There have been a lot of comments on the reflector about the QST review of the FTDX5000. However, no one has noted a significant design flaw in the transmitter that shows up in the review. Refer to Figure 3 that shows the composite transmitter noise. It shows a noise level of -130 dBc/Hz from 10 kHz to 1 MHz (and probably beyond).
This transmitter when combined with a full power amp could wipe weak signals in an entire amateur band for anyone living within a few miles of the transmitter (see the math calculation below). While a number of other transceivers have this problem, I wouldn't expect a top of the line $6K radio to have such lousy composite transmit noise. This problem is created either in the radio's synthesizer or its transmit amplification chain. The K3 was specifically designed to minimize composite transmit noise. The K3 QST review showed a transmit noise level of -155 dBc/Hz at a 100 kHz offset. This is 4 S-units less noise than the FTDX5000 at the same offset. This is not a theoretical calculation. I know of one case of composite transmit noise where an amateur transmitter wiped out weak signal reception across an entire ham band in a receiver located several miles away. -John KI6WX CALCULATIONS Assume that we have a FTDX5000 transmitting CW on 20 meters followed by a 1.5 kW amp. The transmit power is +62 dBm. At a 100 kHz offset, the transmit noise is -68 dBm/Hz. Assume that the FTDX5000 transmit output is fed to an isotropic radiator (0 dB gain) on top of a hill and we have a receiver also with an isotropic antenna in a valley with line of sight to the hill. For directional antennas, the sum of the antenna gains depends on where they are aimed and could be greater or less than the 0 dB in this example. For the moment, we'll place the receiver 1 mile from the transmit antenna. The path loss between the transmit and receive antennas is 60 dB, which implies the receive power of the transmit noise will be -128 dBm/Hz. The normal atmospheric noise on 20 meters is about -144 dBm/Hz, which means that the transmit noise will be 16 dB greater than the normal background noise. This noise will be spread across the entire band whenever the FTDX5000 is transmitting. If it is transmitting CW, the receiver will hear noise modulated in Morse code. If it is transmitting SSB, the noise will vary with the voice modulation peaks. The receiver would have to be more than 6 miles away for the noise to drop to background levels. Another way to look at this problem is how many S-units would the show up in a 500 Hz receive bandwidth. The total power in the noise is -101 dBm in the 500 Hz bandwidth. S4 is -103 dBm, so the noise would be about a S4 signal level. Each time you halve the distance to the transmitter, the noise will increase by 1 S-unit. If you live 1000' from a FTDX5000, you could see a noise level of S7. You can reduce the noise by using a narrower filter, but you would have to drop down to a 100 Hz filter to reduce it by 1 S-unit. This calculation was done with the transmit antenna on top of a hill so we could use free space radiation to calculate the path loss. If both antennas are on a flat surface of earth, the path loss will be somewhat greater, but the exact magnitude requires using antenna radiation software such as NEC-4. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
