Antenna's that won't catch amplifier interference

iceman977th · #1 (**permalink**) 11-01-2012, 1:54 AM

(I'm sorry if this isn't the correct section, feel free to move it if there is a better section)

I'm looking at purchasing a couple NMO mount antennas for my truck, one for my scanner and the other for a (coming soon) VHF P25 mobile radio. I was trying to get an antenna that will suit the needs of both, as they will be on either side of a CB antenna. However, I have a ~2000w stereo system in my truck (yes, I'm one of those "boom trucks") and the scanner likes to pick up interference from the amplifier when it's on. Is there an antenna that will reduce/reject as much of that interference as possible? Right now I have the antennas (which are crap RS antennas which may be the problem) running down the rear pillar's (extended cab Chevrolet pickup) and along the side (opposite of amp power wire) to the scanner. What would you guys recommend? Scanner will be a combination of VHF and UHF, and possibly some 800MHz. Would be nice to monitor AM air band as well without too much trouble...

ka3jjz · #2 (**permalink**) 11-01-2012, 4:29 AM

It's not just the antenna (although it's possible that it's picking up stray RF from the amp) - likely as not you're getting mutual coupling between the scanner and amp. The most likely culprits are the wiring, and I would also consider the possibility of coupling if the scanner and amp are too close together.

You'd be much better off separating the wiring as much as possible. You'd probably need to run some chokes or ferrite beads on both sets of cables. I would also move the antenna as far away from the amp wiring as possible. No antenna will 'reject' that kind of interference. The amp itself no doubt has a fairly hefty RF field around it at 2kw, and you are picking that up.

Not an easy solution here...Mike

iceman977th · #3 (**permalink**) 11-01-2012, 4:54 AM

Quote:

Originally Posted by ka3jjz

It's not just the antenna (although it's possible that it's picking up stray RF from the amp) - likely as not you're getting mutual coupling between the scanner and amp. The most likely culprits are the wiring, and I would also consider the possibility of coupling if the scanner and amp are too close together.

You'd be much better off separating the wiring as much as possible. You'd probably need to run some chokes or ferrite beads on both sets of cables. I would also move the antenna as far away from the amp wiring as possible. No antenna will 'reject' that kind of interference. The amp itself no doubt has a fairly hefty RF field around it at 2kw, and you are picking that up.

Not an easy solution here...Mike

Yeah, it would be a tough one. Most of the time if I'm listening to my scanner the radio is off anyways, mainly heading to the station. I plan on running the wiring across the roof and down the passenger side A pillar. Hopefully it wouldn't cross talk as much then. It seems to be worse across AM air bands & FM rail. Public Safety VHF and UHF are fine. But, every little bit will help. Just need some suggestions for a nice matched pair of NMO antennas, hopefully with the smaller hole possible.

benbenrf · #4 (**permalink**) 11-01-2012, 6:05 AM

Iceman

2Kw is a fair amount of audio output, are these Class D power amps? If so the problem almost certainly arises in the speaker cables. You probably understand that Class D MOSFET power supply rails are designed to switch on/off at high speed to reduce the thermal output associated with the large audio freq power output. By contrast, the power transistors in pure Class A power amps are almost always on – which explains why Class A amps run so hot compared to AB, B, C and D types ….. but that’s another story.

Anyhow, if these are Class D type PA’s this high peak power switching of the power rails emits loads of EMI, which is typically FM'd (frequency modulated) at roughly 10x’s (and higher) the max audio frequency output of the MOSFETs. These “switching freq” pulses leave the amplifier via speaker wire terminals, and use the speaker cable as an antenna to radiate.

So, how to solve the problem?
A low pass filter on each (–) wire of every speaker cable pair is the best solution. The filter should fulfill the following criteria: have a max pass frequency just below the power rail switching frequency (so that all frequencies above are attenuated), offer a good 30dB or more attenuation, and be placed as close as possible to the amplifier speaker terminals, and of course be able to handle the amp peak wattage output.

There are a few other ways, one way is adjusting the slew rate of the amp (i.e. the MOSFET rise & fall time) from the typical 3ns - 5ns that is common of Class D power transistors, to something like 20ns - 25ns. This has a pronounced impact on the EMI radiated. The human ear won't be able to hear any difference in the audio quality 99.9% of the time. The problem here of course is that it requires an intimate knowledge of power amps circuits and electronics, but it can be really effective.

Anyhow, that’s the story in respect of Class D type PA’s.
If your PA’s are A, or AB, or C type, things are different and you’ll have to come back and let us know what type they are. I chose to write-up notes for Class D only because you quoted 2Kw as the power output, and its not all that common that you find 2Kw of audio power been generated with Class A amps in a vehicle because of big battery drains and heat outputs.

jim202 · #5 (**permalink**) 11-01-2012, 6:37 AM

I guess it boils down to shutting off the amp and using the normal radio/stereo or shutting off the scanner.

The choice is yours.

n0nhp · #6 (**permalink**) 11-01-2012, 10:45 AM

Undoubtedly the amps are using switching power supplies to generate the voltages necessary to get that kind of audio power. Unless they are designed to be used in an RF environment, few are properly designed and filtered to suppress noise. Along with what has been said before, it sounds like a double pole double throw switch, audio on one side, RF on the other....

Good luck
Bruce

iceman977th · #7 (**permalink**) 11-06-2012, 12:42 AM

Yes, they are class D amplifiers with switching power supplies. I do plan on removing one shortly as I'm reconing the speakers to work a bit more efficiently with just one amp. But I do plan on replacing the antennas anyways, so could you guys make some suggestions on two NMO mount scanner antennas, one for scanning VHF/UHF/800 and another for a mobile VHF transmit/dual band receive radio? Hopefully two that at least look similar so it doesn't stick out too much...plus, out of curiosity, would there be a NMO mount CB whip yet that would similarly match?

n5ims · #8 (**permalink**) 11-06-2012, 1:46 AM

The Larsen tri-band 150-450-800 for the scanner. A Larsen NMO 2/70 VHF-Hi/UHF for the mobile (this is a ham version cut for those frequencies, if you need commercial frequencies you may need to substitue the similar commercial model - NMO150/450C). If you get the "short" version of the 2/70 (NMO2/70SH) they'll look quite similar in design and height.

For the CB you can get a Larsen NMO27. It won't really match the other two in looks or design, but no CB antenna really will.

The first 2 are on page 20 of this catalog (http://www.pulseelectronics.com/download/3785/asb_11) while the CB antenna is on page 13. All 3 use the same NMO type mount so the installation should be quite similar (just remember which mount goes to which radio!!!).

iceman977th · #9 (**permalink**) 11-06-2012, 3:19 AM

Quote:

Originally Posted by n5ims

The Larsen tri-band 150-450-800 for the scanner. A Larsen NMO 2/70 VHF-Hi/UHF for the mobile (this is a ham version cut for those frequencies, if you need commercial frequencies you may need to substitue the similar commercial model - NMO150/450C). If you get the "short" version of the 2/70 (NMO2/70SH) they'll look quite similar in design and height.

For the CB you can get a Larsen NMO27. It won't really match the other two in looks or design, but no CB antenna really will.

The first 2 are on page 20 of this catalog (http://www.pulseelectronics.com/download/3785/asb_11) while the CB antenna is on page 13. All 3 use the same NMO type mount so the installation should be quite similar (just remember which mount goes to which radio!!!).

I'll probably get the 150/450, as it will mainly be for VHF P25 fire, but I'll receive UHF as well (one of the dual band receive units)

iceman977th · #10 (**permalink**) 11-13-2012, 12:53 AM

Are the Larsen's on Scannermaster any good? $35 for an NMO mount VHF would work fine for what I need. Two of them, one for the scanner and the other for the radio eventually. They'll both be tied in to the scanner for the time being.

Rt169Radio · #11 (**permalink**) 11-13-2012, 8:08 AM

Quote:

Originally Posted by iceman977th

Are the Larsen's on Scannermaster any good? $35 for an NMO mount VHF would work fine for what I need. Two of them, one for the scanner and the other for the radio eventually. They'll both be tied in to the scanner for the time being.

Yes, those should be good. It is still the same brand (Larsen)