Here are a couple of antenna videos that I ran across this weekend:
I’m not saying that it’s not a good idea to use a balun. Even if you cut the elements of a dipole to be exactly the same length, once you get it in the air, there will be some imbalance because the environment won’t be perfectly balanced. How much that imbalance affects the system depends on the particular installation. When you use a balun, you force the currents to be the same in each leg of the dipole, and the current on the outer shield will be zero.
I’m just saying that this video isn’t that conclusive. It is actually an example of how arguments like this get started in the first place. The problem is that even when we try to make measurements, like IZ2UUF has done, the results can be debated. It’s difficult to make these measurements, even with professional-grade test equipment and facilities.
So, armed with inconclusive evidence, hams fall back on their personal experience, and since these experiences can differ widely, opinions differ widely. The ham who has never used a balun with a dipole and gotten good results, with little or no feedline radiation because he or she has managed to get a “good enough” balance, will not see the need for the extra expense of a balun. On the other hand, the ham who really has needed a balun and used one to good effect will sing the praises of using one.
Build your own discone
If you want to design and build your own discone, there are lots of resources on the internet. A search for “discone antenna design” yielded dozens of references, including:
Let me know if you’ve built your own discone or how it goes when you do roll your own.
Rick Barnich says
IZ2UUF is also the author of an excellant Morse Code/CW app for Android devices. I used this while studying for the CWOps CW Academy Level 2 course. I have a number of applicable files I converted for compatability with IZ2UUF and would be happy to share.
Dave New, N8SBE says
Um, a ‘perfectly balanced’ dipole would benefit from a balun, if fed with coax, since the coax is by nature an unbalanced feed line.
This works with any transition from an unbalanced to balanced interface. I use toroidal baluns on the coax feedlines to my quad antenna, since the one-wavelength loops are balanced loads. This stops current from coming back down the outside of the coax shield.
Back in high school, I home-brewed a 2-element quad using bamboo spreaders, etc. Since it was only three bands 10, 15, 20, there was little interaction between the driven elements, so the impedance of the loops was close to the 150 ohms design. To transform the loop impedance to the 50 ohm feedline, I used 1/4 electrical wavelength 75 ohm sections to create the match. This is standard practice for quad designs. I ran each feedline all the way back to the shack. Being high-school poor, the extra RG-58U was cheaper than a remote antenna switch, and current practice counsels against trying to feed all three loops from a single feedline.
What I didn’t know was that I needed to use some sort of balun to transform the balanced loop to unbalanced coax, so I ended up with all sorts of RF problems in the shack. I was using an HW-101 at the time, and kept getting ‘bit’ by RF on the microphone, etc. I thought the problem was caused by bringing all three feed lines into the shacks, etc., but trying to ground the unused lines didn’t seem to help. I never did figure out what the real issue was in those days.
When I decided to re-visit using a quad many years later, I found a nice 5-band design on the web (https://www.qsl.net/ei7ba/Cubical%20Quad.htm), and followed the article’s advice on using toriods on each feedline. I’ve had zero RF problems in the shack, and it covers 20-17-15-12-10 and I added 6 meters on a lark, and that seems to perform OK, although the SWR has been a bit high.
This 2-element quad has a spider boom, so all elements are at optimum spacing, which is also a nice boost from my original design.
And, I use a remote antenna switch (commercial, instead of the home-brewed one in the article), and took advantage of the way it operates to try out a technique that I think worked out peachy. Each feedline from the switch to a driven element is cut to 1/4 electrical wavelength for that band, and the switch leaves the connection open when unused, rather than grounded. Recall that a 1/4 wavelength transforms an open to a short, and vice versa. So every unconnected (at the switch) coax looks like a short at the driven element, thus RF grounding all the unused driven elements, helping to knock down any interaction between elements.
Even then, the close spacing of the elements means the impedance is close to 60-90 ohms, so I just feed it with 50-ohm line (as the article does), and the SWR is usually less than 1.7:1 on most band segments.
Walter Underwood says
If there was one concept I could erase from the ham vocabulary, it would be balanced/unbalanced. It just isn’t useful. Instead, think about differential mode current and common mode current.
A balun is NOT needed when connecting coax to a center-fed resonant dipole. Those FM broadcast stations running 100,000 W? All of them use coax and none of them have a balun.
There are two important uses of “baluns”. One is as a voltage transformer to match the feed line to the antenna. These may be baluns or ununs, but they are always transformers for the differential mode signal. The other use is a 1:1 current balun acting as a choke for common mode currents. When I added one of those to my home dipole, the noise floor dropped 6 dB.
While we are at it, want to know about common mode currents on ladder line? Easy, both wires high against ground, then both wires low. Boom, you now have local noise on your “balanced” feed line.
Davide Achilli IZ2UUF says
Hello Dan and thanks for sharing this video.
The goal of the video was to show what the balun function was to people that confused baluns with impedance matching transformers.
I made that video when participating to a discussion where some people were arguing that they didn’t need a balun since their antenna was presenting already a nice 50 ohm impedance – which is not much related with the role of the balun. I prepared a video where I built a dipole that presents SWR=1 but shows relevant radiation that are stopped by a common mode choke 1:1 balun. The video shows that there is at least one case (the one in the video) where common mode effects are clear and present without a balun and disappear with a balun: this should inspire people to reason, in their own case, why they do not expect common mode current or why they should have negligible effects.
73 de Davide IZ2UUF