Tim, I agree that Mike's theory is very likely (since nobody makes a perfectly clean transmitter), but I will offer a different theory on how your transmitted 2 meter signal was received on the 70 cm band.
To begin with, your ham radio equipment will have far superior sensitivity (and usually a much lower noise floor due to tighter bandwidth) than the typical EMI receiver can attain without significant preamplification. The signal you detected would not pose any risk of failing a CISPR emission standard, especially since it's range was obviously very limited. I believe that your 2 meter signal, likely with an effective radiated power in excess of 50 to over 100 Watts (and at close range). This would by a very strong signal at the Italian car's broadcast receiver (hereafter Radio X). It could be strong enough to perturb Radio X's local oscillator (LO) or maybe (for more modern equipment) the phase lock loop (PLL) - that is synthesizing the LO signal. The LO must be coupled to the first mixer stage - with only one stage, usually a mediocre wideband preamp, providing some isolation from the antenna. (Say maybe 30 to 60 dB worth of isolation, depending on design, quality, age/condition, and proper adjustment of Radio X.) There is a good chance that your signal arrived at Radio X with sufficient strength to modulate/mix with the LO at the mixer stage. Now to 70 cm. I have personally observed that my Jeep's stock FM/AM/Cassette radio, in FM mode, places a CW harmonic in the 70 cm band that is present only at one broadcast frequency. I tune to another broadcast FM station - it is gone - I tune back, there it is again. This is a clearly a harmonic of the LO, strong enough to be notable on a hand-held radio with a poor antenna. So, if that LO harmonic is present, and your 2 meter signal is indeed strong enough to surpass the LO rejection, then you were accidentally forcing Radio X to act as a upconverter for 70 cm (and likely a couple other bands as well). But then, even a rusty tailpipe or bumper (metal) can act as a harmonic upconverter if the Italian car had the wrong two metals in contact (rectifying your 2 meter RF). I've seen AM radio stations in the 0.5 to 1.6 MHz band upconverted to the 2 meter band just by corrosion of the towers guywire cables; a very irksome problem when it happens on a remote mountaintop at 10,500 feet. Best Regards, Eric Lifsey - Callsign AC7K "Tim Haynes G-Net 701 3239 / 3455" <[email protected]> on 06/16/98 09:39:43 AM Please respond to "Tim Haynes G-Net 701 3239 / 3455" <[email protected]> To: emc-pstc <[email protected]> cc: (bcc: Eric Lifsey/AUS/NIC) Subject: Emissions or not? Hello everybody! Isn't experience a wonderful thing? Well, I have just experienced something that makes me wonder about the validity of EMC emission regulations... I am a radio ham and operate on VHF and UHF bands from the car. The other night I was transmitting on 144.8MHz while the UHF receiver was tuned to 433.325MHz (my local UHF repeater). Suddenly, I started hearing a weak signal on UHF and, as it got rapidly stronger, I realised that it was me! At first I thought that somebody was fooling around, but that thought went when I realised that every time I caught up with the green Italian sports machine the signal got stronger and when it pulled away, the signal got weaker. When I stopped transmitting, there was no signal on UHF - no repeater or spurious. I have two thought on this... 1 The Italian car mixed my 144.8 with something internal to it, and retransmitted the resulting product. 2 It has a strong emission that caused mixing in the UHF receiver. I tend to discount the third theory, that the VHF transmitted signal overloads the UHF radio, because I use a duplexer and can receive very weak UHF signals while transmitting VHF. So - any ideas on this matter - and should we start doing emission measurements during immunity tests so that we know what the real world performance of the products will be? Regards Tim [email protected]
