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Author Topic: HW8 receiving broadcast interference.  (Read 946 times)
W4NB
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Posts: 3




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« on: January 26, 2013, 01:52:46 PM »

Recently acquired HW8 sounded great for a few days. Today, Saturday, the receiver was overloaded with a loud spanish speaking broadcast signal across the entire 20M band. Wonder if anyone can help with this? Thanks. Vic W4NB
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KQ6Q
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Posts: 991




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« Reply #1 on: January 26, 2013, 02:43:50 PM »

cycle your pushbutton  band select switches - you may have dirty/intermittent contacts on the 20 meter stack. also check the coax to your antenna - the ground side might be open.
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KZ1X
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Posts: 3229




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« Reply #2 on: January 26, 2013, 05:47:39 PM »

it wasn't WYFR by any chance?  In Maitland, they're still a good bit away from you to create such an overload condition.

Would be helpful to know what you were hearing.  For example, a malfunctioning local AM station could cause this problem.
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WB6BYU
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Posts: 13461




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« Reply #3 on: January 26, 2013, 06:34:14 PM »

What type of antenna are you using?

The LM1496 product detector in the HW-8 is prone to AM pickup, in spite of the
fact that is is balanced.  I found my radio totally useless within 100' of a practice
ELT beacon putting out 50mW on 121.6 MHz.

The first step is to determine the frequency of the interfering signal:  if it is a SW
station near 20m then it will be much more difficult to filter it than if it is an AM BC
station.  In the latter case a high pass filter with a cutoff between 2 and 5 MHz
should help a lot.

If it really is a SW station, then you may need a series-tuned trap in the antenna
lead, but start by making sure all of your antenna connections are in good shape.

As a last resort we might need to increase the standing current in the LM1496
to make it less prone to overload.  But I'd explore the filtering options first.
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W4NB
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Posts: 3




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« Reply #4 on: January 27, 2013, 08:05:58 AM »

The coax cable is fine. I cleaned the band switch contacts. Possibly that helped, because so far this morning I don't hear the interference I had yesterday.
I used a G5RV, a 20M dipole, and a 2 el wire yagi fixed position. Heard it on all antennas.
Don't know the source of the signal. I heard no station ID. Talking in Spanish, then music, then mostly talking. It sounded like some sort of entertainment program rather than ham radio.
If it returns, I will give more information, but for now, it is gone. Thanks for the advice on filters, too.
« Last Edit: January 27, 2013, 08:09:26 AM by W4NB » Logged
W5GNB
Member

Posts: 419




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« Reply #5 on: January 27, 2013, 11:01:06 AM »

The receiver in the HW-8 is a Direct Conversion design, it is very open to out of band interference especially from a STRONG signal like a SW Broadcast station.
About the only thing you can do is perhaps run an antenna tuner between the rig and the antenna to give you a little better rejection of out of band signals.  Sounds like front end overload, a common problem on direct conversion receivers.

73's
Gary - W5GNB
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W4NB
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Posts: 3




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« Reply #6 on: January 27, 2013, 11:08:30 AM »

The interferring station is broadcasting at 11.933 mhz.
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WB6BYU
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Posts: 13461




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« Reply #7 on: January 27, 2013, 02:53:33 PM »

The first question is where the signal is getting detected to audio.  If you short
various points of the RF circuit to ground with a .001uf capacitor, do you still
hear the station?  Try the input and output of the RF amplifier stage and the
input and output of the product detector.  If shorting the RF at that point
does NOT remove the signal, it is getting detected earlier in the circuit.  In
this case possibly in the diode bandswitching or driving one if the RF stages
into a non-linear condition.

Does reducing the RF gain affect the interfering signal?

If shorting the input to the RF amp makes the signal disappear, then you can
try adding some filtering on the 20m side of the input bandswitch, or as part
of the tuned circuit.  It might be that adding a capacitor across the input
winding of L3 to tune it might work.

The long term solution is to find the non-linear element and adjusting the biasing
to prevent it from detecting signals.  A short term fix might be to add a tuned
circuit of some sort to reject this specific frequency.  (A coax stub across the
antenna connector might do the job.)
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