I am wanting to buy some receive preamps and am wondering if I'm looking at
it the correct way. Here are two examples from two different manufacturers,
both for the same band.

#1) .55dB NF, 16dB gain
#2) .9dB NF, 20dB gain

So, do you go for more gain or less NF? Since this is for weak signal
VHF/UHF work, I would say go for less NF but you can tell me what you think.

Also, in this example, one preamp handles more power then the other, but I
don't care since I'm only going to be running 100 watts max. Both preamps
have RF sensing.

"Rod Maupin" wrote in message
. ..
I am wanting to buy some receive preamps and am wondering if I'm looking at
it the correct way. Here are two examples from two different
manufacturers, both for the same band.

#1) .55dB NF, 16dB gain
#2) .9dB NF, 20dB gain

So, do you go for more gain or less NF? Since this is for weak signal
VHF/UHF work, I would say go for less NF but you can tell me what you
think.


Normally you go for the lowest NF. The NF is set by the first amplifier and
it should have enough gain to overide the NF of the next stage. With too
much gain you could overload the receiver, but unless you are in an are with
lots of transmitters it is doubtful you will with 20 db of gain.

The gain can be madeup anywhere in the system but the NF is set by the first
stage and anything after that will not help if the receiver is anything much
to start with.

Unless you are in a quiet location and are mounting the preamps at the
antenna I doubt that you will notice much differance in actual use.

Unless you are in a quiet location and are mounting the preamps at the
antenna I doubt that you will notice much differance in actual use.

Actually, I live in the country and am mounting the preamps close to the
antennas (mast mounting).

Generally speaking, the BEST NOISE figure (factor,
ect. is what you want (best sensitivity),
however, the Gain would be a consideration, IF
you had excessive coax loss. (in this case, in
excess of 16 dB, which would be quite doubtful.
The gain of a vhf (or any amp, for that matter),
only needs to exceed your coax (and your front end
I.E. Reciever's Noise figure, so as to allow the
preamps noise figure to take precidence (establish
your floor sensitivity) I hope I stated that
correctly. And, even then, as long as THAT noise
figure is below the cold sky noise (from other
sources), then the cold sky noise (about 1.8 dB
on 2 meters) is the limiting factor on your
sensitivity, before figureing the SELECTIVITY of
your reciever (half the bandwidth, gain 6 dB,
half again, now gain another 6dB). Probably
as clear as mud, but the best I can describe it--
Jim -NN7K

Rod Maupin wrote:
I am wanting to buy some receive preamps and am wondering if I'm looking at
it the correct way. Here are two examples from two different manufacturers,
both for the same band.

#1) .55dB NF, 16dB gain
#2) .9dB NF, 20dB gain

So, do you go for more gain or less NF? Since this is for weak signal
VHF/UHF work, I would say go for less NF but you can tell me what you think.

Jim - NN7K wrote:

Generally speaking, the BEST NOISE figure (factor,
ect. is what you want (best sensitivity),
however, the Gain would be a consideration, IF you had excessive coax
loss. (in this case, in
excess of 16 dB, which would be quite doubtful.
The gain of a vhf (or any amp, for that matter),
only needs to exceed your coax (and your front end
I.E. Reciever's Noise figure, so as to allow the
preamps noise figure to take precidence (establish
your floor sensitivity) I hope I stated that
correctly. And, even then, as long as THAT noise
figure is below the cold sky noise (from other
sources), then the cold sky noise (about 1.8 dB
on 2 meters) is the limiting factor on your sensitivity, before
figureing the SELECTIVITY of
your reciever (half the bandwidth, gain 6 dB,
half again, now gain another 6dB). Probably
as clear as mud, but the best I can describe it--
Jim -NN7K

Great response, Jim. Only one nit to pick, that Noise Figure and
incoming noise energy are not equivalent or even really related. Also,
a .1 dB change in NF does not imply a .1dB change in signal to noise or
received signal level. It is a non-intuitive relationship to say the least.

tom
K0TAR

On Sun, 17 Sep 2006 18:31:00 -0700, "Rod Maupin"
wrote:

I am wanting to buy some receive preamps and am wondering if I'm looking at
it the correct way. Here are two examples from two different manufacturers,
both for the same band.

#1) .55dB NF, 16dB gain
#2) .9dB NF, 20dB gain

So, do you go for more gain or less NF? Since this is for weak signal
VHF/UHF work, I would say go for less NF but you can tell me what you think.

Rod,

The context in which you intend to employ the preamp is needed to
accurately answer your question.

The industry uses a single metric for fully capture the receive
performance of the entire station, it is the Gain to Noise Temperature
ratio, written as G/T. S/N is directly related to G/T, for every one
dB improvement in G/T, you get exactly 1dB improvement in G/T.

Calculating G/T brings to book the NF of your "receiver", transmission
line etc losses, feed system losses, the gain and noise of your
preamp, and the external noise.

I have just explored options for improving my 144MHz station after
polite on-air advice suggested I needed better feedline and a preamp.
In fact, they are quite costly for little improvement, compared to
stacking another beam. See http://www.vk1od.net/gt/144.htm for the
write up and links to a description of the G/T method and calculation
tool.

A further aspect to keep in mind is that it is easy to build a cheap
preamp that exhibits very low NF and good gain in a shielded room. If
the preamp has poor IMD, it might actually degrade the system
performance depending on your own RF environment. Minimising noise
from IMD means better active devices and better front end selectivity,
both of which are high cost items.

So, for a start, read the page, follow the link to the second article,
download the spreadsheet and start populating the model with your own
scenario.

I would encourage a "try before you buy" evaluation of a preamp to see
what it does in your situation, the IMD noise cannot be simply implied
from published specs.

Owen
--

"Owen Duffy" wrote in message
...
On Sun, 17 Sep 2006 18:31:00 -0700, "Rod Maupin"
wrote:

I am wanting to buy some receive preamps and am wondering if I'm looking
at
it the correct way. Here are two examples from two different
manufacturers,
both for the same band.

#1) .55dB NF, 16dB gain
#2) .9dB NF, 20dB gain

So, do you go for more gain or less NF? Since this is for weak signal
VHF/UHF work, I would say go for less NF but you can tell me what you
think.

Rod,

The context in which you intend to employ the preamp is needed to
accurately answer your question.

The industry uses a single metric for fully capture the receive
performance of the entire station, it is the Gain to Noise Temperature
ratio, written as G/T. S/N is directly related to G/T, for every one
dB improvement in G/T, you get exactly 1dB improvement in G/T.

Calculating G/T brings to book the NF of your "receiver", transmission
line etc losses, feed system losses, the gain and noise of your
preamp, and the external noise.

I have just explored options for improving my 144MHz station after
polite on-air advice suggested I needed better feedline and a preamp.
In fact, they are quite costly for little improvement, compared to
stacking another beam. See http://www.vk1od.net/gt/144.htm for the
write up and links to a description of the G/T method and calculation
tool.

A further aspect to keep in mind is that it is easy to build a cheap
preamp that exhibits very low NF and good gain in a shielded room. If
the preamp has poor IMD, it might actually degrade the system
performance depending on your own RF environment. Minimising noise
from IMD means better active devices and better front end selectivity,
both of which are high cost items.

So, for a start, read the page, follow the link to the second article,
download the spreadsheet and start populating the model with your own
scenario.

I would encourage a "try before you buy" evaluation of a preamp to see
what it does in your situation, the IMD noise cannot be simply implied
from published specs.

Owen
--

Interesting post. I will look at that link you gave.

Thanks,

Rod KI7CQ

On Mon, 18 Sep 2006 03:27:46 GMT, Owen Duffy wrote:


The industry uses a single metric for fully capture the receive
performance of the entire station, it is the Gain to Noise Temperature
ratio, written as G/T. S/N is directly related to G/T, for every one
dB improvement in G/T, you get exactly 1dB improvement in G/T.

Prophetic! Should have read:

The industry uses a single metric to fully capture the receive
performance of the entire station, it is the Gain to Noise Temperature
ratio, written as G/T. S/N is directly related to G/T, for every one
dB improvement in G/T, you get exactly 1dB improvement in S/N.

As you know from HF, changing the NF of your receiver (by swithing in
the attenuator or turning the preamp off) often does not change the
S/N ratio of signals on the lower bands where ambient noise dominates
system performance. G/T gives you a method of predicting what S/N
improvment you will achieve with each of your proposed preamps (model
each at top or mast of at the tx, change out the coax for better stuff
etc)... and when you assess the cost of the improvement, you might
have different thoughts about what is "best" for you.

Owen
--