat, Apr 22, 2017 at 10:35 AM, Michael Babineau <[hidden email]> wrote:

> I agree with almost everything that Ron says in the post, except I would substitute the
> statement  "The very BEST magnetic loops are incredibly inefficient” with “The very BEST magnetic loops are incredibly inefficient on the lower bands”.
>
> Pretty much all of the Magnetic Loops on the market are around 1 meter in diameter, which means that circumference of
> the radiator is about 10 feet.  This size of loop if well constructed can have efficiencies approaching 90% on 10m and something
> in the range of 30 to 40 % on 20m, so they will work reasonably well on these higher bands.
> The wheels start to fall off on 30m and especially 40m, where typically you would be looking at single digit efficiencies.
> This is really not surprising, if you put a 10 foot vertical on 40m you are going to find that it is not very efficient either !
> If you had a 2 meter diameter loop then efficiencies on 40m and 30m would be significantly improved, but you would lose the
> ability to resonate it on 10m/12m and likely on 15m and 17m too.
>
>
> What a 1 meter diameter Magnetic Loop has going for it is extreme portability (at least for those with a foldable radiator),
> quick deployment (typically < 5 minutes for something like the Alex Loop), ground independence (no need for radials), some bi-directionality
> in the plane of the loop but more significantly a very deep null broadside to the loop which can be effective at killing a noise source.
> It also typically provides continuous band coverage from 10m through 40m with 2:1 SWR bandwidths for a well
> constructed loop around 100+ Khz on 10m dropping to something around 10 Khz on 40m, all in a very small package. It is both a reasonable DX antenna
> on the higher bands as well as a usable NVIS antenna on 40m. As with any antenna they work better if higher, but as a minimum they must be 1 loop radius above ground, so
> mounting on a lightweight camera tripod works ok.
>
> It is possible to make NVIS QSOs on 40m using only a few hundred mW of power, often with surprising signal strength, so putting 5 or 10 watts into an
> antenna with  5% efficiency can still yield a usable signal.
>
>
> Cheers
>
> Michael VE3WMB
>
> P.S. The secret to squeezing out the best performance when building a loop is having a solid conductor that is welded/soldered to
> the tuning capacitor (you need to try to avoid mechanicalconnections as they add resistance) and using either a split stator or butterfly air variable
> capacitor or a vacuum variable capacitor. Cheap capacitors with wiper contacts on the rotor will kill your efficiency.
>
> The radiation resistance of a Magnetic Loop is low (typically less than a few ohms) and it drops as you press it into service on lower frequencies. Even small additional
> resistances add up and become significant when compared to the radiation resistance on the lower bands and this is what causes the Loop efficiency to go down the toilet.
>
>
>>From: "Ron D'Eau Claire" <[hidden email] <mailto:[hidden email]>>
>>Subject: Re: [Elecraft] Experiences using a portable HF loop
>>Date: April 21, 2017 at 11:00:09 PM GMT-4
>>To: "'Phil Hystad'" <[hidden email] <mailto:[hidden email]>>, "'Eddy Avila'" <[hidden email] <mailto:[hidden email]>>
>>Cc: "'Elecraft'" <[hidden email] <mailto:[hidden email]>>
>
>
>>The very BEST magnetic loops are incredibly inefficient, yet like any
>>"incredibly inefficient" antenna one can make amazing contacts on them when
> conditions are right.
>
>>The problem is with resistive losses. The day we have room-temperature
>>superconductors, we will have efficient small transmitting loops. Until
>>then, only a few percent of the power applied is radiated.
>
>>For now, they are great when simplicity of setup allows operation where
>>otherwise nothing could be done.
>
>>73, Ron AC7AC
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Re: Experiences using a portable HF loop

michaelstringfellow
I have considerable experience with both commercial and home-brew loops.  

As others have said, it is tough to obtain maximum efficiency - the two largest contributors to loss being resistance in the tuning capacitor and ground losses.  A couple of wires soldered to the tabs of a receiving capacitor with a sliding contact to the rotor will have large losses and several commercial antennas are made this way.  Since small loops use vertical polarization, unless you are over really good soil or sea water, they need to be high enough to minimize earth losses - typically 0.1 to 0.2 wavelengths.

I modified a portable aluminum loop by enlarging it to 5-foot diameter and improving the capacitor arrangement.  It worked acceptably well and I was able to make contacts on 20, 30 and 40 meters with a KX3 barefoot.  However, I find that almost any full-size antenna works better and the difference is usually one to two S points.  When operating portable, I now deploy a telescopic 40-foot fiberglass pole with a wire antenna - vertical or loop.

I continue to use two small transmitting loops in my HOA restricted home shack.

Mike
AF7ON
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Re: Experiences using a portable HF loop

Victor Rosenthal 4X6GP
The AEA Isoloop (I think it is not still being made) was about 3' in
diameter and had a wide aluminum strap for a loop conductor. The loop
was WELDED to the stators of a butterfly capacitor -- that is a
capacitor made with two stator sections and a common rotor. So there was
NO sliding contact or other resistive pieces.

It was fed by a small loop made of coax inside the larger loop with no
physical connection to the main loop. It tuned from 14 to 30 MHz and
would handle up to 100w.

In horizontal configuration I found it just about as good a transmitting
antenna as a full-size dipole, and (probably because of the perfect
balance) quieter on receive. Of course the bandwidth without retuning
was tiny (maybe 10 kHz on 20m) and the tuning was touchy.

But it shows how good a loop can be if the design is optimal.


73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
http://www.qsl.net/k2vco/

On 27 Apr 2017 19:13, michaelstringfellow wrote:

> I have considerable experience with both commercial and home-brew loops.
>
> As others have said, it is tough to obtain maximum efficiency - the two
> largest contributors to loss being resistance in the tuning capacitor and
> ground losses.  A couple of wires soldered to the tabs of a receiving
> capacitor with a sliding contact to the rotor will have large losses and
> several commercial antennas are made this way.  Since small loops use
> vertical polarization, unless you are over really good soil or sea water,
> they need to be high enough to minimize earth losses - typically 0.1 to 0.2
> wavelengths.
>
> I modified a portable aluminum loop by enlarging it to 5-foot diameter and
> improving the capacitor arrangement.  It worked acceptably well and I was
> able to make contacts on 20, 30 and 40 meters with a KX3 barefoot.  However,
> I find that almost any full-size antenna works better and the difference is
> usually one to two S points.  When operating portable, I now deploy a
> telescopic 40-foot fiberglass pole with a wire antenna - vertical or loop.
>
> I continue to use two small transmitting loops in my HOA restricted home
> shack.
>
> Mike
> AF7ON
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Re: Experiences using a portable HF loop

John Huggins, kx4o
In reply to this post by k6sdw
Small loop efficiency is relatively dismal, but as G8HQP once said
concerning this topic...

“That would depend on what you mean by 'efficient.' Probably not efficient
in the usual engineering sense of power out vs. power in, but perhaps
'efficient' in the common meaning of being able to do the job of radiating
something from a small space.”

My small space and portable (or to some lugable) example here...

  http://www.hamradio.me/antennas/small-loop-hf-antenna.html

...performs surprisingly well on 40m despite the losses, hence supporting
G8HQP's premise.

An additional important thing to keep in mind is the seriously high E and
H fields around these things even with QRP power. The H field especially
exceeds MPE limits a couple meters out along a line through the plane of
the loop. Some folks operate next to their loops... not me man.

Small loops are an enigma at first, but part of the fun is unraveling the
mystery through experimentation. Join us.

73
john, kx4o

On Sat, April 22, 2017 01:24, Eddy Avila wrote:
> Greetings all, I'm curious to hear from anyone using a portable HF
> magnetic loop. I've read the theory behind them so I'd like to hear your
> opinion about them? How efficient these antennas are, especially running
> qRP.
>
>
> Thanks all,
>
>



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