Hello Ward,
I think I need to rephrase my email. What I am trying to say is, a small
terminated antenna e.g. a Flag or a Waller Flag is noise limited by the
termination resistor. This is because the resistor has a 3dB noise figure.
The antenna loss or low radiation resistance doesn't matter so much as this
loss can be made up with an LNA. The issue is made worse because most
practical
designs have the matching transformer z that is the same as the termination
resistor. Now if we use amplifier with high input z of say 30k Ohms; using
chemandy.com Return loss and mismatch loss calculator
the resistor noise is miss-matched to the amp. by approx. 10dB.
Thus the antenna noise floor should be lower. However, I am assuming that
the LNA has negligible noise when terminated with the antenna. This could be
a design issue to get the noise figure down to less that 0.5dB!!!
The example I gave using a high z amp. with the K9AY suggests that there is
no loss in gain by using increasing the input z.
73
Andrew G8LUG
-----Original Message-----
From: Ward Silver
Sent: Monday, February 4, 2019 7:50 PM
To: antennaware@contesting.com
Subject: Re: [Antennaware] Antennaware Digest, Vol 117, Issue 1
A flux-coupled transformer with separate windings provides galvanic
isolation between the feed line shield's outer surface and the loop.
Coupling from the loop to the feed line can upset the pattern and couple
noise on the feed line into the antenna.
It is also a good idea to add a choke of several thousand ohms on the feed
line at the antenna to prevent common-mode current on the outside of the
feed line shield from getting into the cable and becoming differential-mode
signals. Same concern at the receiving end although most receivers are
well-shielded.
73, Ward N0AX
On Mon, Feb 4, 2019 at 11:00 AM <antennaware-request@contesting.com> wrote:
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Today's Topics:
1. Fw: source impedance of resistor terminated antennas (Andrew Ikin)
----------------------------------------------------------------------
Message: 1
Date: Sun, 3 Feb 2019 23:04:32 -0000
From: "Andrew Ikin" <andrew.ikin@btopenworld.com>
To: <antennaware@contesting.com>
Subject: [Antennaware] Fw: source impedance of resistor terminated
antennas
Message-ID: <2E4939776E6344ECB4242B584629630C@DESKTOPCPQEP29>
Content-Type: text/plain; charset="UTF-8"
From: Andrew Ikin
Sent: Sunday, February 3, 2019 10:38 PM
To: antennaware@contesting.com
Subject: source impedance of resistor terminated antennas
Is there any reason why most resistor terminated antennas like the K9AY
and the Flag use a matching transformer to the Rx feed line that has the
same impedance as the termination resistance. The reason for the question
is, such antennas have a ideal match to the 3dB noise generated by the
termination resistor and this resistor has no radiation resistance. Hence,
for antennas like a small size Flag, the performance could be limited by
the resistor noise.
However, if the antenna is connected to a high zin amp., then the resistor
noise should be reduced by the miss-match and if the amp. noise figure is
very low, then it may be possible improve the antenna performance?
Winding the clock back 20 years I noticed that the K9AY Rx matching
transformer could be replaced by a high zin amp. with a voltage gain of
11dB. The overall gain increase compared to using 9:1 z transformer was
20dB. This increase in gain is what would be expected (11dB amp. plus 9dB
by replacing the 9:1 transformer). Thus, the antenna gain compared to one
without the amp. had not been degraded by using high zin amp.. However,
then I didn?t consider the effect of resistor noise, but this wouldn?t be
an issue with the K9AY.
Is there something else I need to consider?
73
Andrew Ikin
G8LUG
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