Topband: Loop Antenna Questions

Sat Jan 2 15:45:11 PST 2010

I don't know the parameters or limits that decide if it is a "magnetic loop" or a magnetic loop.  :o)

Literature available limits the circumference (3.1416 times the diameter) to 1/4 wavelenght, because beyond that point stops behaving like a magnetic loop.

It radiates equally on the vertical plane that contains the loop. But it radiates at low angles if not installed away from the ground.

If this is wrong I would like to know it.

My experience with the loop:

While in Spain I built one square with 15mm OD copper tubing, torch soldered. Size was 1,25m per side. 
I lived on a sixth floor and the size of the square was decided by the diagonal of the window from which I took the antenna out about 50 cm from the fassade of the building and used it horizontally on 40 m band, at almost halfwavelenght over the street. The capacitor, a lossy one, was a length of RG213, and SWR started to go up if I held 100 keydown for a few seconds. 

With 50W in the loop I worked several eastern EU stations and the window was right on the opposite side of the building and the whole block of buildings 9 stories high.

Some times with this loop we could copy signals lost in QRM with the inverted vee at 22 meters at the radioclub.

I could tune it from 7 MHz to 14 Mhz.

Of course, pennants, flags and alike are better RX ONLY antennas. But for me in the EA apartment was the difference between TX & RX, or just RX.

My friend HK1N has crossed the pond on 40m with 100W and a 1,90m (c.a. 6' 3") made of 6 meters (20') of 7/8" Heliax. RST's up to 569

I have three pieces o LDF 5-50 that add 22 m, Loopcalc software (http://www.magneticloopantenna.com) estimates efficiency of 37.5% @ 1.825 KHz, that's 4.2 db below 100%, 2:1 bandwidth is  2.7 KHz! I guess something funny must happen on SSB if you are 1:1 at center frequency but +2:1 on your sides. I hope it works on CW and with the losses by its sorroundings it will be less efficient and less Q and more BW. At 3.51 MHz is 85% efficient and 15 KHz 2:1 bandwidth.

I will give it a try here at this urban lot, maybe I won't cross the pond in a long time, but it will get me on the air on top band.

CAUTION: The current density is high, even at low power levels. It can be lethal. The strong RF fields are not to be taken lightly regarding living things.

David  
HK1A
EC5KXA
ex-HK1KXA

> To: richard at karlquist.com; w1ksz at q.com
> Date: Sat, 2 Jan 2010 08:33:13 -0500
> From: w2pm at aol.com
> CC: topband at contesting.com
> Subject: Re: Topband: Loop Antenna Questions
> 
> The so called (poorly) "magnetic loop"  only works when it IS small -  
> .1 wavelength maximum but that is not the optimal size..  According to 
> several texts that should be .037 wavel. which is the 5 ft diameter 
> loop.
> 
> But I think there remains confusion over the term LOOP.   The magnetic 
> loop is a tiny antenna used primarily to null out strong, highly 
> localized single-point noise. Secondarily, you get a general receiving 
> antenna with very poor characterics starting with very low sensitivity 
> and no directivity vertically  - so it won't discriminate between high 
> angle atmospheric noise and low angle DX despite nulling out local crud 
> noise if you can.   It always work well for local work, but is truly an 
> antenna of last resort for DX. I've worked VK with it from noisy NNJ 
> but the conditions were exceptional at those times.  However, without 
> that magnetic loop those days I could not work VK.  None the less it is 
> a VERY POOR receiveing antenna for DX.
> 
> The terminated loops (diamonds, flag, pennant, K9AY) and entirely 
> different.  They are also larger (although there is a mini diamond Flag 
> of 5 ft dimensions).  The nulling they provide are not nearly as useful 
> for nulling out local noise crud - like a bad insulator on a power 
> pole, light dimmer, etc.BUT that null is broader and much more 
> particular to a certain arrival angle. That null is used to increase 
> the S/N ratio but the directional characteristics of the cardiod 
> pattern and the overall pattern has fairly low angle sensitivity. This 
> is a good DX antenna but is much larger than a magnetic loop.  That 
> said you can build a half size Flag (7 by 15 ft) or even a mini diamond 
> config flag of 5 ft per side and still get the same cardiod pattern of 
> the full sized models (15 by 30) but with much lower sensitivity.   
> When these antennas get smaller and their outputs become much lower the 
> chances of feedline pickup and pattern distorion come into play and 
> that can ruin everything but those conditions can be avoided.  ON4UN's 
> book remains the "Bible" and lots of other on line resources out there 
> with this stuff -- including the w8ji.com site.
> 
> 73
> 
> Pete W2PM
> 
> -----Original Message-----
> From: Rick Karlquist <richard at karlquist.com>
> To: RICHARD SOLOMON <w1ksz at q.com>
> Cc: TopBand List <topband at contesting.com>
> Sent: Fri, Jan 1, 2010 10:46 pm
> Subject: Re: Topband: Loop Antenna Questions
> 
> 
> I found that a 40 foot perimeter square loop, roughly
> equivalent to your 12 foot diameter round loop had
> an inferior null to a loop half as big.  It did of cource
> produce about 10 dB more signal (and noise, same SNR).
> 
> Rick N6RK
> 
> RICHARD SOLOMON wrote:
> >
> > I have an NZ4O(KN4LF) Loop Antenna for receiving now. I was wondering
> > what folks thought about using a "bigger" Loop ??
> > By that I mean around 12' diameter instead of the 8' diameter I have 
> now.
> >
> > Would I gain anything by doing this ?
> >
> > Tnx es HNY, Dick, W1KSZ
> >
> > _______________________________________________
> > "160-meters is a band for men, not for sissies!" - SM5EDX
> >
> >
> 
> 
> _______________________________________________
> "160-meters is a band for men, not for sissies!" - SM5EDX
> _______________________________________________
> "160-meters is a band for men, not for sissies!" - SM5EDX

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