Thanks for the good explanation,I figured this was the case. Anyone have an
isolator to sell???
Unfortunately,only the 147.30 is un-coordinated. Now for the politics of the
hobby...
----- Original Message -----
From: Eric Lemmon <[EMAIL PROTECTED]>
To: <Repeater-Builder@yahoogroups.com>
Sent: Sunday, October 03, 2004 4:24 PM
Subject: Re: [Repeater-Builder] Intermod problem? Yes!
>
> Once I realized that you were stating the frequencies as input/output
> (I'm used to output/input; Mea Culpa!), the answer became obvious:
> Third-order intermodulation interference.
>
> If we consider that Repeater A transmits on 146.70 and receives on
> 146.10, and Repeater B transmits on 147.30 and receives on 147.90, we
> have a 2A-B (third order) product on 146.10. That is, 2 times 146.70
> minus 147.30 = 146.10 MHz, the RX frequency of Repeater A. It also
> works the other way, 2B-A, with the product falling on the RX frequency
> of Repeater B. Exactly the same combinations exist for the other two
> repeaters. What a mess!
>
> What's happening here is that the TX carrier from Repeater B is able to
> sail right through the TX cavities on Repeater A, because pass/notch
> duplexers (the most common type) have very little bandpass effect. The
> notch in Repeater A's TX cavities is tuned to notch out 146.10 MHz, so
> it has little effect on the incoming 147.30 carrier. The intruding
> 147.30 carrier mixes with the native 146.70 carrier in a 2A-B manner,
> and a new 146.10 MHz carrier is generated. Unfortunately, the notch
> depth of Repeater A's TX cavities is probably in the 85 to 100 dB range,
> not nearly enough to attenuate this spurious carrier. It simply crosses
> over to the receiver and swamps the desired RX carrier. The same
> process exists on the other three repeaters.
>
> The straightforward solution is to install a dual ferrite isolator on
> each repeater's transmitter. This will prevent intruding carriers from
> reaching the power amplifiers and causing intermod.
>
> As another poster mentioned, the coordinating body should not allow such
> IM-vulnerable pairings to occur within close proximity. I must assume
> that one of each conflicting repeater pair is uncoordinated.
>
> 73, Eric Lemmon WB6FLY
>
> Q wrote:
>
> Trying to troubleshoot an intermod??? problem between two repeaters. The
> 146.10/.70 repeater's receiver gets blasted by the 147.90/.30
> transmitter but only when they are both transmitting. Yes, the transit
> freqs are 600 kHz apart and they are only 5 miles apart. Would a
> circulator help this problem? Also have similar problem between the
> 146.07/.67 and the 147.87/.27 repeaters when they are both keyed up.
> Ideas?
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
