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Author Topic: Receiver problems  (Read 7449 times)
AK7V
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Posts: 267




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« on: September 30, 2013, 04:52:48 PM »

I have a Yaesu FTM-10R dual band rig that I recently connected to a Comet GP-6 base station antenna on the roof of my house.  I had this radio in my car for a while and it worked well.  I don't have much experience with VHF/UHF, though.

Hooked up to the base antenna, it works great except that on many frequencies, I can hear low-audio transmissions that aren't occurring on the tuned frequency.  For example, while tuning the VFO through one part of the 2m band, I hear packet and what I think is a non-ham voice transmission.  On another part of 2m, I hear a repeater output.  I can hear this audio, unchanged, over a wide swath of frequencies as I turn the VFO.  These mysterious signals are strong enough to break the squelch and the audio level is very low.  When a "real" transmission occurs on the tuned frequency, the "ghost" transmission is covered up/goes away.

My guess is that this is intermod and/or overloading of the RX front end, due to using a gain antenna mounted up high.  If that's the case, can I get rid of it?  How?  Attenuation?
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WB2WIK
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Posts: 21754




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« Reply #1 on: October 02, 2013, 10:03:30 AM »

Mobile rigs like this that cover such a wide range of RX frequencies (the 10-R covers AM broadcast, then 76 MHz through 999 MHz continuously) are intended to be used with small mobile antennas where very strong signals are less likely to occur.

Connecting to a larger "home station" antenna can really overload them pretty badly, and it's not just this rig -- it's almost anything with such wide frequency coverage that doesn't use selectable RF filters.  An example of a wide-range receiver that doesn't suffer this problem is this one: http://www.hamradio.com/detail.cfm?pid=H0-006324

There are many others as well, but note that such a receiver costs thousands of dollars, and doesn't transmit. Wink

If you can find the major offenders (stations so strong they're creating the interference problem) and identify their actual frequencies, you might find a simple notch filter in line with the rig-antenna will solve this problem completely.  But if there are "several" sources on widely different frequencies, that may not be possible.

If you want to use the rig only on 2m and 70cm and never use it to "listen" outside those ham bands, then appropriate bandpass filters will work.  DCI makes a good dual-band bandpass/diplexing filter that strongly rejects out-of-band signals and allows operation on both 2m and 70cm using a single antenna.  I know it works well because I have one, model DCI-146-444-DX-DB.  Its data sheet is here: http://www.dci.ca/pdf/DCI-145-435-DB.pdf

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AK7V
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Posts: 267




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« Reply #2 on: October 02, 2013, 01:11:34 PM »

WB2WIK,

Thank you.  This is what I suspected.  That bandpass/diplexing filter looks like a good option.  Alternately, I might look for a ham rig that doesn't have such a wide RX range and hopefully has appropriate RF filters.

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W9RAY
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Posts: 48


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« Reply #3 on: October 03, 2013, 03:04:50 PM »

A few years back... I had similar intermod  problems from a nearby pager transmitter  on  144 and 146 and with my Icom 746 and a  2100..  a good DCI bandpass filter cleared it right up on 2 meters.. I see they have raised their prices if you can even get one anymore.. See if anyone in the local clubs can come over with a spectrum analyzer and see where the strongest offending frequencies are..
  I can plug my frequency counter into my 2meter antenna in and it tells me the frequency of the nearest fm radio station(not that far away)... lol.. that's lots of RF floating in the air...
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AK7V
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Posts: 267




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« Reply #4 on: October 07, 2013, 03:06:17 PM »

That's a good idea - I have a frequency counter I could plug into the antenna and see if anything pops up.
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N9LCD
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Posts: 293




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« Reply #5 on: October 09, 2013, 05:05:03 PM »

A frequency counter isn't a good idea.

Their sensitivity typically is in the  multiple millivolt range.  A receiver may have sensitivity in the microvolt range, 0.001 percent or less than what a counter may need.

A frequency counter doesn't have any selectivity.  Assuming ther'e a strong enough signal within its counting range, it may lock in and count that signal even though it's not the one you're looking for.

N9LCD
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