I realized made an error describing the product of cross-modulation:
If you have only two signals present you will find cross-modulation products
that are related to the harmonics and mixing of the two signals.
Say you have two (about equally strong) signals, one at 20 MHz and one at
20.010MHz. You will find the harmonics 40 MHz and 40.020MHz, the sum of the two
signals at 40.010MHz and the difference 10 Khz. You will, further find mixing
products of these signals, most important the mixing products of the 10 kHz
products. You will, on your spectrum analyzer see signals at 19.990 MHz, 20.02
MHz etc signal that are caused the the rectifying action of the (in this case)
the il-conducting joints between the metal sheets.
Chances that the 10 kHz product will cause any problem is minimal as it will
require a rather large roof to be an efficient antenna for that frequency but
the other products may cause interference in adjacent stations. When you
experience this phenomena in amplifiers the biggest problem are the products
that are close to the signals you want to detect as they are too close to be
easily filtered out.
Sorry for my prior mistake and miscalculation.
Hans - N2JFS
-----Original Message-----
From: Grant Saviers <grants2@pacbell.net>
To: Tower Talk List <towertalk@contesting.com>
Sent: Fri, Aug 13, 2010 6:48 pm
Subject: Re: [TowerTalk] Fwd: Multi-Band Vertical Question
re: metal roof problems: If I look on a spectrum analyzer connected to
a separate antenna, what should I see? Where should I look re the
transmit frequency? Is cross modulation the problem or is it
intermodulation products? What kind of signals am I looking for?
Grant
KZ1W
> It's your (ham) neighbors who would hear it. :)
>
> 73, Jim K9YC
>
>
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