http://groups.yahoo.com/group/UV-3R/message/292 Date: Sun, 05 Jun 2011 04:18:58 -0000 Subject: Re: Serious harmonics problem on VHF! Re: Serious harmonics problem on VHF! I checked my UV-3R on my trusty service monitor (Schlumberger 4031) that has a feature specifically for measuring harmonics - and has been verified correct with other gear (e.g. HP analyzers, etc.) and here is what I found: On 70cm, the 2nd harmonics were at least -60dBc at 420 MHz and better at 450 MHz. On VHF, however, the story was different. At 162 MHz, the 2nd harmonic was -65dBc, but just -32dBc at 144 MHz: Higher-order harmonics were adequately attenuated (better than 65 dB down from the carrier) at both frequencies. Clearly, the folks that built the radio were serious when they specified the 136-174 MHz frequency range, but weren't quite serious enough to add an extra couple of poles to the VHF lowpass filter to to provide an adequate cutoff: In other words, the LPF had to be designed to avoid knocking the carrier down at 174 MHz with its 2nd harmonic at 348 MHz, but it has not quite enough poles to do the job at the 2nd harmonic of 144 MHz (e.g. 288 MHz) *and* still pass 174 MHz. Careful inspection will reveal that the RF PA stage of the UV-3R is pretty simple: A couple of chokes/caps to form a broadband impedance transformation to 50 ohms, perhaps 3 caps and 2 chokes for an LPF, and then another choke/cap for the VHF/UHF diplexer. Inspection also revealed a place for an uninstalled surface-mount chip cap, so this got me to experimenting. Noting that the "missing" cap went from between the RF output and ground on the output of one of the coils (the other cap appearing to be in series - perhaps a blocking cap?) Going by my gut feeling (and also checking my 50 ohm filter tables) I decided that something in the 33-47 range would be OK. Not having any 0400-series chip caps handy, I just used some small disk-ceramics and wedged it in. A 33pF improved the 144 MHz 2nd harmonic rejection (e.g. 288 MHz) from -32 to around -42dBc and actually bumped the power up from 1.65 watts to 1.95 watts. Swapping it out to a 47pF unit resulted in the original 1.65 watts, but the 2nd harmonic (288 MHz) was now at -48dBc. At 162 MHz, the output power was down to about 0.6 watts, but I wasn't planning to transmit there, anyway! Not seeing any other obvious places into which I could cram another disk ceramic, I stopped there, but if I were to be fanatic about it, I might have added some inductance to some of the coils. While not quite the "better than -60dBc" that my other HT's do, the fact that a typical antenna would be nonresonant at the 2nd harmonic, I feel reasonably confident that the radiated harmonics would be quite a ways down - more along the lines of a 70 watt mobile rig with a -60dBc 2nd harmonic... I've posted a picture in the "Photos" section of the modification labeled "UV-3R_LPF_MOD". 73, Clint KA7OEI