Active Hula Hoop Loop Antenna

My primary HF antenna is a horizontal sky loop, with a perimeter of about 670 feet, I also have a 500 ft long beverage aimed toward Europe which is primarily used on the 48 meter band for Europirates. In addition I have a sloping folded dipole for the 43/48 meter band, although that antenna does not get a lot of use. These antenna work great on the lower end of the HF band, but as you might imagine are not ideal for the upper end, especially the 11 meter band which I like to listen to when there are openings to Europe.

I’ve been interested in building an active loop antenna for some time, and recently came across the Active Antenna Amplifier made by LZ1AQ. This amplifier has very good reviews, and LZ1AQ’s site has a wealth of technical information, including suggested designs. Originally I was planning on building a 2/4 crossed parallel loop (and still may one day), but as that is a major product (each of the four loops is 1 meter on a side), I decided to start with something easier to build, the two loops in orthogonal planes, which is described in section 3.4 of this document on his site.

Here is a photograph of my build of the antenna:

It was constructed with two old hula hoops. And yes, it’s not an optical illusion, one is slightly larger than the other. Each loop was wrapped with aluminum foil, to create a large diameter loop, with low inductance. The foil is not wrapped around the entire hoop, there is a small gap of about two inches.

As you can see hookup wire is wrapped around the aluminum foil, and twisted so it is tight and making good electrical contact. They are later covered with tape to be weatherproof. Wire ties are used to secure the loops to an 8 foot long piece of pressure treated 1″ by 2″ furring strip that I happened to have laying around. These wires then go to the amplifier board.

The loop amplifier uses shielded CAT-5e ethernet cable for the connection to the control board back in the shack. This cable carries the power for the board, control signals for switching the loop inputs, as well as the amplifier RF output which eventually goes to the radio. I used some more tape to seal the hole for the ethernet cable, to keep out moisture as well as insects. After the photo was taken, I added a small support arm, made of wood, and attached the ethernet cable to it with a tie wrap, to provide some mechanical support, and remove the strain from the ethernet jack on the amplifier PCB.

This is the rather crude enclosure I made for the control board, using an old plastic enclosure I had laying around.

The functionality of the control board is more completely explained on LZ1AQ’s site, but to summarize:

The first toggle switch selects either dipole or loop mode for the amplifier. I have mostly used loop more in my tests so far, but will experiment with dipole mode eventually. In dipole mode, each loop is used as one arm of an electrical dipole, rather than as a loop antenna.

The second switch selects either loop A or B, when only one loop is used.

The third switch enables crossed mode, when both loops are used at the same time. This can provide some relief from fading, as the two loops are orthogonal to each other. Otherwise, only one loop is used. On the LW and MW bands, this provides different directionality patterns, and works quite well. For example on 1490 AM I can completely switch between two different stations. Obviously this is dependent on the directions to the particular stations on a given frequency. If they are roughly 90 degrees apart and aligned with the loops, you get excellent directionality. If on the other hand they are at 45 degree angles with respect to the loops, you won’t get any.

The last switch is for power.

Below is a waterfall of part of the MW band. Half way through, I switched from loop B to loop A. On many of the channels you can see a significant change in the signal strength, sometimes stronger, sometimes weaker. You can click on the image to view it full size.

It even does a good job on the lower end of the LW band, here is WWVB:

I’m quite pleased with the performance of this antenna, especially on the 11 meter band. And the directionality on the MW band is a bonus. On the lower HF frequencies, the full sized sky loop and beverage antennas perform much better, as you would expect. Still, this is a respectable antenna for the size, and would be very useful for someone with limited space that precludes the use of large antennas.

If you like my loop antenna, you might enjoy some of my radio related Apps

Ferrite Core 1, RFI 0

Once again, a giant ferrite toroid coil saves the day. I have a random wire antenna (about 100 foot long) running into the basement workshop, fed with RG-6 coax (the coax shield is left floating at the antenna end). Reception was horrible, I could barely hear anything, even SWBC stations. I considered that maybe it wasn’t a lack of signal problem so much a signal to noise problem, so I located a large ferrite toroid coil from the junkbox, wrapped as many turns of coax around it as I could (about a dozen), and placed that in series with the incoming coax, just before the radio. Voila, the noise/hash was gone. The choke helps to reduce RFI flowing as currents on the shield of the coax.

The ferrite core was a Fair-Rite 5943003801, 61 mm toroid, type 43 ferrite. I buy mine from Mouser for about $4 each: http://www.mouser.com/ProductDetail/Fair-Rite/5943003801

Here’s a photo showing how the coax is wrapped around the toroid core:

And here are some before and after video recordings. The gap about half way through each is when I disconnected the incoming coax to the radio, and inserted the choke, and then reconnected the coax:

Measuring The Velocity Factor of Coax Using an SDR

Recently I had the need to measure the velocity factor of some coax. The velocity factor of a transmission line is ratio of the actual propagation of radio signals through the cable vs the speed of light in a vacuum.

Here’s the coax in question:

It’s RG-6U, for which I have seen published velocity factors ranging from 0.65 to 0.85, depending on the manufacturer and type of dielectric. This coax was laying in my junk box, and I have no idea who makes it, or what the claimed specifications are. The performance of a lot of lower cost coax often widely varies from published specs, as well.

One technique to measure the velocity factor of a transmission line is to use a piece of it as an open stub, which is a section of transmission line connected to another line via a Tee connector. The added transmission line is open at the other end, hence the term “open” stub. The open stub will act as a notch filter for frequencies with a wavelength close to four times the length, in other words the stub is 1/4 wavelength.

For this measurement, I used an SDR (Software Defined Radio) as the measurement device. In this case an SDR-14. To generate RF I used a noise bridge.

The output of the noise bridge is a good source of wide-band RF.

Here is the Tee. On the left is the RF signal, on the bottom is the connection to the SDR, on the right is the open stub.

With the noise bridge connected, but no stub, here is what the SDR spectrum looks like, click to enlarge:

As you can see it is relatively flat. Next, we’ll connect the 1/4 stub (again, click to enlarge):

You can see the dip in the signal level, caused by the stub.

In this case, the stub was 13 ft (4 meters) of cable. Iif the velocity factor was 1.00 the wavelength would be 16 meters, the frequency 18.75 MHz. The frequency of the center of the notch is 15.7 MHz, so the measured velocity factor is 15.7 / 18.75 = 0.84.

Next I used a 9 ft 3 inch (2.85 meter) cable. The wavelength for a velocity factor of 1.00 would be 11.38 meters, the frequency 26.35 MHz. The frequency of the center of the notch was 21.8 MHz, so the measured velocity factor is 0.83.

Using both cables, the total length is 6.85 meters, the wavelength for a velocity factor of 1.00 would be 27.4 meters, the frequency 10.95 MHz. The frequency of the center of the notch was 9.1 MHz, so the measured velocity factor is 0.83.

For this piece of coax, the velocity factor seems to be 0.83, which is a reasonable value.

Building a Sky Loop Antenna

Due to the continuing interest in the sky loop antenna, I’ve put together some notes and suggestions on the construction of these incredibly well performing antennas. One note – the sky loop is generally used as a receiving antenna. I don’t have any first hand experience using one for transmitting. Typically, the SWR is all over the place, so you’d likely need a tuner if you wanted to transmit with one. For receiving, no tuner is needed. The sky loop covers all of HF, and often down into MW as well, depending on the size.

First, the basics. A sky loop antenna is a large loop of wire mounted in the horizontal plane. How large? Typically, as large as you can make it. Bigger is generally better when it comes to sky loop antennas. Mine has a perimeter of 670 feet, and I am considering enlarging it. Often, this antenna is run around the property line of your yard, maximizing the length of the antenna as well as the cross sectional area. While the antenna is called a loop, this does not imply that it needs to be circular in shape. While a circle does maximize the area for a given length of wire, other shapes work well. Remember, you’re trying to maximize the amount of wire (perimeter of the loop shape) and area, so try to use as much of your yard as possible, although you don’t want to zip zag back and forth too much. You want to make the largest area polygon that you can. The likely constraint will be what trees or other supports you have available for getting the wire in the air. And of course, remember that safety is important – keep the wire far away from any overhead power lines.

Because the antenna is in the form of a loop (a continuous path of wire from one of the transmission line terminals to the other), it is inherently a low noise antenna. Comparing a sky loop to a dipole, you will find that the noise levels are generally much lower.

How to get the wire in the air? There’s lots of ways, what I find that works best is to put rope up into trees around the path I want the antenna wire to run. One end of the rope has an antenna insulator, the rope then goes up over the tree (or a branch or whatever you can manage) and the other end is secured to keep the insulator up in the air. I just put a large nail in the tree, pull the rope taut, and then wrap it around the nail several times to secure it. often I will put a second nail in the tree as well, a few feet below the first, and then coil the excess rope around the two nails, to keep it neat and tidy, and away from rope eating lawn mowers. Untying rope that’s been wrapped around mower blades is no fun. Been there, done that.

I use an EZ Hang to put rope up in trees.

The antenna wire itself then runs through the other end of the insulator. Since the wire is in a loop, this requires some planning, you need to put a bunch of insulators on the antenna wire, as many as you will need, and then use one for each of your antenna support ropes.

Here’s how I do it:

The EZ Hang shoots a fishing weight up and over the tree. There’s fishing line attached to the weight. Once the weight lands on the other side of the tree, I take off the weight, and attach the end of the rope to the fishing line. Then I use the reel on the EZ-Hang to pull the rope back through the tree, until it gets down to the ground at the EZ-Hang. Now I’ve got the rope going up and over the tree and back down. Then I can cut the rope off the spool, and attach an insulator (with the antenna wire already running through the other eye) to one end of the rope, and pull it and the antenna wire up into the air.

If your antenna is relatively small, you may get away with just four or so insulators, one at each corner of the loop. As the antenna gets larger, you end up with a lot of sagging due to the weight of the wire, and you need several intermediate support insulators on each side of the loop, to limit the sagging.

As far as the type of wire to use, there’s several possibilities. First, of course, is normal stranded copper antenna wire. I, however, got a great deal on a 1000 ft spool of #16 insulated wire. I went with that because the antenna wire would be going through trees and leaves, and wanted to minimize the chances of the wire being shorted out to ground. While probably not as important with a receiving antenna like this as with a transmitting antenna, I decided to play it safe. Also, the insulation happens to be green, and I think it does a good job of helping the wire to blend in the trees, making it difficult to see.

There’s a lot of debate as to how high the wire for a sky loop antenna needs to be. Computer modeling shows that the higher it is, the better it is at picking up low angle signals from DX stations. And in general with HF antennas, higher is better. On the other hand, if it is difficult for you to get the wire up very high, a sky loop with low height wire is still going to perform better than no sky loop at all. I started out with several sides of my sky loop being relatively low, 15 ft up or so, because that is what I could easily manage. Then, over time, I have raised those sections as I was able to. I do think the antenna performs better now that it is higher up, but I am not sure that the effects are dramatic. So my rule of thumb would be to get the wire up as high as practical, but I don’t believe there is any magical height you must achieve. Right now, the height varies between about 25 to 50 feet.

As I mentioned at the beginning of this article, the SWR (and feedpoint impedance) of a large sky loop antenna is all over the place. If you think about it, for many SW bands, the antenna is several wavelengths long. In my case, the antenna is about 670 (206 meters) feet in perimeter. So for the 43 meter pirate band (say 6.925 kHz), it is about 4.75 wavelengths long. At 15 MHz, it is over 10 wavelengths long. That said, I do not use an antenna tuner. While you could, I don’t think it is necessary for receiving applications. The antenna collects lots of signal, and I don’t believe you need to squeeze out the last S unit.

Since the impedance is usually quite high at any given frequency, I chose to feed the sky loop antenna with a 12:1 balun. I didn’t choose this based on any calculations, I just happened to have one available. I do keep meaning to try swapping other baluns, such as a 4:1 or even a 9:1, to see if there is any difference in the performance, but I have not gotten around to it. I do think some sort of balun is desired for the sky loop antenna, vs feeding it directly with just coax. You may be able to feed it with ladder line, but I have not tried that.

For the coax, I used RG6, which is commonly available and used for TV. I chose RG6 because it is very cheap, and personally I am sick of putting PL-259 connectors on coax. RG6 has F connectors, and I use an F to PL-259 adapter at the balun, as well as to connect to the radio inside the shack. There’s certainly no requirement to use RG6, you can use any good quality coax.

Performance does suffer once I get down to about the middle of the Medium Wave band. While I can pick up stations all the way down to 530 kHz, the signal strengths are much less than the upper end of the medium wave band. If you assume the antenna is a basic loop, the resonant frequency is about 1460 kHz, so this seems reasonable. I get excellent reception in the X band, 1600-1710 kHz. One of my reasons for wanting to increase the length of the loop is to hopefully get better performance lower down in the MW band. Although simple math shows that even if I doubled the length (which I am not sure I could do), the resonant frequency would still only be about 730 kHz.

Another possibility for worse performance on the lower part of the MW band is the choice of balun, as I addressed above. The impedance of a one wavelength loop antenna is about 120 ohms. With a 12:1 balun, that is reduced to about 10 ohms. And at the lower end of the MW band, the impedance is likely much lower.

If you’ve read my This is why you should disconnect your antenna during a storm article, you’ve seen what happens when you have a thunderstorm nearby. Your antenna is often quite good at collecting that energy, and sending it to your radio. There’s lots of good lightning protection devices out there that you may want to look into. Personally I also disconnect my antenna when there’s a storm, or even the possibility of a storm, especially if I am not going to be around. It takes a few seconds, and can protect your radio. It doesn’t take a direct lightning strike to damage a radio.

I hope this article will motivate several listeners who have the room to consider installing a sky loop antenna. You won’t be disappointed.

Construction of a Helical Antenna for SATCOM Listening

Previously I wrote about the various kinds of transmissions you can heard on the 250 MHz SATCOM satellites. While you can pick these up with a standard scanner antenna, reception is much better with a directional antenna.

This page documents my project to construct a helical antenna for SATCOM listening, 240-270 MHz.

The antenna is based off the design found on this page, which has the specific dimensions and other technical details.

Here are the supplies:
Four 4 ft long strips of steel, four 5 ft long pieces of 1/2″ PVC pipe, one 5 ft long piece of 1 1/4″ PVC pipe for the boom, and window screening for the ground plane.

Here’s a close up of the flange and fitting for the PVC boom:

Here are the four steel strips arranged in the radial pattern:

Next I drilled four additional holes in the flange, so it could be screwed to the eight radials:

#10 hardware was used to attach it:

Here it is with the PVC boom attached, to see the overall size:

And now with the 20 supports for the tubing installed:

The tubing is 1/4 inch diameter:

Here it is with the 5 turns of 1/4″ diameter tubing:

The screening has been added to the reflector. It is sandwiched between the strips for support:

The [mostly] assembled helical antenna. The matching section is made from tin-plate and is cut to be a quarter of a turn, about 60mm wide. It’s soldered or bolted to the ground plane at the connector end, and supported by an adjustment screw at the other end. I’ve honestly not noticed much if any difference in the received signal, by fiddling with it. See http://www.uhf-satcom.com/uhf/uhfantenna.html for more details on the matching section.

Final assembly will be done outside, so everything is not tightly fastened yet:

Here it is outside, mounted on a SG-9120 motor. The motor uses the DiSEqC protocol for control, which is sent over standard coax cable. It is a standard in the satellite TV industry.

The motor is controlled by a Moteck digibox, which sits inside the shack:

Another view:

The angle of the motor is adjusted based on the latitude of the receiving site, so that as the motor turns the satellite tracks across the geostationary orbit.

New Antennas and Diminishing Returns

A few weeks ago, I put up a new antenna, a delta loop for 43 meters. Since it was dedicated for a single band, the performance should be very good. My plan was to write a article here about how well it works, compared to my existing antenna, the 635 foot Sky Loop. That article never materialized, because… the delta loop doesn’t work any better than sky loop. What went wrong?

Shortwave listeners, it seems, are addicted to two types of new things: new radios, and new antennas.

We’re sure that the latest and greatest radio will substantially improve reception, reject QRM, and let us hear lots of stations we could never hear before. And Software Defined Radios promise to do all this and more (just ask Al Fansome). While a new radio often does offer conveniences and advantages over the old one, usually they turn out to be mostly minor improvements (unless you’re switching from say a portable to a desktop communications receiver, or finally giving up that old analog tube radio for a newfangled solid state rig with digital readout).

The same holds true, it seems, for antennas. Sure, if you’ve previously had an indoor antenna, and finally are able to put up your first outside antenna, the improvement will indeed be dramatic. You most likely will hear new stations that you never could pick up before, and the reception of existing stations will be substantially improved. You’ll also end up not hearing some things you previously did, like your plasma TV.

And switching from say a 50 foot random wire to a dipole or T2FD will also produce a noticeable improvement in reception. Not as much of an improvement as going to an outside antenna, but still significant.

But after that, it certainly does seem to be a case of diminishing returns.

When I switched from the T2FD to the Sky Loop, I did notice an improvement in reception, but it was not what one would call amazing. It was better, certainly, and worth the effort. But I went from a 132 ft T2FD to a 635 ft sky loop. Most of the improvement was on the lower frequencies and MW, as one would expect. Reception on the higher frequencies, say above 20 MHz was either the same or worse. Also probably as one might expect.

But, like the gambler looking for that last final big score, we SWLs have to try for the ultimate antenna. The one that will let us hear otherwise impossible DX. Like a pirate on 6925 kHz during the daytime transmitting from Montana. Possibly also being heard in New Zealand. To hell with the laws of physics!

So I ran numerous NEC models on various configurations of the delta loop, optimizing the dimensions and height for the best possible reception. Ignoring the fact that minor changes in things like ground conductivity cause huge changes in antenna performance. And that I have no idea what the ground conductivity is here, anyway. Plus, it probably changes when it rains. Also, the takeoff angle from the antenna varies quite a bit if you change the height of the antenna by a foot or two. Did I mention that my yard is heavily sloped?

But, I did the calculations, cut the wire, shot the fishing line up over the trees to pull up the rope, and installed the new delta loop. Then ran coax to the shack, connected it to the radio, and ran some tests that evening, to see how much better the performance was. It wasn’t. Signal levels were lower than with the sky loop, and more importantly, the signal to noise ratio was the same or worse. Plus, I had an antenna that basically worked for one band, whereas the sky loop is good from MW up.

So, I think I’m going to stick with the sky loop. No need to switch antennas, or use an antenna tuner. It just works. Although, if I take the delta loop and reconfigure it as a horizontal resonant one wavelength antenna… hmm… time to run some NEC simulations!

The Sky Loop Antenna

My present workhorse antenna is a sky loop antenna with a 635 feet perimeter. What exactly is a sky loop antenna? The traditional definition from ham radio circles is that it is a full wave loop antenna, oriented in the horizontal plane. They are often used on 160 and 80 meters. The length or perimeter of a full wave loop antenna is 1005 feet divided by the frequency in MHz. So for 160 meters, say 1.9 MHz, it would be 1005 / 1.9 = 529 ft. The exact size of the loop may be important if you’re transmitting and want a reasonable SWR. For receiving only, it is not as critical, and the “bigger is better” rule usually applies. I ended up with 635 feet because that is the largest length I could easily install.

Here is a diagram showing the dimensions and orientation of the antenna:

Reversing the formula to 1005 / length gives you the resonant frequency, 1.58 MHz in my case, which is the top end of the MW band. From my experience, the antenna works great for the upper end of MW, especially the extended band (1610-1700), adequate for the middle of the MW band, and it produces very weak signals at the lower end of the MW band. I’ve yet to hear any transatlantic longwave stations with it.

The gain of a loop antenna is proportional to the area. While I don’t have enough space to substantially increase the perimeter of the antenna, I could add perhaps 200 feet at the most. An additional 200 feet would drop the resonant frequency to 1.2 MHz, but I’d substantially increase the area, so it may be a worthwhile project.

The height of the antenna varies dramatically, with some points barely 15 ft above the ground, others are around 40 ft. Again, this was what I could easily achieve. Raising sections of the antenna is a planned Spring project, it will be interesting to see what the improvement, if any, is.

The antenna is constructed from #16 insulated stranded wire, and is suspected from trees around the yard. The feedpoint is a 16:1 balun, and 100 feet of 75 ohm RG-6 coax runs from the balun to the shack. I’ve become a big fan of RG-6 coax for my antenna projects. This is the coax used for TV purposes. It’s available everywhere, and is incredibly cheap and low loss. Yes, it is 75 ohm, not 52 ohm, but for receive only antenna like this, who cares?

Running a NEC simulation, the free space resonant frequency is 1.59 MHz, with an input impedance of 140 ohms, which seems reasonable for a loop antenna. Over an average ground, this shifts to 1.55 MHz and 49 ohms, and over a good ground, 1.55 MHz and 27 ohms. Using an average ground, and running NEC simulations for other frequencies gives the following results:

MHz	R	X	Z
1	35	-2421	2421
2	245	1735	1752
3	83	-181	199
4	941	-3196	3331
5	398	1082	1152
6	203	-354	408
7	2233	-1832	2888
8	507	768	920
9	346	-519	623
10	2392	-489	2441
11	542	437	696
12	447	-609	755
13	2113	845	2275
14	487	250	547
15	771	-650	1008
16	1564	786	1750
17	344	157	378
18	1029	-877	1352
19	1132	797	1384
20	470	47	472
21	1338	-998	1669
22	886	708	1134
23	410	-76	416
24	1497	-509	1581
25	748	664	1000
26	480	-173	510
27	1619	-194	1630
28	675	516	849
29	485	-239	540
30	1815	341	1846

R is the real component of the impedance, X is the reactive, and Z is the overall impedance, all values in ohms. As you can see, the impedance values are all over the place. Looking at them in closer detail would show even finer scale variations, but I’m not sure it would be too useful, as this is a simulation, an estimate of the antenna performance, these are not necessarily the impedance values of the actual antenna. Lies, damned lines, and antenna models.

The large Z impedance values over the HF range are why I went with a 16:1 balun, to better match them to the 75 ohm coax. The downside is that the loop impedance over MW is much lower, and the 16:1 balun probably produces a poor match. A 1:1 balun might be best for MW use, but I’m not sure what would happen at HF, I assume a poorer match and weaker signals. I spend most of my time on HF, anyway.

Below is a plot showing the gain of the antenna at three different elevation angles, 30 degrees (low angle radiation, ideal for DX), 60 degrees, and 90 degrees (which would be straight up) for a frequency of 6.9 MHz.

The red circle is the gain for 90 degrees, straight up. This angle for NVIS, where the radio waves are going virtually straight up from the transmitter, and being reflected straight down back to the Earth. The gain is 7.2 dB over an isotropic antenna (an antenna with no gain in any direction). For this case, the antenna has no favored direction, it is equally sensitive in all directions around the compass. For the lower angles, the antenna does have more gain in certain directions, and of course less in others. I find that for NVIS reception of pirates this antenna is excellent, so here’s one case of an antenna model actually approaching reality. DX reception is not bad either, I regularly pick up Europirates, and of course SWBC stations from all over.

One thing I like about the antenna is that it works reasonable well over all of HF and much of MW. I used to have dedicated dipoles for the various HF bands, but it was always a pain to switch antennas when tuning to a different band. And being a loop antenna, the noise levels are much lower than dipoles. I do wish the performance on the lower part of MW was better. I will try enlarging the antenna and see if that improves MW reception.

Don’t let the large size of my build of this antenna discourage you from building your own, if you don’t have the room for one of this size. A full wave loop antenna for 6.9 MHz is 146 feet – that’s a square 36 1/2 feet on a side. Such an antenna should work well from 43 meters on up.

Analyzing Half Wave Dipole Antennas

There are two characteristics that we’re particularly interested in:

First, the radiation pattern. This describes how well the antenna receives (or transmits) a signal in various directions. Below is the radiation pattern for the standard half wave dipole in “free space”, that is, without a ground below it. You can imagine it is in outer space, or so far above the Earth’s surface that there are no effects from the ground.

free space dipole radiation pattern

The antenna wire is oriented east/west. The image on the left is the horizontal pattern. Imagine you’re above the antenna, looking down. This is the pattern around the antenna, all 360 degrees of the compass. There are two main lobes, one to the north, and one to the south. This means that the antenna is particularly sensitive to signals to the north and south, and less so to signals to the east and west. For a transmitting antenna, most of the radiated signal is directed the same way. One rule for antennas is that the radiation patterns are the same for both transmitting and receiving.

The image on the right is the vertical pattern. Imagine you’re at the same height as the antenna, looking at it. The top of the graph represents the signal strength going up, the bottom going down, etc. In this case, there are two sharp nulls directly to the left and right of the antenna. These are in line with the antenna. What this is telling us is that most of the RF energy is directed around the line containing the antenna wire. Here is what it looks like in 3D:
free space dipole radiation pattern 3D

Now let’s make the antenna more realistic by putting it above the ground. In this case, we’re going to put a dipole cut for the 6.9 MHz pirate band 30 feet above the ground, which is probably a typical case for many listeners (and operators). Here’s the resulting radiation pattern:
free space dipole radiation pattern

Here is what it looks like in 3D:
free space dipole radiation pattern 3D

We can think about what happened. The ground obviously blocks reception of radio waves from that direction. Likewise, it absorbs most of the RF energy directed to the ground (some of it is reflected, especially at shallow angles). The resulting antenna pattern is directed upwards.

There’s actually a term for such an antenna – the NVIS – Near Vertical Incident Skywave antenna. Most of the RF energy is directed upwards, where it is then reflected downwards by the ionosphere. Good reception coverage is obtained for a distance of several hundred miles around the antenna, providing the frequency is low enough. If it is too high, the radio waves will pass through the ionosphere without being reflected. NVIS is commonly used below 10 MHz, although higher frequencies are possible with active solar conditions.

Similarly, such an antenna is more sensitive to radio waves coming almost straight down from the ionosphere, that is, from transmitting stations that are several hundred miles away. It’s basic geometry, the more distant the transmitting station is, the lower the angle of radiation.

On the other hand, if you want to reach distant listeners, you need to get more of your radio waves to be directed at a lower angle. If we double the height to 60 feet, here’s what we get:
e dipole radiation pattern

It’s a significant improvement, but the maximum radiation angle is still pretty high. If we triple the height to 90 feet, here’s what we get:
e dipole radiation pattern

That may actually be worse! The radiation pattern changes dramatically with height, often in difficult to predict ways.

A horizontal half wave dipole is still a very useful antenna for shortwave radio, especially for transmitting distances of several hundred miles. Further reception is certainly possible, when conditions are good. In the next entry, I’ll take a look at another type of antenna, the vertical.

Antennas Antennas Antennas

I’ve been an SWL for 30+ years now. In that time, I’ve had probably dozens of HF antennas. My first was a simple Random Wire (aka longwire), about 75 ft long, going from the shack (my second story bedroom as a kid) to a tree. Fed with single conductor wire to the antenna input of my Radio Shack DX-160. It worked reasonably well, I heard lots of stations, and back then there were far fewer sources of QRM in the house. We didn’t have computers or plasma TVs to deal with, nor dozens of switching power supplies. Just horizontal sweep harmonics from TV sets and the occasional dimmer switch.

Eventually, I discovered dipole antennas. By this time I was in to listening to pirate radio stations, so I put one up cut for about 7400 kHz, since that is where most pirates were operating. Dipoles are inherently narrow band antennas, and I eventually had several including one cut for around 6200 kHz for Europirates, since that is where they tended to operate. This was a folded dipole made from standard 300 ohm TV twinlead. The ends were shorted, and a 4:1 balun was connected to the center of the lower conductor, since the antenna was theoretically 300 ohms impedance. From memory, this antenna worked very well.

At one point, when I was more involved with ham radio, I put up a G5RV antenna. I don’t recall spectacular results with it. I spent most of my time on 15 meters CW, so I ended up putting up a 15 meter band dipole, which worked quite well as expected. I had a lot of contacts with Latin America.

Several years ago, I discovered the terminated, tilted, folded dipole (T2FD) antenna. This is a very broadband antenna, with a typical claimed bandwidth ratio between highest and lowest frequency of about 5:1. In my case, I put up a 132 ft long T2FD, which was designed for about 2.5 to 15 MHz. This was at the bottom of the sunspot cycle, so higher frequencies were not of much interest. (Of course, several years later, we still seem to be at the bottom of the solar cycle) This antenna was fed with 75 ohm coax into a 9:1 balun. I got very good results with it for HF, and it worked reasonably well down to the upper end of the MW broadcast band. Performance was very poor, as expected for the rest of the MW band and for longwave.

One thing I immediately noticed about this antenna, vs the various dipoles I had, was that the very low noise. It did not seem to pick up QRM as much as the dipoles. Signal levels of stations were also lower, but, more important, signal to noise ratios were higher. There has been a lot of speculation and claims about the low noise characteristics of various forms of loop antennas. This may explain the excellent results I had years ago with my folded dipole for 6200 kHz.

I finally had an antenna that worked well over most of HF, which meant that rather than switching in various dipoles depending on where I wanted to listen, I could just leave the one antenna connected. Plus I had generally lower noise levels. But I did not have something that worked well down into the MW band.

The next antenna I discovered was the sky loop antenna. The sky loop is a giant loop antenna in the horizontal plane. You run it as high as you can, often around the perimeter of your yard. The exact shape is not important, in my case there are about a dozen or more supports around the various sides of the antenna, and it most closely resembles a trapezoid in shape, with a perimeter of about 635 feet. Yes, it’s a huge antenna. MW reception is excellent as expected, semi-local MW stations are in the S9+60 dB range.

My T2FD was damaged in a storm around the time the sky loop was installed. I hope to get it back up shortly, to run some better comparisons between the two antennas. Also, the T2FD may be better for the higher bands. Should solar activity return to reasonable levels, I may install a shorter T2FD for 19 to 10 meters.

Based on my recent experience, I am certainly sold on loop antennas in their various forms. The lower noise pickup characteristics are reason enough to consider building one the next time you’re considering putting up a new antenna.