3. You Can Tune an Antenna, 
But You Can't Tuna Fish

Tech-Talk
Part 2

Welcome Back!

Last month we talked about what SWR is.  This month we'll explore why we care about it, and what we do about it.  If you need to refresh, or are new to the list, previous installments are located below this one.

Modern transmitters are designed to operate into a 50-Ohm resistive impedance.  We use 50-Ohm coax, a 50-Ohm antenna, all of our power radiates, and the transmitter is happy.  But when the load is something other than 50 Ohms (in other words, the SWR is greater than 1:1, problems can arise.  If the mismatch is bad enough, our solid state final amplifier stage can easily overheat and self-destruct.  So our radios include a protection circuit that lowers the output power as SWR increases.  The circuit generally kicks in at somewhere between 1.5:1 and 2:1 SWR.

So how do we lower the SWR and regain full output power?  There are several ways.


(1) As noted last month, find a spot along the feedline where the SWR is at a minimum.  Tedious, good (at best) on a single band, and not very practical.

(2) Add an appropriate resistor in parallel with the antenna system.  Easy to do, works like a charm, and gets your SWR darn close to 1:1 on all bands.  One slight problem with this approach -- nearly all of your power is dissipated in the resistor, and just about none radiates.  Can you say "Dummy Load"?  Sure you can.

(3) Adjust the antenna length until the SWR is acceptably low.  In theory, a very good solution.  Good in practice as well, if we're looking at a single band antenna, or can set up separate antennas for each band.  Not all of us are blessed with enough real estate for that.  We want to use one antenna on multiple bands and so (drum roll!)...

(4) Use an antenna tuner.  How does a tuner (sometimes called a Transmatch or Matchbox) work?  Quite simply, by putting various values of inductance and capacitance in series or in parallel with our antenna system.  Some combination of these values, plus those of our antenna system, result in a 1:1 SWR at the input of the tuner (Note this... we'll come back to it).  As most of us have found, not all antennas can be matched on all bands.  That's because our tuner has a finite amount of inductance and capacitance.  Given a tuner with infinite values, you could literally tune a thumbtack on 160 meters.  No, it would not work very well... losses in the tuner and feedline would eat up most of the power.  But we'd have a 1:1 SWR.

From this, it should be obvious that by itself, a low SWR does not mean that we have a good antenna.  Neither does a high SWR mean that we have a bad antenna.  We need to look at the entire antenna system -- essentially everything going out from the antenna port on the radio.  That includes (principally) the tuner, the feedline, and the antenna; secondarily, it also includes nearby objects and structures, especially metallic ones.  Later on in this series, we'll delve further into this topic.

Actually, "antenna tuner" is somewhat of a bad name for this device.  It does nothing to improve the antenna.  All it does is make the radio happy so that it can safely generate full power.  A more correct term is "impedance matching network".  But most folks call it a tuner, so we'll stick with that for simplicity.  Just keep in mind what it really does (and does not) do.

That's it for this month.  Next time, we'll take a look at how, and why, your SWR meter may well be lying to you.  And we'll have some facts and figures that may shock and amaze you.

73 for now,
John Bee, N1GNV
Quicksilver Radio

No comments:

Post a Comment