A New Homebrew Magnetic Receive Loop at VE1ZAC
(Edits Nov 26, 2013)
(Edits Nov 26, 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.
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. I have followed the loop construction
guidance on the site.
I have followed the loop construction guidance on the site.
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
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. I chose aluminium flat bar, as it is easier to form it into circles.
I chose aluminium flat bar, as it is easier to form it into circles.
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.
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.
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. All my receive transmission lines have been replaced
with this stuff over the last two years, acting on a tip I received from Dr.
All my receive transmission lines have been replaced with this stuff over the last two years, acting on a tip I received from Dr. Dallas Lankford.
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.
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. ( I have a DXEngineering active
phasor, plus several homebrew passive phasors patterned after the Vic Misek
( I have a DXEngineering active phasor, plus several homebrew passive phasors patterned after the Vic Misek design)
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.
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.
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.
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.
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.
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.
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