Hello ECR Family, and welcome to The Antenna Farm. This is your friendly Antenna Farmer Charles, KC6UFM.

In Article 6 we looked at the simple dipole, however there are a number of common variants of the dipole that make great antennas on the VHF/UHF bands and, in many cases, can even be used in the HF bands. We’ll be looking at four different variants, and they share the relative ease of construction and reliable performance of the simple dipoles, so lets jump in…



The Folded Dipole Antenna

The folded dipole is just what it sounds like…a dipole with two wires in parallel, joined at the ends, and fed in the center of one wire just like a normal dipole. The gain (2.14 dBi) and pattern (broadside to the wires) is the same as a simple dipole. It’s worth noting that some researchers claim the folded dipole does have a small amount of gain over a simple dipole, but the numbers are tiny (around 0.15 dBd) and real world performance doesn’t seem to support even this.

So why on earth use it? There is no advantage! Right?

Wrong.

The additional wire causes an impedance transformation at the feed point. Remember that a simple dipole has an impedance of about 73 ohms and that is not a good match for any commonly available balanced line? Well, you can make the feed point of a folded dipole almost anything you like by shifting the spacing between the two wires and the diameters of the wires. Most commonly, the wires will be the same diameter and the spacing something that is mechanically convenient, and this will give you an impedance transformation of 4 times the simple dipole, or about 292 ohms, very close to the 300 ohm balanced feeder.

Go to https://k7mem.com/Ant_Folded_Dipole.html for one of the best folded dipole calculators on the web. You can enter various wire sizes, spacing info, and operating frequency and see how the parameters impact the impedance. Additionally, for those interested, the related formulae are given as well.

In many cases, folded dipoles make a great driven element for beams, especially if you are stacking antennas to increase gain.

One other big advantage of the folded dipole is that it is much more frequency agile, or able to cover a wider bandwidth than a simple dipole. Typically, a folded dipole will cover about 40% wider bandwidth than a simple dipole.

Many public service and commercial station antennas also use folded dipoles because they are cheap, easy to work with, and can be built to be physically very strong. One popular VHF/UHF folded dipole base station antenna uses 2” aluminum angle stock to build the individual folded dipoles and 8 of the dipoles are installed in various positions around the 6” diameter vertical mast pole to give the desired pattern. And there are no bolts or screws…everything is welded. This antenna is rated for winds up to 500 MPH. The downside? Prices start at around $25,000 or so. You could build something similar, albeit with a wind rating of maybe 150 MPH and a bit less gain, for less than $100.

Another advantage to a folded dipole is that it can be grounded. This will radically reduce the noise seen on receive and so give you quieter signal reception.

One fine example of a phased folded dipole array for 2m can be found at http://www.repeater-builder.com/projects/exposed-dipole.html where you can easily build a 4-dipole array for VHF and higher bands that will give you 6 dBd onmidirectional gain (dipoles arranged around the mast) or 9 dBd gain in an cardioid offset pattern (dipoles all on same side of the mast). This antenna design first appeared in the 1974 ARRL Antenna Book. To cover other bands, you can scale the antenna.

Note also that this antenna uses a “Phasing Harness” to get the RF from the transmitter to the individual dipoles in the proper phase to achieve gain. We’ll be talking more about phasing in later articles.



The Discone Antenna

Odds are, you have seen discone antennas. You may have wondered, “What the heck is that?” Now you know.

As the name implies, a discone is made up of a disk at the top of a cone. Figure 2 shows a typical discone antenna. Technically speaking, a discone works best when the disk and cone are made of some solid material like sheet metal, but the weight and wind loading would be huge, so in practice, most discones are made using rods to “simulate” the disk and cone structures as shown in Figure 2.

The page https://www.electronics-notes.com/articles/antennas-propagation/discone-antenna/discone-basics.php offers a good treatment of discone theory and operation while https://www.changpuak.ch/electronics/calc_11.php is a calculator for discone dimensions.

The big advantage to a discone is that it is extremely broad-banded. If designed for, let’s say, 144 MHz (the bottom of the 2, band), it can reliably work up to at least 1440 MHz (just above the 23cm band). I use a discone daily on every band from 6m up to 23cm, but more on that in a moment. Some authors have said that a discone has a 1:10 receive ratio (that is, it can receive at frequencies 10 times the design frequency) but only a 1:3 transmit ratio. That is clearly against both models and demonstrated performance. The reality is about a 1:10 transmit ratio and a 1:20 receive ratio.

Traditionally, discones have been used mostly for receive antennas for scanners, but they work great for transmitting as well. They have a quite low angle of radiation or “Take Off Angle,” that is, the angle relative to the horizon that the signal is focused at. For VHF and above, you WANT the lowest possible angle of radiation in most cases. It is worth noting that as you move up in frequency toward the discone’s maximum frequency, the angle does begin to rise.

The biggest advantage of the discone, it’s radical frequency agility, is also it’s biggest shortcoming. If your transmitter is a little “dirty” (has some spurious emissions and lacks state of the art filtering as do many of Japanese HTs and ALL of the Chinese HTs), a monoband antenna can—and does—act like a final filter to remove those pesky emissions. A discone, however, will just send them out over the air. At best, your signal will sound horrible. At worst, you may get a letter from the FCC. The lesson here? Do NOT use a discone with an HT. You will be splattering signal all over the place. A 5 minute check with a spectrum analyzer will prove this.

As I mentioned, I use a discone every day. I have the MFJ-1868 unit, and I have had it in the air for well over 15 years. Only issue was that I did strip out one of the rods on the disk. A little JB Weld, and all is well.

I can hear you now…“But, Charles! You build all your antennas!!” Well, I build MOST of my antennas, but discones are not one of them. I have, in 31 years, built just exactly one discone. They are a real pain from a mechanical point of view. Honestly, you need some special tools (like a small machine shop) to build a good discone, but if you have a metal lathe and a decent drill press with the skill to use them, it can be fairly easy. But when you can buy them so cheap, it’s a path of least resistance thing. Another advantage to the MFJ discone is that it can receive down to 25 MHz and transmit as low as 50 MHz. This is done with a “stinger” at the center of the disk. If we leave off the 6m stinger, it’s easy to see that in my everyday use the MFJ discone has a 1:10 transmit ratio. According to my VNA, I could use the antenna up to about 2.5 GHz before things start getting strange.

Also keep in mind that the discone is just a dipole. Look at the images, and you’ll see that we have the droopy radials of a ground plane and the vertical element replaced by a disk. You will get just exactly 2.14 dBi of gain. Not a drop more.

Lastly, there have been discones built for use at HF. The biggest one I have personally ever seen was for the 40m band. They used a 75’ tower for the mast, but it worked! In fact, it worked all the way up to the 6m band. I have heard rumors of a military discone back in the 1950s that could work down to 400 KHz.



The J-Pole Antenna

At first glance, it’s hard to see that a J-pole is actually a form of dipole. If you have never seen one, they are bit odd looking. See Figure 3 (this image is from the site https://www.hamuniverse.com/jpole.html which also has a good calculator). For the remainder of the J-pole discussion I will use the nomenclature given in Figure 3.

The name J-Pole should be obvious from the drawing. It looks a bit like a J. But where is the dipole bit?

Look at Dimension A, the tall part of the J. This is about ¾ wave length, the same as the total length of a dipole plus another ¼ wave. Remember in Article 6 when we learned that the ends of a simple dipole have a very high impedance? Well, that’s where Dimension B, the short part of the J (about ¼ wave length), comes in. Dimensions A and B, combined with the spacing between them as shown as Dimension D, form a balanced open wire feed line segment that is ¼ wave long and shorted at the lower end and open on the other end. This “feed line” is then attached to a ½ wave element at the top. A feed line that is ¼ wave is shorted at one end, the open end will show a very high (theoretically infinite) impedance. Dimension C is the approximate point where you will connect the coax feed line coming from your transmitter. More on that in a moment.

The ¼ wave feed line matching network does two things for us:

1) It transforms the very high impedance at the end of the dipole section to a lower impedance to match your coax, and

2) It bridges between the balanced antenna and the unbalanced coax. (Some writers say that the J-pole is unbalanced. I suggest they do the calculus and models.)

To tune a J-pole, you just slide the coax connections up and down the matching section until you find the lowest SWR. It’s as easy as that. Once you find the right place, make the connection permanent. The point you are looking for is where the impedance is 50 ohms, and the transformation is possible because we are working up and down the ¼ wave matching section seeing an impedance that varies from extremely high to very low.

There are two other areas of disagreement on J-poles, and let’s look at those…

First, does the coax center conductor connect to the tall leg or the short leg? Obviously, the shield will go to the other leg. There are armies of people on each side of this argument. The fact is—as supported by mathematical analysis, modeling, and real world performance—it just doesn’t matter. Pick one. As for me, over the dozens of J-poles I’ve built and used, I connect the center conductor to the tall leg. No reason for that other than I do the same thing every time, so that’s one less thing to remember.

Second, should the J-pole be grounded? What we’re talking about is that little stub below the crossover point between the tall and short legs that is used for mounting the J-pole. Again, there are many people on both sides of this argument and most only say what they do because that is what they were told. That is, they have no actual data to back up their position. To a large degree, this also doesn’t matter, except for one small detail: Noise. Most noise (QRN and QRM) is vertically polarized. The J-pole is vertically polarized. Like every other vertical antenna, J-poles will pick up more noise than a horizontal antenna. However, if you ground your J-pole, it gets rid of the noise (usually dropping it by 30 dB or more) AND it makes the antenna safer during storms. I have built J-poles and models with some grounded and others not. In the models and the performance, I could see no difference. The mathematical analysis, however, does show that grounding the antenna might raise the take off angle by 0.5-1 degree, probably not enough to notice. One thing worth mentioning…if you tune your J-pole while it is NOT grounded, when you ground it during mounting, the tuning will change. So make sure you tune the antenna either grounded or not as it will be installed.

The J-pole itself has a few variants…

The Slim Jim is essentially a folded dipole version of the J-pole.

There are many “roll up” antennas made from 300 ohm (or other) twin lead that are just J-poles or Slim Jims. You can get by with this because the Dimension D is not all that critical.

Like all dipoles, the J-pole and it’s own variants are fairly narrow banded. However, the J-pole (as presented on the sites above) lend themselves to what is usually called “Plumber’s Delight” construction using ½” (or larger) copper pipe. This fairly large diameter element gives you great bandwidth. On every band (except 6m) from 10m up to 23cm you can cover the entire FM portion by designing your J-pole for the center of the FM segment. On 6m, you can cover about 3 MHz of the band, so for most FM work, aim for 53 MHz and you’ll cover the top ¾ of the band.

Note that the bandwidth discussion above does NOT apply to any kind of roll up antenna. They are made from wire and the bandwidth is quite narrow.

Then there are the so-called Super J-Poles. Take another look at Figure 3, and imagine another ½ wave extension on top of the tall element with a ½ wave “Delay Section” between the two tall segments. The Delay Section is simply a conductor ½ electrical wave long and mounted so as to minimize radiation from the delay section…usually coiled up so the fields cancel. The idea is to end up feeding the top and bottom ½ wave elements in phase. You are effectively stacking two elements, and you will see about 3 dB gain over the normal J-pole. You can continue to add delay sections and elements as you like, limited only by the resistance losses in the element conductors. Well, and the physical construction…Extended Super J-poles can get big in a hurry!

The oddest J-pole I have ever built was a 3-band design (6m, 2m, 70cm) made from ¼” aluminum rod with nylon spacers to keep the long and short legs in the right places. It had three separate feed lines (one per band) and was designed and built to be used mobile on my VW Rabbit. It worked great! And yes, it did look a little odd going down the freeway.

Right now, I have a 6m J-pole up made from 1” copper pipe. With the large element size, I get full band coverage at 2:1 SWR and below.

For single band VHF/UHF operations, a J-pole is a good choice that can be easily built and be able to outperform the majority of commercial antennas.



The End Fed Half Wave Antenna

Usually called an EFHW, these antennas look a bit like a simple dipole, but are fed at the end like a J-pole. See Figure 4 for a typical design.

Again recall when we talked about simple dipoles, we saw that the ends of a dipole has a high impedance and that a simple dipole will work well on odd harmonics of the design frequency. Well, the high impedance of the feed point can be handled in a number of ways (like a ¼ wave matching section as used in the J-pole) and for an EFHW we can use a matching transformer. And the biggest benefit of feeding a dipole at the end is that it will still work well on the odd harmonics AND it will also work on the even harmonics! Again, look back at Part 6 of The Antenna Farm and while a simple dipole for 40m will also work well on 15m (an odd harmonic), it fails at 20m (an even harmonic). The 40m EFHW will also work on 15m, but it performs well at 20m, too.

This fact, and the placement and harmonic relationships of the Ham bands, allows a properly designed EFHW antenna to cover every band from 160m-6m, albeit some bands (like 17m and 12m) might need an external antenna matching device.

One big advantage to the EFHW is that it is fed at the end of the wire instead of at the center. You can mount the impedance transformer to the outside wall of your shack, run coax inside to your rig, and then run the wire itself to some other point. Being end fed, the EFHW can greatly simplify installation.

There are many commercially available EFHW antennas out there and the price is all over the board. Personally, I have had good luck with the MFJ offerings.

Right now, we’re not going to get into the design and construction of EFHW antennas. Instead, we’ll look closer at these in a later article when we also get into the various Off-Center Fed Dipoles (OCFD or OCD) like the G5RV and others. This family of antennas are based on the EFHW and share many common attributes.

For now, let’s just say that you CAN design and build an EFHW. The antenna itself is very straightforward if a bit tedious in the math department to get good harmonic relationships, but the matching transformer can give you fits. Essentially, you will need to know more about matching networks than what has been presented so far here on the Farm to be successful at this.


That’s it for this article, my budding farmers, but next time we’ll look at something we have mentioned a few times in passing…grounds. And rest assured we will look closely at all THREE kinds of grounds that a Ham operator needs to understand because they are VERY important for many reasons on several fronts.


Take Care & 73
de KC6UFM
Charles