Antenna Analyze, or System Analyze?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by CHUCKSTEIN, Oct 8, 2021.

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1. W9IQHam MemberQRZ Page

A couple of corrections to your thought process.

SWR is defined as either a ratio of voltages or currents, but not the ratio of forward to reflected power.

SWR can be calculated from a term called the reflection coefficient. This is the percent of voltage or current reflected from the load (antenna) back toward the transmitter. To convert this from a field value to a power value, the magnitude of the reflection coefficient must be squared. There are related metrics, such as return loss (RL), that are based on the square of the reflection coefficient magnitude but this is expressed as a decibel (dB) which isn't intuitive to the casual user. The RL is equivalent to the reflected power divided by the incident power expressed in decibel form.

The reflected power indicated on the meter does not need correction. The forward power on the meter is equal to the transmitter output power (also called incident power) plus the reflected power. So to confirm your transmitter output power, you subtract the reflected power from the forward power. Then remember that your transmit power may be reduced due to the SWR. So for examples:

The first case would indicated that your incident (transmitter output) power is 10-2 or 8 watts. The second case would indicate that your incident power is 20-4 or 16 watts.

The SWR in both of these cases should be read on the meter at the point where the two needles intersect. What were these two values? By my calculation, these result in the ~6 dB RL which is equivalent to an SWR of ~3 (or 3:1 if you prefer). So I don't see any apparent errors in your measurements.

- Glenn W9IQ

K9AXN likes this.

Very nicely done Glen, and well articulated Chuckstein.

Regards Jim K9AXN

3. CHUCKSTEINQRZ Member

Mixed in 70cm,1.25m,2m
UV-5X3, UV-5RIII, D878UV, KG-UV9P

4. CHUCKSTEINQRZ Member

I look at my needle meter, it shows fwd and rev "power", and then some lines for SWR. It says nothing about volts or currents. Surely the crossing dual needles and some trickery on the silkscreen converts voltage or current rations into a readable swr format.

Another issue with this MFJ-842, and perhaps same for others like it., I can drag my finger across the plastic face window and the needles will move, presumably from the static charge I am creating. Not good for a needle meter.

Another issue, the ticks on the scales are not even linear, making it very difficult to know where the needle is at when the ticks on hi scale are 0-5 5-10 10-20 20-50 and the gap between each is no multiple of any other. The lo-pwr scale too is not linear, but easier to read.

Last edited: Oct 15, 2021
5. CHUCKSTEINQRZ Member

Those #'s I gave were just as-examples of what I should expect to see simply by changing power by factor of 2.
Tx, fwd, rev, were all 2x from power-A to power-B.

So yes, swr remains the same regardless of power change, well, we expect it to, but for many reasons I guess, not always the case in real world.

6. W9IQHam MemberQRZ Page

The red scale is performing a division. The non-linear power scales are converting voltage to power.

Electrostatic meter movement is common with analog meters.

- Glenn W9IQ

7. CHUCKSTEINQRZ Member

My old Simpsons did not have needle moves, guess the things were shielded in some way.

8. W9IQHam MemberQRZ Page

Cheap vs quality. Plastic vs glass.

- Glenn W9IQ

CHUCKSTEIN likes this.