Measured forward power less than "usual"

Question

I've just tried making my own J pole antenna. While matching for lowest SWR on the SWR meter, I noticed that my FTM400 didn't show 5 W forward power.

I hardly saw the reflected power needle move so I increased the power to 20 W just to have a quick peek. There it showed a forward power of about 12 W.

On my normal antenna I get 5 W for 5 W input and 20 W for 20 W input, but this has me stumped.

The homebrew is a J pole for 146 MHz and SWR should be in the ballpark. At 20 W I get a SWR of about 1.3.

Answer 1

Let's consider this system

Transmitter --> SWR meter --> Antenna

A (V)SWR of 1.3 means that the magnitude of your reflection coefficient $\Gamma$ between your transmitter and your antenna is

$$|\Gamma| = \frac{SWR-1}{SWR+1} = \frac{0.3}{2.3}\approx 0.13$$

which is pretty good, actually, meaning that but 13% of the field strength your transmitter inserts into the antenna is reflected by the antenna, or that 87% actually reach the antenna!

You can convert that into a relation of power inserted and reflected, simply by squaring it – then things look like this: The reflection dampening factor (i.e. how much of your energy is reflected is $$a=\frac{1}{|\Gamma|^2}\approx59\approx 17dB$$.

Now, for a power amplifier to reach maximum power output, it must be connected to a device that matches its source impedance $Z_s$. In that case, 0% of power is reflected. In all other situations (e.g. $Z_s=50\,\Omega$, $Z_a\ne50\, \Omega$, output power is below the maximum output. I don't actually know how your device displays forward power – that's something non-trivial to measure – but the fact that we don't have a unity SWR indicates your PA can't sink it's full output.

It's known that

$$\begin{align} \Gamma &= \frac{Z_s-Z_a}{Z_s+Z_a}\\ \implies\\ 0.13 &= \frac{|Z_s-Z_a|}{Z_s+Z_a}\\ 0.13(Z_s+Z_a) &= |Z_s-Z_a|\\ 0.13Z_s+0.13Z_a &= |Z_s-Z_a|\\ 0.13Z_s+0.13Z_a &= Z_s - Z_a &\vee&& 0.13Z_s+0.13Z_a &= Z_a - Z_s\\ -0.87Z_s&= - 1.13 Z_a &\vee&& 1.13 Z_s &= 0.87 Z_a \\ Z_a&\approx 0.77 Z_s &\vee&& Z_a &\approx 1.30 Z_s\text{ .} \end{align}$$ I'm going to assum $Z_s = 50\,\Omega$, so your antenna impedance is either $38.5$ or $65.0\,\Omega$.

That doesn't really explain the power loss your device is displaying, but it's common for HF measurement devices to not be on identical terms.

Answer 2

There are various considerations which could influence your measurements.

• Antenna height and location: compared to your (unspecified) "normal antenna" did you test the J-pole at exactly the same height and location ?
• Feed line: did you use the same, or same lengt/type of, feed line ?
• Weather condition: did you test in the same weather conditions: temperature, humidity, rain, wind, snow, hale can all influence measurements

In my experiments with J-poles I have a good range of failures and some successes. This is (IMHO) to do with the fact that a J-pole is an:

• Unbalanced half-wave radiator
• with a balanced matching section
• fed with an unbalanced coaxial cable

This will cause all kinds of wonderful effects, most common: common-mode. Which may or may not be influencing the measurements you are taking.

The best results I have established over the years is to use a 4:1 balun, which then needs to be fed "higher" on the matching section to get a 200 Ohm match, combined with a good number of beeds on the coax.

You can easily create 4:1 match using something like this:

This will at least transform your unbalanced 50 Ohm feed into 200 Ohm balanced, for the balanced matching section of the J-pole. It still is a unbalanced half-wave radiator, nothing you really can do about that.

Personally I have given up on J-poles, while with the above method I had some success, I really think that a standard quarter-wave GP, or a vertical half-wave dipole is much easier and I had better results.

YMMV