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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> 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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> End of Antennaware Digest, Vol 117, Issue 1
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