Antennas cancelling signals?

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vocoder

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I have 2 of the same type of discones at the same height, 2ft apart same axis, and 1 scanner per antenna. It seems at times they cancel each other out and cause rx troubles. Any thoughts on frequency cancelling or interfering with each other? Weakening signals? Scanner IF's conflicting?
 

ChrisABQ

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The antennas are "coupling" which means they are sharing the transmissions they receive. Think of a black hole eating a neighboring star, materials are streamed slightly to one another. This causes them to de-sense. I'm not sure what size they are ,but they need to be much further apart, at least 6 feet.
 

jonwienke

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If you only have 2 scanners, you don't really need 2 antennas, especially if they are identical. The loss from a splitter isn't enough to affect reception in most cases.
 

RFI-EMI-GUY

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I have 2 of the same type of discones at the same height, 2ft apart same axis, and 1 scanner per antenna. It seems at times they cancel each other out and cause rx troubles. Any thoughts on frequency cancelling or interfering with each other? Weakening signals? Scanner IF's conflicting?
Can you tell us how this problem manifests itself? What are the symptoms?

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vocoder

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The discones are 108mhz-1xxxmhz 18inch tallest element. They seem to corrupt data streams on p25 CC,s
Scanners cannot decode properly and noise when parked on CC. Some conventional frequencies are affected too. Sometimes rx signals seem to be muted out or turn into non-modulated signals. Kinda hard to put into words, reminds me of when my ham radios would get affected by a computer powered up or low watt tx were near the hams. Something seems to increase the scanners sensitivity values.
1 scanner / ant performs as intended with the other powered down.
 

RFI-EMI-GUY

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The discones are 108mhz-1xxxmhz 18inch tallest element. They seem to corrupt data streams on p25 CC,s
Scanners cannot decode properly and noise when parked on CC. Some conventional frequencies are affected too. Sometimes rx signals seem to be muted out or turn into non-modulated signals. Kinda hard to put into words, reminds me of when my ham radios would get affected by a computer powered up or low watt tx were near the hams. Something seems to increase the scanners sensitivity values.
1 scanner / ant performs as intended with the other powered down.
OK, do you have preamps on these antennas? If so, there could be an unintended oscillation. If no preamp, there could be local oscillator interference from one receiver to another. Why not remove one antenna and install a four port multicoupler to provide some isolation between receivers?

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majoco

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It might be a problem that occurs mainly when you have two receivers close by each other tuned to different frequencies. The tuned circuit at the antenna input of one receiver is a high impedance to ground at the tuned frequency, but a low impedance to everything else so the unwanted signal gets sucked to ground. If the receiver antennas are close enough, which it sounds like yours are, then perhaps the low impedance at the receiver will be transferred up its antenna and 'suck out' all the signal from the other antenna too, bearing in mind that discones are untuned.

The suggestion to get them further apart is a good one.
 

byndhlptom

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two ant/two radios

With a specific description as to the exact symptoms, we will most be speculating.....

I haven't heard the "sucks out the signal" theory before, although you can get IF interference with adjacent radios.

It could be that there is just a difference in signal level between the two antennas at different frequencies. That is why some systems use "Diversity receivers" to try and "cancel" signal fade....

A little more information as to symptoms would be helpful...

$.02
 

rivardj

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I concur with RFI-EMI-GUY. Sounds like local oscillator interference between the two radios when both are powered on due to the proximity of the antennas.

I would find a way to separate the antennas. Vertical stacking might be an option on the same mast and should provide fair isolation with proper spacing.
 

vocoder

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Thank you to all, i appreciate all the help and ideas on this matter, any others, please chime in!!

After i get things in working order, i would like to know what caused it so i dont run into anything like it again!
Climbing gets old real quick, lol

No preamps are used, but when i tried amps, More troubles arise.
Power down=turn off 1 scanner.
Antennas are electrically insulated from one another.

If i put stock antenna on 1 mobile scanner, leave the other connected to a discone, and scan with both mobile scanners, the trouble is not noted. The only thing is, i do not get the distance i could with an external vs stock.

Thanks for multicoupler suggestion.
 
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ecps92

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Actually it would be best to isolate/separate the Radios, not the Antennas [Jusy my experience] Altho, even being in the same residence has not always isolated the issue.
I concur with RFI-EMI-GUY. Sounds like local oscillator interference between the two radios when both are powered on due to the proximity of the antennas.

I would find a way to separate the antennas. Vertical stacking might be an option on the same mast and should provide fair isolation with proper spacing.
 

nanZor

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If the radios aren't intefering with each other when using the typical junk oem supplied whips, you may have a common-mode ground / coax issue.

Try using ferrite beads of the appropriate frequency range up near the feedpoint of both. Quick try use them at the shack end.

Try running one from a battery. A common ground could be introducing an interference path to the other, and this might help point that out. If so, perhaps run on from a different outlet (hoping it's not on the same circuit.)
 

jonwienke

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The tuned circuit at the antenna input of one receiver is a high impedance to ground at the tuned frequency, but a low impedance to everything else so the unwanted signal gets sucked to ground.
No. That might have been true of early radios that didn't even have crystals, but is not true of any modern PLL oscillator controlled receiver. Especially given that we are talking about wideband scanner receivers.
 

prcguy

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Even in an older receiver with tunable RF section, it should be a low impedance to ground at the antenna terminal at resonance and high impedance everywhere else. I think.
prcguy

No. That might have been true of early radios that didn't even have crystals, but is not true of any modern PLL oscillator controlled receiver. Especially given that we are talking about wideband scanner receivers.
 

jonwienke

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But not a wideband receiver with PLL oscillator control, like the scanners being discussed.
 

vocoder

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Yes
These are two 996p2's.
CC for tdma phase 2: 770.71875
CC for p25 phase 1: 854.9625 sys 1, 859.7125 sys2
Also, multi vhf hi and 400 mhz uhf being monitored
 

vocoder

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I swapped out a scanner with a psr600 and the 600 had even more noise on CC and could not decode p25 phase1. I swapped it out just for s$&?! and giggles, the other scanner RXing remained unchanged.
 
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