Keeping an NVIS Antenna Legal on 60 Meters
This article is the seventh in a series on Portable NVIS Communication. This series focuses on short-haul communication during a disaster.
What Hath God Wrought?
Earlier in this series, I explained why I included wire elements for the 60 Meter Band in my NVIS antenna.
That decision opened a set of problems.
First, no amateur station can transmit with effective radiate power (ERP) greater than 100 watts on the 60 Meter Band.
Fine, except to compute ERP, antenna gain must be relative to a half-wave dipole antenna. An NVIS antenna has a lot more gain than a dipole.
Next, my antenna is not commercially made. The Commissions Rules at 97.313(i) require me to calculate the gain of my homebrew antenna.
Finally, once I know the antenna gain, I have to know how much (or little) transmitter power I can drive into my antenna to keep it below 100 watts. To determine that number, I need to know how much loss is in my antenna system.
Let’s get started.
Modeling an NVIS Antenna
The EZNEC model of my NVIS antenna at 5405 kHz suggests a 4.97 dBi gain at 90.0-degree angle elevation.
Calculating Antenna Gain as dBd
The FCC wants amateurs to measure power as it relates to a dipole antenna, or dBd. This requirement forces us to convert our EZNEC results from the isotropic measurement.
The formula to convert antenna gain from dBi to dBd is
dBd = ( dBi – 2.15 )
Therefore,
dBd = 4.97 – 2.15
dBd = 2.82
As a result of this calculation, actual power at the antenna feed point must be less than 52.235 watts in order to keep ERP below 100 watts.
Calculating Total Antenna System Loss
My antenna system consists of 75-feet of RG-213 between the transceiver and the antenna matching network. There is an additional 25-feet of RG-213 between the antenna match and the feed point. Here are my calculations for system loss.
Frequency |
Coax Loss |
1.2:1 SWR Loss |
Total Line Loss |
5332 kHz |
0.450 dB |
0.007 dB |
0.457 dB |
5348 kHz |
0.451 dB |
0.007 dB |
0.458 dB |
5358.5 kHz |
0.451 dB |
0.007 dB |
0.458 dB |
5373 kHz |
0.452 dB |
0.007 dB |
0.459 dB |
5405 kHz |
0.453 dB |
0.007 dB |
0.459 dB |
Calculating Maximum Transmitter Power & ERP
Doing the math, we find that Transmitter Power Output (TPO) of 58 watts will keep us below both of the feed point (52.235 watts) and ERP (100 watts) limits.
Frequency |
TPO (Watts) |
Total Line Loss (dB) |
Feed Point (Watts) |
ERP (Watts) |
ERP (dbW) |
5332 kHz |
58 |
0.457 |
52.21 |
99.943 |
19.998 |
5348 kHz |
58 |
0.458 |
52.20 |
99.924 |
19.997 |
5358.5 kHz |
58 |
0.458 |
52.195 |
99.915 |
19.996 |
5373 kHz |
58 |
0.459 |
52.187 |
99.899 |
19.996 |
5405 kHz |
58 |
0.459 |
52.170 |
99.867 |
19.994 |
Street Legal, Baby!
It appears that my NVIS antenna will be in compliance with the Commission’s Rules when TPO does not exceed 58 watts.
Since amateur transmitters cannot control transmitter power with such accuracy, I think it will be a good practice to set TPO a few watts lower just to be safe.
Have you compared your NVIS antenna to an 87’3″ dipole antenna, modeled using the same software both times? This would be more of a fair comparison, and I think you will be surprised at how overly-complicated you have made this. And it would be more compliant with the regulation that you are trying to satisfy, that is, “antenna gain relative to a dipole”. It doesn’t say anything about an isotropic radiator in 97.313(i). Ground loss, coax cable loss, connector loss, dipole antennas have these things, so when you start with an isotropic antenna, you are changing the equation and thereby not doing the thing that they are describing in these rules. What I am saying is that if you model a (very standard) dipole antenna using your configuration, you will find that it has gain over a dipole. Since that makes no sense on its face, you might want to re-think your basic premise.
I’m glad that you actually deployed the antenna instead of sitting around looking at a model on-screen, but it would have been awesome if you actually took some field strength measurements and compared that to a dipole, or really any kind of qualitative measurement instead of saying you were “59” somewhere.
Awesome. Can you describe your antenna? I’m curious about the assertion that NVIS antennas have gain over a dipole — mine certainly doesn’t!