5
$\begingroup$

I've grown up using SWR/power meters. They're "simple" devices: They show forward and reverse power, and show the standing wave ratio. They, however, do not show antenna efficiency. New hams are traditionally instructed to tune their antennas so as to minimize the SWR.

On the other hand, I've been reading about the RF amperemeters. Older radio sets used to have them, and there are people who recommend tuning for maximum antenna current, instead of minimum SWR. Manuals for some of them have statements such as "Tuning for maximum feedline current for any given antenna always gives maximum radiated power.", without going into details why.

So with what extra information does the amperemeter provide me, compared to a regular SWR meter?

$\endgroup$
  • $\begingroup$ "Tuning for maximum feedline current for any given antenna always gives maximum radiated power" is correct. Please see my answer below. $\endgroup$ – Mike Waters Jan 24 at 23:26
2
$\begingroup$

In a way, an SWR measures voltage and current simultaneously, and that's how it's able to separate forward and reverse power. W2AEW has an excellent video on how directional couplers work. One way to convert an SWR meter into an ammeter is to disconnect one of the transformers.

Older equipment included an RF ammeter because that older equipment used vacuum tubes. Tube PAs have a characteristically high output impedance, and thus require a matching network to drive typical loads. Thus in practice tube equipment has what amounts to an antenna tuner built in. The ammeter is a useful tool for tuning the matching network, but the tuning procedure usually involves measuring currents within the PA, not just feedline current.

I would say an ammeter is of little to zero practical use if you already have an SWR meter. If you have tube equipment, there's likely already a ammeter built into it. And if you have solid-state equipment, it has a fixed matching network which is designed for a 50 ohm load. Deviating from 50 ohms will result in one or more of:

  • activation of protection circuitry, reducing power output or shutting down the transmitter
  • an increase in nonlinear distortion
  • overheating and eventual destruction of the transmitter

Measuring feedline current is a tricky business, since it depends on where the meter is placed. You probably know that a VSWR greater than 1:1 means there will be high and low voltage nodes on the feedline. Those high voltage nodes are also low current nodes, and vise-versa.

On the other hand, an SWR meter indicates the same thing regardless of where it's placed1. It can do this because it measures voltage as well as current.

It is true that if feedline length, meter position, and frequency are held constant, then more feedline current means more radiated power. However the SWR meter also measures forward power, and more forward power also means more power to the antenna. If you want to tune for maximum power without regard to anything else, the SWR meter allows that.

But given the issues with improperly loading the transmitter above, I would not recommend it. Hams decades ago may have gotten away with it due to the generally more robust equipment and relaxed spurious emission regulation of era, but the times have changed, standards are higher, and there's newer equipment available.


1 modulo transmission line losses. As the line loss increases, more of the reflected power is absorbed by the line, and the indicated SWR approaches 1:1. For typical feedlines with low loss and moderate SWR the effect is negligible.

$\endgroup$
  • $\begingroup$ Ah, but if your antenna can't be changed, and the finals can be tuned, I think an SWR meter would be useless - it'll always read the same thing, and won't help you tune. The ammeter is the only thing to have if it's between the tuner and antenna, the SWR meter is the only thing useful between the (matched) transmitter and the tuner. (I mean a single needle SWR meter of course) $\endgroup$ – tomnexus Jan 26 at 17:52
  • $\begingroup$ @tomnexus Good point, I didn't consider the case of putting the meter after the match, or single-needle SWR meters. I'm assuming the crossed needle kind that show forward and reverse power, and in that case the indication of forward power should be just as useful as an ammeter. $\endgroup$ – Phil Frost - W8II Jan 26 at 20:12
1
$\begingroup$

A VSWR meter does not tell you the current. An RF ammeter does. Tuning for maximum current = maximum power to the antenna.

Having said that, if we have to choose between them the former is always the best choice. An ammeter cannot tell you the VSWR, and a low SWR reading is synonymous with the lowest transmission line losses.

RF ammeters can be placed anywhere, because the current in a series circuit is always the same. However, if the SWR is not 1:1 at the antenna, the reading will be inaccurate unless the feedline is exactly one-half of an electrical wavelength long (or an even multiple thereof).

RF ammeters were invented before VSWR meters. See How did hams manage to tune their antennas before VSWR meters?

$\endgroup$
  • $\begingroup$ "However, if the SWR is not 1:1 at the antenna, the reading will be inaccurate unless the feedline is exactly one-half of an electrical wavelength long" Huh? Putting the meter at the transmitter does mean feedline losses make the SWR look better than it is, but being a half wavelength has nothing to do with it... $\endgroup$ – Phil Frost - W8II Jan 25 at 13:00
  • $\begingroup$ @PhilFrost-W8II With a mismatched load, its actual impedance repeats every multiple of a half-wavelength, with some inaccuracy depending on the loss in the transmission line. $\endgroup$ – Mike Waters Jan 25 at 17:17
  • $\begingroup$ Yes, but of all the impedances you might get from a mismatched load, they all have the same SWR. So an SWR meter will indicate the same number, regardless of line length (modulo losses). $\endgroup$ – Phil Frost - W8II Jan 26 at 14:42
  • $\begingroup$ Good point that max current = max power, if the ammeter is between tuner and antenna. If the ammeter is before the tuner then no, you could be tuning for a short circuit. But in the case of a valve radio with adjustable matching, and a fixed antenna, the ammeter would be useful (and the SWR meter useless - it wouldn't change as you tune the finals). Just one complaint - it's not a series circuit, the transmission line has substantial parallel capacitance, and the tuner probably has an element in parallel. The current is unlikely to be the same everywhere. $\endgroup$ – tomnexus Jan 26 at 17:45
  • $\begingroup$ @tomnexus Won't the SWR meter show a change in forward power as the finals are tuned, even in the finals -> match -> meter case? Is there a circumstance where tuning for maximum current gives a different result than tuning for maximum forward power? $\endgroup$ – Phil Frost - W8II Jan 26 at 20:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.