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Author Topic: AM/FM/XM Radio Overload  (Read 5806 times)
K3SGB
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« on: October 28, 2015, 11:53:46 AM »

Most new Hondas have a spiral wrapped antenna stalk on the rear of the roof area -- it's about 6-8" in length.  At first I thought this was only for XM Radio, but now I wonder if it also serves as a combo antenna for AM and FM as well.  So in thinking about setting up a mobile installation for such a car, whether it be VHF/UHF at the 50-70W level, or possibly HF running 100W, or both, I worry about how much power the car's default antenna will pick up.  And whatever the amount, whether the Honda's built in radio gear would be in jeopardy or front end burn out.  Front end overload might be acceptable if it's a totally recoverable situation, but I have no clue about how robust these factory radios are and how resistant they are to front end burnout.  Even though the factory antenna is small and has a small capture area, it will wind up being VERY close to those ham antennas.  Has anyone ever characterized the above, actually made measurements, etc.?

73 - John
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K0BG
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« Reply #1 on: October 28, 2015, 02:37:42 PM »

The AMFM antenna is in the back glass, or under a plastic cover on some models, like the Ridgeline. If you mount an antenna on a lip mount, next to the back glass, overload will always be a problem. This issue IS NOT unique to Hondas and Acuras.
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K3SGB
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« Reply #2 on: October 28, 2015, 03:25:02 PM »

Hi Alan-

Oh I have no doubt that what you say is true, but I am much more worried about front end burn-out than just overload.  Even if the mfgr tries to minimize this possibility with back-to-back diodes at the front end (and I am not at all certain that they do this), who knows how much power is too much, etc.  These modern car radios are WAY too expensive to gamble with!

Thanks for your thoughts.......

73 - John
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K0BG
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« Reply #3 on: October 29, 2015, 06:31:20 AM »

Well, one of my VHF/UHF antenna is less that 6 inches away from the AMFM antenna. I haven't had one issue, and it has been there for near 10 years!
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K3SGB
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« Reply #4 on: October 29, 2015, 08:25:33 AM »

That's good to hear!  If I may ask a bit more....  I notice you did not mention having XM, so how modern a car?  Also roughly how much RF power output do you run on VHF a/o UHF?  Tnx for your help!
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K0BG
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« Reply #5 on: October 29, 2015, 08:34:41 AM »

It is a 2006 Honda Ridgeline, and yes it does have XM radio. And no, it doesn't have issues either.

This subject is an age-old debate. I've been mobile, both high-power HF and VHF for nearly 45 years. I have NEVER had an issue where an AMFM radio, or any other failed due to fronted overload. The question remains. Could it happen. Yes, however, the chances are very rare and anecdotal for the most part.
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G8YMW
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« Reply #6 on: October 29, 2015, 05:22:05 PM »

Just a side question. What is XM? Is it similar to DAB (Digital Audio Broadcasting) we have in the UK?
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73 de Tony
Windows 10:  Making me profane since March 2017
K0BG
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« Reply #7 on: October 29, 2015, 06:35:55 PM »

Same animal, just a different name.

We used to have XM radio and Sirius as separate entities, but they are now combined. I'm not too sure, but I think your's and our's use the same spectrum space, but on different birds.
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G8YMW
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« Reply #8 on: October 30, 2015, 03:53:54 AM »

With you talking about "birds" I assume sattelite, if so not the same.
DAB is transmitted from local(ish) sites on VHF band 3 ( former TV band at the top end of VHF) 174-240 MHz
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73 de Tony
Windows 10:  Making me profane since March 2017
K3SGB
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« Reply #9 on: October 30, 2015, 05:00:33 AM »

It is a 2006 Honda Ridgeline, and yes it does have XM radio. And no, it doesn't have issues either.

This subject is an age-old debate. I've been mobile, both high-power HF and VHF for nearly 45 years. I have NEVER had an issue where an AMFM radio, or any other failed due to fronted overload. The question remains. Could it happen. Yes, however, the chances are very rare and anecdotal for the most part.
\\

I have no idea which front end would be more susceptible - AM, FM, or XM -- but it nice to hear that you've had no problems so far.  I assume you are running (and have run) relatively high VHF/UHF power?
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K3SGB
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« Reply #10 on: October 30, 2015, 05:14:39 AM »

It is a 2006 Honda Ridgeline, and yes it does have XM radio. And no, it doesn't have issues either.

This subject is an age-old debate. I've been mobile, both high-power HF and VHF for nearly 45 years. I have NEVER had an issue where an AMFM radio, or any other failed due to fronted overload. The question remains. Could it happen. Yes, however, the chances are very rare and anecdotal for the most part.

I did some calculations yesterday.  I assumed 60-70W at 146MHz and a 0dB gain 2m antenna with 3 ft separation to the auto antenna(s).  I had no idea about what gain to assign to the car antennas at this ham freq.  Assuming 0 dB gain as a worst (but not necessarily ridiculous) case, the calculation showed that car antenna intercepted 1-2 WATTS of power, that is, 30 dBm or more using this gain estimate.  That's at least 20dB above my threshold of comfort, but of course the straw-man here is the estimate of the effective gain at 146 MHz.  However, a typical 28-30" car antenna does represent a very decent capture area.  I wish I had some real technical data to deal with here -- maybe someone of this forum has characterized some of this.....?
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K0BG
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« Reply #11 on: October 30, 2015, 06:31:46 AM »

Well, sort of. The power out on 2 meters is a bit more than 250 watts, if the amplifier is turned on. I think the last time I used it, was talking to some guy simplex on my way to Dayton. The UHF is 35 watts, and no amplifier. Last time I used UHF was probably 5 years ago. The antenna is question is a Larsen NMO2/70.

The HF antenna is about 2 feet from the AMFM and SAT antenna, and the power out hovers around 550 watts PEP. You can talk on HF (any band), and listen to the radio (no matter the band) without interference.

The only time the AMFMXM ever has an issue is when the VHF amp is on. That causes a hiss in the background, but the radio doesn't stop working.

I should add that I have proper common mode chokes on ALL of the antennas.
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M0GVZ
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« Reply #12 on: October 30, 2015, 08:00:35 AM »

Hi Alan-

Oh I have no doubt that what you say is true, but I am much more worried about front end burn-out than just overload.  Even if the mfgr tries to minimize this possibility with back-to-back diodes at the front end (and I am not at all certain that they do this), who knows how much power is too much, etc.  These modern car radios are WAY too expensive to gamble with!

Thanks for your thoughts.......

73 - John

My VHF/UHF antenna sits a few inches away from the car's radio antenna and running 50W it has not caused a problem in 3 years of use. My HF antenna is a couple of feet away from the car's radio antenna and running 200W out of it has not caused a problem.

The manufacturers cannot use diodes on the RF input stages or the radio won't work.
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K3SGB
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« Reply #13 on: October 31, 2015, 07:51:39 AM »

Well, sort of. The power out on 2 meters is a bit more than 250 watts, if the amplifier is turned on. I think the last time I used it, was talking to some guy simplex on my way to Dayton. The UHF is 35 watts, and no amplifier. Last time I used UHF was probably 5 years ago. The antenna is question is a Larsen NMO2/70.

The HF antenna is about 2 feet from the AMFM and SAT antenna, and the power out hovers around 550 watts PEP. You can talk on HF (any band), and listen to the radio (no matter the band) without interference.

The only time the AMFMXM ever has an issue is when the VHF amp is on. That causes a hiss in the background, but the radio doesn't stop working.

I should add that I have proper common mode chokes on ALL of the antennas.

With those kinds of Po levels (in most case HIGHER than I had pondered), seeing no problems is great to hear.  So relative to the calculations I did, all I can figure is: 1) my guess about the gain I plugged in for of the AM/FM/XM antennas was WAY higher than real life, or 2) that there might be matching networks or actual filters in the car antenna lines which GREATLY knock down these out of band signals.  Were it not for one or both of these guesses, I can't see how the car radio would be happy with multiple watts at their RF input terminals.  On your common mode chokes...,  I can see adding ferrite cores around the ham antenna feed lines (typical current baluns, as some call them), but doing this on the car radio antennas seems like an entirely different matter.. Can you explain further and why you think this helps the situation?  I have my own thoughts -- I'd just like to hear your input on this.......  Thanks!

I sure wish I could easily get to the car antenna feed likes easily to do some actual measurements.

73 - John
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K3SGB
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« Reply #14 on: October 31, 2015, 07:58:13 AM »



My VHF/UHF antenna sits a few inches away from the car's radio antenna and running 50W it has not caused a problem in 3 years of use. My HF antenna is a couple of feet away from the car's radio antenna and running 200W out of it has not caused a problem.

The manufacturers cannot use diodes on the RF input stages or the radio won't work.
[/quote]

I don't get why the diodes are a no-no.  My recollection is that a 0 dBm signal at a 50 ohm impedance level works out to 223 mV rms (think that's right).  Okay, maybe the car is other than 50 ohms (probably the case), but at least 223 mV is starting point.  Two silicon diodes would touch this, even two Schottky's might not see that voltage level.  I am under the impression that many ham rigs with superb sensitivity use diodes at the front end (but maybe that's old info).  Can you explain a bit?  Thanks

73- john
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