More Thoughts on Antennas

I recently corresponded briefly with a guy on the Elecraft mailing list about his difficulty in understanding antennas. As I’ve pointed out in the past, I’m no expert, but my purchase of an antenna analyzer has certainly helped, and I advised him to also buy one. He replied that he did have one, but…

I’m clueless whether those values of Z, L and/or C are good/bad when it comes to trying to maximize ERP.

This really bugs me because I KNOW the antenna is the most important element (“a dime in the antenna is worth a dollar in the transmitter”). Yet for me my antenna system is the least understood portion of my equipment.

If I could just find a ‘dummies guide’, something that just ‘boils it down’ in laymans terms to give me a foundation to start from I’m sure I could then progress to the more technical explanations.

My response was,

The short answer is that, for the most part, it doesn’t really matter. As long as an antenna’s SWR is less than 1.5:1 (and some would say 2:1), then it will radiate just fine. You know, there are many factors which contribute to an antenna’s effectiveness, and for most amateur installations, you’re not going to be able to optimize all of those factors.

How well an antenna matches the feedline and the transmitter is only one of those factors. Other things, such as the height above ground, location of nearby metal structures, etc. will all have an effect. Look at it this way — connecting a 50-ohm resistor to the transmitter output will give you a 1:1 SWR, but you’re not going to be radiating a lot of power with it.

To me, the big reason for trying to keep the SWR down is to reduce feedline losses when you’re using coax. Coax was designed for systems where the transmitter output impedance matches the feedline impedance and the feedline impedance matches the antenna input impedance. When there’s a mismatch, then the coax contributes greatly to the losses.

Another thing about coax, I think, is that the quality can vary quite a bit. I’m just getting into this, but I think it’s a good idea to actually measure how lossy a particular piece of cable is. I haven’t done this yet, but there are articles out there that describe how to do this.

You can, of course, avoid these losses by using open-wire feedline or ladder line. Because of the way they are constructed, they are not as lossy as coax, even when there’s a high SWR on the line. How cool, eh? The trick, however, is that you have to use an antenna tuner to make sure that the impedance at the transmitter input is 50 ohms. The reason for this is not because the antenna won’t radiate effectively, but rather so you don’t damage your transmitter.

The more I think about it, the more I think that coax is really the culprit here. Sure, it’s easy to work with–it’s flexible and you can run it just about anywhere. To get the best results, however, you have to use qualiy coax and use it with an antenna whose input impedance is close to the characteristic impedance of the coax. If your coax is less than optimal, or the SWR on the line is high, then you’ll get poor results.

I think this is one reason that the guys who have K1s and KX1s and use them with random wires and the internal antenna tuners seem to have such good results. The output power may only be 3-5 W, but nearly all of that power is going into the antenna. There’s no coax to suck up precious watts.

My next antenna project is going to be a loop antenna fed with 450 ohm ladder line. I’m going to string up as much wire as I can around the backyard and run the ladder line down to my antenna tuner. I should have enough space for a full wave on 40m and maybe even 80m. Should be fun.