On Fri, 2006-03-31 at 22:56 -0600, Duane A Calvin wrote:
> Gerald - isn't that the purpose of attenuators? I'm not sure what
> benefit is to be gained by mismatching RX antenna impedance, although I
> agree it's not highly critical they be exactly matched.
Some receivers have no attenuator. Those that do have only one. Under
some condition that can be too much attenuation and under other
conditions not enough. And sometimes the attenuation is more than
attenuation. Like in the Corsair II where the attenuator switch actually
removes the low noise RF stage hooking the double balanced mixer
directly to the antenna (through bandpass filters) so the receiver gain
goes down and the NF rises. In some situations, that double whammy isn't
workable. In receivers like the S-line detuning the RF and mixers
sometimes was most effective, though tossing the ringing mechanical
filters was more effective for standing up to spherics.
>
> I was thinking Merle might have a point - I'm trying to set up the
> conditions where I saw the attenuation on a different band when
> accidentally working cross-band, but I'm not able to replicate that
> result at the moment. However, on 30m, where my G5RV Jr is definitely
> non-resonant, 4.5 - 5:1 SWR, as a result of using the tuner, band noise
> goes from S 3.5 to S5 (relative S-meter reading.) If the tuner is not in
> the RX antenna path, how would this be explained (single VFO, simplex
> operation)? Selecting a different band on the TX VFO causes the tuner to
> turn off, but the S-meter still reads S 3.5. I'm on an original Orion
> with 2.56 level firmware in case anyone else is also trying this. If I
> had an RF signal generator, I could do better measurements, but I'll have
> to live with noise level at the moment.
Surely the receivers use the transmitter low pass filters as a minimum
of RF selectivity. How fast can the filter selection relays change, fast
enough for cross band listening between transmissions. A memory tuner
might switch as fast, though when trying to receive on two bands at the
same time the single tuner might flip its wig or just switch out of the
circuit. It is conceivable that each receiver front end could have its
own bandpass filters. That could help my FT-857D which hears 5 MHz BC
stations around 10.1 MHz when the 5 MHz paths are strong and 10 MHz is
about closed. Switching in attenuation (internal or external) or IPO
cleans it up at the cost of the really weak signals left on 10 MHz of
evenings lately. I find harmonics and beats between stations. And
judging by reports, I'm not the only one hearing them.
>
> 73, Duane
>
>
> On Fri, 31 Mar 2006 21:55:10 -0600 "Dr. Gerald N. Johnson"
> <geraldj@storm.weather.net> writes:
> > On Fri, 2006-03-31 at 22:35 -0500, Carl Moreschi wrote:
> > > Of course what you said is true, but it is still better to have a
> > matched
> > > antenna on receive. There is always the possibility of very low
> > atmospheric
> > > noise on a given night and the extra signal just might make the
> > difference,
> > > especially on 10 meters. Why do people insist on using a 9 to 1
> > balun on
> > > receive only beverage antennas?
I heard an engineer from Collins talking at a CSVHF Conference saying
going better than 2 dB NF at 2m was a waste. He was more concerned with
signals that were reliable 99.9% and so would never try a path with the
chances for random quiet and signal heard only 2% of the time. I wanted
to stand up and point out that we hams weren't concerned about the 99.9%
reliability paths, those we considered local QRM, while we waited and
searched for the 2% reliability paths and didn't want receiver noise to
prevent their use so we strove for fractional dB NF at 2m and higher.
The signal from a beverage is really small, its an inefficient antenna
and the NF of receivers has generally been neglected on beverage bands
because its well known that atmospherics are far stronger than the
simple receiver NF when the antenna is a resonant dipole. Sometimes to
hear on the beverage requires a low noise preamp and good matching.
Fact is, "one size fits all" doesn't make the receiver that works best at
10m and at 160 when the extremes of antennas are used, and when the receiver
is used on 40 meters in Europe with the local in band BC stations.
> > >
> > > Carl Moreschi N4PY
> > > 121 Little Bell Drive
> > > Bell Mountain
> > > Hays, NC 28635
> >
> >
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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