[NLRS] Preamp questions and contest results

[Dr. Gerald N. Johnson, electrical engineer]

Preamps are good and they are bad. As you noticed, the relay cost gets
significant. And if the relay isolation isn't good enough at the antenna
side you find preamps smoked too often. RF sensing is poor, hard on the
relays and lets short bursts of RF get to the preamp. Then a preamp is
generally not supplied with a narrow RF tuning (even if it has a narrow
filter after the gain stage) so that its more easily overloaded by out
of band signals, like pagers, APRS, packet, and in band repeaters. Some
2m preamps had maximum gain at about 200 MHz enhancing their overload by
high band TV signals.

To preserve the preamp its necessary to set up sequenced switching. That
way the PTT (or key closure) keys the antenna relay first, then after
enough delay for the relay to operate (20 or 30 milliseconds), then it
can enable the PA, then a little later it can key the exciter. Rigs that
supply a switched output like the FT-726 and FT-736 often supply RF at
the same time or slightly before the relay keying output so you can't
use their inverted sequencing. In the 726, the RF is diode and
transistor switched (nano second switching speed) while the external
keying output is a relay driven by a transistor switched line. So the
keying output is 10 milliseconds or so AFTER the RF. Which is deadly on
good preamps. Been there, too many times. Worked fine for SSB PTT, and
so long as I remembered to hit the PTT before I hit the key for CW, but
on FM it was good by preamp the first time.

You can save on relays by using only one large on at the antenna, then
run a separate receive only feedline to the exciter with transmit and
receive separated, or to a small relay between the exciter and the PA.

Because of selectivity issues, I'm looking at adding gain inside my
FT-736. The first mixer is improperly terminated into a crystal filter.
In my 220 module last week, I added an IC (MAV-11) between the mixer and
the crystal filter. I can't yet prove it helped on the TV intermod, but
it sure made the receiver sound better and I heard very well on 222 so
long as I wasn't pointed at the channel 13 tower. I suspect some of what
I hear is not my receiver but is channel 11 and channel 13 intermod at
the transmitters. They share the same antenna. I did have to use a
double bandpass cavity to calm the receiver before and after
modifications when looking south (towards the TV towers).

The FT-736 is deficient on gain before the IFs there's something like 30
dB of signal range between detecting a signal at maximum gain and the
threshold of AGC by my ears. That's way too much. Properly terminating
the mixer and adding 9 or 10 DB with the MAV-11, I claim is a great
improvement. I've not modified the 2m side so as I get the bench
arranged for intermod and blocking range tests, I'll be able to compare
the unmodified 2m to the modified 222 that otherwise have the same
circuits and components.

It may prove necessary to change the RF stage to something that can
handle more signal with a greater bias current. I'll look into that as I
get the measuring capabilities working. Perhaps a 300CF. Perhaps their
sort of discrete first mixer is junk.

Many rigs are deficient in handling strong signals. Adding a preamp
makes them worse by exactly the preamp gain. To try to retain strong
signal handling, its important to set the preamp gain (often by adding
attenuators between the preamp and the receiver) so that the system NF
is the best that can be obtained at the minimum preamp gain.

Lots of rovers, though many got out of my range and neglected to look
this direction. Many NLRS members around MSP neglected to look south and
probably missed me on three bands if not four.

My first counts look like: 6m 11 grids in 17 contacts. 2m 28 grids in 54
contacts, 22 17 grids in 29 contacts, 432 17 grids in 30 contacts. Maybe
184 contact points in 73 grids for a score of 13,432. 

Sure was quiet Sunday afternoon with N0DQS too far away to be heard.
Could have taken a nap all afternoon and not changed my score.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.