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Author Topic: Hum on Receive after Transmitting  (Read 778 times)

Posts: 2

« on: April 16, 2009, 10:15:32 PM »

Hello all. I have been licensed for about a year, and just recently upgraded to General and have started to explore HF. I recently acquired an FT-897D with the attached FC-30 antenna tuner. I've made a few contacts without any trouble but I've noticed some odd behavior.

Sometimes (seemingly at random, though it appears to be becoming more frequent) following a transmission (any transmission, including an antenna tunning) what sounds like a 60 cycle hum appears on the receive side, with about an S7 signal strength. I've mainly been working 40 meters, recently using a W3FF Buddistick, before I was using simple wire dipole, with the same problem. The noise will also be present on the bands surrounding the one I initially transmitted on. The noise is usually (though not always) eliminated, by the radio's build in Noise Blanker feature. So for example: If I were tune the antenna on 40 meters, this hum would appear, I could switch down to 80 meters, the hum would still be present, and activating the Noise Blanker feature would bring the S7 hum down to S0 background noise. The noise will sometimes disappear, just as it began, following another transmission.

The hum will disappear if the antenna is disconnected, though if any load at all is applied to the antenna port (my finger for example) it will reappear. I'm primarily using a Radio Shack 15 Amp (I've only been transmitting at 50 watts) switching power supply, this was my first thought as to the origin of the problem. However, I can turn off the radio (after the hum has started), disconnect the radio's power, connect it to another power supply, and the hum will still be present. If I leave the radio off and alone for a few hours, and come back the noise will have disappeared and wont reappear until I transmit again. The station is well grounded.

I am at a total loss for what could be causing this, if anyone has any input at all it would be greatly appreciated.


Posts: 16

« Reply #1 on: April 17, 2009, 03:43:40 AM »

I  would see what happened if it was connected to a pure dc source such as a car battery. If the problem went away, you would know where to look next.

73,   Bill

Posts: 9749


« Reply #2 on: April 17, 2009, 03:52:44 AM »

So if you never transmit, and just leave the receiver sit there for hours listening, the hum does not come and go??

When you tune the receiver around, is the hum about the same strength over smaller frequency changes and gradually rolling off as you change frequency away from the antenna's resonant frequency?

Is it a low pitched hum, or is it a sharp buzz? Noise blankers generally do not work well with low pitched hum like pure 60 Hz, they only work with a sharp or raspy buzz with high frequency content like a spark or arc. The sound description is very important.

73 Tom

I would not waste time, at the moment, looking at power supplies. I would try to define the noise better to prevent a wild goose chase.

Posts: 6646

« Reply #3 on: April 17, 2009, 03:57:38 AM »

What happens on a known good dummy load?


Posts: 14499

« Reply #4 on: April 17, 2009, 10:45:55 AM »

I'd try it on a dummy load or resonant antenna without the tuner. I'm thinking it may be the processor in the tuner waking up during transmit and generating the noise. After a while in receive it goes back to sleep mode and the noise stops. I don't know about the FC30, but the processor in many automtic tuners work in that manner.

Bob  AA4PB
Garrisonville, VA

Posts: 3289

« Reply #5 on: April 17, 2009, 08:10:28 PM »

I'm wondering if the Tx/Rx relay is dirty or sticking.


Posts: 2

« Reply #6 on: April 19, 2009, 04:08:33 PM »

Thank you all for your suggestions and input. I've created a webpage (forgive the cheesy googlepages formatting) with some data, i.e.,  audio files and spectrograms of the noise, the address is at the bottom of the post. After looking at the spectrogram, I am fairly confident this a 60 cycle impulse noise of some sort.

I did take the rig out to my car, drive out to the middle of nowhere and do some testing (unfortunately no laptop along for empirical analysis). When transmitting, I did notice a similar type of strange signal appear, however it showed up in the way of an increase on the S Meter, not an audible noise. That is, the S meter was showing 4 or 5 background noise, but increased to 7 (this time with no really noticeable audio change) following transmission. Application of the NB brought the S meter back down to 4 or 5, but again no readily noticeable change in audio.

I have repeated these experiments with and without the antenna tuner, with the same results, so I do not think that is the cause.

In terms of the exactly location of the noise on the band. It appears to focus on the frequency on which I transmit, initially covering almost the entire band. Overtime though (maybe 20 minutes), the center of the signal appears to move up in frequency until it finally disappears.

So, any further input would be greatly appreciated, and thank you all for getting back to me. Sorry about the delay in my follow up post.

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