--- On Tue, 16 Jun 1998 15:39:43 +0100 (BST) Tim Haynes G-Net 701 3239 / 3455 <[email protected]> wrote:
> 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? > [email protected] Tim: My first thought is that 433.325MHz is darn close to being the third harmonic of 144.8MHz, so I would assume that you are simply detecting your own third harmonic energy. The requirement for proximity of the other vehicle is interesting. Perhaps it is reflecting the 433MHz signal back toward you. And maybe simple reflection isn't all. Maybe some part of that speific vehicle's structure was resonant at 433MHz. You shouldn't assume too much about your transmitter's harmonic content; why don't you drag a spectrum analyzer outside and check it out yourself? A more interesting possibility might be the generation of harmonics via current through non-linear discontinuities on the other vehicle. Having had the priviledge of owning a small Italian sports car (during a previous lifetime), I am fully informed of the number of rusty joints and loosly bonded parts which those joyous vehicles contain. This is the legendary "Rusty Bolt" problem, which is a major concern on military systems like ships. Perhaps that green patina was actually corrosion? Ed -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 06/16/98 Time: 11:36:23 --------------------------
