---------- Forwarded message ---------- From: "Dennis" <[email protected]> Date: Nov 3, 2015 07:24 Subject: BCB Interference To: <[email protected]> Cc:
I have a difficult BCB interference that I'm trying to resolve for a fellow ham. It is present on both a K3 and a 7800 at the station. When switching to 160M, there is a constant noise level of about S9. When using a narrow CW filter, it sounds like local noise, but if you use an AM filter you can hear different BCB radio stations mixed together. Any station below S9 can't be heard, and stations above S9 are difficult to copy as they seem to be "noise modulated". When using the K3 with neither the preamp nor the attenuator on, and switching from any band to 160M, the attenuator turns itself on; this indicates a signal at the input of +13 to +20dBm. The antenna system is a 160m inverted V through a 403A High Power LPF for 160M. When switching to 80M, there is a constant noise level of about S5. Similar to the 160M noise, when using an AM filter you can hear different BCB radio stations mixed together creating the noise. On 80M, the noise threshold is S5, and stations S5-S7 are difficult to copy due to the "noise modulation". When using the K3 on 80M, if you turn the preamp "on" it will quickly turn itself off; this indicates a signal at the input of +3 to +10 dBm. The antenna system is an 80M 3el beam through a 4O3A High Power BPF for 80M. On 40M, the noise can be heard, but at much less amplitude. It only makes the weakest signals difficult to copy. On 20/15/10m, you can also hear the noise, but it is just an annoyance. The station owner purchased an ICE 402x BCB filter several years ago, but reported that it made no difference in being able to hear. A 4-square listening array was also tried, but the interference was there also. I plan to attack this on 2 levels. I made 2 Rx-only BCB filters at home; each filter gives about 55dB attenuation on the BCB. I have scoped them out on a VNA, and also put them on another station and can verify that they provide about 55 dB attenuation on an AM signal at 1420KHz. I also have a step attenuator that can provide another 60dB of attenuation, so between the 3 devices I have 170dB of available attenuation. If the culprit is simple front end overload, I suspect this should be enough attenuation to show it. After reading the article by N0AX in QST early this year, there may be rectification going on in one or more pieces of equipment. Since switching diodes in different gear could cause a mixing problem, I'm also planning to remove the feedlines from the back of all amplifiers and looking for the problem to go away. I have a handful of disc caps to bypass rotor, stackmatch, and other control cables. I'm also bringing an RF probe, so I can listen for IMD products at various pieces of equipment. Am I overlooking anything? Are there other sources that could be feeding into the problem? Suggestions appreciated. Dennis W1UE _________________ Topband Reflector Archives - http://www.contesting.com/_topband
