First published in the SOTABEAMS Newsletter 26 July 2016. If you like this article, why not sign up to our newsletter? It comes out about monthly.

A few thoughts on Antenna Tuners

When the subject of antenna tuners comes up in any ham forum, almost inevitably people start to talk about losses and someone will say they only ever use resonant antennas to avoid those losses. Few of these conversations are informed by any objective measurements; they mostly rely on repeated opinions. Recently I read a thread about end-fed half-wave tuners which suggested that these were particularly lossy because:

• they have a high impedance transformation ratio
• losses in toroidal coils are high
• losses in film-dielectric (Poly Vari Con) capacitors are high.

One contributor had made some measurements and although he suggested that his methodology was not rigorous.

The exchange set me wondering what the losses might be in our popular Mountain Tuners. These use a toroidal inductor and a film-dielectric capacitor. I made the measurements and filmed them in a little video clip. Click the image below to watch it.

The Mountain Tuner

The measured loss of a Mountain Tuner at 14 MHz was 0.8 dB. This is one tuner on one frequency and it would be wrong to assume that such a measurement would apply to all tuners and all frequencies. In fact in the internet thread I was reading, the author measure the loss of his (different) end-fed tuner as 4 dB.

So how bad is a 0.8 dB loss?

On a perfectly quiet telecommunications circuit few people can detect a 1 dB change in level. On a typical radio circuit, 3 dB is the smallest noticeable change (half an "S" unit) so a loss of less than 1 dB will not be noticeable. That is not to say that it is not relevant; all losses should be minimised for optimum performance.

Comparing losses

In the case of the Mountain Tuner it might be interesting to see how this loss might compare with a resonant antenna. A big advantage of end-fed antennas for portable operating is the ability to use them with a short co-axial cable, when compare to a dipole. This is because with an end-fed half-wave, the feed point is often most convenient when close to the ground. Co-axial cable is fairly heavy and bulky. For a dipole we want the feed point as high as possible which means that a longer co-axial cable will be required.

Let's compare the losses:

• For the end-fed halfwave we have the feeder loss (say 2 m of RG174) plus the tuner loss
  • feeder loss = 0.2 dB, tuner loss = 0.8 dB so the total loss of the feed system is 1 dB.
• For the dipole we just have the feeder loss - say 12 m RG174
  • feeder loss = 1.2 dB.

Different lengths and different cables would give different losses but the overall conclusion must be that for this aspect of the antenna system, there really is not that much difference.

Conclusions

As with most things, your choice of gear will depend upon your style of operating. In some cases, the convenience of an antenna tuner may actually be more important than any inherent losses.

Tuner losses need not be high, even when using components some people consider to be lossy.

73 Richard G3CWI

First published in the SOTABEAMS Newsletter 26 July 2016. If you like this article, why not sign up to our newsletter? It comes out about monthly.