Antenna Theory and Line Transformation from DX Engineering

This probably goes to show how much of a nerd I am with Ham radio, but I just read through the 22 pages of Tom W8JI of DX Engineering's PDF "Baluns: Choosing the Correct Balun" and found it a very worth while read. Anyone that operates HF should read through this. There are too many people out there that don't fully understand their antenna systems and line transformation. Admittedly, I only know so much when it comes to the subject but I know I think about antennas, tuners, and efficiencies more than a lot of Hams that toss a dipole in the air, feed it with coax, and slap a tuner and amp on it.

I will hold on to this PDF for sure and use it as a reference. There is too much in this article for me to soak up all at once but I just realized how much I don't understand with the subject. Maybe I am just to the point that I know enough to understand the ideas presented in the article, but getting it to be forefront in my mind will take some application of the ideas to see it first hand.

I checked the manual of my Palstar AT1500CV (now discontinued, replaced by the AT2K) to see what kind of balun it uses - it is a 4:1 Ruthroff Voltage type. From the article Voltage type baluns are not good because they don't handle current well, especially in mis-matched systems. Current baluns are ideal. Another issue with 4:1 baluns, regardless of whether it is a Current or Voltage design, is that the impedance transformation is from high to low. The impedance after the balun is 1/4 of the impedance before it. Tuners do a better job with transforming moderate to high impedances. When you take a moderate impedance and divide it by 4 you now have a low impedance - which is harder, and less efficient, for a tuner to match.

So much to learn and it will only be absorbed in my head so fast. This is the great thing about ham radio - you can think about the subject for an eternity and never learn everything there is.

10 Meter Contest - Lots of Stations Today!

I am surprised to hear that 10 meters is doing pretty well today. I have worked all over Texas this afternoon, in to Pennsylvania, and Florida. Not too bad for the bottom of the solar cycle...

My buddy Jason N8XE worked New Zealand on 10m too. Though, having a 600 watt amp and a beam antenna works better than my 100 watts and attic ground plane antenna... I can't wait until I can have good antennas!

Get on the band and make some noise! You might be surprised with what you are able to do.

Coax Loss vs. Impedance Mis-Match

I posted a thread on the QRZ.com forums asking the question of what happens to coax attenuation characteristics as the impedance mismatch increases. The reason for my question is I know the attenuation increases when the impedance mismatch increases, such as when an operator uses a "tuner" to make an antenna load up any band they want (which a LOT of hams do).

One reply came back with this online calculator. Based on this calculator, surprisingly the attenuation isn't that drastically different, within reasonable limits, with high SWR.

For example - 100' of LMR400 (a common high quality RG8 cable) is still about 71% efficient with a 5:1 SWR. If you have a 40 meter dipole that has the whole band with under a 3:1 SWR and you feed it with 50' RG-8x you will have at least 87% efficiency. At the resonant frequency of that dipole the efficiency of the feedline will be about 92% efficient. Not too much of a difference.

However, here is where the problem of using coax comes in: If you use that 40 meter dipole on 20 meters with a tuner and the SWR is, say, 8:1, the feedline is now only 58% efficient. That means for your 100 watt input power only 58 of those watts are getting to the antenna.

I am not sure what the impedance of the 40 meter dipole really would be on 20 meters, but click on the link above and plug and chug some numbers and see for yourself! This is a fun tool...