A lot of Hams hate ‘antenna tuners’, and it’s quite understandable given that there are some common and very bad practices concerning tuner usage.
The June 2015 QST Article ‘Don’t Blow Up Your Balun’ explains this better than I ever could – it explains when and how to use a tuner without causing huge losses. I’ve summarized some of those concepts here, and included some other advantages of using a remote tuner.
Antenna Tuners and ‘Loss’
Back in the days of ladder line, having a tuner in the shack wasn’t such a big problem as the ladder line of the day often had an impedance in the order of 600 ohms, and balanced feed lines are less lossy (have less resistive losses) than coaxial cable when mismatched (being driven into a high SWR). When you combine their high impedance, (which basically means a lower SWR in a mismatch), with their lower losses under mismatch, having a tuner in the shack was not much of an issue.
However when we switched to coax feeds, we should have moved the tuner. Coaxial cable has a relatively low impedance of 50 ohms, which means that when there is a mismatch between the antenna and the feed line, the SWR is generally higher. Coaxial cable also has higher losses (higher resistive losses) under mismatched conditions than ladder line, so when you combine these two things, you can burn up a considerable amount of power just in your coaxial cable. The solution is to move the tuner, or as we should properly call it, ‘matching network’, to the antenna feed point.
Here is a few photos of my working conditions at the moment. Basically, I’m operating from the rooftop of a 5 story apartment. My antenna is literally a piece of Bamboo with a bit of wire taped to it! Its roughly 1/4 on 15m, and it works a treat to be honest. Total cost was about $5! I don’t have a power source up there yet, so I’m at the mercy of both my laptop battery and the car battery running the transceiver. Click on the link below for some more photos.
Just thought I’d share some notes I’ve made regarding Antenna Theory. Believe it or not, it took many hours of scouring the web and consulting books such as the ARRL Antenna Handbook to come up with this list.
Antenna Resonance is the point / frequency at which an antenna produces a purely resistive load. That is to say, there is no complex impedance component, eg 73 + j0 Ohms.
Non Resonant antennas do not necessarily make bad performers, but care must be taken to ensure that matching components are correctly placed and efficient.
Antenna Efficiency is directly related to how much ‘radiation resistance’ an antenna has. The shorteran antenna with respect to the driven frequency, the less radiation resistance it will have.
Maximizing the current across the element (linear metal segment eg, whip) of an electrically short antenna will result in the greatest possible efficiency (all other things equal). This is achieved by correct placement of matching components.
‘Short’ Antennas (with respect to wavelength) present a capacitive impedance,they require the use of an inductor or ‘loading coil’ for matching. The optimal location electricallyis at the top.
‘Long’ Antennas present an inductive impedance, they require the use of a capacitor or ‘condenser’ for matching. The optimal location electrically is at the top.
And that concludes the basics. Next time we’ll explore the affect matching has on antennas, some of the tradeoffs & other design considerations.