Meeting 60M FCC requirements
The FCC apparently requires...
"Transmissions shall not exceed an effective radiated power (ERP) of 50 W PEP. For the purpose of computing ERP, the transmitter PEP will be multiplied with the antenna gain relative to a dipole or the equivalent calculation in decibels. A half-wave dipole antenna will be presumed to have a gain of 0 dBd. Licensees using other antennas must maintain in their station records either manufacturer data on the antenna gain or calculations of the antenna gain.
If I am using a homebrew resonant loop for 60M, what "calculations" should I have in my shack?
You should either measure or model it relative to a half-wave dipole in its position. (Be sure the main lobes of each are oriented in the same directions. ) Enter the measurements taken, or the calculations or software used and the results or output, in your station log.
Originally Posted by K3UK
I am very bad at trying to use modeling software for home brewed antennae , tried several times and just do not "get" it. The software I have does have some standard antennas already in there. Would that suffice if there was a standard loop, or would the FCC expect exact modeling of my "odd" loop that zig-zags in elevation (varying from 10 feet to 40 feet) and is "mostly" rectangle ?
Originally Posted by KA5S
My half-baked guess is that their standard loop is probably optimal and your loop likely will not overperform it.
What does the software say about a standard loop?
I would suggest that would qualify as a good faith effort anyway.
Right. A good faith effort is enough. If you assume you have 2.1 dB gain over a dipole (a circular full wave loop) you will not exceed 50W ERP if you turn down the RF to 30 watts at the antenna terminals. You can calculate coax loss if necessary to see how much that takes at the transmitter. Enter the published loop gain* and calculated RF loss in the feedline those numbers in the log. If you do model your rectangle you may get less gain and could then run more power.
*Lots of material on the Web, as the full wave loop has been discussed for a long time.
The 60 meter power limit spec is difficult to figure out. Their method of figuring ERP is not the same as is normally done for antennas. It' a very confusing spec and technically inaccurate. You have to read between the lines to figure out the intent.
They say a half wave dipole is assumed to have a gain of 0 dBd. That means a half wave dipole has the same gain as a half wave dipole. DUH. We normally use dBd when comparing an antenna to a dipole when both antennas are located at the same height. That is not their intent. For their statement to make any sense at all, you have to assume they mean that the reference for 0 dBd is free space. Assuming anything else would allow the reference number to be anything between about 0 dBi and about 6.9 dBi, and even a lot greater range than that if you consider uneven terrain. That's not much of a reference and would be impossible to accurately calculate.
So to figure the power limit on your antenna you first have to calculate its free space gain, compare that to 0 dBd (really 2.15 dBi) then apply the appropriate power correction factors. The gain of a 60 meter square loop is 3.15 dBi if fed in the center of one side. It is 3.07 if fed at a corner. That assumes some copper loss (#12 wire). So you have to reduce power 1 dB below 50 watts or to 39.7 watts.
Notice that these specs don't include ground gain for an antenna since they are referenced to free space. So whatever gain you get from a change in height (or vertical/horizontal antenna orientation) is free. You can put your dipole almost on the ground and have a gain of -2 dBi or put it at 100 ft and have a gain of 6.8 dBi, and the power limit for both is 50 watts. You could bring your dipole to my house and put it at 100 ft and get 8 dBi gain to Europe and still have a 50 watt limit.
From the previous example you can see that you are almost always better off using a horizontal antenna so that you can take advantage of the free ground gain. Most verticals will have a gain of about 1.4 dBi in free space. So you could run about 59.4 watts to most verticals. The gain of the vertical over real ground might be -6 to +1 dBi depending on your radial system. Compare that to a dipole that could have a gain of 6 dBi or more.