Jim
I have used double shielded coax and haven't had too much problem with
leakage.
73
Brian
Jim Brown wrote:
I have found that the main problem in using a talkie
as the receiver for tuning the notch in a duplexer is
the possible leakage of RF between the signal source
directly into the talkie. Most talkies are not
shielded at all, and any leakage will cause you to
tune a combination of the signal through the duplexer
and the direct signal leaking into the talkie.
Years ago when tuning the old Prog Line tube type
radios I came up with a 'poor boy' signal source using
a hand held Bearcat scanner. It had a 10.8 meg IF and
by calculating the offset frequency a high level
synthesized signal source (the LO) was available.
Here again, signal leakage could be a problem and I
solved that by putting a BNC feedthrough in a metal
cake pan and making an aluminum cover to slide over
the cake pan once the scanner was set on frequency.
External 10 and 20 dB BNC pads were used to bring the
signal level down to a usable level for tuning a
radio. The signal from the scanner inside the cake
pan was coupled from the antenna connection to the
feedthrough.
I now have a knock-off of a Wavetek service monitor,
and it does great in all areas except for tuning
duplexers. Signal leakage direct from the service
monitor makes the notch tuning inaccurate. You can
test for this condition by putting your hand around
the verious cables in your test lash-up and watching
the signal on the talkie. If you see any variation,
you have a multipath signal at the talkie, and only
one of those paths is going through the duplexer.
I solved that problem by using an old Motorola analog
signal generator for my signal source. Using double
shielded cables from the Motorola to the 6 dB pads in
series with the input and output of the duplexer shows
no signal leakage as touching the cables has no effect
on the signal at the talkie. I do have a 50 ohm load
on the unused port of the duplexer when it is being
tuned.
The ultimate isolation I have seen is my old
scanner-in -the-cakepan lash up, but it does not have
enough output level to tune a notch accurately. With a
few clothes line pins around the two ends (to clamp
the ends of the aluminum cover) and a 50 ohm load on
the feedthrough, the internal signal cannot be heard
on a talkie with its antenna held around the outside
of the cake pan.
The Motorola sig gen has an attenuatior on a track
that slides in and out of the oscillator cavity (a
large round brass housing) which makes an arm
stretcher out of it, but it has very low exteral
leakage.
73 - Jim W5ZIT
--- skipp025 <[EMAIL PROTECTED] <mailto:skipp025%40yahoo.com>> wrote:
> Hi Cort,
>
> Just something to keep in mind...
> You can use a regular radio as a signal generator
> just as you
> can use another for your receiver/detector function.
>
>
> Duplexer adjustment is more easily done with some
> RF-Pads
> (attenuators) placed on each coax port. I use some
> 3dB pads
> fairly easy to purchase off Ebay cheap enough. There
> are
> even some very nice low power step and variable
> attenuators
> found (on Ebay) for more than fair prices.
>
> I've even built basic 1 and 3dB pads from common
> values found
> in ARRL and similar Handbooks. Only a few
> resistors, a proper
> size box/tube (with connectors) and the time to
> construct them
> right... There's also a step attenuator in some ARRL
> Handbooks
> and FAR Circuits probably still sells the pc-board
> kit to make
> them.
>
> So... one might assume your portable radio has a low
> power
> output setting, which should be still be more than
> enough
> generated signal to see with your receiver.
>
> The key to all of this is to at all costs avoid
> transmitting
> anything more than a very low (fractional) power
> signal toward
> the receiver. Using enough of the right size series
> placed
> RF-Pads should be the trick.
>
> One "days of old when knights were bold" trick was
> to solder a
> very low value fuse inside with a 3dB constructed
> RF-Pad. Easy
> enough to replace a 1/8 or 1/16th amp fuse versus
> the front
> end of an early Service Monitor Output Section. Many
> a smart
> radio service shop had external RF-Pad boxes (with
> internal
> pico fuses) in series with the coax from the Service
> Monitor
> Signal Generator Port.
>
> One other thought might be to find/buy/borrow a
> small amplifier
> for your Service Monitor, which would be something
> like a Mini
> Circuits wide-band box. Once again cheap enough if
> you search
> Ebay at the right time & place.
>
> Many things are possible
>
> chow for now...
> s.
>
> > Cort Buffington <[EMAIL PROTECTED]> wrote:
> > Thanks Skipp025!
> >
> > I have an old Lampkin service monitor and I've
> been tuning the
> > duplexer by using my handheld as a receiver with
> the Lampkin as the
> > generator. It just doesn't have enough oomph for
> proper tuning of the
> > reject portion of 2 cans in series.... So I'm
> working "somewhere" in
> > the notch... not good enough. I'll be looking for
> use of a proper
> > service monitor, and probably keep using the
> MSR2000 unit -- it fits
> > my installation better than the Wacom.
>
>
>
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