1/4 wave has more noise than 5/8 wave

[WB4CS]

So, this one has me stumped. Maybe one of y'all can figure it out...

I have a drilled NMO mount on the roof of my car, 2012 Impala. Yesterday I purchased a new antenna, Diamond 72BNMO, to replace my Larsen NMO150. The new antenna is a 1/4 wave on 2-meters and the old antenna is a 5/8 wave.

On the new 1/4 wave when I scan across 2-meters my radio stops constantly on noise. I'm not sure if it's RFI or intermod - just random frequencies with about S3-S9 white noise. When I switch back to the 5/8 wave the entire band is quiet and my radio only stops on frequencies in use. I've even tried a different 1/4 wave, a Larsen NMO-Q, and that one picks up the same interference.

I may have to pull out my old MFJ 5/8 wave from the closet to do more testing because I'm stumped! Why would a 1/4 wave be picking up more noise/interference than a 5/8 wave?

Anyone have the answer, or a good theory?

 

[k3cfc]

Maybe your 1/4 wave is Resonant for the interference your getting. if you have something that works why fix it?

 

[WB4CS]

The interference is on about 6 different random frequencies on 2-meters. Sounds almost like when you get an HT too close to a computer, that kind of noise.

As for why switching antennas, I'm about to start parking inside a garage and needed to be able to clear the door. A 5/8 on the top of the car wouldn't fit Also, I got the 5/8 when I traveled a lot, but now my travel time has been cut drastically and so now all I need is an antenna that can get into the local repeaters.

 

[jhooten]

The noise is coming in from a higher angle that is in the radiation pattern of the 1/4 wave while being in the lower gain area of the 5/8 radiation pattern, perhaps.

 

[N8IAA]

Have you tried tightening up the squelch? Or, putting some attenuation on the affected frequencies?
Larry

 

[WB4CS]

Have you tried tightening up the squelch? Or, putting some attenuation on the affected frequencies?
Larry

To my knowledge I don't have a way to add attenuation. The squelch on my radio is set the same with both antennas. The radio is a Kenwood TM-281A which the squelch is menu driven. It's currently set at "2" which is enough to let signals above S1 through. I could turn it up a little, but generally try to keep it set low because the area I live in is pretty mountainous and there are quite a few dead spots. When I get back in the car I'll try raising the squelch up a notch and see what that does.

 

[k9rzz]

Maybe it's telling you that the 5/8 wave antenna is junk!

Is it "noise"? (hiss) or a birdie? (electronically generated signal?)

Probably coming from your car? Maybe you can nip it in the bud and get rid of the source. That would be the ultimate solution of course.

 

[dksac2]

If you have no luck, find an antenna that will bend over by pulling it up and folding it over. Pain in the rear, but anything is better than the noise.
I use a 5/8 wave duel bander, the Comet CA 4X4. It's an excellent antenna, great gain and has a very wide bandwidth. It's made for EMCOMM work so you can run other radios without to high of an SWR such as a GMRS radio and not have to adjust from the 2m/70CM. It folds over easily, but you do have to get out of your vehicle.
I've never run into this, but the possibility of it picking up more noise such as electrical noise from power poles because of the higher take off angle could be a cause.
The only other thing I can think of is if it's a newer vehicle, is perhaps it's interacting with the computer in your vehicle.
If you find a cure, let us know as it would be good info for all to have.
My Vehicle is a 90 Blazer 1500, so I don't have any problems other than having to get rid of electrical noise in the power wire, which was easy to do.

Good Luck, John

 

[WB4CS]

Well update to the issue....

It's a birdie from something, and I think it's related to the electronics in the car as it only picks up the birdies when the car is running. I have no idea why a 1/4 wave is picking it up and a 5/8 wave isn't, but it's there. I've also narrowed it down to just one frequency, 145.590. I thought it was on multiple frequencies, but it's just the one. I can squelch it out, but that requires turning the squelch too high for my preferences since I enjoy scanning the band looking for band openings. So, I've just learned to live with it for the time being.

As for the fold down antennas, no thanks lol. Folding up and down an antenna isn't something I'm prepared to do twice or more a day. If it comes down to it, I'll order the base spring for my Larsen 5/8 antenna and see how that works going in and out of the garage.

Thanks all for the suggestions and theories. Later on if I decide to revisit the issue I'll update this thread.

 

[n6dlh]

The noise is coming in from a higher angle that is in the radiation pattern of the 1/4 wave while being in the lower gain area of the 5/8 radiation pattern, perhaps.

This^

Take off angle is much lower on the 5/8 wave antenna. I suspect that you may be picking up clock frequencies on some of the CPU's in the vehicle on the 1/4 wave. Best thing to do is note the frequencies and run the car with a hand held on one of the offending frequencies. This will give you an idea if it is an external or internal problem.

Ok never mind just saw the update. I will look at the TSB's on the vehicle tomorrow if you like, if you give me the engine size, I can check if there are any issues with clock frequency interference on that model and year. There have been many TSB's sent on newer vehicle that are also used for public service. This is an a common problem, you might even want to run some testing on the 5/8 wave on the trunk lid. This may be an alternative.

Dave

 

[popnokick]

Do you have any 12V lighter socket chargers plugged in to charge cellphones or other devices? I had a "white noise" problem that would open up the squelch but just sound like the normal background hiss. Was mostly in the 30-50mHz range, but had some in milair and 2m range, too. Drove me crazy until I started unplugging accessories in the car. Oh... interesting side note: The noise only appeared if there was a cellphone or other device plugged in and actively charging. In fact, the lower the battery the worse the RFI. The charger alone plugged into the 12V lighter socket has no effect.

 

[WB4CS]

Ok never mind just saw the update. I will look at the TSB's on the vehicle tomorrow if you like, if you give me the engine size, I can check if there are any issues with clock frequency interference on that model and year. There have been many TSB's sent on newer vehicle that are also used for public service. This is an a common problem, you might even want to run some testing on the 5/8 wave on the trunk lid. This may be an alternative.

Dave

Yeah Dave that would be great, thanks! It's a 2012 Chevy Impala with 3.6L V6.

Prior to doing the permanent mount on the roof, I had the 5/8 on the trunk lid and did not have any issues. I may have (but can't remember) had a 1/4 wave on the trunk at one time too, and don't recall any issues either.

I had almost forgot about this post, since it seems to just be on one frequency I've gotten used to it lol. Luckily it's on a frequency that's not in use in my area, so the radio only stops on it if I scan the entire band.

Do you have any 12V lighter socket chargers plugged in to charge cellphones or other devices? I had a "white noise" problem that would open up the squelch but just sound like the normal background hiss. Was mostly in the 30-50mHz range, but had some in milair and 2m range, too. Drove me crazy until I started unplugging accessories in the car. Oh... interesting side note: The noise only appeared if there was a cellphone or other device plugged in and actively charging. In fact, the lower the battery the worse the RFI. The charger alone plugged into the 12V lighter socket has no effect.

No, nothing plugged into the lighter/charging ports. That almost sounds like a bad ground. Hope you got that fixed or looked at.

 

[popnokick]

Not a bad ground... just the 12V to USB converter (forgot to note that in my original reply). The circuitry used in the 12V to USB lighter socket converter devices.... most of them, not all.... generate the noise.

 

[n6dlh]

Yeah Dave that would be great, thanks! It's a 2012 Chevy Impala with 3.6L V6.

Prior to doing the permanent mount on the roof, I had the 5/8 on the trunk lid and did not have any issues. I may have (but can't remember) had a 1/4 wave on the trunk at one time too, and don't recall any issues either.

I had almost forgot about this post, since it seems to just be on one frequency I've gotten used to it lol. Luckily it's on a frequency that's not in use in my area, so the radio only stops on it if I scan the entire band.



No, nothing plugged into the lighter/charging ports. That almost sounds like a bad ground. Hope you got that fixed or looked at.

Ok, No direct TSB's that cover this. Now I have heard of problems like this on GM vehicles with roof mounted antenna's. Being in the center of the roof (Where it should be) puts it close to the On Star module and create problems. Many of the police SEO's eliminate the On Star module.

There is however A TSB about RFI on this vehicle when the Rear window defroster is on, and it is due to microscopic cracks in the heating element traces on the rear window.

Normally when a Fleet company runs into this issue they deal with the manufacture engineers once the offending module has been identifies. Normally it is a clock frequency of a processor that radiates the RFI, and normally a hardware or software change is made to fix it. Hams on the other hand are unlikely to get this level of treatment, and if it is in fact the onstar module, then your only course of action is to disable it, or move your ham antenna. Or you can find a way to allow the radio to skip this frequency.

Dave

 

[WB4CS]

Thanks for the info Dave!

The antenna is mounted on the roof directly above where the interior light is, which puts it almost center on the roof.

There is an on-star system in the car, but the module is located in the trunk. I've researched disabling/bypassing it, but that will cause other parts of the car to not function correctly. GM got smart by routing the AM/FM radio, remote keyless entry, and alarm system through on-star so it can't be disabled. Grr.

Next time I catch the noise I will see if I am running the rear defroster. Here lately I've been using it a bit due to high humidity and foggy mornings, but not sure if it's been on when I've encountered the noise.

Thanks again!