Re: Need some antenna advice (for my CM15A, of course)

[Dave Houston <ydobon@xxxxxxxxxxx>]

 >>If I understand correctly, your unit had a built-in signal strength
 >>meter. I submit that's really the only way to lend some objectivity
to >>claims of enhanced reception.  A lot of the posts I've read have
been >>lacking that real world quantifier:  is the signal really
stronger >>after an antenna change?  Since you've got an iron-clad way
to test >>for improvement, once again I have to defer to your knowledge.
  How >>much better is the eggbeater than the turnstile in practical
terms?  >>Is the BX24-AHT still available anywhere?

 >The RSSI measurement of the receiver linear output made with the
 >BX24-AHT wasn't really useful for making definitive comparisons
between >antennas because the RF receiver module had AGC which reduces
the gain >as the signal gets stronger. It was useful for fine tuning the
 >receiver. Lacking expensive lab style equipment, the most reliable
 >measurement is to compare range.

I forgot that one of the pages on my web site has some screenshots that
might be helpful in understanding this.

      http://www.mbx-usa.com/rf-noise.htm

The first picture shows both the linear and digital outputs of the RF
receiver with a relatively weak RF signal. I used one of the BX-24 ADC
inputs to measure the amplitude of the lead-in pulse.The RSSI
measurement was only output if the signal that followed was a valid X-10
signal. (The one shown was not.)

You can readily see the effects of the AGC circuits in the slope of the
pulse top. With stronger signals, the linear output looks more and more
like the digital output. Comparisons between strong signals is difficult
because there is no simple way to know where the threshold is nor what
the gain level is.

I did make comparisons between antennas by running tests where the
antennas were placed in the "same" location and I sent signals from
certain locations about the building, always using the same Palmpad and
using the "same" locations. The locations were far enough away from the
antenna to guarantee a moderate signal level. From these it seemed clear
that the eggbeater was better but there are too many uncontrolled
factors to supply a definitive percentage of "betterness". Minor
differences in the antenna and Palmpad orientations and things like air
temperature and humidity can affect signal strength so it was impossible
to make a definitive measurement.

Anyway, an MMIC similar to that used in the TenTec preamp retails for
just over $1 in small quantities. They are cascadable so it's both easy
and cheap to add enough gain that there are no weak signals at the RF
receiver. (You do need a receiver with good out-of-band rejection to
avoid overwhelming it but those are also cheap (under $5 for QTY 1).