[Antennaware] Antennaware Digest, Vol 117, Issue 1

[Andrew Ikin]

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 at 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 at contesting.com> wrote:

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>    1. Fw: source impedance of resistor terminated antennas (Andrew Ikin)
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> Message: 1
> Date: Sun, 3 Feb 2019 23:04:32 -0000
> From: "Andrew Ikin" <andrew.ikin at btopenworld.com>
> To: <antennaware at contesting.com>
> Subject: [Antennaware] Fw: source impedance of resistor terminated
>         antennas
> Message-ID: <2E4939776E6344ECB4242B584629630C at DESKTOPCPQEP29>
> Content-Type: text/plain;       charset="UTF-8"
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>
>
> From: Andrew Ikin
> Sent: Sunday, February 3, 2019 10:38 PM
> To: antennaware at 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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