Re: [Elecraft] KXB3080 - Same output power on 40m as 20m - How to fix?
Hi, All. Thanks to all who responded about the power output being the same between 20m and 40m. I was able to run some tests with a recently calibrated HP8591A spectrum analyzer, and I thought I'd share my results. (I also had a chance to calibrate the KXAT1 it was reading about 0.25W high). My battery voltage was: 9.1V key-up and 8.2V key down. I was hooked up to the analyzer through a recently certified precision 10dB attenuator and a recently certified low loss coaxial feedline. (I've adjusted the fundamental power below by 10dB for the pad). Fundamental 2nd 3rd 4th 5th BandPowerHarmonic Harmonic HarmonicHarmonic (Adjusted Pad)Delta Delta DeltaDelta 80m 30.6dBm (1.15W) -45dB -66dB-76dB -76dB 40m 29.4dBm (0.87W) -39dB -52dB -59dB-64dB 30m 30.4dBm (1.09W) -40dB -70dB -72dB-71dB 20m 30.1dBm (1.02W)-60dB -66dB -70dB-69dB Observations: The harmonic rejection is within specification, at least at 1W out. I assume that the level of harmonic suppression (delta) will remain the same regardless of the power out, I'd be interested in comments on that assumption as I did not have the right connector to run the radio on 12V to hand when I made the measurement. I do NOT have the 20m power being 0.3-0.6W lower than 40m (any more - it was before the KXB3080 mod), in fact I have almost the opposite (0.15W more power on 20m than 40m), and I assume that difference would be more pronounced at higher output powers. The manual states that my harmonic rejection may therefore be sub-optimal: If both bands have adequate output power but 20 meters is higher, you should adjust the windings of toroidal inductor L2. To do this, squeeze the turns of L2 together slightly so that they occupy only about 70% of the core's circumference. With L2 at its optimal turns spacing (for best harmonic rejection), 20 meter output will be about 0.3 to 0.6 watts lower than 40 meters. The worst case harmonic rejection seems to be 40m with the 1st harmonic being -38.7dB down from the fundamental, but that's close enough to the specification (-40dB) I think. Should I be trying to squeeze my turns to improve it? I'm not sure. I did compare my results to John AE5X's measurements for his KXB30 KX1: KX12nd 3rd4th 5th OwnerBandHarmonic Harmonic HarmonicHarmonic Delta Delta DeltaDelta AE5X40m -40dB -53dB -60dB-66dB VA3SIE 40m -39dB -52dB -59dB-64dB AE5X 30m -35dB -63dB -71dB-68dB VA3SIE 30m -40dB -70dB -72dB-71dB AE5X 20m -58dB -60dB -73dB-70dB VA3SIE 20m -60dB -66dB -70dB-69dB So the KXB3080 low pass filters apparently offer superior harmonic rejection compared to the KXB30 except on 40m where it's slightly worse, although in practice the 1dB difference is probably close enough. Perhaps I could improve the 40m harmonic rejection by squeezing turns, but it probably isn't worth it for 1dB. I hope others can benefit from these measurements. 73 Martin. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] KXB3080 - Same output power on 40m as 20m - How to fix?
Martin VA3SIE wrote: I placed a scanner across the room (with a 2m rubber duck antenna) from the KX1 driving a magnetic loop antenna tuned to 7040kHz and transmitted on 7040kHz. The scanner showed full scale deflection. I then tuned the scanner to the 1st harmonic 14080kHz and the scanner 'S' meter was maybe 4/5ths deflected. I know that isn't very scientific, it may be telling me the harmonic suppression isn't good or maybe it's not. I don't have anything sophisticated like a spectrum analyzer to do a proper analysis of the relative strength of the 1st harmonic versus the fundamental. - Martin, Your magnetic loop antenna could be confusing your measurement using the scanner because a magnetic loop exhibits narrow bandwidth, and in normal use would act as a filter reducing the radiated level of the 20m harmonic. Before I am flamed on this I know that there are many 'buts' and 'ifs' :-). When using your scanner it would be better to drive the Tx into a dummy load, and position the scanner plus rubber duck so that you get * no more* than half scale deflection of its 'S' meter on 7040 kHz.to avoid any possibility of overloading the scanner, then compare that reading with the scanner's 'S' meter reading on 14080 kHz. It would be better to have the Tx in its case when doing this to avoid any direct radiation from the PA etc being picked up by the scanner. This measurement will give you a useful indication but not a precise figure. Sorry but I am unable to comment on the KXB3080 since I do not have one. 73, Geoff GM4ESD ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] KXB3080 - Same output power on 40m as 20m - How to fix?
Martin, You may be reading too much into the instructions. It is not required that the power on 20 meters be lower than on 40, it just usually happens that way. The fact that you have the same power output on 20 as you have on 40 is not necessarily an indication of a lack of 2nd harmonic suppression. Look at the schematic and you will see that there are two 1000 pF capacitors added to the LPF on 40 meters, and the cutoff frequency when tuned to 40 will be lower than when tuned to 20. The green winding is used on 40, 30 and 20 while both the red and green windings are used on 80. Normally the red winding should be squeezed a bit and the green winding spread out somewhat. Count the number of turns to be certain L1 and L2 are wound correctly. Count the number of times the wire goes through the center of the core rather than counting around the outside. I would not trust the front end of a scanner to determine the relative strength of a 2nd harmonic. If you wish to do that, put a tuned circuit between the antenna and the input to the scanner to give the front end of the scanner some selectivity - it likely is wide open for broadband coverage. The KX1 power display has a lot of granularity, but you can obtain a smooth relative reading from the KXAT1 by reading the voltage from the forward side of the wattmeter with a DMM. One easy to reach place for this voltage is pin 5 of P3. 73, Don W3FPR -Original Message- I completed my KXB3080 modification tonight but during testing I discovered that I am getting pretty much the same transmit power out (1.8W with 10V) on 40m as 20m (according to the KXAT1 this is the only watt-meter I have) into a 50 ohm load. I took the board out and squeezed the green turns on L2 of the LPF1 board together as tightly as I could and replaced the board back into the case, but it didn't make any difference I still get the same power out on 40m as 20m. I understand that this may be an indication that the harmonic suppression is not working correctly as I should be getting about .6W lower power on 20m (I certainly was before the mod!) ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
