Idea for Jim Weir

[Don Tuite]

Hey Jim. How about an antenna-line pad (maybe 40 dB or so) that you
could switch in when listening to unicom? The local guys would still
break squelch, but the guys 100 miles away wouldn't. And even with
step-ons, I suspect the hetrodyne squeal would be reduced. Make it
selectable by a pushbutton on the intercom. For safety, maybe it
could automatically switch itself out when you changed frequency or
radios. (How? I dunno. Maybe sense a change in the L.O. freq?)

Don

[[email protected]]

On 8-Feb-2004, Don Tuite > wrote:

> Hey Jim. How about an antenna-line pad (maybe 40 dB or so) that you
> could switch in when listening to union? The local guys would still
> break squelch, but the guys 100 miles away wouldn't.


Well, I'm not Jim, but what the hey, it's a public forum.

Like you, I am often frustrated by congestion on unicom frequencies in and
around urban areas, BUT...

Unfortunately, the difference in received signal level from equal power
transmitters 10 miles away and 100 miles away is only 20 dB (assuming
line-of-sight). There is probably 10 dB variability in effective unicom
station transmit power owing to different antenna gains, cable losses, etc.
That leaves only a 10 dB "window" to separate "desired" signals from
"undesired" ones, and only about 4 dB if the undesired is a more realistic
50 miles away. An additional negative would be the two coax relays or other
type of RF switches required to switch the pad in and out. These would add
considerable cost and reduce reliability, but most significantly would hurt
receiver sensitivity (since there will be some signal loss in the switches.

It's an imaginative idea, though.

-Elliott Drucker

[Tom Pappano]

Don Tuite wrote:
> Hey Jim. How about an antenna-line pad (maybe 40 dB or so) that you
> could switch in when listening to unicom? The local guys would still
> break squelch, but the guys 100 miles away wouldn't. And even with
> step-ons, I suspect the hetrodyne squeal would be reduced. Make it
> selectable by a pushbutton on the intercom. For safety, maybe it
> could automatically switch itself out when you changed frequency or
> radios. (How? I dunno. Maybe sense a change in the L.O. freq?)
>
> Don

You can also just tighten the squelch. The heterodynes will still be
there though, even if you rigged up a pad.

Tom Pappano, PP-ASEL-IA

[Don Tuite]

On Tue, 10 Feb 2004 03:59:00 GMT, wrote:

>Well, I'm not Jim, but what the hey, it's a public forum.
>
>Like you, I am often frustrated by congestion on unicom frequencies in and
>around urban areas, BUT...
>
>Unfortunately, the difference in received signal level from equal power
>transmitters 10 miles away and 100 miles away is only 20 dB (assuming
>line-of-sight). There is probably 10 dB variability in effective unicom
>station transmit power owing to different antenna gains, cable losses, etc.
>That leaves only a 10 dB "window" to separate "desired" signals from
>"undesired" ones, and only about 4 dB if the undesired is a more realistic
>50 miles away.

It'd be worth an experiment if you could get the hardware.

>An additional negative would be the two coax relays or other
>type of RF switches required to switch the pad in and out. These would add
>considerable cost and reduce reliability, but most significantly would hurt
>receiver sensitivity (since there will be some signal loss in the switches.

I was hoping one could find old Transco "D" switches on the surplus
market. (I worked for Transco in the early '70s.) At VHF and UHF,
they had exceptionally low insertion loss and VSWR and exceptionally
high isolation in a small form factor. Generally, the conectors were
SMA.

Maybe that's a pipe dream.

Don