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Poor reception while moving

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#1
I'm fairly new at this, so please bear with me..

I just mounted this Larsen antenna to roof of my car (2006 Pontiac Vibe) with a mag mount.

NMO150/450/800 - Larsen Antennas - HamCity / Ham Radio Equipment


I'm using a Pro-164, scanning mostly 400 and 800mhz channels and get an odd static while in motion. When parked or stationary at a light, it is crystal clear. With the rubber duck antenna there is no issue. The noise if I can describe it is like a quick burst of static every second to two seconds. I've tried revving the engine and that doesn't seem to have any effect, it seems to just be while the car is actually moving.

Any ideas or suggestions will be greatly appreciated.
 
Joined
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NJ USA (Republic of NJ)
#2
I'm fairly new at this, so please bear with me..

I just mounted this Larsen antenna to roof of my car (2006 Pontiac Vibe) with a mag mount.

NMO150/450/800 - Larsen Antennas - HamCity / Ham Radio Equipment


I'm using a Pro-164, scanning mostly 400 and 800mhz channels and get an odd static while in motion. When parked or stationary at a light, it is crystal clear. With the rubber duck antenna there is no issue. The noise if I can describe it is like a quick burst of static every second to two seconds. I've tried revving the engine and that doesn't seem to have any effect, it seems to just be while the car is actually moving.

Any ideas or suggestions will be greatly appreciated.
You have attached a very good antenna to a very sensitive receiver.

You may be experiencing front end overload - try your attenuator and see
if it helps reduce a possible overload problem.

or

Could be the vehicle computer or some other electronics putting out a high
noise floor. Do you have the scanner hooked into the vehicle dc or are you
running it off the scanner batteries?

Your gonna have to experiment around to see what may be causing this
problem.
 
Joined
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United Kingdom
#4
.... a multitude of possible causes here, but I'll share with you one possibility against the background of experience I have.

Some years back when involved in comm's setups for armoured personel vehicles I experienced a similar problem i.e. no interference when the vehicle was static, but as soon as it started to move interference above squelch level occured on a regular basis every 15-20secs or so.

The cause (after a lot of head scratching): a single electrolytic capacitor that pulsed on a regular basis to drive a fuel lift pump.

The solution: a simple can structure soldered over over the cap to act as an rf screen.

The question you need to address once the cause is id'd, is the periodic interference been introduced to your receiver through its DC power supply, or is it entering through the antenna? A good quality hifi type noise filter on a troublesome dc input will not remove interference getting into the receiver via the antenna. By the same token, good quality rf screening around any electrical component on the car is not going to stop interference which is introduced via the receiver 12vdc power supply.

In short the cause needs to be id'd first, but I guess no harm in adding a good quality dc bourne noise filter (avaliable from any shop/dealer that sells "top shelf" car audio gear - usually takes the form of a good sized dense grade ferrite doughnut or hollow square core through which or around which the dc power supply wires are wound several times ).

If that doesn't address the problem chances are the interference is getting in through the antenna - what happens if there is no antenna plugged into the receiver/scanner and it is running on batteries? - still getting interference?

Does your scanner/receiver have different bandwidth settings? - if the noise is rf, chances are the level will drop as the bandwidth setting is widened (receiver front-end sensitivity decreases as bandwidth is increased).

Good luck
 
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#5
This sounds like a classic example of Rayleigh fading, usually more noticable at higher frequencies. Usually manafists itself as a swishing sound at slow speeds and static at higher speeds, normally occous in areas where you have buildings or other reflective structures close by, better reception in open areas. It's not an antenna problem or a problem with your receiver, just how radio signals behave. Another name that perhaps better discribles the effect is Phase Cancellation, the same problem you experience while listening to a distant AM broadcast station at night when the signal fades in and out.
 
Joined
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#8
Any the above could be correct, or it could be wheel static. Try rolling down hill while listening - with the ignition off. (It's dangerous, so do it where you have a wide road and no traffic.) If you still get noise it's most likely either Rayleigh fading or wheel static.

If the fade is approximately every half wavelength (7.3" on 800, 13" on 450, 39" on 150) of motion, it's Rayleigh fading.
 

trap5858

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Feb 22, 2003
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Doylestown PA
#10
What about the possibility of a bad ground? It is a magnetic mount and possibly loses some connectivity while the vehicle is moving. I have the same antenna and mag mount on a Subaru Forester but do not experience any of the symptoms you are having. Just for laughs, try relocating the mount and see if the problem is duplicated. Another thought is maybe the shielding in the coax is damaged where it passes over a metal edge.
 
Joined
Dec 25, 2008
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New Zealand
#11
Rayleigh fading - commonly known as 'picket fencing' - caused by re-radiation of the signal from objects near to the moving receiver - you receive the signal from two or more paths - the path length changes as you drive along - the signals arrive in phase and get stonger or out of phase and cancel.

Wheel static - wheels and brake discs not grounded to the vehicle body through the wheel bearings. Rotating wheels cause a static build-up which makes the interference. Probably not in this case as the OP says "every two seconds".

I'd be inclined to run the power for the radio right back to the battery, both power and ground. I suspect that currents from all sorts of things through the vehicle chassis and power leads are getting through to your radio. Fuel pumps are the likely suspect - they run intemittantly with the excess fuel dumped back to the tank - they're not a 'on demand' type on more modern vehicles.
 

burman

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Premium Subscriber
Joined
Jul 24, 2009
Messages
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#12
I'm fairly new at this, so please bear with me..

I just mounted this Larsen antenna to roof of my car (2006 Pontiac Vibe) with a mag mount.

NMO150/450/800 - Larsen Antennas - HamCity / Ham Radio Equipment


I'm using a Pro-164, scanning mostly 400 and 800mhz channels and get an odd static while in motion. When parked or stationary at a light, it is crystal clear. With the rubber duck antenna there is no issue. The noise if I can describe it is like a quick burst of static every second to two seconds. I've tried revving the engine and that doesn't seem to have any effect, it seems to just be while the car is actually moving.

Any ideas or suggestions will be greatly appreciated.


I had the same problem and nothing I tried would work,so I decided to take the mount apart and found that the factory forgot to solder the center coax cable wire to the center terminal in the mount. Now that I fix what the factory forgot I do not have the problem any more.
 
Joined
Dec 26, 2004
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Tulsa
#14
Perhaps he is busy building a dual diversity receiver adaptor for his scanner. FYI some of the original AMPS cellular mobiles has dual diversity front ends but must customers didn't like the 2nd antenna.
 
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