Measuring VSWR and Power Output for VHF Marine Band Antennas and Transmitters

[jimh]

In December 2024 I took some great advice (see details below) about four components being sold on Amazon.com that would be very useful for measuring the voltage standing wave ratio (VSWR) on a coaxial cable transmission line feeding a VHF Marine band antenna by using a 25-Watt fixed-mount VHF Marine Band radiotelephone. The four components were:

• a directional wattmeter;
• two adaptors to allow N-series and UHF-series connectors to mixed in the cabling;
• a short coaxial cable cable to allow the directional wattmeter to be inserted in the transmission line; and,
• a 50-Ohm dummy load to facilitate testing the transmitter power output into a proper load.

In addition, the advice offered very specific suggested products. I thought the selection of these four products was so very well chosen that I could not resist buying them, even though I already owned other instances of all of them. The recommended products were so particularly appropriate for the task of measuring VSWR at 156-MHz using a 25-Watt transmitter that they would be much more easily used than the gear I already had. So I spent about $100 on Amazon to buy them, as an early Christmas present to myself.

The four products had to sit in their boxes until just a few days ago when I got the chance to put them to use as intended: measuring the VSWR on my antenna with my 25-Watt radio. As anticipated, the four product worked beautifully together and made a compact and portable measuring system that could easily be carried to and used aboard boats.

Here are the details of each product and a link to Amazon.com where you can find them, as well as the current price:

DIRECTIONAL WATTMETER
Surecom SW-102 digital wattmeter, $58
125 to 525-MHz, 120-Watts maximum
N-series receptacle connectors
includes two adaptors, see below
https://www.amazon.com/dp/B01D86IKIQ


ADAPTORS
The two necessary UHF receptacle to N-type plug adaptors are included with the wattmeter.

SHORT CABLE
uxcell UHF PL259 to N-type male, 2-feet, $16
50-Ohm RG400 double shielded, silver plated, PTFE jacket
https://www.amazon.com/dp/B07VCYXBPQ?re ... title&th=1

For the cable specifications see http://www.rosnol.com/upload/product/RG-400%EF%BC%8F%EF%BC%B5.pdf


DUMMY LOAD
BECEN 25-Watt BC-TE25-1 500-Ohm Dummy load, $17
N-plug connector, 0 to 3-GHz Max. VSWR 1.2:1
https://www.amazon.com/dp/B08K2ZGYZ3

The original thread in which these devices were proposed was authored by a very knowledgeable guy going by the user-ID s/v Jedi on cruisersforum.com at

https://www.cruisersforum.com/forums/f1 ... ost3951595

I thought this would be a good place to share his recommendation and report on how well they worked together.

[jimh]

In June and July of 2025 I finally got around to being able to use the directional wattmeter, dummy load, adaptors, and cable that I ordered in December 2024. The delay was because all my VHF Marine Band radios and their antennas and the boat were up North.

Before I got the boat out of storage, wanted to measure the VSWR of my shore station antenna. That antenna is a three-element loop-fed Yagi antenna that I bought about ten years ago (or longer) in order to be able to monitor AIS boat traffic in the Detroit River. The antenna was originally mounted on the roof of my house down there, but when we sold that house the antenna came down. Eventually I re-installed it at the cabin (on Lake Michigan shoreline) in the attic. The transmission line from the antenna to the VHF Marine Band radio is 55-feet of Times MIcrowave LMR-240-UF cable. According to the manufacturer, the loss in that length of cable is -2 dB. (See the very useful transmission line loss calculator on their website.)

The first use of the new wattmeter was to measure the power output and VSWR into a 50-Ohm dummy load. The radio under test was a Standard-Horizon GX2150 (my usual boat radio). However, to allow measurement at the 25-Watt power setting, I had to buy a new 120-VAC bench power supply. (See a separate article for details of the particular power supply I chose.)

As expected, into the 50-Ohm dummy load the power output was about 26-Watts, and the VSWR was extremely low, 1.01 or lower. That was expected.

I then connected the Yagi antenna and tested its VSWR. Again, the results were very low, perhaps 1.05 or lower. I was initially somewhat suspicious of the wattmeter, as it seemed to always measure an almost perfect VSWR. I was expecting the antenna to have a somewhat higher VSWR than a perfect or almost perfect 1.0 or 1.01 VSWR.

Of course, the reason the antenna looked so darn good was the loss in the transmission line. While the incident power measured at the transmitter end was 26-Watts, with 2-dB loss in the line, only 16-Watts was reaching the antenna. Supposing, for example, the antenna VSWR was actually 1.2:1, the return loss (amount of reflected power) would be -20.8 dB. With 16-watts at the antenna, the reflected power would then be only 0.13-Watts.

Those 0.13-Watts would then travel back to the wattmeter to be measured, but would again be attenuated by the line loss by -2 dB, so only 0.08-Watts would arrive.

The directional wattmeter would see incident power 26-Watts, reflected power 0.08-Watts, a return loss of -25 dB and then compute the VSWR as 1.12:1.

However, at a power level of 0.080-Watt, we are getting close to the ability of the watt meter to detect such a low power level. The VSWR calculated by the wattmeter is thus 1.01:1, which is an insanely good VSWR and much better than expected. And, again, this is mostly due to the masking effect of the transmission line loss of -2 dB. Also, this situation demonstrates the general problem of measuring any VSWR that is lower than about 1.3:1, as the amount of reflected power become very low. (More on this topic to come in a follow-up post.)

The next chance I had to use the new wattmeter came a few days ago, when I had the boat in the driveway. Testing the boat antenna, a MORAD VHF-156HD with only 15-feet of transmission line, the highest reading encountered across the VHF Marine Band was 1.5:1, a more typical and more expected outcome. At an incident power of 27-Watts, a VSWR of 1.5 is produced if there is 1-Watt of reflected power, and 1-Watt of reflected power would be expected to be well within the detectable range of the inexpensive Surecom directional wattmeter which is being used at a 25-Watt power level. (The Surecom wattmeter is rated for use with a maximum power of 125-Watts.)

[jimh]

Further complicating the measurement of extremely low (thus extremely good) VSWR is there is a limitation imposed by the directional coupler itself. The design of the coupling mechanism in the directional wattmeter must be able to properly differentiate between the forward power waves and the reflected power waves. Simply put, the power sensed as the forward wave should not include any of the reflected wave power, and correspondingly, the power sensed as the reflected wave should not include any forward wave power.

To explain the reasons behind this with some detail, I asked the artificial intelligence agent GROK.COM to explain it. The AI did a very good job. (By the way, it is said that AI will have a big impact on the job market for technical writers, which is how I made a living for several years.) Here is a transcript of that interaction:

Q: What directivity is needed in a directional coupler to be able to measure accurately a VSWR of less than 1.5 to 1?

To accurately measure a Voltage Standing Wave Ratio (VSWR) of less than 1.5:1 using a directional coupler, the directivity of the coupler is critical because it determines the ability to distinguish between the forward and reflected power. A directional coupler with insufficient directivity can introduce errors in the measurement due to unwanted coupling of the reflected signal into the forward port or vice versa.

Key Concepts:

VSWR is related to the reflection coefficient (Γ) by the formula:

(1) VSWR = [ (1 + Γ) / (1 - Γ)]

For a VSWR of 1.5:1:

(2) 1.5 = [ (1 + Γ) / (1 - Γ)]

Solving for |Γ| (the absolute value of Gamma):

(3) |Γ| = (VSWR - 1)/(VSWR + 1) = (1.5 -1)/(1.5 + 1) = 0.5/2.5 = 0.2

The reflection coefficient magnitude is 0.2, or -14 dB since:

(4) 20 LOG10(0.2) ≅ -14 dB

Directivity of a directional coupler is the ratio of the power coupled to the desired port (e.g., forward power) to the power coupled to the undesired port (e.g., reflected power) when measuring in one direction. It is typically expressed in dB:

(5) D = 10 LOG10(Pforward / Pundesired)

High directivity ensures that the reflected power measurement is not contaminated by the forward power.

To measure the reflected power accurately, the directivity must be significantly higher than the return loss (in dB) corresponding to the VSWR. The return loss RL is:

(6) RL = -20 LOG10|Γ| = -20 LOG10(0.2) ≅ 14 dB

Directivity Requirement:

For accurate VSWR measurements, the directivity of the coupler should be at least 10 to 15 dB higher than the return loss to minimize errors due to leakage between ports. For a VSWR of 1.5:1 (return loss ≈ 14 dB):

A directivity of 25–30 dB is typically recommended to ensure accurate measurement of the reflected power without significant interference from the forward power.

This means the coupler should have a directivity of at least 25 dB, but 30 dB or higher is preferred for better accuracy, especially in precision applications.

Practical Considerations:

Measurement Error: If the directivity is too low (e.g., close to the return loss of 14 dB), the coupled reflected power signal will include contributions from the forward power, leading to inaccurate VSWR calculations.

Coupler Selection: A directional coupler with a directivity of 30 dB or more is common in RF test equipment for measuring low VSWR values (e.g., 1.5:1 or better).

Frequency Range: Ensure the coupler’s directivity is specified over the frequency range of interest, as directivity can vary with frequency.

Calibration: Proper calibration of the measurement setup is essential to account for any residual errors, even with high-directivity couplers.

Final Answer:

To accurately measure a VSWR of less than 1.5:1, a directional coupler with a directivity of at least 25–30 dB is required, with 30 dB or higher preferred for better accuracy.

ASIDE
To present the text of the GROK answer, I had to spend about an hour and a half editing the text to permit the proper display of the math symbols in the formulas in the normal format I use on the website and then proofreading it. So it was not just "a cut-and-past." GROK did an excellent job of answering the question. The intelligence that AI is showing in technical writing is beginning to be very good, and probably significantly better than what is normally found on web discussion forums.

[jimh]

To solve for Γ (Gamma) in equation VSWR = (1+ Γ)/(1 - Γ) the method may not be immediately apparent to all readers. The process is shown below:

Start with the given equation

VSWR = (1+ Γ)/(1 - Γ)

1. Multiply both sides by (1 - Γ) to eliminate the denominator:

VSWR(1 - Γ) = (1+ Γ)

2. Distribute VSWR on the right side:

VSWR - (VSWR・Γ) = 1 + Γ

3. Move all terms involving Γ to one side and constants (1's) to the other

VSWR -1 = Γ + (Γ・VSWR)

4. Factor out Γ on right side

VSWR - 1 = Γ(1 + VSWR)

5. Divide both sides by (1 + VSWR) to get answer (assuming Γ ≠ -1):

Γ = (VSWR - 1) / (1 + VSWR)

This solution is only valid if VSWR ≠ -1, which it never could be. Also, the initial equation had denominator (1 - Γ), where Γ≠ 1, which is not allowed.

[jimh]

A further problem in measuring VSWR at the transmitter is the masking of the actual antenna VSWR due to line loss.

For example, if a directional wattmeter at the transmitter output measures

Powerforward = 100
Powerreflected= 6.25

the corresponding VSWR is 1.67:1

If the transmission line has a loss of -3dB, what has occured is that of the 100-Watts at the transmitter, only half or 50-Watts reaches the antenna. The amount of reflected power at the antenna must also travel through the transmission line and have lost half its power, so the true reflected power was 12.5-Watts. The antenna VSWR was then

Powerforward = 50
Powerreflected= 12.5

and the actual antenna VSWR is 3:1

A faster method to determine the actual antenna VSWR is to begin noting that the apparent Return Loss (RL) includes twice the line loss of -3 dB loss or an additional -6 dB.

Using the same values as in the earlier example and computing the RL at the transmitter

Powerforward = 100
Powerreflected= 6.25

The return loss seen at the transmitter is -12.0 dB.

Subtracting the transmission line loss of -6 dB from the Return Loss of -12 dB gives the actual RL = -6 dB. The VSWR for a RL of -6 dB is 3:1, the actual VSWR at the antenna.

An even more illustrative example is a case where the measured VSWR at the transmitter is 3:1 or a RL = -6 dB. If the line loss were -3 dB, then reducing the RL by -6 dB gives the true RL of 0 dB at the antenna indicating all power was reflected ( a VSWR of infinity). This result would be typical if there were a short circuit or an open circuit at the far end of the transmission line.

[jimh]

Recently I used the three items I mention above to measured the VSWR of an antenna installation which has been in place for about 20-years. The directional coupler indicated at the transmitter the VSWR was 4.2:1.

A VSWR of 4.2 indicates a RETURN LOSS of -4.22 dB.

In this installation the transmission line length was approximately 20-feet of what appeared to be RG-8X, which could have (in best case) a line loss of -3.8 dB per 100-feet. For 20 feet the loss would be (20 /100) * -3.8dB = -0.76-dB. The cable is quite old. and it has been continuously exposed to weather for 20 years and has been painted-over perhaps more than once, so let's call that -1 dB loss.

Assuming a line loss of -2 dB for a round trip, then the real return loss at the antenna would have been more like -4.22 -(-2) = 2.22 dB, indicating a VSWR at the antenna of 7.9:1. (Here we subtract the line loss from the return loss to remove the masking effect of the line loss making the VSWR at the transmitter look better than it actually is at the antenna.)

An antenna VSWR of almost 8:1 indicates there is a significant problem in the antenna system.