| Most important is for you to establish a single point ground for the feeds for both antennas, and for your station AC power, phone lines, wired Ethernet, etc. Installing current baluns (ferrite beads) on the coax for each antenna will also help minimize currents induced from one antenna onto the feed coax of the other. If the U/VHF antenna has a DC ground design, this will essentially short out any pickup from the HF antenna. In my experience with a similar arrangement, this was not a problem. You would need an RF voltmeter to measure how much HF energy is getting into the U/VHF antenna feed to see if there is a real problem. One way to fake an RF voltmeter is to use an RF power meter and a dummy load. Put a HF wattmeter on your U/VHF coax and on a dummy load, then you can measure the power picked up by the U/VHF antenna when you transmit on HF. If it is less than 1 Watt, it should not be a problem. Hope this helps! 73, Avery WB4RTP |
The May/June 2021 issue of QEX, in the article on page 3, "QO-100: Working the First Ham Satellite in Geostationary Orbit", included the following editorial comment:
"Words of warning: do not stand in front of the dish while transmitting. Even with 5W or less from the upconverter/amplifier, the dish with 20 or 40 dBi of gain will result in an EIRP of hundreds or even thousands of watts of microwave energy. This can cause blindness and other serious harm."
In my opinion, this statement is wrong on several details. First, antenna gain does not create energy. 5W in to an antenna is still 5W radiated. Effective Isotropic Radiated Power is the effect of the power density concentration relative to an isotropic radiator and only applies in the far field. The actual power in the beamspot is still the same or less than the actual power into the antenna (the difference is antenna losses, atmospheric absorption, sidelobes, etc.). Standing right in front of the dish, the power density is the feed power dispersed over the area of the dish. The bigger the dish, the lower the power density. For example, a 25 cm dish has an area of about 2000cm2, so 5W will result in power density right in front of the dish of 2.5mW per cm2, which is below the microwave long term exposure limit of 10mW per cm2. This is how microwave near field exposure should be calculated. I like to keep in mind that 10mW per cm2 is the same as 100W per m2, and this is 1/10th sunlight. Also, eye damage usually not a problem below 100mW per cm2, which is 1000W per m2, the same as sunlight. I would like to see a correction included in a future issue of QEX. |
