An implementation of the the

shielded loop antenna

by Ralph Klimek Nov 2009

abstract: The shielded loop is the ideal reception antenna for long wave and medium DX reception. It has a highly directional, deep, null
and rejects most noise sources.
keywords: shielded loop, redirectional reception antenna, 160 meters, medium wave DX reception, deep nulls

The shielded loop antenna was first used in WW2 for direction finding utilizing  its extremely sharp null.  The depth of the null depends on a number of factors, one of them is symetry of the active element, the Q of the resonant system and spacing of the loop with respect to other conducting surfaces.  It has no real lower frequency limit, save that of the Q of the system given the large number of loop turns required to achieve resonance.   Loop antennas also featured in early crystal set designs, however, they mostly failed to notice the value of resonance in the loop.

There is essentially only one differance between the shielded and unshielded loop as a reception antenna.  The unsheilded loop will pick up electromagetic radiation, that is true EM waves (photons)  and also responded to electrostatically induced noise. This noise in the average industrial of suburban envoiroment will typically come from flourescent lights, brush motors, corona discharge from HV power lines, and the radiated rubbish emitted by the line scan of TV sets.  Increasingly  the plasma screen TVs  radiate broad spectrum garbage that has all but destroyed short wave reception.  The shielded loop , on the other hand,  is allmost insensitive to electrostatically induced noise.  Nearly all of the energy content of long wave and medium wave interference locally generated is electrostatic. Very little of that noise energy gets transformed to real EM radiation (photons).  

The sheilded loop is capable of remarkable feats of long or medium wave reception, on paths that "should not exist", in envoiroments that are beset by electrical noise.  They have moderate directivity, but their true merit lies in their nulls which on practical antennas can give 60db attenuation to an unwanted transmitter.  It is even possible to use the shielded loop antenna to completely null out the groundwave radiation on a local medium wave broadcast station and listen to it via its ionospheric echo during the day.  It sounds exactly like a short wave broadcast!

Radio hams are only dimmly aware of this remarkable antenna for working 80 and 160meter bands.  many who try them superficially, discard them in favour of long wire antennas. The reason is that the loop is an extremely inefficient and low gain antenna. A long wire allways picks up more signal, but it cannot discriminate against the noise.  The true merit of the loop is its profound ability to reject noise, and nothing else.  If your station is in a quiet rural site, then the loop gives you nothing that you dont allready have. ( no noise!)  If you are located in a city or suburban area and you are restricted in the size of your antennas and beset by noise then the loop is essential for long/medium wave boradcast and 160/80 meters.  To be used effectively you require a low noise preamplifier or receiver with a generous AGC margin. The reciever should be a fully sheilded communications radio because of the extreme requirement to reject noise.

The sheilded loop antenna , because it is so good, must preforce come with some caveats ! The main limitation is the upper frequency limmit.  Because they are typically constructed with coaxial cable, the loop inductance, being proportional to the square of the diameter, resonates with the distributed capacitance of the cable at a surprisingly low frequency.  A successfull shielded loop is a compromise between sensitivity which depends on the number of turns, the band of frequencies to be covered and self resonance.  The loop at resonance has an extremely high Q, and you will discover that the depth of its power of rejection is strongly related to its Q.  

The loop is also inconvieient because it requires tuning with a variable capacitor which makes mounting and using the loop cumbersome. For this practical reason the loop must be used as close as possible to the reciever.

The loop should be wound with the largest coaxial cable that you can find. It should have large conductance, that means 50 ohm, RG8 or larger if available.  Large diameter Cable TV hardline would be ideal. It should have a foam dilectric to minimise distributed capacitance.
You will find that the most frustrating limitation is self resonance, and for that we need 75ohms coax with its inherently reduced distributed capacitance, but oh bother! 75 Ohm coax has smaller conductors, limiting the loop Q.  We just cannot win.  I have found, in practice, lower distributed capacitance is the thing to aim for, as this increases the usefull frequency range of your loop. Problem is "you cant get the wood"!  Large diameter 75Ohm cable is just not available.  You may have to compromise and just use 75Ohm foam dilectric CATV cable like  RG6.

In practicc, the maximum usefull upper frequency is determined by self resonance in any real system, and this is reached at about 4 Mhz wound with any real world coax.  There is also little to be gained with a loop above this frequency because the loop can really only null out groundwave radiation. Ionospheric reflected  radiation cannot really be nulled out because it has low spatial coherence...I mean, it does not have a clear "point source".  Nearly all "usefull" long and medium wave signals only have a groundwave component, and that means an interfering transmitter can be eliminated ( as with the noise) which is great for chasing medium wave dx.

My loop featured in the pictures  has a mean diameter of one meter, low capacitance , heavy solid copper armour/shield instrumention cable  wound with only two turns series connected inside the shield  is self resonant at 2.7Mhz. For that result I kick myself because I really wanted this to work at 80 meters!  It is a superb antenna for 160 meters. The lower limit is set by my  1200pf variable capacitor and that occurs just at 1Mhz.

Performance wise.   At midnight at my QTH in an outer Melbourne suburb the main noise sources are general lighting, my neighbour's wretched plasma screen (only 10 meters away too!) , the shack computers and leakage from the cable TV system.  I picked up some "rare dx" on 160 meters, VK2,5 hams on CW, I'm in Melbourne, VK3.  This was impossible on the long wire.  Special broadcast stations from VK2 in the 1700Khz special broadcast allocation were clearly audible and well  above the noise, and completely inaudible on the long wire. Lower down the band , the 1Khz heterodynes  with international medium stations on 10Khz spaced allocations ( Australian medium wave stations are on a 9Khz spaced allocation, hence the distinctive heterodynes)
were audible.  Stations in south australia and NSW were clearly distinguishable and seperable, even on the same frequency by nulling out the unwanted station.  The maximum depth of null on my loop was somewhat dissapointing, I have built better performing loops in the past.
I think this is due to the central pickup coil not being shielded, and as you can see from the pictures, loop symmetry is far from perfect.
My other loops have allways had the sheild symmetrically broken. As this is impractical on this design, the outer conductor runs continuously around the loop.  Some argue about the neccesity of a symmetic shield, but I think this is still required to get the ultimate noise rejection and null depth.

Dont forget, the sheild must be split, otherwise its just a shorted turn!

This loop can be rotated and resonated from the operating position! Its outdoors, up in the sky. So far my neighbours have been too polite or shy or apathetic to comment about it.  It also repels vampyres!

I strongly urge radio hams to seriously experiment with the sheilded loop, it is non critical, easily reproduced and the only real way to contemplate operation  on 160 and 80 meters in a suburban situation.  Note well, its only for RECEPTION. Sheilded Loops make a profoundly ineffective transmit antenna.  Long wires are ok for transmitting, lousy for reception.

cross arms are one meterpickup coil has 3 turns13.7 vampyres successfully repelled!
connections to the loop, shield,pickup coilinductive pickup loopmounting post on roof
underoof mounting on roof trussunderoof mounting on roof trusshandle on the "armstrong rotator"
thumb.imgp4676-loopant.jpgThe fully rotatable mount consists of two concentric tubes, the inner can rotate and the conductors are threaded through this. There is a homemade thrust bearing at the top. The central pole also mechanically supports the loop.  

construction details.
The loop is wound with Hubner and Suhner "Radox" (tm)  industrial instrumention cable.  This expensive solid copper armoured and shielded cable is used in industry for control box cabling that need armouring and shielding.  I found half used discarded drum of this expensive and excellent cable in a dumpster at work. The solid copper sheath surrounds two conductors insulated with mica and a high temperature insulator with a heavy gauge pretinned conductors.  Sorry, you cant have any !  Its very stiff and the solid copper sheild makes it allmlost self supporting. This stuff will form the basis of the transmit loop, in the fullness of time. Distributed capacitance is between that of RG8 and RG6. The conductors have been joined to make a two turn coil. The loop conductors and pickup coil are brough down in the inner rotating tube to a variable capacitor mounted just under the roof.

The fully rotatable mount consists of two concentric tubes, the inner can rotate and the conductors are threaded through this. There is a homemade thrust bearing at the top. The central pole also mechanically supports the loop.  The loop arms are made from 25MM PVC electrical conduit which has just bearly enough mechanical strength for this job.  The central spider is perspex.  
Pre-amp for 160M loop antenna

I have found that a low noise pre amplifier for 160m reception through this loop is absolutely essential. The very low efficiency of the loop antenna is only compensated by its ability to reject noise , which in the suburban envoironment is extreme.  The loop will resolve signals  that are totally inaudible with the main 160m vertical antenna.

The common base configuration  gives this amplifier an extremely low input impedance, of the order of 5 ohms,  so it makes a particularly good match for the low inductance pickup coil in the center of the sheilded loop.  At this low frequency just about any modern solicon transistor will do,  I chose a 2n2219A because these have usefull gain up to 150Mhz and their 1 Watt rating means that a sensibly large amount of collector current may flow, which gives good  immunity to broadcast band cross modulation.  The ferrite bead in the collector is required to prevent VHF oscillation.  The presence of parasitic oscillation is indicated if there is a huge amound of broadband noise and no evidence of real signals.

I have used 0.1uF chip capacitors, their low parasitic inductance makes them ideal bypass and RF coupling capacitors.  The collector inductor was made from a mystery found toroid  that exhibited a usefull Q at 2Mhz and sufficient turns wound  to make up 27uH.  I used this method , first , to ensure that the coil would work as required.  The Q is sufficiently high to require the use of a variable capacitor on the output to peak tune  the whole 160m band.  There is sufficient capacitance reserve to permit tunign down to the top of the broadcast band to permit hunting for 
broadcast band DX on the loop.
Images of the the little preamp. PCB was quickly made with the Dremel Method, just like the paddy boards, but with less fiddly work.  Notice extensive use of surface mount chip capacitors.  These can be obtained in large numbers from dead consumer durables  using a heat gun to desolder them. They are highly recommended for RF bypassing service. Capacitors that are meant to be part of resonators should  be silver mica.


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added 160m preamp Thu Apr 28 10:32:10 EST 2011