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Receive interference on dual conversion receivers only?

loosecannon

Sr. Member
Mar 9, 2006
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Ever since putting my antenna higher up in the air a few months back, after having no antenna up for at least two months prior to that, i have had a solid S-4 noise floor 24 hours a day 7 days a week.
no buzzing or anything, just general static at S-4 all the time.
This was on two different receivers with different circuitry, both using a 455khz second IF frequency.
one of them is a SSB radio and i noticed that the noise level dropped off significantly on SSB, but just wrote it off to differences in AM and SSB modes.


So, i happen to be working on a radio that uses a single conversion AM receiver with a 7.8mhz IF, and was amazed to find that when i connected it to my antenna, i was only reading 1/2 an S unit of static.
this is what i was used to seeing on all my radios many months back with my old antenna set up.
I thought it might just be a weak receiver but local stations are coming in at the same strength as always.

The only thing i can figure is that the difference lies in the 455khz IF.

Has anyone else had any interference that seems to only apply to radios using a 455khz 2nd IF?

if so, or really anyone who wishes to hazard a guess, what do you guys think this interference could be from?

I doubt it's from an actual AM broadcast station, as no one is building any new ones these days, and i have lived at this location for almost 8 years with no issues like this.

thanks in advance for any advice and information on where i should start looking for this interference source.
LC
 

Ever since putting my antenna higher up in the air a few months back, after having no antenna up for at least two months prior to that, i have had a solid S-4 noise floor 24 hours a day 7 days a week.
no buzzing or anything, just general static at S-4 all the time.
This was on two different receivers with different circuitry, both using a 455khz second IF frequency.
one of them is a SSB radio and i noticed that the noise level dropped off significantly on SSB, but just wrote it off to differences in AM and SSB modes.


So, i happen to be working on a radio that uses a single conversion AM receiver with a 7.8mhz IF, and was amazed to find that when i connected it to my antenna, i was only reading 1/2 an S unit of static.
this is what i was used to seeing on all my radios many months back with my old antenna set up.
I thought it might just be a weak receiver but local stations are coming in at the same strength as always.

The only thing i can figure is that the difference lies in the 455khz IF.

Has anyone else had any interference that seems to only apply to radios using a 455khz 2nd IF?

if so, or really anyone who wishes to hazard a guess, what do you guys think this interference could be from?

I doubt it's from an actual AM broadcast station, as no one is building any new ones these days, and i have lived at this location for almost 8 years with no issues like this.

thanks in advance for any advice and information on where i should start looking for this interference source.
LC


Excuse my ignorance, but what type chassis radios use the 455khz second IF frequency?
 
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That's a new one on me. There is a lot of isolation between the antenna socket and the first 455 kHz circuit in the dual-conversion radios.

A wide scan of the radio spectrum with a spectrum analyzer connected to your antenna might reveal what's the deal.

Would take a buttload of signal level at 455 kHz to leak into the radio enough to hear.

73
 
This may be useless information, but I will put it out there. There maybe a UHF repeater out there with an open channel. 453.xxx MHz frequencies are used by a lot of public services such as utilities, transit companies, some fire, police, EMS depending on terrain. It’s used by the county EMA agency and is connected with a statewide repeater system here. I just remembered this because is would use the EMA channel on my old MT1000 to test it, because you could key the repeater for that channel.
 
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spill the beans LC what rigs are we talking about ?,
some modern radios are more prone to picking up noise & some older rigs are less prone,

having s4 of noise is nothing unusual over here, crappy switch mode supplies spew out garbage,
home hubs that send internet over your house mains wiring can also cause major problems with noise all across HF
both of those are the type of noise i have had luck reducing for people,

you already told us you did something that can in some situations increase noise level in raising it higher but not for the reason most people would think,

what antenna are you using, where is it & how is it installed ?
 
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thanks for all the replies.

i am a certified bone head, and should have checked my facts better before posting, so i apologize to all of you who i led in the wrong direction.

I still don't know the source of the interference, but i do know it has nothing to do with a 455khz 2nd IF.

how do i know that?
because one of the radios i was using as a test is actually a single conversion receiver.
DUH!

Here's the interesting thing, both radios that are experiencing the interference use a 10.695mhz first IF!

one of them is basically a base station version of a 29LTD and as such, uses a 10.695 first IF, and a 455khz 2nd IF. (uniden Zachary T with the 2816 chip)
the second radio is a upd2824 SSB chassis which uses a single IF at 10.695.
(president AX-144)

the radio im using now that does't have the interference is a upd858 chassis that uses a single 7.8mhz IF.


so at this point, i have to figure this interference has to do with the 10.695 mhz IF.

I only figure that because i can't explain it any other way, and this 858 radio definitely DOES NOT get the interference that the other two radios get.

im at a loss, and again, sorry for steering everyone in the wrong direction.
this is too weird.
LC


PS- Bob85, im using the CTE top one astroplane copy, on a fiberglass mast, choked at 9 feet below the hoop, with the tip sitting at about 38 feet.
 
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That's a tricky one LC,

astroplanes are probably the worst antenna out there for current on mast & coax,
which makes them worst for noise picked up on the mast & coax if its not installed optimally,

since you have no metal mast the coax is doing all the work,

things you could try,

drop a 1/2wave wire or coax down the inside of the fiberglass mast connected to top bracket to simulate some kind of 1/2wave mast & share current with the coax braid,

wrap turkey foil HomerBB style around the top 1/2wave of mast to create a 1/2wave conductive mast,

try another choke 1/4wave down from the first choke,

try some clamp on 43 or 61mix ferrites & slide them along while watching the noise level,

add a 1/4wave jumper to the coax to invert the common mode impedance of the braid.

for any of that to work the noise source will have to be closer to the coax than it is to the antenna.
 
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I can try adding more chokes below the ones i already have on there.

I was concerned about the coax as well because im running LMR-240, and wasnt sure if it would act right, but the SWR is very low, around 1.3 or so, and 2 to 1 span is way beyond the CB band.
I don't have an antenna analyzer with which to see what the reactance is like, but the SWR doesn't change in the wind, and it really does seem to be working quite well.

either way, it is just so strange to me how two different radios with a 10.695 IF have an S-4 noise level, and the one with a 7.8 IF is half an S unit.

not really sure what i expected to get in the way of advice when posting this thread, more just wanted to put this weirdness out into someone else's brain for a while because im tired of thinking about it LOL.

the coax does run down the roof slope and then along the eaves of the house that are about 8 feet off the ground, and i have a neighbor whose house is only 10 feet from there or so. could be something in their house IDK.

I think i need to do more experimenting.
LC
 
keep your fingers crossed that its coming in via the coax braid from a source close to it, that can be reduced or even eliminated.

can't help with the different rigs IF frequency other than some rigs are much worse than others for noise as if the front end tuned circuit before the front end amp is as wide as a barn door & others seem tighter less prone to noise.

some don't like been peaked for maximum signal on the front end but most are ok.
 
10.695?

It may not be your fault, your 7.8MHz being quiet - even the old Cobra 148 Grant XL and Dare I say - Washington's - use 10.7 but also 7.8.

You may have to look around to try another receiver but if the radios use a POSITIVE offset to pick up an image, it may even be the 455kHz one that is your noise source - for if the 455kHz image "rejection" being high or lower portion of the mixing result product you see it in two radios because they use the same down-mix conversion, but a third misses it completely. See if you can locate a receiver that is SSB, but is a dual conversion using a different image frequency.

Have you tried the local source noise testing method yet?

The old, Test Probe banana plug to SO-239 center - and see if the noise can be gleaned from local sources?

If you get some noise, then you may have leaky IF mess in the home - the antenna is just the better pickup sensor for local interfering noise.

If you don't get it inside, can you make the radio portable to locate other sources outside - drive around or a walk around sort of approach?
 
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Thanks for the reply Andy.

I do have a mobile direction finding setup in the works, and will be trying to triangulate the source at some point.

I'll probably be moving in the next six months or so, so whatever it is, i'll know at that point if i brought the problem with me or left it in my old neighborhood LOL.

thanks to all for replying.
LC
 

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