Last week I wrote about SWR, and the video referenced in that post clearly shows how standing waves are set up on a transmission line. The basic idea is that if the load at the end of a transmission line is greater than or less than the impedance of the transmission line, standing waves with a ratio greater that 1:1 will be set up on that transmission line. Now, let’s take that idea a step further.
Many antennas that we use in amateur radio, such as the 43-ft. vertical, will have an impedance other than 50 Ω on the amateur bands. That’s OK. In fact, that’s how it was designed to be. They were designed to be used with an antenna tuner. The antenna tuner will match the impedance of the antenna system to the 50 Ω output of your transceiver.
So, if purchase a 43-ft. vertical, you’ll also have to purchase (or already own) an antenna tuner. The question then is, where do you put the antenna tuner? If you put the antenna tuner at the shack end of the coax, you could be losing a lot of signal. The reason for this is that coax can be quite lossy when there is a high standing wave ratio.
Let’s say that you’re using 100-ft. of RG-8X coax, and your antenna has an SWR of 5:1 on 14 MHz. If you plug those numbers into a popular coax loss calculator, you’ll find that the coax loss is 2.28 dB, and about 1.2 dB of that is because of the high SWR. A 2.3 dB loss translates to a loss of more than 40%.
If the SWR is 10:1, the loss jumps up to 3.6 dB, with 2.5 dB the result of the high SWR. A 3.6 dB loss means you’re losing nearly 60% of your signal.
On the other hand, if you locate the antenna tuner at the antenna end of the coax, the antenna will appear to be a 50 Ω load, and the SWR losses will disappear. You’ll still have the resistive losses associated with the coax, but 100-ft. of RG-8X coax only has a resistive loss of about 1.1 dB.
Of course, there are some challenges associated with locating a tuner remotely at the antenna feedpoint. You’ll have to devise a way to route power out to the antenna tuner, assuming that you’ll be using an automatic antenna tuner, for one thing. The antenna tuner will also have to be housed in a weatherproof enclosure. Some tuners come with this type of enclosure; to use models that don’t, you’ll have to purchase one.
AD5X wrote an product review of three remote antenna tuners that appeared in the March 2010 issue of QST. You can read that article on his website, http://www.ad5x.com/images/Presentations/Remote%20Tuners.PDF.