Why Remote Antenna Tuners Are Not Evil

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.

Antennas, Matching Networks & Correct Antenna Tuner Placement

The key to remember is that all antennas use ‘matching networks’ so that they appear as 50 ohm loads and we can feed them nicely with our transceivers and linears.  For example:

  • An ‘ideal’ ¼ wave vertical antenna would present a 36 ohm resistive load
  • An ‘ideal’ dipole would present a 72 ohm resistive load
  • Folded Dipoles often present a 300 ohm resistive load
  • Yagis often present a ~20 ohm resistive load

What do all these antennas have in common?  Matching networks, although some are simpler than others.  For example, some people intentionally shorten a ¼ dipole or play around with the radials to bring the impedance at resonance up to 50 ohms.  Most ½ Wave Dipoles are actually cut slightly shorter to lower their impedance at resonance to 50 ohms (at the cost of performance, although slight).  Many antennas such as OCF Dipoles & Windoms either use 4:1 Baluns as matching devices, or sections of ladder line.  Yagis often use a ‘gamma match’, which is the sliding rod assembly on the driven element.

In Fact, there are only a few antennas that come to mind that inherently have a 50 ohm impedance at resonance; the Moxon Rectangle & the Hexbeam.

The key insight is that an antenna feed point mounted remote tuner is nothing more than a matching network, and almost all antennas have matching networks.  Certainly all the common ones do.

If the tuner is placed at the antenna feed point, it takes the place of traditional matching network, such as coils, 4:1 baluns, capacitors etc, and performs about equally as well (perhaps a few % less efficient).  The big gain however, is that at the press of a button, the tuner based matching network can be changed to make the antenna present a low SWR on other bands, or re-tune the antenna if it has been moved.

 

A Few Caveats (Remote Tuner Or  Otherwise)

Of course, there are some catches, but they have nothing to do with whether you use a remote tuner at the antenna or a traditional matching network.  Namely, they revolve around the fact that a poorly shaped (designed) and/or located antenna will be a poor antenna regardless of what SWR you see.  Here are some examples:

  • Matching networks generally have a very limited effect on radiation pattern, so if you’ve got a sky heater (a dipole/OCF/Windom/Inverted-V too close to the ground), it will be a sky heater regardless of whether you use a 4:1 balun or a remote antenna tuner. The only solution is to lift the antenna up into the air. 
  • Matching networks do not make an antenna more efficient. If your antenna is 5 foot long and you want to use it on the 160m band, don’t expect it to radiate much power – it is simply too short.  You can however, expect that wire to get quite warm! (And to be honest, this kind of task is going to be pretty harsh on any matching network).

 

So Aside From Matching, What Else Can A Remote Tuner Offer?

Remote tuners allow for more creative designs; they allow for more flexibility.  With a remotely controlled matching network (tuner), I can:

  • Concentrate on my radiation patterns without worrying about the antenna’s impedance. There is nothing magical about a ‘resonant’ antenna in terms of performance, it is just a quality people desire because it helps them easily match an antenna to their radio, but that’s another lengthy story!
  • Build multiband antennas without having to spend literally days designing ‘static’ matching networks for each band I want to use. This is why many commercial multiband verticals are such a funny looking critters.  They are full of capacitors and inductors, although they may be hard to spot.  Those spikes coming out the top half way up?  That’s a capacitor!  The coils are usually easier to spot.
  • Relocate an antenna and not have to worry about retuning. Have you ever built and tuned up a ¼ vertical?  If you move the damn thing, it will need to be re-tuned.  I’m about to head to the beach with my vertical, and I’d much rather push a button and work DX, than spend hours shortening and lengthening parts here and there or changing the taps on the base loading coil.

Recently I designed a vertical for an IOTA Activation.  I spent about 1 hour optimizing the length of the antenna, the height of the antenna’s feed point, etc.  I did this so that the radiation pattern looked nice on all the bands I wanted to use the antenna on.  If I made the antenna 1 meter longer, it would result in the 15M band radiating a significant portion of my power straight up at the clouds!  So I made it shorter, but this came at a cost too – my antenna won’t be as efficient on 40m. I was happy to make this trade, as 40M is a pretty hard band to work DX from in Cambodia anyway. I sure am glad I won’t have to worry about designing static matching networks!

 

Should I Replace My Traditional Matching Network With A Remote Tuner?

On a commercial antenna that is working well, permanently installed and doesn’t need a shack tuner generally no.

The advantages and caveats mentioned in the previous sections should be at the forefront of your considerations if you’re tempted to rush out and replace your traditional matching network with a remote tuner, so I’ve included a hypothetical scenario that is applicable to real life.

Let’s say you have a traditional, mono band, 20M 5/8 Wave vertical antenna, and you decide to replace the big loading coil with a tuner so you can use it on other bands.  It will work, sort of.  You will be able to tune (‘match’) your previous monobander up on whatever frequency you like, and transmit to your heart’s content.

However don’t expect it to automatically become a great antenna on other bands – remember the tuner can’t change the physical shape or location of the antenna, and it’s the physical shape and location that determines the radiation pattern.

 

So maybe I’m unconventional, or enlightened, only time will tell.  But i’ll end with one last bit of advice:

 
XU7AGA’s Maxim #4: Remote antenna tuners make very expensive matching networks for fixed position, monoband antennas!

 

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