A New Homebrew Magnetic Receive
Loop at VE1ZAC
October 2013
On
my list of new antennas to experiment with is a magnetic loop with a broadband
current amplifier in lieu of the usual resonating components. One can buy a
product like this from Wellbrook http://www.wellbrook.uk.com/ , such as their ALA1530 series loops and
Pixel Technologies http://www.pixelsatradio.com/products/antennas/ and
their AM-2 loop.
I
elected to build my own and utilized a very nice broadband amplifier kit and
technical information from Chavdar LZ1AQ, http://active-antenna.eu/ . There is also
a wealth of technical information on active loop antennas on his pages. His amplifier kit is an excellent value and
has worked out very well for me.
My
antenna utilizes an aluminium 1 meter diameter loop with dual “Hoops” to make it
look fatter and attain a lower self-inductance. In fact, mine measures at 1.48 uHy which is
apparently quite good for a loop like this. It is mounted on a 2 M mast and
sits on a TV rotator allowing remote control from inside the shack.
Why a new antenna?
My
existing receive antenna collection consists of a K9AY, a pair of phased active
whips at 100 foot spacing, and a cut
down dual active delta flag array (DADFA). The DADFA and whips have actually
outperformed the K9AY in most instances, plus my particular K9AY shows no directivity
on 7 MHz or above. The K9AY works quite well on AM broadcast band, 160M and 80M. The performance of an active magnetic loop
has high praise from other users and the information on LZ1AQ’s website is very
complete. So, a new antenna project has been engaged. I have a spot for it that
is within the K9AY footprint and for testing purposes it is very easy to drop
the K9AY to the ground to prevent interference. That has been done and
comparison with the other antennas at my QTH is underway. The rotation of the
magnetic loop allows moving it’s rather sharp null around to engage noise
sources or other QRM, especially plasma TV RFI. The antenna has equal and
opposite nulls (nominally).
Claims for this antenna state that it should function from 20 kHz to over
50 MHz, with nulls approaching 30 dB deep. (I have tested the loop from LW to
the 30M ham band.) Further, a magnetic loop engages
with the H field of a radio signal more so than the E field, allowing a
different reception “view” of signals. Some rather clever switching also allows
this amplifier kit to function as several different kinds of antenna, switch
selected from the control end. (More on this later.)
Building the loop
Without
going into the justification, an optimum loop maximizes surface area vs. inductance. Loop resistance is not critical. The optimum
loop size for this amplifier and frequency range is
approx. 1 Meter diameter. It can be more, or less. However, inductance is
critical. The ideal loop is fat with very low inductance. Aluminium is a
particularly useful loop material, but building a low inductance loop is not a
simple exercise. I have machine shop
capabilities so decided to go one step better than a compromised “plumbing’
loop.
A
single loop of aluminium tube or flat bar of reasonable dimensions can attain an
inductance of 3 to 4 uHy. Fatter is
better, but it is not strictly necessary to have a full metal loop to get the
improved low inductance result. A second loop place parallel to the first and
offset by about 8” produces a lower inductance through mutual coupling. I used
two aluminium flat bars 3.1 meters long, 1 ¼” wide and 1/8” thick. (my apologies for mixing up systems of units… it will
keep you on your toes to convert them !) To get really
good connections, I decided to weld all my joints. This included butt joints in
the flat bars to increase their length. A big problem with welding however is
the loss of temper in the aluminium after welding. It is impossible to
bend these bars into nice rounds with the loss of temper/strength in the weld
areas. I solved that problem by adding thin aluminium straps over the welded
areas and holding them in place with pop rivets. A little experimenting was
required, but the original strength of the aluminium was re-created in the
joints and the circles formed perfectly.
My
finished loop tested out at 1.48 uHy.
Here
is a view of the joint reinforcement strap:
My
loops are spaced 8” apart, or 9” on centers. After mounting the end bars to a
PVC plastic base, I measured 1.48 uHy. This would appear to be as good as
professional loops. However, if you decide to build one like this.. it
is not a simple task! If you must join aluminium
sections, I recommend flat bar. If you want to do it with one length, then
either flat bar or tubing is best. Since all my
joints are welded, I can virtually ignore internal joint resistance in the
aluminium. Note, these aluminium bars are commercial DIY
items from a large hardware store. They come in 8 foot lengths and are
anodized. The anodizing has to be removed in areas where welding or connections
are made.
Could
you make a loop like this without welding? Of course and it will probably work
very well. However, I have aluminium welding capability and wanted to eliminate
a source of performance variability in the joints.
My
mast is assembled from a few left over pieces of aluminium tubing that have been
fitted together to make up an appropriate height. The mast is insulated from
the loop and the rotator base.
There
is a PVC plastic stiffener in the center of my loop. The top and ends are
joined with the same size flat bar. The two electrical connections are made to
the exact center of the end cross pieces to insure the two loops are as similar
as possible, RF and inductance wise. I had to remove the anodizing where the
leads to the amplifier connect. I used 'NO-ALUX' aluminium joint compound to cover the attachment
area.
One
may notice that I have two guy lines attached to the unit. These are attached
to the shaft via a loose PVC sleeve so the shaft can turn and adds to the
stability of the unit. I may add a couple more of these lines if required.
The amplifier
LZ1AQ
has excellent documentation and how-to tips on his website. No need for me to
recreate any of that information here. See http://active-antenna.eu/
The
amplifier actually has three receive modes built in, all controlled from the
remote shack end switches. You can have one loop, a second crossed loop or
other combinations. Plus, the built in switching allows using your two loops,
or one loop and a short wire as a small dipole connected to a voltage
amplifier. In my case I used the loop mast for the second element. It works
very well. The entire system is well made and arrived complete and intact in a
timely fashion via airmail. The units are checked out and adjusted before
leaving Bulgaria, and there is excellent documentation showing how to hookup
and test the system. All connectors and hardware are provided. You are
responsible for the loop, some shielded CAT5 cable to hook the unit up, and a
power supply. I used my system power supply with some additional filtering in
the control box. I also added some common mode chokes to the lead in cable as a
matter of course.
I
am totally in favour of CAT5 shielded cable for receiving antennas. This provides
a balanced 100 ohm transmission line, shielded, with additional control lines.
All of my receive antennas around the property now use shielded CAT5 cable. I
have cable with an outdoor rated jacket. I also used a piece of unshielded CAT5
for the rotor cable.
You
may note in the picture above that I ran the feed line inside the loop support
mast. This made it easier to utilize the mast as a dipole element.
I
already have a receive antenna selector switch under my main radio (an IC7700)
and a splitter output for other radios, SDR, etc., so it was no trick to bring
the antenna on line. My system also allows instant use with a phasor that uses
one of my active verticals for the other antenna input. A loop and a vertical
is an excellent combination for phasor use.
Note,
in my comparison discussion below, I am ignoring the ‘short vertical’ mode of
this antenna. It does work well, and as advertised, but is not competition for
my other receive antennas. If you build one of these as a primary receive
antenna, I do recommend you include this feature, however.
A quick comparison to my other receive antennas
Local noise sources
Like
most urban radio folks these days, I have some nasty local noise sources. The
worst one is a local plasma TV. Tuning into 3535 kHz with the magnetic loop, I can
rotate a null on this signal of… 5 S units. That’s approximately 30 dB, which
is exactly predicted! By using the phasor and an active vertical element, I can
take the plasma signal into the noise floor. Last evening when I tried this, I
unmasked 4 CW signals that were buried in the plasma noise. So far so good.
I
tried the same stunt on a local washing machine with a noisy VFD drive, plus
another unknown low frequency noise source. I had happy results with all of
them. Note that I can also null the
plasma source with the verticals and with the dual active delta flag array that
I have erected with the null pointed right at the plasma TV. The DADFA produces 40
to 50 dB of null with the flip of a switch on AM broadcast, 160M and 80M.
Receive antenna comparison
170 to 250 kHz, long wave AM
Occasionally
I have a listen to the 6 or 7 long wave stations from Europe that I can hear in
Halifax in the late evenings. Usually I have quite a lot of local noise to deal
with. My regular ham antennas are useless for this band but the active whips,
small DADFA and this magnetic loop have better results. The best antenna for
this was the full sized Lankford style DADFA I started experimenting with. It
was pointed towards Europe and really did a fine job on the LW stations too.
However, I replaced that with a smaller one as a 2nd experiment and
it lost its LW capability. (Although it is still a remarkable MW , 160M and 80M directional antenna.)
The
magnetic loop does a good job of nulling my local noise sources, but I usually get
better SNR results by phasing the two active verticals or one active vertical
and the magnetic loop. The phasing results are about equal between a DX
Engineering active phasor and my modified Misek/Lankford unit.
The
older full sized DADFA in its non-directional mode was better than the directional mode, but
there is little to be gained by phasing it with a vertical, at this location.
The
K9AY was not very useful on LW when it was up. Overall, the phased verticals
are the best for LW here, with the magnetic loop a close second, especially
when you null out the worst local noise source and phase it against an active
vertical.
MW DXing
The
small DADFA is very hard to beat for DXing transatlantic MW stations Any evening I can suppress NA stations by 40 dB or so,
across the band, which really makes the Euro and Mid East stations stand out.
Normally I can copy 40 to 50 of these across the bands from late afternoon till
late at night and early morning. None of my other antennas come close to this
performance. However, the phased verticals and the magnetic loop would be tied
for second place. Local noise sources are not as big a problem on MW for me, so
the nulling capability of the magnetic loop is more usefull for directional
suppression.
160M
The
DADFA is exceptionally good on this band. In 160M contests I use this antenna
exclusively with a simple switch for omni versus NE reception use. When in it’s NE mode, it really suppresses
the NA signals behind me and helps pull out the 160M stations from Europe. The magnetic loop is a close second and
the phased verticals a distant third. My K9AY was very good on 160M as well,
but not better than the magnetic loop.
80M
The
nulling and generally good SNR of the three receive antennas are pretty much a
tie on 80M. The magnetic loop can be rotated to null out a pesky local plasma
source for 80M CW, which I can also do with the DADFA and the phased verticals
to a certain extent. However, I will grade the magnetic loop 1st on
this band and the DADFA and phased verticals as second. The K9AY was good on
80M as well, but not as good as the magnetic loop is now.
40M
Besides
40M contesting, I also like to listen to the pirate SW broadcast activity
around 6950 kHz. I also find receive arrays most useful on this band
when conditions are noisy, which is a lot of the time in early evening it seems. The DADFA in non-directional,
the magnetic loop and the phased verticals are all about the same performance.
However for nulling a noise source or depressing a particular direction for
reception, the magnetic loop and phased verticals are the best.
30M
While
all these antennas are capable of low SNR receiving on this band, I have not
spent much time using it. Generally I spend any time on this band DX hunting,
and I have not had a lot of time to check out the reception characteristics of
my receive arrays vs my transmit antennas. Since I
don’t spend a lot of time here, this activity has not had a high priority.
Conclusions:
The
magnetic loop is very good antenna, with the lowest of low footprint and space
requirements. Compared to the K9AY, it is a winner for me. If I had to pick one
receive array of my three current ones… hmmm, it would really be a tough call. For
directional use, nothing I have seen, including beverage antennas can beat a
DADFA. And when you figure in that the DADFA only needs 100 feet, not 800 or
1000 feet, it is an incredible performer for low band use. Right behind that is the magnetic loop and the phased active vertical whips,
plus they have the advantage of being able to rotate the null. By
the way, these whips are only 15 feet high, and do not have radials and neither
does the magnetic loop or the DADFA. All
of these antennas have specialized common mode noise rejection schemes, utilize
shielded CAT5E transmission lines and have some electronics as active gain
elements or remote gain amplification when needed. But the trade-off gain against
lengthy wire antennas is huge! The footprints are so small, it really levels
the low band receive capability on small lots with those folks who have acreage
for their antenna farms.
Active receive antennas will be with me for the rest of my radio days, even if I have large amounts of space for antennas in the future. The new magnetic loop has remarkable performance for such a small footprint antenna. It is a keeper !
Partial Acronym List:
DADFA Dual Active Delta Flag Array
NE North East ( Europe is NE of Halifax, nearly all of USA is SW , the reciprocal)
LW Long Wave
MW Medium Wave
SNR Signal to Noise Ratio
M Meters (as in 80 Meter band)
QRM RF noise, man made
QTH Home location
SW South West
kHz Kilo Hertz
uHy Micro Henry
CAT5E Network cable spec, 4 twisted pairs.
CW Continuous Wave ( normally refers to Morse Code transmission)
PVC Poly Vinyl Chloride plastic
RFI Radio Frequency Interference