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Old March 17th 09, 06:16 PM posted to rec.radio.amateur.antenna
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Default Receiver antenna/amplifier matching question

I'm trying to figure something out here...

Assume that I have a 2m antenna that (through whatever means) happens to have
an impedance in the ballpark of 50ohms. Also assume that this antenna will
generate 1uV open-circuit.

Assume that I have a MMIC amplifier that has FET inputs and therefore has a
high input impedance... let's say it's 500ohms.

If I directly connect the antenna to the amplifier, the amplifier input
effectively sees ~0.909uV and then amplifies it.

If I build a matching network to make the amplifier "appear" as 50ohms, at the
input of the matcing network I'll have 0.5uV, but since energy has to be
conserved (my matching network is assumed to be lossless),
(0.5uV)^2/50=Vamp^2/500 -- Vamp=1.58uV. This is a gain of
20log(1.56/0.909)=4.8dB over a direct connection.

Is that all correct?

Now let's go down to HF. My antenna is still 50ohms, but at such low
frequencies parasitics aren't that bad and I just use a JFET or MOSFET and
have a 5kohm input impedance. In this case, without the matching network, the
amplifier sees 0.99uV. With the matching network, it sees 5uV (!), a gain of
14.1dB over a direction connection.

Yet many designs for HF/shortwave antenna amplifiers don't bother with a
matching network, just feeding the antenna directly into a FET. So why is
this? Is the extra gain just not needed on HF? Or too unwiedly to build when
you're trying to cover everything from 1MHz-30MHz?

Thanks for the help,
---Joel Koltner




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Old March 17th 09, 08:08 PM posted to rec.radio.amateur.antenna
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Default Receiver antenna/amplifier matching question

On Mar 17, 10:16*am, "Joel Koltner"
wrote:
I'm trying to figure something out here...

Assume that I have a 2m antenna that (through whatever means) happens to have
an impedance in the ballpark of 50ohms. *Also assume that this antenna will
generate 1uV open-circuit.

Assume that I have a MMIC amplifier that has FET inputs and therefore has a
high input impedance... let's say it's 500ohms.

If I directly connect the antenna to the amplifier, the amplifier input
effectively sees ~0.909uV and then amplifies it.

If I build a matching network to make the amplifier "appear" as 50ohms, at the
input of the matcing network I'll have 0.5uV, but since energy has to be
conserved (my matching network is assumed to be lossless),
(0.5uV)^2/50=Vamp^2/500 -- Vamp=1.58uV. *This is a gain of
20log(1.56/0.909)=4.8dB over a direct connection.

Is that all correct?

Now let's go down to HF. *My antenna is still 50ohms, but at such low
frequencies parasitics aren't that bad and I just use a JFET or MOSFET and
have a 5kohm input impedance. *In this case, without the matching network, the
amplifier sees 0.99uV. *With the matching network, it sees 5uV (!), a gain of
14.1dB over a direction connection.

Yet many designs for HF/shortwave antenna amplifiers don't bother with a
matching network, just feeding the antenna directly into a FET. *So why is
this? *Is the extra gain just not needed on HF? *Or too unwiedly to build when
you're trying to cover everything from 1MHz-30MHz?

Thanks for the help,
---Joel Koltner


You didn't mention noise figure, and that's important. At UHF and
above, atmospheric noise is typically very low, and you benefit from a
really low noise figure. In that case, you want to match the source
to the receiver input in such a way as to achieve the best noise
figure, which in general is NOT the same as getting maximum power
transfer into the receiver. Data sheets for parts intended as input
receiver amplifiers generally have a graph and/or table of optimal
source impedance for lowest noise figure versus frequency. Some will
have noise contours on a graph for some critical frequency or
frequencies.

At HF, atmospheric noise is generally very high, and you also have to
put up typically with some very large signals. So the design of the
receiver input is driven by different needs. You want to keep
distortion low, to avoid problems from those large signals, and you
want to achieve a "reasonable" noise figure -- but reasonable might
well be 15dB, an absolutely terrible noise figure for a 440MHz
receiver front end. And you might well put in a pad you can switch in
to lower distortion, and out for better noise figure.

Also, in some cases such as a car radio, the antenna impedance isn't
anything close to 50 ohms (for the AM broadcast band). The design
there would of course be different than if you were dealing with a 50
ohm source.

Cheers,
Tom
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Old March 17th 09, 09:05 PM posted to rec.radio.amateur.antenna
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Default Receiver antenna/amplifier matching question

Hi Tom,

That's a most informative post; thanks.

At work we have a design that uses these:
http://datasheets.maxim-ic.com/en/ds...71-MAX2373.pdf -- right after an
antenna and a matching network. Too bad that Maxim didn't choose to provide
any clues on what antenna impedance it would like to see for the lowest
nose -- I just matched it to 50ohms. (And truth be told, we have a slightly
wretched antenna to work with anyway... the main reason it's workable at all
is due to the antenna designer -- and outside firm -- sticking a 160ohm
resistor in parallel across the input leads.)

---Joel


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Old March 17th 09, 09:16 PM posted to rec.radio.amateur.antenna
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Default Receiver antenna/amplifier matching question

"Joel Koltner" wrote in
:

Yet many designs for HF/shortwave antenna amplifiers don't bother with
a matching network, just feeding the antenna directly into a FET. So
why is this? Is the extra gain just not needed on HF? Or too
unwiedly to build when you're trying to cover everything from
1MHz-30MHz?


Joel, you might hope that a good receiver provides some degree of band
pass filtering prior to the RF amplifier for IMD protection.

For best receiver weak signal performance at SHF, you would usually match
for optimum noise figure rather than best front end gain.

At HF, receiver internal noise is usually lower than ambient noise, so
receiver NF is less an issue, but... intermodulation performance is very
important. "Noise" can be created in the RF amp due to intermodulation
distortion, so designs often focus on optimisation of IMD rather than NF
directly, or front end gain. For example, I note that the TS2000 does not
use a high performance signal FET for the HF RF amp, but uses a couple of
power FETs.

The 144MHz case you mention is somewhere between the two, ambient noise
in the city is usually much higher than a state of the art LNA, and often
higher than a modern transceiver with a NF of 5dB. In my experience, IMD
noise is a bigger issue on 144MHz than NF measurements made in a shielded
room. A high gain amplifier with poor NF or high IMD noise delivers a
high volume low S/N output from the receiver. (Try an IC706IIG on a
discone antenna in the city.)

Owen
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Old March 17th 09, 09:45 PM posted to rec.radio.amateur.antenna
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Default Receiver antenna/amplifier matching question

Joel Koltner wrote:


Assume that I have a MMIC amplifier that has FET inputs and therefore has a
high input impedance... let's say it's 500ohms.

I don't consider 500 Ohms to be "high impedance". I was taught that
MMICs were 50 Ohms in and out.


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Old March 17th 09, 11:19 PM posted to rec.radio.amateur.antenna
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Default Receiver antenna/amplifier matching question

"dave" wrote in message
...
I don't consider 500 Ohms to be "high impedance".


Compared to 50 ohms it's high. :-) Granted, this is a bit subjective, but in
my mind a "high impedance RF probe" is often 500 ohms (a 450 ohm resistor +
the 50 ohm terminator in a scope"), and the implication is that it doesn't
significantly load down whatever it is that you're probing.

(Note that a "1Mohm in parallel with 10pF" "high impedance" probe ends up
being ~110ohm by the time you hit 144MHz...)

I was taught that MMICs were 50 Ohms in and out.


True, most are reasonably close in the bands they were designed for. I
probably shouldn't have said "MMIC amplifier" there -- just plain "amplifier."


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Old March 18th 09, 01:54 AM posted to rec.radio.amateur.antenna
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Default Receiver antenna/amplifier matching question


"Joel Koltner" wrote in message
...
I'm trying to figure something out here...

Assume that I have a 2m antenna that (through whatever means) happens to
have an impedance in the ballpark of 50ohms. Also assume that this
antenna will generate 1uV open-circuit.

Assume that I have a MMIC amplifier that has FET inputs and therefore has
a high input impedance... let's say it's 500ohms.

If I directly connect the antenna to the amplifier, the amplifier input
effectively sees ~0.909uV and then amplifies it.

If I build a matching network to make the amplifier "appear" as 50ohms, at
the input of the matcing network I'll have 0.5uV, but since energy has to
be conserved (my matching network is assumed to be lossless),
(0.5uV)^2/50=Vamp^2/500 -- Vamp=1.58uV. This is a gain of
20log(1.56/0.909)=4.8dB over a direct connection.

Is that all correct?

Now let's go down to HF. My antenna is still 50ohms, but at such low
frequencies parasitics aren't that bad and I just use a JFET or MOSFET and
have a 5kohm input impedance. In this case, without the matching network,
the amplifier sees 0.99uV. With the matching network, it sees 5uV (!), a
gain of 14.1dB over a direction connection.

Yet many designs for HF/shortwave antenna amplifiers don't bother with a
matching network, just feeding the antenna directly into a FET. So why is
this? Is the extra gain just not needed on HF? Or too unwiedly to build
when you're trying to cover everything from 1MHz-30MHz?

Thanks for the help,
---Joel Koltner


Receiving antenna matching is the same as a transmitter loss, except in
reverse. As you said if you start with a 50 ohm antenna and transmission
line and feed it into a 500 ohm receiver you will have a 10:1 swr and the
additional loss of the transmission line will come into effect. If it is
very short , it will not mater too much . If the line is long it will.
Above around 140 mhz the noise is low enough you need to use a low loss line
of a very short length. The noise figure is most important. Usually there
is a mismatch when tuned for lowest noise figure. Short line , no problem,
long or lossey line will cause much signal loss.

On the lower frequencies it usually does not mater too much what you do as
the noise comming in from the antenna will usually be the major factor.


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