[SMCARA] 64 Repeater

[Clarke, Tom AIR4.0P NATOPS]

Hi Guys and Gals,

Just a quick update on the repeating machine. Some of this is a basic
explanation aimed at our new users and may bore some of the old heads!
Dwayne, WD8OYG and I went to the site Thursday, the 8th and did a little
PMS (Preventative Maintenance System, not the humanoid affliction) on
K3HKI.

We have been experiencing some noise/grunge coming in on the squelch
tail.  Dwayne adjusted the squelch a bit and that improved the squelch
action.

FYI a squelch tail is the short time period after you release your mike
button and the time that the squelch circuit mutes the audio (and trips
the beboop indicating the "alligator" timer has reset). Technically what
is happening here is that the receiver has a COS, or carrier operated
signal that tells the controller that the input signal has dropped and
to mute the audio to the transmitter. Due to a phenomenon called
"hysteresis" the system waits a tick or two before closing and the noise
sneaks in during that window.  Some controllers have an audio delay that
delays the received audio 500 to 750 ms and the squelch mutes the audio
before the noise gets transmitted.  We do not have audio delay on our
repeater. The dropping of the PL (CTCSS) also can mute the audio, but we
are not discussing that at this point.

We also replaced the existing duplexer (the bit of magic that permits a
transmitter and receiver only 600 KHz apart to simultaneously share the
same antenna) with a different type.  Since the noise may be coming in
from another service (pagers, etc) we need to keep that noise from
getting to the receiver. The previous duplexer notches out the transmit
signal(146.64) to the receiver and puts a narrow notch on the transmit
side tuned to 146.04 to eliminate transmitter noise. Imagine a "W" with
the two valleys at the bottom occurring at 146.04 and 146.64. This type
of duplexer provides greater than 100 db of isolation, but does not
reject out of band signals like the pager or other vhf signals. The new
duplexer we are using features band pass band reject and provides low
pass to the receiver and high pass to the transmitter side. This type
has slightly less isolation (notch depth), but offers greater immunity
to signals outside the repeater frequency.  It also exhibits greater
loss (affecting range) through the cavities, but this is offset somewhat
by a low noise preamplifier in the receive chain. The system is
engineered to balance these losses/gains.

Our initial observations are that the range is good and the noise has
been reduced. The squelch action is smoother, and the annoying Brrrap!
during the squelch tail has been lessened. The squelch in the GE
receiver we use has a feature that causes the squelch tail to be shorter
on strong signals into the machine and a bit of a longer tail on weak
ones. It's that hysteresis guy again! 

We still will have holes in the coverage (i.e., the Great Mills gulch,
Northern county, and the hills and dales of Calvert), but you can't fool
mother nature!  Well, you can , but it involves remote voted receiver
sites...maybe someday! 

I overheard a conversation the other day where someone gave a signal
report that he had a station with "full bars" on the meter, but the
signal was noisy and hard to copy. Remember your radio is showing the
strength of the repeater (50 watts on a 150 foot antenna) and not the
strength of the input signal! Garbage in Garbage out applies!

It appears that some of our new hams are using their new HTs (thanks
Dwayne!) and it is great to hear them on along with our friends on the
other side of the Potomac/Ches Bay.  A couple of stations tried to check
in last night to the net and never made it.  I suspect they were not
within the HT footprint of the repeater. That rubber dummy load, err, I
mean Rubber Ducky isn't a very efficient antenna*. We need to encourage
them to walk around the house, upstairs too, and find the "sweet spot"
if possible.  An antenna party might be in order, as well.  We could
sling together some JPoles or SlimJims and help them put up an outside
antenna and get a good signal into the repeater.  Might be a good club
project some Saturday morning when we are not giving exams at the
museum.  It flies in the face of tradition to do antenna work in nice
weather, but we probably should not wait until winter to do it! 

Keep your eyes and ears on the repeater and let us know how it is
working or if any problems pop up.  Especially if they are cyclical.
Our repeater shares the site with a Navy AM repeater (yes there is such
a thing!  The first ham repeater in the US was AM up in CT) that
operates in the 200-300 Mhz range and is used to permit
aircraft/vehicles on the ground at Webster Field talk to the Baywatch
controllers at Pax River.  We have looked at the 64 repeater while the
AM repeater is on the air and there is no interaction at all.

Pass this on to any newbies, or better yet, get them on the list for the
SMCARA reflector.

73 de Tom/W4OKW

* Try this:  Put a 19 inch piece of wire onto a crocodile clip
(alligator clip will work, but the croc jaws open a bit more) and clip
that to the metal base of the HT antenna.  Some ducks have rubber
insulation that precludes this, so try a crimp lug big enough to fit
over the antenna connector and the wire.  Unscrew the duck, put the lug
on and reinstall the duck.#

# Clarke's First Rule of Antennas. "If it sounds stupid, but it works,
it ain't stupid!"