> My station has 4 Rx antennas. Each has a different signal
> output. Each has
> unique pattern characteristics that I can use to my
> benefit under various
> circumstances. Controlling the signal output of each
> antenna to make them
> comparable is important to maximizing S/N while minimizing
> the number of
> knobs and switches.
That's what I do here. I attenuate the TX antenna externally
so it matches the noise level (on a quiet night) of the
receive antennas, which are all amplified because of the
long feedlines or very small antennas.
>I'm thinking that I need to install a preamp on each
> antenna to bring the signal levels up to a point where the
> noise floor is S0
> on each--all while leaving the ATT setting on the MKV
> alone.
Personally I would run the MK V
http://www.w8ji.com/ft1000mk_v.htm
at full input with no IPO and no attenuation. I would do
this even if I had to patch an external pad into the RX line
when using the TX antenna.
I also would NOT do anything to the 70MHz IF system, any
gain you add there decreases close-spaced IM3 dynamic range.
I would run the receiver IF gain as high as needed and
reduce the TX IF gain to about 3 on all bands.
> may need 10dB. Another 20dB. A fixed output on the
> preamp is useful for
> gain, but must then be followed with a variable
> attenuation to make the gain
> precise and the antennas comparable.
That will hurt dynamic range. Two things you never want to
do with a preamp when the signal level is too strong are:
1.) Reduce transconductance in an FET (like lowering forward
bias on an FET gate). This significantly lowers INPUT
dynamic range of the preamp.
2.) Pad the output. This also lowers the input dynamic range
of the preamp.
160 is so noisy even in the quietest locations that noise
figure generally isn't an issue at all. So the best way as
an almost universal rule is to attenuate before the preamp
if you have to tweak gain.
160 and other lower HF bands are a very unique situation.
There are so many very strong signals that overload is a
real concern. The ideas (and dreams) that work so well on
EME do not apply to 160 unless you are on a remote island
somewhere without BC stations and other 160 ops. This isn't
VHF or EME.
Consider the preamp power. The preamp and receiver has to
handle hundreds of very strong signals (even more in
contests) and still hear noise floor signals without
generating IM. One of the big problems is the AM BC band.
All those strong signals sap available headroom (power) from
the amplifier (just like they would use the "bits" up in a
direct conversion receiver). Unless you do something with
incredible power handling and reasonably low noise floor,
you will need to reduce some of the unwanted signal levels.
You don't want to amplify and then attenuate UNLESS the
antenna is not setting the noise floor. You want to do the
gain reduction and any filtering before you amplify if at
all possible.
This is why my amplifiers, even with 3000 ft feedlines, are
in the house. The cable attenuation helps reduce signals,
and I can notch the BC band or pad the input of the preamps
if needed. http://www.w8ji.com/coaxial_cable_leakage.htm
Be very careful what you do with gain distribution or adding
adjustable gain to preamps! The last thing you want is to
reduce gain while making strong signal handling worse.
My bet is you can do it all with input attenuators, some
filters, and one strong IP3 preamp for the really low level
antennas. My preamps are all in the mid-40's or higher
output IP3. I then pad and filter the inputs as required. I
find I only need remove the AM BCB by about 10-20dB to
eliminate problems. Without doing that I can dimly light a
12v dial light on the preamp outputs just from AM BCB
stations!
73, Tom
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