Radiation angle - This is influenced by -among other things - the loop height above ground (also depending on the quality of ground) and the
ground at a large radius distance from the antenna which tends to characterise the performance of the loop at low vertical angles in the plane
of the loop. A lot of anti-loop proponents and some loop builders disagree with most of this; often quoting reams of maths, theory and antenna
modelling outputs to claim that a magnetic loop is an NVIS antenna only. Therefore they also disagree that the placement of the tuning capacitor
or feed-point location on a magnetic loop has any effect on the radiation angle. I wonder if sometimes they can't see the forest for the trees.
Tuning Capacitor top or bottom? - There is an enormous amount of stored energy in the electric field of the capacitor and gap region of the
loop. You do not want to couple / perturb this electric field symmetry and spatial distribution by the close proximity of the ground, building or
metal mast. Keep the capacitor element well clear of all such conducting surfaces by placing it at the top of the loop.
Feed point - Any feed point near the top of the loop will leave you with the problem of the feed line influencing the symmetry of the loop and
having even more problems with common mode currents on the coax caused by the coax in the radiating field of the loop. The radiation pattern
will go to ruin and you'll compromise those highly useful sharp pattern nulls for rejecting local QRM. I can null a +20dB local carrier into the noise.
Yes! I have seen people running feed lines down the centre axis of the loop. They say that it did not affect their loop. I notice anything placed
in this position on this loop, the impedance changes, the matching has to be readjusted and the bandwidth gets wider as the Q goes down due
to loss and once again you may compromise the sharp pattern nulls. Running the control cables for the capacitor tuning unit inside the copper
loop- Ted Hart ARRL ANTENNA Handbook - works perfectly, even under QRO power. The multicore cable in my loop is not even shielded, it works!
The Ground Plane - From VK5KLT Leigh Turner
In all my years of magnetic loop experimentation in both a professional and amateur radio context I have not yet experienced cases where a
good ground plane / set of radial wires / ground mat under the loop has not resulted in increased performance and greater independence from
the surrounding environment. The other factor at play here is the impreciseness of loop calculator programs.
The Q on 40m is higher and the BW lower than predicted, probably due to the metal shed roof / ground plane influence. (My Aerial - VK4AMZ)
What you report raises questions about the accuracy of magnetic loop calculator programs posted on the internet; such discrepancies do not
surprise me. I’ve never seen one that is accurate in all aspects of properly modeling / predicting real-world loop parameters and behavior over
the whole HF band. The biggest influence and variable is the proximity to ground which has profound influences on the loop’s reflected loss
resistance, Q, and attainable efficiency. As I’d previously mentioned, all of the programs under-estimate the loop’s radiation resistance; sometime
by huge amounts! This is because the traditional equations relied upon in such calculators are incomplete and do not factor in all of the complex
additional frequency dependent modes that contribute to EM field creation and radiation. - Thanks Leigh.
Leigh - VK5KLT was responding to the data I had sent him on the measured bandwidth and Q values of this loop in its final operating position and
as to why the Q and bandwidth on 40m and the Q, bandwidth and efficiency on 160m are so different to conventional calculation predictions.
Note: I have found the following with this loop. If you are going to rotate the loop then the ground plane needs to be a counterpoise or ground
mat, otherwise the SWR will be all over the place as the loop rotates over any wide spaced radials. A gently sloping metal roof is fine.
160m performance recap -We normally have ten or more stations on the 160m nets, five within 60km of my location and all but one using
dipoles, inverted-V's and G5RV's. That one station was 15km from me and running a short top-loaded vertical mounted over salt water. In bad
conditions he and I were the only two stations with a readable signal on 160m to interstate stations. One station close to the vertical was
running a dipole at a height of 40 to 50ft over salt marsh with a radial system as well.
Limited space - How many of us can put up a full-size 160m dipole on a suburban block? let alone get it to a low compromised height of 40ft
and have a salt marsh ground with full length radials to reduce ground losses. How many can erect a shortened vertical on 160m over a full size
counterpoise or salt water ground plane? I am always received interstate on these nets and usually stronger that the big dipoles, especially on
long distance contacts. On local contacts the vertical - obviously - was not even in the race. I had less noise and stronger received signals than
the vertical even on DX stations and received signals equal to and, quite often stronger than the dipoles.
A good magnetic loop correctly built and sited is not just an NVIS antenna - as some will have you believe.
The Toy aerial (a loop is often refered to as this) versus the dipole. (The following is nicely stated again by Leigh VK5KLT)
While dipoles may well be full sized in respect of wire resonant length, they are not high enough in terms of wavelength distance from the lossy
ground; this is the limitation of such E-field antennas on the lower HF bands. It's all about keeping return current paths and radiator element
image RF currents out of the lossy ground. Large expansive ground mats and radial systems (in the case of monopoles / verticals) are the order
of the day for achieving efficient aerials for the low HF bands. The small magnetic loop with its highly localised / contained magnetic and electric
fields neatly sidesteps and circumvents most of these problematic issues with big wire antennas deployed at restricted heights.
I'll reiterate the biggest influence and variable to any low-HF antenna is the deleterious proximity to ground which has profound influences on
both the dipole's and loop's reflected loss resistance and attainable efficiency. All of the software programs and formula under-estimate the
loop's true radiation resistance; sometimes by huge discrepancies! This is because the traditional equations relied upon in small loop parameter
calculators do not factor in all of the complex modes that contribute to EM field creation and radiation.
When a traditional half-wave or long wire dipole antenna is placed horizontally above ground, its "image" in the ground is of the opposite phase.
As a consequence, if the height above ground is reduced to less than a quarter wavelength, fairly high system losses develop due to a rapid
decrease in radiation resistance concurrent with a rapid rise in loss resistance resulting from dissipation of power within a less than perfect
ground. A classic double-whammy scenario conspires to produce a poor radiator; particularly on 160m or 80m where achieving the requisite height
By way of contrast, the oscillatory RF currents associated with the image of a small vertical loop antenna above ground are "in-phase" with those
of the loop. Therefore the effect of ground on the performance of a vertically oriented loop is relatively small. In fact, because the magnetic
component of an electromagnetic wave is maximum at the boundary between the ground and the space above, loop performance is usually best
when the loop is located near the ground at a distance just outside of the loop's close-in induction field (only a loop diameter or two). However, if
nearby conductive objects such as power lines or buildings exist in the direction of transmission / reception; it is normally preferable to choose a
height above ground which will provide the loop with a clear and unobstructed view of the intended signal path. - Leigh.
Antenna testing - I feel that you should carry out extended long term testing between an antenna and an alternative construction or type of
aerial to really check for the difference under all conditions. The problem I see with A/B testing, especially with a loop, is not allowing sufficient
spacing between antennas. Antennas will reradiate or absorb RF energy if erected within the field of the antenna under test. You cannot preform
a true A/B test on a small suburban block unless you can quickly detune and retune each loop to a different band when switching between loops
and each aerial must be placed an appropriate distance apart. All other antennas that are harmonically related in any way or in the field of the
loop must be removed. This should be normal practice when testing any aerial. Yet I still see hams placing one aerial inside another for testing?
I drop my other aerials to the ground when testing the loop or testing between aerials. However this can still interact with the efficiency and
radiation pattern of the antenna under test as the aerials on the ground may distort the existing ground plane.
On air testing - I have joined in the various nets for over two years, testing each aerial that I have built for weeks at a time. During this time
A/B testing combined with real world results have painted a revealing picture of the performance of this magnetic loop antenna.
There are unusually ten or more stations on air, some a few km from my location, others over 2000km away. This testing was carried out with the
same stations, those close to me operating under similar band conditions week in and week out. I feel that this is a real indication of an aerials
value. After all, is it not the performance under various operating conditions over varying distances when compared to the same known stations
on air over a sustained period of time that gives a truer indication of the worth - or not - of an antenna?
The Death Ray - Some people have claimed that loop antennas emit death rays. An answer from Leigh VK5KLT
Being in the near-field region of a magnetic loop antenna, or any antenna for that matter while running under QRO drive levels can be immensely
dangerous and not to be underestimated or taken lightly.
There are Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) and IEEE C95.1xx standards for safe SAR levels of human
exposure to RF fields and these electromagnetic radiation guidelines can easily be greatly exceeded in terms of both V/m and A/m when one
approaches within a loop diameter or two of the radiating loop or too close to an electric dipole or whip. Unlike elevated E-field horizontal dipole
antennas, QRO driven loops operated down at ground level would be particularly hazardous to humans and animals. Likewise getting too close to
the near-field of a short vertical whip antenna is also dangerous under QRO drive power. All antennas, irrespective of their operating and EM field
creation mode are equally hazardous in this respect.
Loops are particularly dangerous in their near-field region because of the very highly localised spatial concentration of the E and H fields.
Any closed-path conductors falling within the loop's magnetic induction field will have a considerable RF current induced in them (like a loosely
coupled transformer winding) and could easily develop sufficient voltage to cause a visible arc to ground such as that reported in the article
with the nearby wrought iron table in the vicinity of the loop. - Leigh.
In other words, would you bring the base of your vertical into the shack, place on the desk next to you and run 100 watts or more into it?
No! Then why are people doing this with a loop and then claiming that loops are dangerous? Any aerial is dangerous in this situation, more
care is needed with a loop as stated above. With a well elevated loop - in my case on the roof of the shack - there should be no reason for
any concerns about exposure to unsafe RF field strengths. Use common sense and think before you "fire her up on the desk" with any aerial.
Loop Bandwidth - The bandwidth limit is usually the transmit SWR bandwidth on the lowest operating frequency of the loop.
The signal bandwidth is actually a bit wider than this so there are usually no problems passing SSB or even AM. A valve linear or valve output
stage should have no trouble with a 2:1 swing in SWR with modulation. My old TS930s doesn’t complain either. You can almost forget about
harmonic distortion / radiation with a TX loop. The largest measured out of band harmonic distortion product from my TS930s is almost 70dB
down with the loop.
Loop Tuning - Unless an aerial is easy to tune it will only be a short time before you tire of using it. As this is my only aerial I was determined
from the start to make this loop easy and quick to tune.
The only motor I will use is a stepper motor. With a stepper motor you have the ability to use a low cost rotary encoder to adjust the tuning
capacitor a fraction of a degree forward or back without any overrun. When driven correctly, a stepper motor will only supply torque up to a
point where they stall, after which there is very little torque produced, no damage is caused to the motor and this will in most cases remove
the need for limit switches and prevent damage to any reduction drive or the tuning capacitor.
The rotary encoder allows you to rock the cap backward and forward with precision and feel. Combined with fast forward and reverse buttons
you can tune the loop with ease. Incorporated in the design is an optional LCD display that shows the position of the capacitor as it rotates.
This makes it easy to quickly go to a different band and get close to the correct tuning position. When tuning across the band you only need to
use the rotary encoder. I intend to design and write the code for a completely automatic loop tuner when I get the chance, for now this manual
tuning works so well that I have put that off for a while. (Too many other projects to build)
The other piece of essential gear for loop tuning is a combined RF bridge + dummy load. This is an easy and low cost piece of gear to build that
allows you to simply throw a switch and drop the RF from the rig into the dummy load while at the same time a sample of RF (a few milliwatts) is
fed to the bridge, the loop is also switched into one leg of the bridge. The loop is then adjusted for bridge balance (50 Ohms).
This allows you to tune up on the frequency without causing interference - except to a very close station - When you throw the switch back
you are on the air with the loop resonated and the SWR correct.
With a bit of gain after the bridge the power level could be reduced to microwatts which should cause no interference even to close in stations.
There will be no change in tuning or the need to retune when full power is switched back to the loop; provided you have built the loop correctly
with no component or construction resistive losses or heating that could lead to a change in impedance or resonance in the loop with power.
Once you tune a loop or other tuneable aerial in this way you will wonder how you opperated without it. I will add the circuit and construction
for this low cost unit soon. This is quite an old idea and I am surprised it is not used more often.
The Controller - The circuit and Hex file for the loop stepper motor controller will be posted at a later time. If you are interested in trying the
controller in its current state then please send me an email.
Click Image to
Observations - The Loop in use and info that matches my findings